Optical Engineering 49(11) 111121 (November 2010)

Applications of midinfrared quantum cascade lasersto spectroscopy

Gus HancockGrant RitchieJean-Pierre van HeldenRichard WalkerThe University of OxfordDepartment of ChemistryThe Physical and Theoretical Chemistry LaboratorySouth Parks RoadOxford OX1 3QZ United KingdomE-mail grantritchiechemoxacuk

Damien WeidmannThe University of OxfordDepartment of ChemistryThe Physical and Theoretical Chemistry LaboratorySouth Parks RoadOxford OX1 3QZ United Kingdom

andSTFC Rutherford Appleton LaboratorySpace Science amp Technology DepartmentHarwell Science and Innovation CampusDidcot OX11 0QX United Kingdom

Abstract We review the use of both pulsed and continuous wavequantum cascade lasers in high-resolution spectroscopic studies ofgas phase species In particular the application of pulsed systems forprobing kinetic processes and the inherent rapid passage structure thataccompanies observations of low-pressure samples using these rapidlychirped devices are highlighted Broadband absorber spectroscopyand time-resolved concentration measurements of short-lived speciesrespectively exploiting the wide intrapulse tuning range and the pulsetemporal resolution are also mentioned For comparison we also presentrecent sub-Doppler Lamb-dip measurements on a low-pressure sampleof NO using a continuous wave external cavity quantum cascade lasersystem Using this methodology the stability and resolution of this sourceis quantified We find that the laser linewidth as measured via the Lamb-dip is ca 27 MHz as the laser is tuned at comparably slow rates butdecreases to 13 MHz as the laser scan rate is increased such that thetransition is observed at 30 kHz Using this source wavelength modulationspectroscopy of NO is presented Ccopy 2010 Society of Photo-Optical InstrumentationEngineers [DOI 10111713498770]

Subject terms quantum cascade laser rapid passage modulation spectroscopyLamb-dip midinfrared reaction kinetics

Paper 100328SSR received Apr 14 2010 revised manuscript received Jun 232010 accepted for publication Jun 24 2010 published online Nov 22 2010

1 IntroductionQuantum cascade lasers (QCLs) are rapidly becoming theradiation source of choice for practitioners working withinthe midinfrared (mid-IR) (4 to 14 μm) and at longer wave-lengths The devices whether pulsed or continuous offerhigh output power room temperature operation single-modecharacter and wide tunability making them ideal tools forhigh-resolution spectroscopy with a wide variety of applica-tions Recent work has seen the application of QCLs to breathanalysis atmospheric sensing plasma spectroscopy andreaction kinetics1ndash12 These devices are progressively takingover the mid-IR laser market and will continue to do so astheir reliability output power wall plug efficiency and tun-ability continue to improve and their emission wavelengthsdecrease toward the 3-μm region where many moleculeshave strong CmdashH NmdashH and OmdashH stretching vibrationsIn this paper we discuss some applications of these devicesfor measurements of interest to gas phase kinetics and non-linear spectroscopy which are enabled by the high chirp rateand high output power of mid-IR QCLs

2 QCLs Operated in the Pulsed Mode

21 Rapid Passage EffectThe majority of QCL-based studies that have been conductedhave employed pulsed sources in which Joule heating leadsto rapid changes in the optical length of the laser cavity andhence the laser frequency towards lower frequencies ie thelaser output is chirped The chirp rate is often of the order of10minus3 cmminus1 nsminus1 as can be seen from the spectrum shown

0091-32862010$2500 Ccopy 2010 SPIE

in Fig 1 and directly impacts on the spectral resolutionof spectrometers based on pulsed QCLs This limitation hasbeen described in the literature3 13 where the spectral reso-lution ν is given by

v =(

Cdv

dt

)12

(1)

where dνdt is the chirp rate of the QCL and C is a constantdepending on the characteristics of the acquisition systemThe chirp rate is not constant across the duration of theapplied current pulse and therefore neither is the spectralresolution

In addition to this instrumental linewidth which is oftenlarger than the contribution due to Doppler broadening theuse of chirped radiation to probe samples at reduced pressureleads to measured spectral lines that are not symmetric aswould be observed in conventional absorption spectroscopybut displaying both absorptive and emission components asdepicted in Figs 1 and 2 This is due to the rapid passage(RP) effect where the laser frequency is chirped through themolecular resonance on a timescale that is short compared tothe time between collisions as such the sample is polarizedand it is the interference between this induced polarizationand the chirped laser radiation field that results in the ob-served signal3 14ndash16 The RP effect however can be removedby increasing the overall pressure by the introduction of aninert buffer gas typically of the order of 100 Torr such thatthe collision rate is increased and the sample polarizationandor coherence is destroyed on a timescale shorter thanthat of the signal acquisition system12 An example of suchbehavior is shown in Fig 2

Optical Engineering November 2010Vol 49(11)111121-1

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

-50 0 50 100 150 200 2500

10

20

30

40

50

60

Wavenumber (cm )-1

Sign

al (m

V)

Time (ns)

12545 12540 1253012535

Fig 1 Raw intra pulse spectrum of 130 mTorr of methane in a 40-cmpath length showing P(8) and P(9) transitions in the fundamentalband of the triply degenerate ν4 bending vibration in the range 1253to 1254 cmminus1

A pulsed distributed feedback (DFB) QCL (Alpes Laser)housed and driven by a Q-MACS system from Neoplas Con-trol operating in the intrapulse mode (see later) was uti-lized A single current pulse of 190 ns length was appliedto the chip and current detuning occurred between 12533and 12542 cmminus1 during this period with a chirp rate vary-ing between 225 and 160 MHz nsminus1 producing an aver-age pulse power of ca 20 mW The radiation was directedthrough a 40-cm-long cell containing 130 mTorr of CH4and onto a thermoelectrically cooled mercury cadmium tel-luride detector (VIGO PVI-2TE-106) with a fast preampli-fier (Neoplas control) The resulting data were recorded on a 2Gsamples 350-MHz bandwidth digital oscilloscope (LeCroyWavesurfer 434) At low pressure the characteristic oscilla-tion between absorption and emission signals associated withrapid passage is readily apparent When the pressure andhence the collision rate is increased by the introduction of100 Torr of a buffer gas (Ar) however the interference effectsare washed out and the spectral line shape becomes a Voigtas one would expect for traditional direct absorption spec-

Fig 2 Spectrum of the P(8) transition in the fundamental ν4 bandof CH4 showing 130 mTorr of CH4 with and without the addition of100 torr buffering Ar gas The high pressure data is shown over-laid with a Voigt fit showing the return to lsquonormalrsquo behaviour at thispressure

troscopy Note that under these higher pressure conditionsboth the number density and the linewidth match the expectedvalues12 The rapid passage however does little to affect thesensitivity of the system and for the unbuffered data a sensi-tivity of 12times10minus5 cmminus1 Hzminus12 was achieved in 5000 aver-ages something that could likely be improved by an order ofmagnitude by pulse normalization17 18

The standard method for modeling RP signals is via theoptical Bloch equations in which both the real and imaginaryparts of the complex refractive index and the associated pop-ulation transfer are calculated as the rapidly swept radiationfield interacts with the (simplified) two-level system19 Cor-rect implementation of the optical Bloch equations requiresknowledge of both the population and polarization decayrates in gas phase studies it is common to make the approx-imation that these two rates are very similar and equal tothe rate of collisional relaxation However detailed measure-ments of RP effects and how they are affected by the presenceof different perturbers clearly indicate that this assumptionis too simplistic and that this form of coherent spectroscopycan provide intriguing insights into the roles of elastic andinelastic collisions and any velocity-dependent effects20

