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Organic Semiconductor Lasers and Optical Amplifiers
G.A. Turnbull, Y. Yang, D. Amarasinghe, G. Tsiminis,A. Ruseckas and I.D.W. Samuel
Organic Semiconductor CentreSchool of Physics and Astronomy, University of St Andrews
www.st-and.ac.uk/[email protected]
Organic Semiconductor Centre
►Light-emitting dendrimers for OLEDs
►Photophysics – fs to µs luminescence
►Exciton diffusion/solar cells
►Lasers and optical amplifiers
►A new light source for medicine
Outline: Part I
►Introduction – towards polymer photonics
►Review of laser physics
►Gain measurements
►Feedback structures
►Distributed feedback lasers
►Exercise (for you!)
Organic Semiconductors
►Conjugated molecules
►Novel semiconductors
►Easy to process
►Can tune properties
►Can emit light
►Flexible
Laser Physics
Light
Amplification byStimulatedEmission of Radiation
Generic LASER:optical amplification
with feedback
Excitation
Output
mirrors
lνpν
Laser Rate Equations
( )cav
ppN
L
cl
dt
dp
τσ −+= 1
Amplification Decay cLcav βτ /2=
L
lβ
υσ
τ h
INN
dt
dN −−Λ=
ExcitationSpontaneous
emissionStimulated emission
ΛΛΛΛN2
N1
ττττ
12 NNN −=
Steady-state Lasing Characteristics
ΛΛΛΛ
ΛΛΛΛ
N
p
► Increased spectral and spatial coherence ►Narrowing of emission spectrum►Directional output beam
ΛΛΛΛTH
►Population inversion (spontaneous emission) clamped at threshold level by stimulated emission
►Power rises linearly with pumping
Threshold and Slope Efficiency
►Gain exceeds loss
►Photon mode concept►106-1010 spatial and spectral photon modes►Above threshold all energy stimulated into one
mode (or at least very few modes)
Threshold:
( )THINRp
loout PPP −= η
νν
ββ
Efficiency:
Fraction of useful losses
Photon energies
Radiative efficiency
Pump rate above threshold
βσ >= Nl2gain
Pulsed Operation
► Dynamic solution of rate equations
► Gain medium has fluorescence lifetime τ to build population inversion
► If tΛ < τ -pump energy density important► If tΛ > τ -pump intensity important
ΛΛΛΛpN
ΝΝΝΝTH
► Pulse build-up time
► Time required for pulse to be macroscopically amplified
► 1 photon (= 3x10-19 J) to 1 nJ pulse - 100 dB gain required
► Build-up time must be less than excited state lifetime/pump duration
► Short cavities required
time
Pulsed Operation: A Femtosecond Polymer Laser
2D DFB laser with MEH-PPV as active medium
Pumped by 100 fs pulses
Pulse width decreases at high excitation density
Minimum pulse width of 700 fs
0 1 2 3 4 50.00
0.25
0.50
0.75
1.00
Nor
mal
ised
Inte
nsity
(ar
b)
Time (ps)
Response 2.8 µJcm-2
4.8 µJcm-2
6.2 µJcm-2
13 µJcm-2
Potential for Polymer Lasers +Amplifiers
► Absorption and emission separated - 4 level system
► Strong absorption ~105 cm-1
- Enormous gain
► Little concentration quenching
► Broad spectra - broad bandwidth
► Compatible with polymer fibretransmission windows (500–560 and ~660 nm),
► Scope for low cost manufacture
► Possibility of electrical pumping
3.2 3 2.8 2.6 2.4 2.2 2 1.8
C8H17H17C8
n
O
O
n
n
Pho
tolu
min
esce
nce
(arb
.)
Energy (eV)
400 500 600 7000.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
O
O
n
Abs
orpt
ion
(105 /c
m)
Wavelength (nm)
Photolum
inescence (arb.)
