Efficient Optical to Terahertz Wave Conversion through ... · [7] M. Jarrahi, "Advanced Photoconductive Terahertz Optoelectronics based on Nano- Antennas and NanoPlasmonic Light Concentrators",

Forum for Electromagnetic Research Methods and Application Technologies (FERMAT)

Efficient Optical to Terahertz Wave

Conversion through Plasmonic Antennas

Mona Jarrahi Associate Professor of Electrical Engineering, University of California Los Angeles

66-147E Engineering IV, 420 Westwood Plaza, Los Angeles, CA 90095 Phone: (310) 206-1371, Fax: (310) 825-8282, Email: [email protected]

URL: http://www.seas.ucla.edu/~mjarrahi/ Abstract: Although unique potentials of terahertz wave for chemical identification, material characterization, biological sensing, and medical imaging have been recognized for quite a while, the relatively poor performance, higher costs, and bulky nature of current terahertz systems continue to impede their deployment in field settings. In this talk, I will describe some of our recent results on developing fundamentally new terahertz electronic/optoelectronic components and imaging/spectrometry architectures to mitigate performance limitations of existing terahertz systems. In specific, I will introduce new designs of high-performance photoconductive terahertz sources that utilize plasmonic antennas to offer terahertz radiation at record-high power levels of several milliwatts – demonstrating more than three orders of magnitude increase compared to the state of the art. I will describe that the unique capabilities of these plasmonic antennas can be further extended to develop terahertz detectors and heterodyne spectrometers with single-photon detection sensitivities over a broad terahertz bandwidth at room temperatures, which has not been possible through existing technologies. To achieve this significant performance improvement, plasmonic antennas and device architectures are optimized for operation at telecommunication wavelengths, where very high power, narrow linewidth, wavelength tunable, compact and cost-effective optical sources are commercially available. Therefore, our results pave the way to compact and low-cost terahertz sources, detectors, and spectrometers that could offer numerous opportunities for e.g., medical imaging and diagnostics, atmospheric sensing, pharmaceutical quality control, and security screening systems.

Mona Jarrahi received her B.S. degree in Electrical Engineering from Sharif University of Technology in 2000 and her M.S. and Ph.D. degrees in Electrical Engineering from Stanford University in 2003 and 2007. She served as a Postdoctoral Scholar at University of California Berkeley from 2007 to 2008. After serving as an Assistant Professor at University of Michigan Ann Arbor, she joined University of California Los Angeles in 2013 as an Associate Professor of Electrical Engineering and the Director of the Terahertz Electronics Laboratory. Her research group focuses on Terahertz, Millimeter-Wave Electronics and Optoelectronics; Imaging and Spectroscopy Systems; and Microwave Photonics. Prof. Jarrahi has made significant contributions to the development of ultrafast electronic and optoelectronic devices and integrated systems for

mailto:[email protected]

http://www.seas.ucla.edu/%7Emjarrahi/

terahertz and millimeter-wave sensing, imaging, computing, and communication systems by utilizing novel materials, nanostructures, and quantum well structures as well as innovative plasmonic and optical concepts. In recognition of her outstanding achievements, Prof. Jarrahi has received several prestigious awards in her career including the Presidential Early Career Award for Scientists and Engineers (PECASE); Early Career Award in Nanotechnology from the IEEE Nanotechnology Council; Outstanding Young Engineer Award from the IEEE Microwave Theory and Techniques Society; Booker Fellowship from the United States National Committee of the International Union of Radio Science (USNC/URSI); Lot Shafai Mid-Career Distinguished Achievement Award from the IEEE Antennas and Propagation Society; Grainger Foundation Frontiers of Engineering Award from National Academy of Engineering; Friedrich Wilhelm Bessel Research Award from Alexander von Humboldt Foundation; Young Investigator Awards from the Army Research Office (ARO), the Office of Naval Research (ONR), and the Defense Advanced Research Projects Agency (DARPA); Early Career Award from the National Science Foundation (NSF); the Elizabeth C. Crosby Research Award from the University of Michigan; and best-paper awards at the International Microwave Symposium, International Symposium on Antennas and Propagation, and International Conference on Infrared, Millimeter, and Terahertz Waves. She has also been named a Kavli Fellow by the National Academy of Sciences. Prof. Jarrahi is actively involved in several professional societies and has been on program committees of several conferences from IEEE, OSA, and SPIE societies. She is a senior member of IEEE, OSA, and SPIE societies and serves as a member of the Terahertz Technology and Applications Committee of IEEE Microwave Theory and Techniques, an editorial board member of Journal of Infrared, Millimeter and Terahertz Waves, a Distinguished Lecturer of IEEE Microwave Theory and Techniques Society, a Traveling Lecturer of OSA, and a Visiting Lecturer of SPIE. In addition, she serves as a panelist and reviewer for National Science Foundation (NSF), National Institutes of Health (NIH), and Department of Energy (DOE). Keywords: Terahertz Source, Terahertz Detector, Terahertz Imaging, Terahertz Spectroscopy, Plasmonics References: [1] S.-H. Yang, R. Salas, Erica M. Krivoy, Hari P. Nair, S. R. Bank, M. Jarrahi,