22 Broadband Absorber SpectroscopyPulsed QCLs are traditionally used in two modes of oper-ation the interpulse and the intrapulse The first of theseseeks to minimize the effects of the inherent frequency chirpby utilizing current pulses of the order of lt20 ns and oftenusing relatively high pressure samples The frequency tun-ing is then achieved by adding a subthreshold low-frequencycurrent ramp to the pulse train Conversely the intrapulsemethod embraces the frequency chirp and seeks to lengthenthe current pulse so that as wide a range of frequencies aspossible can be probed in a single pulse in this mode arange of ca 1 to 2 cmminus1 can be covered using a 02- to2-μs-long pulse The length of pulse is usually limited bythe gain andor temperature dissipation of the QCL chip andoften under the conditions of a longer pulse (gt500 ns) thefinal laser output power has diminished by 60 to 70 fromthat at the beginning of the pulse Also as the temperaturechanges so does the rate of heating and toward the end ofthe pulse the chirp rate can be as low as a few megahertz pernanosecond providing an increased spectral resolution

As mentioned the intrapulse method enables a relativelywide wave number range to be accessed and this can oftenprovide sufficient spectral range over which one can un-ambiguously identify relatively ldquolargerdquo molecules whose IRspectra exhibit broadband features and are not rotationallyresolved Figure 3 shows an example of such a case for themolecule isoprene C5H8 for excitation of its out of plane(CndashH) wagging vibration around 10 μm (Ref 21) The samecell and detection system were utilized as for the CH4 workbut now in combination with a prototype laser developed byJ Cockburnrsquos group at the University of Sheffield UnitedKingdom Current pulses of 500 ns were applied to the gainmedium at a rate of 5 kHz and light was emitted between9913 and 9925 cmminus1 during this time with a chirp rate inthe range 50 to 100 MHz nsminus1 Note that even in the caseof this rotationally unresolved spectrum the effects of RPcan be clearly seen with a number of asymmetric line shapessuperimposed on a continuous background and once againaddition of some buffer gas removes this effect

Optical Engineering November 2010Vol 49(11)111121-2

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

Fig 3 Typical intrapulse spectrum of isoprene obtained around10 μm Despite the lack of rotationally resolved features RP struc-tures are still evident in the low-pressure spectrum

23 Time-Resolved Measurements of RadicalConcentrations

One suitable application of these pulsed devices is in moni-toring time-dependent processes such as in studies of reactionkinetics and dynamics and of ignition and perturbations inplasma and combustion systems In such cases concentra-tions may change on timescales of the order of 01 to 10 μsWe have for example demonstrated the use of a pulsed QCLto study the kinetics of a photolysis event and the ensuingmolecular reaction

The 8-μm output from the source previously mentionedwas spatially overlapped with a counterpropagating 266-nmNdYAG laser (Spectra Physics LAB-130) through a 50-cm-long cell with CaF2 windows The simplistic and easy con-trollable nature of the acquisition process in intrapulse modeenables a complete spectrum to be measured on a timescaleconducive to monitoring both nascent and relaxing fragmentconcentrations and thus the concentration and energy dis-tribution of CF3 radicals produced from the photolysis ofCF3I could be determined11 The CF3 radical concentra-tion was recorded as a function of delay between photol-ysis and probe pulses and the CF3 radical concentrationresolved over several hundred microseconds and radical re-combination was observed An example of typical data ob-tained is shown in Fig 4 as the CF3 spectrum at 12534to 12540 cmminus1 evolves over time following the initial pho-tolysis pulse Once analyzed and matched to the reactionscheme the returned radical-radical recombination rate con-stant for the formation of C2F6 was found to 39times10minus12 cm3

moleculeminus1 sminus1 in excellent agreement with previous litera-ture values22ndash25

3 QCLs Operated in the cw Mode

31 Lamb-Dip SpectroscopyDevelopment of more efficient QCL structures has meant thathigh-power (gt100 mW) continuous wave (cw) single-modeoperation is now routinely achievable at room-temperatureTherefore the resolution of room-temperature QCL absorp-tion spectrometers are no longer dominated by the fast fre-quency chirp inherent to pulsed operation and as such high-resolution spectroscopy can now be undertaken with far less

0100

200300

400500

12534125351253612537125381253912540

0

002

004

006

008

Wavenumber (cm )mdash1

Time smicro

Abs

orba

nce

125341253612535

125371253912538

12540

008

006

004

002

000

0100

200300

400500

Fig 4 Evolution of the IR spectrum of CF3 between 12534 and12540 cmminus1 as a function of time after the 266-nm photolysis eventThe time-dependent absorption enables the CF3 radical recombina-tion rate constant to be determined

complex systems In addition the development of broad-band QCL structures to be used as the gain medium in ex-ternal cavity systems has led26 27 to increasingly wide tuningranges (gt15 of the central frequency) The cw externalcavity QCLs sources (EC-QCL) are commercially available(Daylight Solutions) and the spectroscopic studies presentedhere were obtained with a system reaching over 190 cmminus1with a central frequency of 535 μm Larger tuning rangeshave been reported in the literature28ndash30 and through the com-bination of multiple-cascade QCL structures EC sourceswith tuning ranges up to 430 cmminus1 have been demonstrated31

In addition to high-resolution studies32 on NO and DBraround 53 μm (Ref 33) the high output power of the EC-QCL source has been exploited to investigate nonlinear ef-fects

For example measuring the Lamb-dip width providesa good estimate of the free-running laser linewidth of theEC-QCL as well as its evolution in various frequency tun-ing schemes Figure 5(a) shows an example of the Lamb-dip spectra observed when the EC-QCL beam propagatesthrough a 70-cm-long cell containing a low pressure of NO isretroreflected with a mirror and directed onto a MCT detector(VIGO PVI-2TE-6) The two spectral lines shown probe both-doublets (e f ) of the v = 0 J = 135 state on the R(135)12transition around 19207 cmminus1 The laser frequency is sweptthrough the wave number range at a frequency of 40 Hz byapplying a voltage to the piezoelectric transducer controllingthe external cavity grating The (nearly) counterpropagat-ing beams are aligned such that feedback into the EC-QCLwhich will cause frequency and amplitude instabilities isminimized

The data plotted in Fig 5 were obtained by averaging80 individual scans As the laser frequency fluctuations dueto both mechanical stability of the cavity and low-frequencynoise in the current controller were too large to enable obser-vation of individual Lamb-dips using scope-based averagingthe 80 individual scans were postprocessed recentered andthen the averaging was performed The experiment was car-ried out under the following conditions a laser output powerof 160 mW as the ldquopumprdquo beam a beam waist of 06 mm aNO partial pressure of 37 mTorr and a residual air pressureof 38 mTorr

Optical Engineering November 2010Vol 49(11)111121-3

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

(a)

(b)

(c)

Fig 5 (a) Lamb-dip signals produced from 37 mTorr of NO in a70-cm-long cell and probing both doublets of v = 0 J= 135 within the 212 spin-orbit state on the R(135) transitionand (b) and (c) close-ups on the Lamb dips of both transitions andcorresponding global fits and residuals Experimental data pointsfitted model solid line and residual (lower panel) values

Following Demtroderrsquos34 formalism the saturation pa-rameter S (applicable to strong laser fields) is given by

S = 2Ipμ2τ 2

t

ε0ch2 (2)

where μ is the transition dipole moment Ip is the pumpintensity τ t is the transit time ε0 is the permittivity of freespace c is the speed of light and h is Planckrsquos constant Thelineshape α(ω) accounting for the Lamb-dip is then given asa function of the unsaturated Doppler line shape αD(ω) by

α(ω) = αD(ω)

B(ω)1 minus [2(ω minus ω0)A(ω) + B(ω)]212

(3)

with

A(ω) =radic

[(ω minus ω0)2 + 2]12

and

B(ω) =radic

[(ω minus ω0)2 + (1 + 2S)2]12

where (ω minus ω0) is the frequency detuning from line center is the homogeneous half width given by the root mean squaresum of the transit time broadening and the collisional broad-ening and was calculated to be 240 kHz Using this modelthe theoretical transmission spectra were generated and thenconvoluted with a Gaussian line shape to account for theeffective laser linewidth Since the homogenous broadeningis much smaller than the laser linewidth the experimentallyobserved Lamb-dip widths are a direct measurement of thelaser linewidth The Lamb-dip spectra observed on the doublet-resolved transitions appearing in Fig 5(a) were fit ina global routine as is seen in Fig 5(b) and 5(c) and widths of24 and 32 MHz were returned for the lower and higher wavenumber transitions respectively with an average transitiondipole moment of 23times10minus3 D Similarly Gaussian profilesfitted directly to the Lamb-dips returned widths of 23 plusmn 01and 31 plusmn 01 MHz respectively Both the transition dipolemoments and laser bandwidth-dominated linewidths are inreasonable agreement with those in the literature35ndash37 anddiscrepancies between values are likely due to nonlinearitiesin the scanning frequency