Chemical Reviews 107 1272 (2007)
Organic Semiconductor Laser Materials
► Conjugated polymers
► solution processed
► wide range of materials
n
C8H17C8H17
O
O
n C6H13
n
C6H13
C10H21
C10H21
O
N
CNNC
AlO
N
O
N
O
N
► Dye doped Alq3
► co-evaporated
► conventional laser dyes
in energy transfer host
O
O O
O
► Dendrimers
► Spiro molecules
► Truxenes►Small solution processed
molecules
(4)
(1) (3)
(2)
Organic Laser Energy states
3.0 2.5 2.0 1.50.0
0.2
0.4
0.6
0.8
1.0
400 500 600 700 800
Abs
orba
nce
/P
hoto
lum
ines
cenc
e (a
rb.)
Energy (eV)
Wavelength (nm)
O
O
n
0S
1S
(4)
(3)
(2)
NS
NT
1T
Measuring Gain 1:Transient Absorption
►Excite sample with short laser pulse►Measure change in transmission with time-delayed probe►Gain (if present) will increase in transmission
SampleOptical delay
ChopperExcitation
Probe Detector
∆T/T
Probing Optical Gain (Stimulated Emission) in Materials
►Excite with short pulse to S1 state.
►Send a probe pulse resonant with PL.
► Probe is amplified by stimulated emission
S0
S1
∆T/T~ N1(σSE- σESA ) - Npσp
SE
ESA
Transient Absorption ~1997
► Snapshot of absorption ∆Ainduced by pump pulse
► Photo-induced absorption at low energies
► Gain (-ve absorption) mirrors PL spectrum
► Gain present for a few hundred picoseconds
-0.004
-0.002
0.000
0.002
1200 1000 800 600
t=200 fs
Laser
PL
∆A
Wavelength, nm
Photon energy, eV
Abs
1.0 1.5 2.0 2.5
n( )S
Data from Arvydas Ruseckas
Measuring Gain 2: Waveguide Amplifiers
Polymer film (100-400 nm) forms
optical waveguide
Pump λ absorbed in ~100 nm
Total internal reflection confines light
in polymer film
Signal amplified by 1000 times in 1
mm propagation through film
Amarasinghe et al., App Phys Lett 89, 201119; Adv Mat 21, 107 (2009)
Pump Input Probe
polymer
Silica
Amplified output
0.4 – 1 mm
0 50 100 1500
10
20
30
1022 µm
822 µm
422 µm
Gai
n (d
B)
Pump energy density (µJ/cm2)
ADS233YE F80.9BT0.1
Measuring Gain 3: ASE
► Excite stripe on film and observe emission from edge
Helitois et al., Appl Phys Lett 81, 415 (2002)
n
C8H17C8H17
► Above characteristic energy:
► Super-linear increase in output intensity
►Dramatic narrowing of spectrum
► Signatures of gain narrowing via amplified spontaneous emission
Increasing pump power
420 440 460 480 500 5200.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ised
inte
nsity
Wavelength (nm)
Measuring Gain 3: ASE
►Net gain and waveguide losses can be inferred from the SLN measurements
►Gain: output related to stripe length
►Loss: measured by varying length of unpumped region
( ) ( )( )[ ]1−∝ lgp e
g
II λ
λλ
l
( ) ( )zeI λαλ −∝
z
Ip
Measuring Gain 3: ASE
►Net gains of 10 cm -1, 21 cm-1 and 30 cm-1 (up to 130 dBcm-1) for different pump densities
►Loss coefficient: 3 cm-1
in unpumpedwaveguide
105
2 105
3 10 5
0 0.02 0.04 0.06 0.08 0.1
Experimental DataExponential Fit
Ou
tpu
t S
ign
al (
a.u
.)
Stripe Position (cm)
1000
10 4
10 5
0 0.5 1 1.5 2 2.5
E=3.3µJE=2.1µJE=1.3µJ
Ou
tpu
t S
ign
al a
t λA
SE (
a.u
.)