"Characterization of ErAs:GaAs and LuAs:GaAs Superlattice Structures for Continuous-Wave Terahertz Wave Generation through Plasmonic Photomixing", Journal of Infrared, Millimeter and Terahertz Waves, 36, DOI: 10.1007/s10762-016-0255-z, 2016

[2] S.-H. Yang, R. Watts, X. Li, N. Wang, V. Cojocaru, J. O'Gorman, L. P. Barry, M. Jarrahi, "A compact terahertz radiation source using a bimodal laser and plasmonic photomixer", SPIE Newsroom, International Society for Optics and Photonics, DOI:10.1117/2.1201603.006389, 2016

[3] S.-H. Yang, M. Jarrahi, "Frequency-Tunable Continuous-Wave Terahertz Sources based on GaAs Plasmonic Photomixers", Applied Physics Letters, 107, 131111, 2015

[4] S.-H. Yang, M. Jarrahi, "Spectral Characteristics of Terahertz Radiation from Plasmonic Photomixers", Optics Express, 23, 28522-28530, 2015

[5] S.-H. Yang, R. Watts, X. Li, N. Wang, V. Cojocaru, J. O'Gorman, L. P. Barry, M. Jarrahi, "Tunable Terahertz Wave Generation through a Novel Bimodal Laser Diode and Plasmonic Photomixer", Optics Express, 23, 31206-31215, 2015

[6] N. T. Yardimci, R. Salas, Erica M. Krivoy, Hari P. Nair, S. R. Bank, M. Jarrahi, "Impact of Substrate Characteristics on Performance of Large Area Plasmonic Photoconductive Emitters", Optics Express, 23, 32035-32043, 2015

[7] M. Jarrahi, "Advanced Photoconductive Terahertz Optoelectronics based on Nano-Antennas and Nano-Plasmonic Light Concentrators", IEEE Transactions on Terahertz Science and Technology, 5, 391-397, 2015

[8] N. T. Yardimci, S.-H. Yang, C. W. Berry, M. Jarrahi, "High Power Terahertz Generation Using Large Area Plasmonic Photoconductive Emitters", IEEE Transactions on Terahertz Science and Technology, 5, 223-229, 2015

[9] S.-H. Yang, M. R. Hashemi, C. W. Berry, M. Jarrahi, "7.5% Optical-to-Terahertz Conversion Efficiency Offered by Photoconductive Emitters with Three-Dimensional Plasmonic Contact Electrodes", IEEE Transactions on Terahertz Science and Technology, 4, 575-581, 2014

[10] C. W. Berry, N. T. Yardimci, M. Jarrahi, "Responsivity Calibration of Pyroelectric Terahertz Detectors", arXiv:1412.6878v1, 2014

[11] C. W. Berry, M. R. Hashemi, S. Preu, H. Lu, A. C. Gossard, M. Jarrahi, "Plasmonics enhanced photomixing for generating continuous-wave frequency-tunable terahertz radiation", Optics Letters, 39, 4522-4524, 2014

[12] C. W. Berry, M. R. Hashemi, S. Preu, H. Lu, A. C. Gossard, M. Jarrahi, "High Power Terahertz Generation Using 1550 nm Plasmonic Photomixers", Applied Physics Letters, 105, 011121, 2014

[13] C. W. Berry, M. R. Hashemi, M. Jarrahi, "Generation of High Power Pulsed Terahertz Radiation using a Plasmonic Photoconductive Emitter Array with Logarithmic Spiral Antennas", Applied Physics Letters, 104, 081122, 2014

[14] C. W. Berry, N. Wang, M. R. Hashemi, M. Unlu, M. Jarrahi, "Significant Performance Enhancement in Photoconductive Terahertz Optoelectronics by Incorporating Plasmonic Contact Electrodes", Nature Communications, 4, 1622, 2013