That the Lamb-dip is best modeled by a Gaussian profile isa result mirrored in the literature35 36 38 and is most likelydue to injection current fluctuations Investigation on theinfluence of the frequency scan rate on the observed laserlinewidth has confirmed this In this case the frequency tun-ing rate was increased by application of a sine wave mod-ulation to the injection current of the QCL and Lamb-dipsmeasured using the methodology presented earlier with thesubstitution of a faster detector (VIGO PVI-2TE-106) Eventhough the QCL bias tee and the acquisition electronics limitthe range of accessible scan rates to between 5 kHz and2 MHz a general linear decrease in laser linewidth wasobserved down to 13 MHz at 30 kHz when our detectorbandwidth became the limiting factor While the averagingprocess effectively lengthens the measurement time and thuswould allow more laser jitter to affect the results the mea-sured Lamb-dip widths for both single shot and averageddata gave essentially identical values Thus averaged datawhich has a lower uncertainty due to a higher SNR wasdetermined appropriate for use Extrapolating the measuredvalues for the linewidth obtained at kilohertz scan rates to

Optical Engineering November 2010Vol 49(11)111121-4

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

Fig 6 Second-harmonic WMS spectra of the Q(35) -doubletof NO within the 232 spin-orbit state at 187559 cmminus1 as a functionof applied modulation voltage Here 1 mV corresponds to a 02-MHzmodulation amplitude

dc gives a linewidth value of ca 25 MHz This is consistentwith our ldquoslowrdquo tuned measurements obtained through exter-nal cavity mechanical modulation and reflects the increase inmeasured linewidth when the transition was scanned over alonger time period Note also that in this case the accessiblescan range was far less than at the lower scan rates and themeasurements were frequency calibrated using the width ofone of the doublet transitions The low-frequency jittercancellation methodology already mentioned also enabledthe analysis of the variation in the measured separation ofthe Lamb-dips on this pair of transitions and we could de-duce the tuning stability of the system returning a deviationof about 3 from the expected value of 277 MHz as given39

in HITRAN

32 Wavelength-Modulation SpectroscopyAll of the sensitivity-enhancing techniques that havepreviously been employed for absorption studies usingtelecom diode lasers can be adapted to QCL spec-troscopy and the literature contains many examplessuch as noise-immune cavity-enhanced optical heterodynemolecular spectroscopy40 (NICE-OHMS) photoacousticspectroscopy41 and Faraday rotation spectroscopy42 Perhapsthe most straightforward methodology is wavelength modu-lation spectroscopy (WMS) which can be readily achievedby modulating either the external cavity or the injection cur-rent of the EC-QCL Figure 6 shows typical WMS signalsobtainable with a low pressure Doppler broadened sampleof NO and modulation of the laser injection current at afrequency of 15 kHz A 5-cm-long cell with CaF2 win-dows was utilized containing 140 mTorr of NO and thee and f components of the -doublet of Q(35) transition at187559 cmminus1 were monitored The greatest sensitivity wasachieved at a modulation amplitude of 140 MHz correspond-ing to a modulation index of 22 (defined as the ratio of themodulation amplitude to the half width half maximum of thetransition) and resulting in a sensitivity of 37times10minus5 cmminus1

Hzminus12 an enhancement of ca 23 times on the sensitivity ofour direct measurements of the same sample32 In this in-stance we conducted demodulation of the second harmonicalthough the signal amplitude decreases as the harmonic isincreased higher harmonics often lead to flatter baselinesmdash an effect that can be very beneficial especially when

employing multipass cells in which etalon fringing can bea limiting factor on both sensitivity and selectivity In thismanner WMS has been used to probe OCS within a 20-m White cell and a sensitivity of 13times10minus8 cmminus1 Hzminus12clearly employing a longer-path-length Herriot cell wouldincrease this sensitivity further43 Further to the results pre-sented here greater output power and smaller frequency andintensity instabilities in a newer version of this laser have ledto an order of magnitude improvement on our base WMSsensitivity

4 Summary and OutlookQCL spectroscopy is already becoming a well-establishedfield of research with many applications already demon-strated The number of applications will continue to growin the future as ever cheaper more reliable more widely tun-able and short-wavelength devices become available Evennow the production of high-fidelity short-wavelength de-vices operating around 3 μm is a rapidly developing fieldThe QCL and its sister device the intersubband cascade laserare thus well placed to be strong contenders for the marketcurrently employing experimentally nontrivial methods suchas difference frequency generation and optical parametricoscillators The combination of high power broadband tun-ability and room-temperature operation promises the exten-sion of optical cavity techniques to the mid-IR and alsofurther development in the field of trace species detectionin a number of environments such as atmospheric moni-toring plasma processing and breath analysis Similarlythe next few years will see QCLs further utilized for stud-ies of condensed phases and interfaces Other applicationsof such high-powered systems can be found in fundamen-tal studies of chemical dynamics where output powers arenow sufficiently high to contemplate using these devices foroptical pumping therefore the preparation of vibrationallyexcited molecules that are alignedoriented in the laboratoryframe44

AcknowledgmentsThe authors would like to thank J Cockburn for the con-tribution of the 10-μm QCL laser chip This work is con-ducted under the Engineering and Physical Sciences Council(EPSRC) program Grant No EPG00224X1 New Horizonsin Chemical and Photochemical Dynamics RJW would liketo thank the EPSRC for the award of a postgraduate stu-dentship

References1 Y A Bakhirkin A A Kosterev C Roller R F Curl and F K Tittel

ldquoMid-infrared quantum cascade laser based off-axis integrated cavityoutput spectroscopy for biogenic nitric oxide detectionrdquo Appl Opt43(11) 2257ndash2266 (2004)

2 Y A Bakhirkin A A Kosterev R F Curl F K Tittel D A YarekhaL Hvozdara M Giovannini and J Faist ldquoSub-ppbv nitric oxide con-centration measurements using cw thermoelectrically cooled quantumcascade laser-based integrated cavity output spectroscopyrdquo Appl PhysB 82(1) 149ndash154 (2005)

3 G Duxbury N Langford M T McCulloch and S Wright ldquoQuantumcascade semiconductor infrared and far-infrared lasers from trace gassensing to non-linear opticsrdquo Chem Soc Rev 34 921ndash934 (2005)

4 K Namjou C B Roller and G McMillen ldquoBreath-analysis using mid-infrared unable aser pectroscopyrdquo Proc IEEE Sensors 1ndash3 1337ndash1340(2007)

5 R Provencal M Gupta T G Owano D S Baer K N Ricci and J RPodolske ldquoCavity-enhanced quantum-cascade laser-based instrumentfor carbon monoxide measurementsrdquo Appl Opt 44(31) 6712ndash6717(2005)

Optical Engineering November 2010Vol 49(11)111121-5

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

Fig 3 Typical intrapulse spectrum of isoprene obtained around10 μm Despite the lack of rotationally resolved features RP struc-tures are still evident in the low-pressure spectrum

23 Time-Resolved Measurements of RadicalConcentrations

One suitable application of these pulsed devices is in moni-toring time-dependent processes such as in studies of reactionkinetics and dynamics and of ignition and perturbations inplasma and combustion systems In such cases concentra-tions may change on timescales of the order of 01 to 10 μsWe have for example demonstrated the use of a pulsed QCLto study the kinetics of a photolysis event and the ensuingmolecular reaction