Excitation Length (mm)
Helitois et al., Appl Phys Lett 81, 415 (2002)
n
C8H17C8H17
Polymer Laser Resonator Geometries
► Solution processing- high quality waveguides
► Sub-micron structures- laser microresonators
► Strong absorption – transverse pumping possible
microspheremicroringmicrocavity
1-D DFB2-D DFB /
photonic bandgaprandom scatterers
Typical Experimental Configuration
Pumplaser
Variable grey filter
Lens
Polymer laser in vacuum chamber
CCD spectrograph
Energy meter/ beam profiler
Polymer Microcavity Lasers
► Resonant wavelengths:► Few spectral modes
► At high pump energies emission is stimulated preferentially into one spectral mode
Tessler et al., Nature 382, 695 (1996)
dm m 2=λ
Surface-emitting Distributed Feedback Lasers
Air
Polymer (100nm)
Silicaλ
2nd order scattering provides distributed feedback
1st order scattering provides surface output coupling
Air
Polymer (100nm)
Silicaλ
Pulsed pump laser
excitation
2-D distributed feedback
1µm1µm
e.g. Phys Rev B 67,165107 (2003); J Appl Phys 98, 023105 (2005)
4 µµµµm
Photoluminescence dominated by Bragg-scattered emission from waveguide modes
Angle Dependent Photoluminescence
Photonic stopband evident at normal incidence
-10 -5 0 5 10
580
600
620
640
660
-10 -5 0 5 10Angle (deg)
660
640
620
600
580
Wav
elen
gth
(nm
)
540 560 580 600 620 640 660 680 7000.0
0.2
0.4
0.6
0.8
1.0
10o
5o
0o
UntexturedMEH:PPV (x 5)
Inte
nsity
(ar
b.)
Wavelength (nm)
O
O
n
AirMEH-PPVSilica
φ
θ
Distributed Feedback Lasers
Air
Polymer
Silica
Λ
► Wavelength scale corrugations cause Bragg scattering
► Counter-propagating waveguide modes couple together at λB
Λ= eff2nm Bλ
Questions – For You to AnswerYou wish to make an MEH-PPV DFB laser operating at 640 nm. The refractive index of the waveguide mode is 1.6.
(a) Write down the Bragg condition for the lasing modes(b) For the first two diffracted orders
(i) Calculate the grating period required(ii) Determine whether the laser is surface or edge-emittingWhat are the advantages and disadvantages of each configuration?
(c) What would the emitted beam look like for(i) A one-dimensional grating?(ii) A two-dimensional grating?
1-D versus 2-D feedback
n
C8H17C8H17
Blue lasing in PFO2-D mode confinement leads to lower threshold lasing and improved slope efficiency
G Heliotis et al., Adv Fun Mat 14, 91 (2004)
Outline: Part II
►Tuning polymer lasers, simple fabrication►Reducing size of pump – towards practical lasers►Indirect electrical pumping►Sensing explosives►Polymer optical amplifiers►Optical switching of lasers and amplifiers
Tuning a Polymer Laser by Microstructure
Array of 15 grating periods from 350 to 420 nm
95 nm grating depth, unity mark to space ratio
420 415 410 350355360405 365
3 mm
5µm
5µm
0.2µm
Atomic force microscope image of grating
Tuning of Laser Emission
• Laser wavelength tuned by translating the substrate
• Stopband wavelength changes linearly with grating period
• Laser emission on band edge ~4nm longer in wavelength
• 20 nm tuning range using a single substrate (1 mm translation)
390 395 400 405 410 415 420605
610
615
620
625
630
635
Grating Period (nm)W
avel
engt
h (n
m)
Laser EmissionBragg DipTheory
Elastomeric mould
Simple Fabrication of Polymer Nanostructures► Hot embossing► Solvent assisted micro-moulding► UV-nanoimprint lithography
Appl Phys Lett 81, 1955 (2002); Appl Phys Lett 82, 4023 (2003);
Polymer film
Solvent
Elastomeric mould
120
seco
nds
AFM image of 400 nm period, 90 nm deep eggbox corrugation
Shrinking Polymer Laser systems
1995 Regenerative amplifier(Tessler)
~2000 Q-switched Nd:YAG
2003 Microchip laser2006 Diode
pumped2008 LED pumped
Yang et al., App Phys Lett 92, 163306 (2008)
► Pulsed microchip pump laser
► 1 ns pulses at 532 or 355 nm
► Up to 100 nJ to 1 µJ pulse energy
► 5 kHz rep rate
Compact Polymer Lasers
Materials Today 7, 28 (2004)Chemical Reviews 107 1272 (2007)
Diode laser pumped polymer lasers► GaN diode laser
► λ = 407 nm► Epulse = 0.7 nJ
► tpulse ~ 1.2 nsecEMISSIONEMISSION
20 µm cavity
C-102 / MEH-PPV (120nm thick)
Silica
GaN diode
EMISSIONEMISSION
20 µm cavity
C-102 / MEH-PPV (120nm thick)
Silica
GaN diode
620 630 6400
2
4
30 µµµµm
outp
ut in
tens
ity (
a.u.