The 8-μm output from the source previously mentionedwas spatially overlapped with a counterpropagating 266-nmNdYAG laser (Spectra Physics LAB-130) through a 50-cm-long cell with CaF2 windows The simplistic and easy con-trollable nature of the acquisition process in intrapulse modeenables a complete spectrum to be measured on a timescaleconducive to monitoring both nascent and relaxing fragmentconcentrations and thus the concentration and energy dis-tribution of CF3 radicals produced from the photolysis ofCF3I could be determined11 The CF3 radical concentra-tion was recorded as a function of delay between photol-ysis and probe pulses and the CF3 radical concentrationresolved over several hundred microseconds and radical re-combination was observed An example of typical data ob-tained is shown in Fig 4 as the CF3 spectrum at 12534to 12540 cmminus1 evolves over time following the initial pho-tolysis pulse Once analyzed and matched to the reactionscheme the returned radical-radical recombination rate con-stant for the formation of C2F6 was found to 39times10minus12 cm3

moleculeminus1 sminus1 in excellent agreement with previous litera-ture values22ndash25

3 QCLs Operated in the cw Mode

31 Lamb-Dip SpectroscopyDevelopment of more efficient QCL structures has meant thathigh-power (gt100 mW) continuous wave (cw) single-modeoperation is now routinely achievable at room-temperatureTherefore the resolution of room-temperature QCL absorp-tion spectrometers are no longer dominated by the fast fre-quency chirp inherent to pulsed operation and as such high-resolution spectroscopy can now be undertaken with far less

0100

200300

400500

12534125351253612537125381253912540

0

002

004

006

008

Wavenumber (cm )mdash1

Time smicro

Abs

orba

nce

125341253612535

125371253912538

12540

008

006

004

002

000

0100

200300

400500

Fig 4 Evolution of the IR spectrum of CF3 between 12534 and12540 cmminus1 as a function of time after the 266-nm photolysis eventThe time-dependent absorption enables the CF3 radical recombina-tion rate constant to be determined

complex systems In addition the development of broad-band QCL structures to be used as the gain medium in ex-ternal cavity systems has led26 27 to increasingly wide tuningranges (gt15 of the central frequency) The cw externalcavity QCLs sources (EC-QCL) are commercially available(Daylight Solutions) and the spectroscopic studies presentedhere were obtained with a system reaching over 190 cmminus1with a central frequency of 535 μm Larger tuning rangeshave been reported in the literature28ndash30 and through the com-bination of multiple-cascade QCL structures EC sourceswith tuning ranges up to 430 cmminus1 have been demonstrated31

In addition to high-resolution studies32 on NO and DBraround 53 μm (Ref 33) the high output power of the EC-QCL source has been exploited to investigate nonlinear ef-fects

For example measuring the Lamb-dip width providesa good estimate of the free-running laser linewidth of theEC-QCL as well as its evolution in various frequency tun-ing schemes Figure 5(a) shows an example of the Lamb-dip spectra observed when the EC-QCL beam propagatesthrough a 70-cm-long cell containing a low pressure of NO isretroreflected with a mirror and directed onto a MCT detector(VIGO PVI-2TE-6) The two spectral lines shown probe both-doublets (e f ) of the v = 0 J = 135 state on the R(135)12transition around 19207 cmminus1 The laser frequency is sweptthrough the wave number range at a frequency of 40 Hz byapplying a voltage to the piezoelectric transducer controllingthe external cavity grating The (nearly) counterpropagat-ing beams are aligned such that feedback into the EC-QCLwhich will cause frequency and amplitude instabilities isminimized

The data plotted in Fig 5 were obtained by averaging80 individual scans As the laser frequency fluctuations dueto both mechanical stability of the cavity and low-frequencynoise in the current controller were too large to enable obser-vation of individual Lamb-dips using scope-based averagingthe 80 individual scans were postprocessed recentered andthen the averaging was performed The experiment was car-ried out under the following conditions a laser output powerof 160 mW as the ldquopumprdquo beam a beam waist of 06 mm aNO partial pressure of 37 mTorr and a residual air pressureof 38 mTorr

Optical Engineering November 2010Vol 49(11)111121-3

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

(a)

(b)

(c)

Fig 5 (a) Lamb-dip signals produced from 37 mTorr of NO in a70-cm-long cell and probing both doublets of v = 0 J= 135 within the 212 spin-orbit state on the R(135) transitionand (b) and (c) close-ups on the Lamb dips of both transitions andcorresponding global fits and residuals Experimental data pointsfitted model solid line and residual (lower panel) values

Following Demtroderrsquos34 formalism the saturation pa-rameter S (applicable to strong laser fields) is given by

S = 2Ipμ2τ 2

t

ε0ch2 (2)

where μ is the transition dipole moment Ip is the pumpintensity τ t is the transit time ε0 is the permittivity of freespace c is the speed of light and h is Planckrsquos constant Thelineshape α(ω) accounting for the Lamb-dip is then given asa function of the unsaturated Doppler line shape αD(ω) by

α(ω) = αD(ω)

B(ω)1 minus [2(ω minus ω0)A(ω) + B(ω)]212

(3)

with

A(ω) =radic

[(ω minus ω0)2 + 2]12

and

B(ω) =radic

[(ω minus ω0)2 + (1 + 2S)2]12

where (ω minus ω0) is the frequency detuning from line center is the homogeneous half width given by the root mean squaresum of the transit time broadening and the collisional broad-ening and was calculated to be 240 kHz Using this modelthe theoretical transmission spectra were generated and thenconvoluted with a Gaussian line shape to account for theeffective laser linewidth Since the homogenous broadeningis much smaller than the laser linewidth the experimentallyobserved Lamb-dip widths are a direct measurement of thelaser linewidth The Lamb-dip spectra observed on the doublet-resolved transitions appearing in Fig 5(a) were fit ina global routine as is seen in Fig 5(b) and 5(c) and widths of24 and 32 MHz were returned for the lower and higher wavenumber transitions respectively with an average transitiondipole moment of 23times10minus3 D Similarly Gaussian profilesfitted directly to the Lamb-dips returned widths of 23 plusmn 01and 31 plusmn 01 MHz respectively Both the transition dipolemoments and laser bandwidth-dominated linewidths are inreasonable agreement with those in the literature35ndash37 anddiscrepancies between values are likely due to nonlinearitiesin the scanning frequency

That the Lamb-dip is best modeled by a Gaussian profile isa result mirrored in the literature35 36 38 and is most likelydue to injection current fluctuations Investigation on theinfluence of the frequency scan rate on the observed laserlinewidth has confirmed this In this case the frequency tun-ing rate was increased by application of a sine wave mod-ulation to the injection current of the QCL and Lamb-dipsmeasured using the methodology presented earlier with thesubstitution of a faster detector (VIGO PVI-2TE-106) Eventhough the QCL bias tee and the acquisition electronics limitthe range of accessible scan rates to between 5 kHz and2 MHz a general linear decrease in laser linewidth wasobserved down to 13 MHz at 30 kHz when our detectorbandwidth became the limiting factor While the averagingprocess effectively lengthens the measurement time and thuswould allow more laser jitter to affect the results the mea-sured Lamb-dip widths for both single shot and averageddata gave essentially identical values Thus averaged datawhich has a lower uncertainty due to a higher SNR wasdetermined appropriate for use Extrapolating the measuredvalues for the linewidth obtained at kilohertz scan rates to

Optical Engineering November 2010Vol 49(11)111121-4

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

Fig 6 Second-harmonic WMS spectra of the Q(35) -doubletof NO within the 232 spin-orbit state at 187559 cmminus1 as a functionof applied modulation voltage Here 1 mV corresponds to a 02-MHzmodulation amplitude

dc gives a linewidth value of ca 25 MHz This is consistentwith our ldquoslowrdquo tuned measurements obtained through exter-nal cavity mechanical modulation and reflects the increase inmeasured linewidth when the transition was scanned over alonger time period Note also that in this case the accessiblescan range was far less than at the lower scan rates and themeasurements were frequency calibrated using the width ofone of the doublet transitions The low-frequency jittercancellation methodology already mentioned also enabledthe analysis of the variation in the measured separation ofthe Lamb-dips on this pair of transitions and we could de-duce the tuning stability of the system returning a deviationof about 3 from the expected value of 277 MHz as given39

in HITRAN

32 Wavelength-Modulation SpectroscopyAll of the sensitivity-enhancing techniques that havepreviously been employed for absorption studies usingtelecom diode lasers can be adapted to QCL spec-troscopy and the literature contains many examplessuch as noise-immune cavity-enhanced optical heterodynemolecular spectroscopy40 (NICE-OHMS) photoacousticspectroscopy41 and Faraday rotation spectroscopy42 Perhapsthe most straightforward methodology is wavelength modu-lation spectroscopy (WMS) which can be readily achievedby modulating either the external cavity or the injection cur-rent of the EC-QCL Figure 6 shows typical WMS signalsobtainable with a low pressure Doppler broadened sampleof NO and modulation of the laser injection current at afrequency of 15 kHz A 5-cm-long cell with CaF2 win-dows was utilized containing 140 mTorr of NO and thee and f components of the -doublet of Q(35) transition at187559 cmminus1 were monitored The greatest sensitivity wasachieved at a modulation amplitude of 140 MHz correspond-ing to a modulation index of 22 (defined as the ratio of themodulation amplitude to the half width half maximum of thetransition) and resulting in a sensitivity of 37times10minus5 cmminus1

Hzminus12 an enhancement of ca 23 times on the sensitivity ofour direct measurements of the same sample32 In this in-stance we conducted demodulation of the second harmonicalthough the signal amplitude decreases as the harmonic isincreased higher harmonics often lead to flatter baselinesmdash an effect that can be very beneficial especially when

employing multipass cells in which etalon fringing can bea limiting factor on both sensitivity and selectivity In thismanner WMS has been used to probe OCS within a 20-m White cell and a sensitivity of 13times10minus8 cmminus1 Hzminus12clearly employing a longer-path-length Herriot cell wouldincrease this sensitivity further43 Further to the results pre-sented here greater output power and smaller frequency andintensity instabilities in a newer version of this laser have ledto an order of magnitude improvement on our base WMSsensitivity

4 Summary and OutlookQCL spectroscopy is already becoming a well-establishedfield of research with many applications already demon-strated The number of applications will continue to growin the future as ever cheaper more reliable more widely tun-able and short-wavelength devices become available Evennow the production of high-fidelity short-wavelength de-vices operating around 3 μm is a rapidly developing fieldThe QCL and its sister device the intersubband cascade laserare thus well placed to be strong contenders for the marketcurrently employing experimentally nontrivial methods suchas difference frequency generation and optical parametricoscillators The combination of high power broadband tun-ability and room-temperature operation promises the exten-sion of optical cavity techniques to the mid-IR and alsofurther development in the field of trace species detectionin a number of environments such as atmospheric moni-toring plasma processing and breath analysis Similarlythe next few years will see QCLs further utilized for stud-ies of condensed phases and interfaces Other applicationsof such high-powered systems can be found in fundamen-tal studies of chemical dynamics where output powers arenow sufficiently high to contemplate using these devices foroptical pumping therefore the preparation of vibrationallyexcited molecules that are alignedoriented in the laboratoryframe44

AcknowledgmentsThe authors would like to thank J Cockburn for the con-tribution of the 10-μm QCL laser chip This work is con-ducted under the Engineering and Physical Sciences Council(EPSRC) program Grant No EPG00224X1 New Horizonsin Chemical and Photochemical Dynamics RJW would liketo thank the EPSRC for the award of a postgraduate stu-dentship

References1 Y A Bakhirkin A A Kosterev C Roller R F Curl and F K Tittel

ldquoMid-infrared quantum cascade laser based off-axis integrated cavityoutput spectroscopy for biogenic nitric oxide detectionrdquo Appl Opt43(11) 2257ndash2266 (2004)

2 Y A Bakhirkin A A Kosterev R F Curl F K Tittel D A YarekhaL Hvozdara M Giovannini and J Faist ldquoSub-ppbv nitric oxide con-centration measurements using cw thermoelectrically cooled quantumcascade laser-based integrated cavity output spectroscopyrdquo Appl PhysB 82(1) 149ndash154 (2005)

3 G Duxbury N Langford M T McCulloch and S Wright ldquoQuantumcascade semiconductor infrared and far-infrared lasers from trace gassensing to non-linear opticsrdquo Chem Soc Rev 34 921ndash934 (2005)

4 K Namjou C B Roller and G McMillen ldquoBreath-analysis using mid-infrared unable aser pectroscopyrdquo Proc IEEE Sensors 1ndash3 1337ndash1340(2007)

5 R Provencal M Gupta T G Owano D S Baer K N Ricci and J RPodolske ldquoCavity-enhanced quantum-cascade laser-based instrumentfor carbon monoxide measurementsrdquo Appl Opt 44(31) 6712ndash6717(2005)

Optical Engineering November 2010Vol 49(11)111121-5

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

(a)

(b)

(c)

Fig 5 (a) Lamb-dip signals produced from 37 mTorr of NO in a70-cm-long cell and probing both doublets of v = 0 J= 135 within the 212 spin-orbit state on the R(135) transitionand (b) and (c) close-ups on the Lamb dips of both transitions andcorresponding global fits and residuals Experimental data pointsfitted model solid line and residual (lower panel) values

Following Demtroderrsquos34 formalism the saturation pa-rameter S (applicable to strong laser fields) is given by

S = 2Ipμ2τ 2

t

ε0ch2 (2)

where μ is the transition dipole moment Ip is the pumpintensity τ t is the transit time ε0 is the permittivity of freespace c is the speed of light and h is Planckrsquos constant Thelineshape α(ω) accounting for the Lamb-dip is then given asa function of the unsaturated Doppler line shape αD(ω) by

α(ω) = αD(ω)

B(ω)1 minus [2(ω minus ω0)A(ω) + B(ω)]212

(3)

with

A(ω) =radic

[(ω minus ω0)2 + 2]12

and

B(ω) =radic

[(ω minus ω0)2 + (1 + 2S)2]12

where (ω minus ω0) is the frequency detuning from line center is the homogeneous half width given by the root mean squaresum of the transit time broadening and the collisional broad-ening and was calculated to be 240 kHz Using this modelthe theoretical transmission spectra were generated and thenconvoluted with a Gaussian line shape to account for theeffective laser linewidth Since the homogenous broadeningis much smaller than the laser linewidth the experimentallyobserved Lamb-dip widths are a direct measurement of thelaser linewidth The Lamb-dip spectra observed on the doublet-resolved transitions appearing in Fig 5(a) were fit ina global routine as is seen in Fig 5(b) and 5(c) and widths of24 and 32 MHz were returned for the lower and higher wavenumber transitions respectively with an average transitiondipole moment of 23times10minus3 D Similarly Gaussian profilesfitted directly to the Lamb-dips returned widths of 23 plusmn 01and 31 plusmn 01 MHz respectively Both the transition dipolemoments and laser bandwidth-dominated linewidths are inreasonable agreement with those in the literature35ndash37 anddiscrepancies between values are likely due to nonlinearitiesin the scanning frequency

That the Lamb-dip is best modeled by a Gaussian profile isa result mirrored in the literature35 36 38 and is most likelydue to injection current fluctuations Investigation on theinfluence of the frequency scan rate on the observed laserlinewidth has confirmed this In this case the frequency tun-ing rate was increased by application of a sine wave mod-ulation to the injection current of the QCL and Lamb-dipsmeasured using the methodology presented earlier with thesubstitution of a faster detector (VIGO PVI-2TE-106) Eventhough the QCL bias tee and the acquisition electronics limitthe range of accessible scan rates to between 5 kHz and2 MHz a general linear decrease in laser linewidth wasobserved down to 13 MHz at 30 kHz when our detectorbandwidth became the limiting factor While the averagingprocess effectively lengthens the measurement time and thuswould allow more laser jitter to affect the results the mea-sured Lamb-dip widths for both single shot and averageddata gave essentially identical values Thus averaged datawhich has a lower uncertainty due to a higher SNR wasdetermined appropriate for use Extrapolating the measuredvalues for the linewidth obtained at kilohertz scan rates to