)
wavelength (nm)
1.25 x threshold 1 x threshold 0.8 xthreshold
DBR microcavity with a few resonant modes
Above threshold (8µJ/cm2), lasing peak grows rapidly
Single lasing peak dominates spectrum at higher pump powers
See also: Riedl et al. Appl. Phys. Lett. (2006)Karnutsch et al. Phot Tech Lett (2007)
Vasdekis et. al., Opt. Express 14, 9211 (2006)
Electrically Pumped Organic Lasers?
Main Challenges
► High projected threshold current density (> 100s A/cm2)
► Low carrier mobilities
► Losses from metal contacts
(10X to 100X intrinsic waveguide loss)
► Polaron and triplet absorption losseshttp://en.wikipedia.org/wiki/Laser
Chemical Reviews 107 1272 (2007); Nature Photonics 3, 546 (2009)
Why?
Novel, tuneable, simple to fabricate, (flexible) lasers
Without need for expensive (and bulky) separate pump laser
Alternative: Indirect Electrical PumpingTry a Nitride LED as the Pump
www.lumileds.com
Advantages
► Exploits high mobility of InGaN
► Separates charge injection from gain medium
► Much lower cost than laser pumped system
► Very compact, electrically controlled
All the benefits from direct electrical pumping
www.modding-faq.de/
Challenges
► Incoherent light source hard to focus
► CW intensities < 30 W/cm2
0 20 40 60 80 100 120 140 1600
50
100
150
200
250
Pea
k in
tens
ity (W
/cm
2 )
Peak drive current (A)
Lumiled K2 emitter cw spec: (475mW @ 1A at 455 nm)
Under ns pulsed operation:
► 47 ns pulses
► Up to 255 W/cm2 (275 nJ) for 160 A drive current (8 kAcm-2)
High Power Pulsed Nitride LEDs
www.lumileds.com
-50 0 50 100 150
0.0
0.5
1.0
1.5
2.0
Drive current (normalised and offset )
LED emission (normalised)
Time (ns)
Yang et al., App Phys Lett 92, 163306 (2008);
Polymer Gain Medium
500 550 600 6500.0
0.2
0.4
0.6
0.8
1.0
Ligh
t Em
issi
on (n
orm
.)
Wavelength (nm)
PL ASE
0.0 0.5 1.0 1.50.1
1
Nor
mal
ised
PL
Time (ns)
PLQY: 80%
lifetime: 3.4 ns
Requirement for gain medium
► Absorption matches LED emission
► Long exciton lifetime
Yang et al., App Phys Lett 92, 163306 (2008);
300 350 400 450 5000.0
0.5
1.0
1.5
2.0
2.5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Abs
orba
nce
Wavelength (nm)
LE
D e
mis
sion
(nor
m.)
ADS 233YEF80.9BT0.1
►1D Distributed feedback resonator►355 nm period grating► In-plane feedback and surface
output coupling
Polymer DFB Lasers
Air
ADS233YE (350 nm)
Silica
560 570 580 5900
1
2
3
4
wavelength (nm)
0 5 10 15 20 25 30 35 400
1
2
3
0 50 100 150 200 250 300
outp
ut a
t 568
nm
(arb
.)
pump energy (nJ)
Pump intensity (W/cm 2)
►Pumped with 450 nm from 4 ns OPO
►DFB lasing in TE0 and TM0
transverse modes
Low Threshold Polymer DFB Lasers
►Dependence on pump beam area
►Threshold intensity falls with increasing excitation area
►Saturates for areas above ~1 mm2
0.1 1 100
2
4
6
8
10
Thre
shol
d in
tens
ity (
kW/c
m2 )
Pump beam area (mm 2)
0 10 20 300
1
2
3
4
5
6
70 50 100 150 200
Out
put (
arb.
)
Pump energy (nJ)
Pump intensity (W/cm 2)
►Minimum threshold ~200 W/cm2
►Threshold reduced to 160 W/cm2
by double-passing pump light550 560 570 580
0
1
2
3
568 nmPumped by OPO at 450 nm
Wavelength (nm)
LED Pumped Polymer Lasers
Light emitting area: 1.5 mm by 1.5 mm
InGaN LED
Laser Output
Pulsed current source
CYTOPADS233YE
Silica
Pump filter
InGaN LED
Laser Output
Pulsed current source
CYTOPADS233YE
Silica
Pump filter
Yang et al., App Phys Lett 92, 163306 (2008); Lupton, Nature (N&V) 453, 459 (2008)
LED Pumped Polymer Laser
0 25 50 75 100 125 1500
100
200
300
400
Ligh
t em
issi
on (
arb.
)
Peak drive current (A)
568 nm (TE0 mode)
555 nm (TM0 mode)
Onset of lasing around 150A
► Sharp laser peak at 568 nm
► Change in output slope
550 560 570 580
0
100
200
300
400
500
600
700
800
900
L
ight
em
issi
on (
arb.
)
160 A
156 A
152 A
148 A144 A120 A
80 A
40 A
Wavelength (nm)
Yang et al., App Phys Lett 92, 163306 (2008); Lupton, Nature (N&V) 453, 459 (2008)
HYPIX
Hybrid organic semiconductor-GaN-CMOS smart pixel arrays
(St Andrews, Strathclyde, Edinburgh, Imperial College)
GA Turnbull, IDW Samuel
µLEDarray
CMOS device
2 mm
B.R. Rae et al J. Phys. D 41 (2008) H.X. Zhang et al Opt. Exp.16 (2008) APL 92, 163306 (2008);
Optical Concentrators for Organic Lasers
Aim: increase power density from LED
► Use luminescent optical concentrators as used in solar panels
► Miniaturised version designed for
concentrating LED power density
Yang et al., Adv Mat 21, 3205 (2009)
Pump beam
Dye doped polymer
Silica
CY
TO
PM
EH
-PP
V
Sili
caLa
ser
outp
ut
► 5-fold reduction in lasing threshold for optimised concentrators
Fluorescent explosives detection
►Many common explosives include TNT, DNT etc
►Nitroaromatic compounds are strong electron acceptors
► Introduction of nitroaromatic molecule leads to dissociation of exciton and quenches emission
S0
S1
S0
S1
n
C8H17C8H17
Swager et al., Chem Rev 107, 1339 (2007) Richardson et al., APL 95 063305 (2009)
Sensitivity enhancement with polymers
►Reversable binding of nitroaromatic molecule with fluorophore
►Only fluorescence from the bound molecules is quenched
Toal et al., J Mater Chem, 16, 2871
►Conjugated polymer- series of connected fluorescing sites
►One nitroaromatic molecule can quench emission from whole chain
Toal et al., J Mater Chem, 16, 2871
Fluorescence sensing with polyfluorene
►Polyfluorene film exposed to ~10ppb dinitrobenzene vapour in air
►15% drop in fluorescence
►Fluorescence recovers to original value when purged in nitrogen
400 450 500 550 6000.0
0.2
0.4
0.6
0.8
1.0
Flu
ores
cenc
e
wavelength (nm)
before exposure after 2min exposure
n
C8H17C8H17
PFO
DNB
Polyfluorene Laser Explosives Sensor
► Polyfluorene DFB laser exposed to ~10ppb DNB vapour in air
► 85% drop in emission
► Factor 3 change in slope efficiency
► Lasing recovers to original performance when purged in N2
0 5 10 15 20 25 300.0
0.2
0.4
0.6
0.8
1.0
Rel
ativ
e ou
tput
pow
er (
arb.