Optical Engineering November 2010Vol 49(11)111121-4

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

Fig 6 Second-harmonic WMS spectra of the Q(35) -doubletof NO within the 232 spin-orbit state at 187559 cmminus1 as a functionof applied modulation voltage Here 1 mV corresponds to a 02-MHzmodulation amplitude

dc gives a linewidth value of ca 25 MHz This is consistentwith our ldquoslowrdquo tuned measurements obtained through exter-nal cavity mechanical modulation and reflects the increase inmeasured linewidth when the transition was scanned over alonger time period Note also that in this case the accessiblescan range was far less than at the lower scan rates and themeasurements were frequency calibrated using the width ofone of the doublet transitions The low-frequency jittercancellation methodology already mentioned also enabledthe analysis of the variation in the measured separation ofthe Lamb-dips on this pair of transitions and we could de-duce the tuning stability of the system returning a deviationof about 3 from the expected value of 277 MHz as given39

in HITRAN

32 Wavelength-Modulation SpectroscopyAll of the sensitivity-enhancing techniques that havepreviously been employed for absorption studies usingtelecom diode lasers can be adapted to QCL spec-troscopy and the literature contains many examplessuch as noise-immune cavity-enhanced optical heterodynemolecular spectroscopy40 (NICE-OHMS) photoacousticspectroscopy41 and Faraday rotation spectroscopy42 Perhapsthe most straightforward methodology is wavelength modu-lation spectroscopy (WMS) which can be readily achievedby modulating either the external cavity or the injection cur-rent of the EC-QCL Figure 6 shows typical WMS signalsobtainable with a low pressure Doppler broadened sampleof NO and modulation of the laser injection current at afrequency of 15 kHz A 5-cm-long cell with CaF2 win-dows was utilized containing 140 mTorr of NO and thee and f components of the -doublet of Q(35) transition at187559 cmminus1 were monitored The greatest sensitivity wasachieved at a modulation amplitude of 140 MHz correspond-ing to a modulation index of 22 (defined as the ratio of themodulation amplitude to the half width half maximum of thetransition) and resulting in a sensitivity of 37times10minus5 cmminus1

Hzminus12 an enhancement of ca 23 times on the sensitivity ofour direct measurements of the same sample32 In this in-stance we conducted demodulation of the second harmonicalthough the signal amplitude decreases as the harmonic isincreased higher harmonics often lead to flatter baselinesmdash an effect that can be very beneficial especially when

employing multipass cells in which etalon fringing can bea limiting factor on both sensitivity and selectivity In thismanner WMS has been used to probe OCS within a 20-m White cell and a sensitivity of 13times10minus8 cmminus1 Hzminus12clearly employing a longer-path-length Herriot cell wouldincrease this sensitivity further43 Further to the results pre-sented here greater output power and smaller frequency andintensity instabilities in a newer version of this laser have ledto an order of magnitude improvement on our base WMSsensitivity

4 Summary and OutlookQCL spectroscopy is already becoming a well-establishedfield of research with many applications already demon-strated The number of applications will continue to growin the future as ever cheaper more reliable more widely tun-able and short-wavelength devices become available Evennow the production of high-fidelity short-wavelength de-vices operating around 3 μm is a rapidly developing fieldThe QCL and its sister device the intersubband cascade laserare thus well placed to be strong contenders for the marketcurrently employing experimentally nontrivial methods suchas difference frequency generation and optical parametricoscillators The combination of high power broadband tun-ability and room-temperature operation promises the exten-sion of optical cavity techniques to the mid-IR and alsofurther development in the field of trace species detectionin a number of environments such as atmospheric moni-toring plasma processing and breath analysis Similarlythe next few years will see QCLs further utilized for stud-ies of condensed phases and interfaces Other applicationsof such high-powered systems can be found in fundamen-tal studies of chemical dynamics where output powers arenow sufficiently high to contemplate using these devices foroptical pumping therefore the preparation of vibrationallyexcited molecules that are alignedoriented in the laboratoryframe44

AcknowledgmentsThe authors would like to thank J Cockburn for the con-tribution of the 10-μm QCL laser chip This work is con-ducted under the Engineering and Physical Sciences Council(EPSRC) program Grant No EPG00224X1 New Horizonsin Chemical and Photochemical Dynamics RJW would liketo thank the EPSRC for the award of a postgraduate stu-dentship

References1 Y A Bakhirkin A A Kosterev C Roller R F Curl and F K Tittel

ldquoMid-infrared quantum cascade laser based off-axis integrated cavityoutput spectroscopy for biogenic nitric oxide detectionrdquo Appl Opt43(11) 2257ndash2266 (2004)

2 Y A Bakhirkin A A Kosterev R F Curl F K Tittel D A YarekhaL Hvozdara M Giovannini and J Faist ldquoSub-ppbv nitric oxide con-centration measurements using cw thermoelectrically cooled quantumcascade laser-based integrated cavity output spectroscopyrdquo Appl PhysB 82(1) 149ndash154 (2005)

3 G Duxbury N Langford M T McCulloch and S Wright ldquoQuantumcascade semiconductor infrared and far-infrared lasers from trace gassensing to non-linear opticsrdquo Chem Soc Rev 34 921ndash934 (2005)

4 K Namjou C B Roller and G McMillen ldquoBreath-analysis using mid-infrared unable aser pectroscopyrdquo Proc IEEE Sensors 1ndash3 1337ndash1340(2007)

5 R Provencal M Gupta T G Owano D S Baer K N Ricci and J RPodolske ldquoCavity-enhanced quantum-cascade laser-based instrumentfor carbon monoxide measurementsrdquo Appl Opt 44(31) 6712ndash6717(2005)

Optical Engineering November 2010Vol 49(11)111121-5

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

Fig 6 Second-harmonic WMS spectra of the Q(35) -doubletof NO within the 232 spin-orbit state at 187559 cmminus1 as a functionof applied modulation voltage Here 1 mV corresponds to a 02-MHzmodulation amplitude

dc gives a linewidth value of ca 25 MHz This is consistentwith our ldquoslowrdquo tuned measurements obtained through exter-nal cavity mechanical modulation and reflects the increase inmeasured linewidth when the transition was scanned over alonger time period Note also that in this case the accessiblescan range was far less than at the lower scan rates and themeasurements were frequency calibrated using the width ofone of the doublet transitions The low-frequency jittercancellation methodology already mentioned also enabledthe analysis of the variation in the measured separation ofthe Lamb-dips on this pair of transitions and we could de-duce the tuning stability of the system returning a deviationof about 3 from the expected value of 277 MHz as given39

in HITRAN

32 Wavelength-Modulation SpectroscopyAll of the sensitivity-enhancing techniques that havepreviously been employed for absorption studies usingtelecom diode lasers can be adapted to QCL spec-troscopy and the literature contains many examplessuch as noise-immune cavity-enhanced optical heterodynemolecular spectroscopy40 (NICE-OHMS) photoacousticspectroscopy41 and Faraday rotation spectroscopy42 Perhapsthe most straightforward methodology is wavelength modu-lation spectroscopy (WMS) which can be readily achievedby modulating either the external cavity or the injection cur-rent of the EC-QCL Figure 6 shows typical WMS signalsobtainable with a low pressure Doppler broadened sampleof NO and modulation of the laser injection current at afrequency of 15 kHz A 5-cm-long cell with CaF2 win-dows was utilized containing 140 mTorr of NO and thee and f components of the -doublet of Q(35) transition at187559 cmminus1 were monitored The greatest sensitivity wasachieved at a modulation amplitude of 140 MHz correspond-ing to a modulation index of 22 (defined as the ratio of themodulation amplitude to the half width half maximum of thetransition) and resulting in a sensitivity of 37times10minus5 cmminus1

Hzminus12 an enhancement of ca 23 times on the sensitivity ofour direct measurements of the same sample32 In this in-stance we conducted demodulation of the second harmonicalthough the signal amplitude decreases as the harmonic isincreased higher harmonics often lead to flatter baselinesmdash an effect that can be very beneficial especially when

employing multipass cells in which etalon fringing can bea limiting factor on both sensitivity and selectivity In thismanner WMS has been used to probe OCS within a 20-m White cell and a sensitivity of 13times10minus8 cmminus1 Hzminus12clearly employing a longer-path-length Herriot cell wouldincrease this sensitivity further43 Further to the results pre-sented here greater output power and smaller frequency andintensity instabilities in a newer version of this laser have ledto an order of magnitude improvement on our base WMSsensitivity

4 Summary and OutlookQCL spectroscopy is already becoming a well-establishedfield of research with many applications already demon-strated The number of applications will continue to growin the future as ever cheaper more reliable more widely tun-able and short-wavelength devices become available Evennow the production of high-fidelity short-wavelength de-vices operating around 3 μm is a rapidly developing fieldThe QCL and its sister device the intersubband cascade laserare thus well placed to be strong contenders for the marketcurrently employing experimentally nontrivial methods suchas difference frequency generation and optical parametricoscillators The combination of high power broadband tun-ability and room-temperature operation promises the exten-sion of optical cavity techniques to the mid-IR and alsofurther development in the field of trace species detectionin a number of environments such as atmospheric moni-toring plasma processing and breath analysis Similarlythe next few years will see QCLs further utilized for stud-ies of condensed phases and interfaces Other applicationsof such high-powered systems can be found in fundamen-tal studies of chemical dynamics where output powers arenow sufficiently high to contemplate using these devices foroptical pumping therefore the preparation of vibrationallyexcited molecules that are alignedoriented in the laboratoryframe44

AcknowledgmentsThe authors would like to thank J Cockburn for the con-tribution of the 10-μm QCL laser chip This work is con-ducted under the Engineering and Physical Sciences Council(EPSRC) program Grant No EPG00224X1 New Horizonsin Chemical and Photochemical Dynamics RJW would liketo thank the EPSRC for the award of a postgraduate stu-dentship

References1 Y A Bakhirkin A A Kosterev C Roller R F Curl and F K Tittel

ldquoMid-infrared quantum cascade laser based off-axis integrated cavityoutput spectroscopy for biogenic nitric oxide detectionrdquo Appl Opt43(11) 2257ndash2266 (2004)

2 Y A Bakhirkin A A Kosterev R F Curl F K Tittel D A YarekhaL Hvozdara M Giovannini and J Faist ldquoSub-ppbv nitric oxide con-centration measurements using cw thermoelectrically cooled quantumcascade laser-based integrated cavity output spectroscopyrdquo Appl PhysB 82(1) 149ndash154 (2005)

3 G Duxbury N Langford M T McCulloch and S Wright ldquoQuantumcascade semiconductor infrared and far-infrared lasers from trace gassensing to non-linear opticsrdquo Chem Soc Rev 34 921ndash934 (2005)

4 K Namjou C B Roller and G McMillen ldquoBreath-analysis using mid-infrared unable aser pectroscopyrdquo Proc IEEE Sensors 1ndash3 1337ndash1340(2007)

5 R Provencal M Gupta T G Owano D S Baer K N Ricci and J RPodolske ldquoCavity-enhanced quantum-cascade laser-based instrumentfor carbon monoxide measurementsrdquo Appl Opt 44(31) 6712ndash6717(2005)

Optical Engineering November 2010Vol 49(11)111121-5

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

6 G D Stancu N Lang J Ropcke M Reinicke A Steinbach and AWege ldquoIn situ monitoring of silicon plasma etching using a quantumcascade laser arrangementrdquo Chem Vapor Deposit 13(6ndash7) 351ndash360(2007)

7 M L Silva A OrsquoKeefe D M Sonnenfroh D I Rosen and M GAllen ldquoIntegrated cavity output spectroscopy measurements of nitricoxide levels in breath with a pulsed room-temperature quantum cascadelaserrdquo Appl Phys B 81(5) 705ndash710 (2005)

8 B W M Moeskops H Naus S M Cristescu and F J M HarrenldquoQuantum cascade laser-based carbon monoxide detection on a secondtime scale from human breathrdquo Appl Phys B 82(4) 649ndash654 (2006)

9 D Weidmann W J Reburn and K M Smith ldquoGround-based prototypequantum cascade laser heterodyne radiometer for atmospheric studiesrdquoRev of Sci Instrum 78(7) 073107 (2007)

10 D Weidmann W J Reburn and K M Smith ldquoRetrieval of atmo-spheric ozone profiles from an infrared quantum cascade laser hetero-dyne radiometer results and analysisrdquo Appl Opt 46(29) 7162ndash7171(2007)

11 G Hancock S J Horrocks G A D Ritchie J H van Helden andR J Walker ldquoTime-resolved detection of the CF3 photofragment usingchirped QCL radiationrdquo J Phys Chem A 112(40) 9751ndash9757 (2008)

12 J H van Helden S J Horrocks and G A D Ritchie ldquoApplicationof quantum cascade lasers in studies of low-pressure plasmas charac-terization of rapid passage effects on density and temperature measure-mentsrdquo Appl Phys Lett 92(8) 081506 (2008)

13 A A Kosterev F K Tittel C Gmachl F Capasso D L SivcoJ N Baillargeon A L Hutchinson and A Y Cho ldquoTrace-gas detec-tion in ambient air with a thermoelectrically cooled pulsed quantum-cascade distributed feedback laserrdquo Appl Opt 39(36) 6866ndash6872(2000)

14 J H van Helden R Peverall G A D Ritchie and R Walker ldquoRapidpassage effects in nitrous oxide induced by a chirped external cavityquantum cascade laserrdquo Appl Phys Lett 94(5) 051116 (2009)

15 M T McCulloch G Duxbury and N Langford ldquoObservation of satu-ration and rapid passage signals in the 1025 micron spectrum of ethy-lene using a frequency chirped quantum cascade laserrdquo Molec Phys104(16) 2767ndash2779 (2006)

16 G Duxbury N Langford M T McCulloch and S Wright ldquoRapidpassage induced population transfer and coherences in the 8 mi-cron spectrum of nitrous oxiderdquo Molec Phys 105(5ndash7) 741ndash754(2007)

17 D Weidmann A A Kosterev C Roller R F Curl M P Fraser andF K Tittel ldquoMonitoring of ethylene by a pulsed quantum cascadelaserrdquoAppl Opt 43(16) 3329ndash3334 (2004)

18 G Wysocki M McCurdy S So D Weidmann C Roller R F Curland F K Tittel ldquoPulsed quantum-cascade laser-based sensor for trace-gas detection of carbonyl sulfiderdquo Appl Opt 43(32) 6040ndash6046 (2004)

19 L Allen and J H Eberly Optical Resonance and Two-Level AtomsDover New York (1987)

20 N Tasinato G Duxbury N Langford and K Hay ldquoAn investigationof collisional processes in a Dicke narrowed transition of water vapor inthe 78 μm spectral region by frequency down-chirped quantum cascadelaser spectroscopyrdquo J Chem Phys 132(4) 044316 (2010)

21 M Traetteberg G Paulen S Cyvin Y Panchenko and V MochalovldquoStructure and confromations of isoprene by vibrational spectroscopyand gas electron-diffractionrdquo J Molec Struct 116(1ndash2) 141ndash151(1984)

22 N Selamoglu M Rossi and D Golden ldquoAbsolute rate of recombina-tion of CF3 radicalsrdquo Chem Phys Lett 124(1) 68ndash72 (1986)

23 N Basco and F G M Hathorn ldquoThe electronic absorption spec-trum of the trifluoromethyl radicalrdquo Chem Phys Lett 8(3) 291ndash293(1971)

24 A B Vakhtin ldquoThe rate constant for the recombination of trifluo-romethyl radicals at T = 296 Krdquo Int J Chem Kinet 28(6) 443ndash452(1999)

25 C E Brown J J Orlando J Reid and D R Smith ldquoDiode LaserDetection of Transient CF3 radicals formed by CO2 laser multiphotoninduced dissociation of halocarbonsrdquo Chem Phys Lett 142(3ndash4) 213ndash216 (1987)