)
Pump energy (uJ)
Laser output before exposure after 20 mins exposure after recovery
0 5 10 15 20 25 300.4
0.6
0.8
1.0
Fra
ctio
nal d
rop
in
em
issi
on
Pump energy (uJ)
0 10 20 30 400.0
0.2
0.4
0.6
0.8
1.0
Purge
ExposureExposure
Lase
r int
ensi
ty (n
orm
alis
ed)
time (min)
Yang et al., Adv. Func. Mater. (2010)
Some Future Applications
► Integrated tunable lasers sources for spectroscopy
► Intrinsic chemical sensing
► Optical amplifiers/modulators
A. Rose et al., Nature 434, 876 (2005)
0 50 100 150
0
10
20
30
40
Inte
nsity
(ar
b)Time (ps)
control off control on 2
400 nm
580 nm
800 nm800 nm
Amarasinghe et al., APL 92, 083305 (2008)
Advanced Polymer Photonics:Optical Amplifiers
Pump Signal
IN Signal OUT
Grating coupled
Need for low cost amplifiers e.g compensate splitting losses
Compatibility with polymer optical fibre, planar lightwavecircuits
Scope for very broad amplification bandwidth
Solution Polymer Optical Amplifier
Nitrogen Laser337 nm (0.5 ns)
TuneableOscillator 580–630 nm
PolymerOptical Amplifier
Amplify a weak light pulse to a strong one
First demonstrated in solution
Solution Polymer Amplifier: High Gain, Broadband
APL 80, 3036 (2002)and 85, 6122 (2004)
540 560 580 600 620 6400
10
20
30
40
50
PPV derivativeSt Andrews
Gai
n (d
B)
Wavelength (nm)
F8BTImperialSt Andrews
Gain over 30 dB in 1 cmBandwidth >50 nmor 50 THz
Solid State Polymer Amplifiers
560 580 600 620 640 660 6800
5
10
15
20
Red-FImperialSt AndrewsMEH-PPV
St Andrews
F8BT St Andrews
Gai
n(dB
)
Wavelength (nm)
Pump Signal
INSignal OUT
Grating coupled
Gain up to 21 db: signal amplified 100 times
Wide wavelength range
A Polymer Optical Amplifier Using F8BT : Amplification of a Pulse Sequence with 70 ps Spacing
Gain characteristics of multiple pulsesσ=6x10-17 cm2
0 20 40 60 80 100 120 140 1600
100
200
300
400
P o
w e
r (
a r
b .
u n
i t s
)Time(ps)
Three pulses in a 140 ps window. Amplification > 20 dB. Repetition rate 5 kHz. Pulse shape not affected.
Pump Signal
IN Signal OUT
Grating coupled
Optical Switching of Gain in a Co-polymer
-3 -2 -1 0 1 2 3 40.0
0.5
1.0
τ2=1 ps (a
2=0.14)
τ1=0.05 ps (a
1=0.86)
Push
∆T/T
(ar
b un
its)
Time (ps)
Full recovery in 2 ps.
Amarasinghe et al. Adv. Mater (2009)
Optical Switching of Copolymer Amplifier
0 50 100
0
10
20
30
0 50 100
0
10
20
30
Inte
nsity
(a.
u.)
Time(ps)
Amplified Push blocked- amplified again
Input
Switched off by push pulse
APL 92, 083305 (2008).
Organic Semiconductor Lasers and Optical Amplifiers: Conclusion
►Organic Semiconductor Lasers:►Compact, tuneable visible lasers►Simple fabrication►Direct pumping by InGaN LED demonstrated
►Polymer Optical Amplifiers►Strong gain >20 dB/mm over broad bandwidth►Potential compatibility with POF
►Optical Switching►All-optical switching of gain and lasing demonstrated►Promising building blocks for polymer photonics
See Chemical Reviews April 2007; Nature Photonics Oct 2009