26 G Wysocki R Curl F Tittel R Maulini J Bulliard and J FaistldquoWidely tunable mode-hop free external cavity quantum cascade laserfor high resolution spectroscopic applicationsrdquo Appl Phys B 81(6)769ndash777 (2005)

27 L Hildebrandt R Knispel S Stry and J Sacher ldquoExternal cavity QCLsystemsrdquo Tech Messen 72(6) 406ndash412 (2005)

28 M Beck D Hofstetter T Aellen J Faist U Oesterle M Ilegems EGini and H Melchior ldquoContinuous wave operation of a mid-infraredsemiconductor laser at room temperaturerdquo Science 295(5553) 301ndash305(2002)

29 G Wysocki R Lewicki R F Curl H Q Le S S Pei B Ishaug and JUm ldquoWidely tunable mode-hop free external cavity quantum cascadelasers for high resolution spectroscopy and chemical sensingrdquo ApplPhys B 92(3) 305ndash311 (2008)

30 J N Baillargeon G P Luo C Peng H Q Le S S Pei B Ishaug andJ Um ldquoGrating-tuned external-cavity quantum-cascade semiconductorlasersrdquo Appl Phys Lett 78(19) 2834ndash2836 (2001)

31 A Hugi R Terazzi Y Bonetti A Wittmann M Fischer M Beck JFaist and E Gini ldquoExternal cavity quantum cascade laser tunable from76 to 114 μmrdquo Appl Phys Lett 95(6) 061103 (2009)

32 G Hancock J H van Helden R Peverall G A D Ritchie andR J Walker ldquoDirect and wavelength modulation spectroscopy usinga cw external cavity quantum cascade laserrdquo Appl Phys Lett 94(20)201110 (2009)

33 R J Walker J H van Helden and G A D Ritchie ldquoQuantum cascadelaser absorption spectroscopy of the 1-0 band of deuterium bromide at5 μmrdquo Chem Phy Lett (in press)

34 W Demtroder Laser Spectroscopy Basic Concepts and Instrumenta-tion Springer BerlinNew York (2003)

35 N Mukherjee and C K N Patel ldquoMolecular fine structure and transi-tion dipole moment of NO2 using an external cavity quantum cascadelaserrdquo Chem Phys Lett 462(1ndash3) 10ndash13 (2008)

36 J T Remillard D Uy W H Weber et al ldquoSub-Doppler resolution lim-ited Lamb-dip spectroscopy of NO with a quantum cascade distributedfeedback laserrdquo Opt Express 7(7) 243ndash248 (2000)

37 S Bartalini S Borri P Cancio et al ldquoObserving the intrinsic linewidthof a quantum-cascade laser beyond the Schawlow-Townes limitrdquo PhysRev Lett 104(8) 083904 (2010)

38 S Borri S Bartalini I Galli et al ldquoLamb-dip-locked quantum cascadelaser for comb-referenced IR absolute frequency measurementsrdquo OptExpress 16(15) 11637ndash11646 (2008)

39 L S Rothman A Barbe D Chris Benner et al ldquoThe HITRAN molec-ular spectroscopic database edition of 2000 including updates through2001rdquo J Quant Spectrosc Radiat Trans 82 5ndash44 (2003)

40 M S Taubman T L Myers B D Cannon and R M WilliamsldquoStabilization injection and control of quantum cascade lasers andtheir application to chemical sensing in the infraredrdquo SpectrochimActa Part A 60(14) 3457ndash3468 (2004)

41 B A Paldus T G Spence R N Zare et al ldquoPhotoacoustic spec-troscopy using quantum-cascade lasersrdquo Opt Lett 24(3) 178ndash180(1999)

42 H Ganser W Urban and A M Brown ldquoThe sensitive detection of NOby Faraday modulation spectroscopy with a quantum cascade laserrdquoMolec Phys 101(4ndash5) 545ndash550 (2003)

43 L Ciaffoni R Peverall and G A D Ritchie ldquoLaser spectroscopy onvolatile sulfer compoundsPossibilities for breath analysisrdquo J BreathResearch (in press)

44 N Mukherjee ldquoMolecular alignment using coherent resonant excita-tion a new proposal for stereodynamic control of chemical reactionsrdquoJ Chem Phys 131(16) 164302 (2009)

Gus Hancock received his first degree fromTrinity College Dublin and his PhD degreefrom Cambridge University He was a twopostdoc with the University of CaliforniaSan Diego and was then appointed asWissenschaftlicher Angestellte in the Univer-sitat Bielefeld Germany In 1976 he was ap-pointed to a post in physical chemistry at theUniversity of Oxford where he is now pro-fessor of chemistry and heads the Physicaland Theoretical Chemistry Laboratory His

awards include the Corday Morgan Medal and Prize Royal Societyof Chemistry in 1982 Royal Society of Chemistry Reaction KineticsAward in 1995 the 14th Italgas Prize for Science and Technologyfor the Environment in 2000 the Polanyi Medal of the Gas KineticsGroup of the Royal Society of Chemistry in 2002 and the ChemicalDynamics Award from the Royal Society of Chemistry in 2010

Grant Ritchie has worked in the generalarea of gas phase chemistry for the past 10years applying laser techniques to the studyof reaction dynamics and trace gas detec-tion He completed his DPhil degree in 1999and was awarded a Ramsay Memorial Re-search Fellowship (2000 to 2003) and thena Royal Society University Research Fellow-ship (2004 to 2009) Dr Ritchie has beena lecturer in chemistry at the University ofOxford since 2006 Current research in the

Ritchie group is centered on reaction dynamics optical manipula-tion and analytical spectroscopy In the latter area their researchinterests lie in combining diode lasers and quantum cascade laserswith a range of sensitivity enhancing techniques such as modu-lation spectroscopy and optical-cavity-based absorption methodsfor a variety of applications including measurement of trace atmo-spheric species probes of the composition of a variety of etching and

Optical Engineering November 2010Vol 49(11)111121-6

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms

Hancock et al Applications of midinfrared quantum cascade lasers to spectroscopy

deposition plasmas and analysis of the contents of human breathand air pollution

Jean-Pierre van Helden received his MScdegree in applied physics in 2001 and hisPhD degree in applied physics in 2006 bothfrom the Eindhoven University of Technol-ogy the Netherlands In 2007 he joined theRitchie group at the University of Oxford asa post-doctoral research assistant He con-ducts research focused on the developmentof compact fast and ultrasensitive laser-based optical sensors for trace gas detec-tion by means of cavity-enhanced absorption

techniques using diode and quantum cascade lasers and their appli-cation to atmospheric gas monitoring and diagnostic breath analysisHis current research interest is the application of quantum cascadelasers to problems in nonlinear optics chemical kinetics and reactiondynamics

Richard Walker received his MChem degreefrom the University of Oxford in 2007 and iscurrently in his third year reading for his DPhildegree with the Ritchie group His researchis funded by the Engineering and PhysicalSciences Council (EPSRC) and is primarilyconcerned with the application of quantumcascade lasers to spectroscopy nonlinearoptics and reaction kinetics

Damien Weidmann graduated in physics in1998 and earned his PhD degree in appliedphysics in 2002 both from the Universityof Reims France He subsequently workedas a postdoctoral researcher in the LaserScience Group of Rice University Hous-ton Texas In 2004 he joined the SpaceScience and Technology Department of theRutherford Appleton Laboratory UnitedKingdom to initiate develop and lead thelaser spectroscopy activity within the Spec-

troscopy Group Since 2009 he has been with the Physical and The-oretical Chemistry Laboratory through a joint appointment with theUniversity of Oxford United Kingdom His research interests are high-resolution tunable laser spectroscopy for remote and in situ molecularsensing as well as the development of corresponding instruments andapplications

Optical Engineering November 2010Vol 49(11)111121-7

Downloaded from SPIE Digital Library on 26 Nov 2010 to 130246132178 Terms of Use httpspiedlorgterms