[15] S.-H. Yang, M. Jarrahi, "Enhanced light-matter interaction at nanoscale by utilizing high aspect-ratio metallic gratings", Optics Letters, 38, 3677-3679, 2013

[16] N. Wang, M. Jarrahi, "Noise analysis of photoconductive terahertz detectors", Journal of Infrared, Millimeter and Terahertz Waves, 34, 519-528, 2013

[17] C. W. Berry, M. R. Hashemi, M. Unlu, M. Jarrahi, "Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters", Journal of Visualized Experiments, 77, e50517, doi:10.3791/50517, 2013

[18] C. W. Berry, M. Jarrahi, "Principles of impedance matching in photoconductive antennas", Journal of Infrared, Millimeter and Terahertz Waves, 33, 1182-1189, 2012

[19] C. W. Berry, M. Jarrahi, "Terahertz generation using plasmonic photoconductive gratings", New Journal of Physics, 14, 105029, 2012

[20] B-Y. Hsieh, N. Wang, M. Jarrahi, "Toward Ultrafast Pump-Probe Measurements at the Nanoscale", Special Issue of "Optics in 2011" Optics & Photonics News, 22, 48, 2011

[21] B-Y. Hsieh, M. Jarrahi, "Analysis of periodic metallic nano-slits for efficient interaction of terahertz and optical waves at nano-scale dimensions", Journal of Applied Physics, 109, 084326, 2011

[22] M. Jarrahi, "Terahertz radiation-band engineering through spatial beam-shaping", Photonic Technology Letters, 21, 2019620, 2009

*This use of this work is restricted solely for academic purposes. The author of this work owns the copyright and no reproduction in any form is permitted without written permission by the author.*

Terahertz Electronics Laboratory1

Efficient Optical to Terahertz Wave Conversion Through Plasmonic Antennas

Mona Jarrahi

Electrical Engineering DepartmentUniversity of California Los Angeles


Unique specifications of terahertz waves Rotational & vibrational frequencies of most molecules lie in terahertz region Terahertz waves can see through opaque materials with spectral fingerprints Several absorption lines for water lie in terahertz range Terahertz radiation is non-ionizing & non-destructive

Applications of terahertz waves Chemical sensing, material characterization, security screening, medical

imaging and diagnostics, atmospheric and space studies

Significance of terahertz waves


Medical Imaging Security Screening

Pharmaceutical Industry

Biological and Genomic studies

Industrial Quality Control Atmospheric /space studies

Significance of terahertz waves

T. G. Phillips et. al., Proc. IEEE 80,(1992)Taday, Pharma. Sci. 92 (2003)

Nagel et. al, (RWTH Aachen) http://www.teraview.comWallace et. al, Faraday Discuss (2004)

Hu et. al, Optics Lett. 20 (1995)

Terahertz Electronics Laboratory

Physical limitations of existing terahertz sources

Electronic Sources Optical Sources

Source

Gate

Drain

Vac~

J

p-type n-type

Ec

Ev

gE

=> power roll-off 41 f

AlGaAs

GaAs

GaAs

Artificially engineered lower bandgap energies

using quantum wells

tP

RCtr 21&

21At least two poles for current:

No radiation at photon energies lower than the lowest natural bandgap energy of ~ 40 meV (~ 10 THz)

4

Same physical limitations exist in optical & electrical terahertz detection schemes


Physical limitations of existing terahertz sources

Survey on the output power of various terahertz sources

M. Tonouchi, Nature Photonics 1 (2007)

5


Conventional photoconductive terahertz sources

Limitations of conventional schemes Extremely low quantum efficiencies due to long

carrier transport path to bias electrodes Contact electrode spacing is limited by

capacitive loading and diffraction limit Maximum output power limitation due to the

carrier screening effect and thermal breakdown within small device active areas


Plasmonic photoconductive terahertz sources

Limitations of conventional schemes Extremely low quantum efficiencies due to long

carrier transport path to bias electrodes Contact electrode spacing is limited by

capacitive loading and diffraction limit Maximum output power limitation due to the

carrier screening effect and thermal breakdown within small device active areas

Advantages of plasmonic contact electrodes Ultrafast response is achieved by reducing

photo-carrier transport path to contact electrodes.

High output power levels can be achieved by using large nano-aperture active areas without increasing the capacitive loading to the terahertz antenna


Multiple guided modes supported by subwavelength gratings

8

Design of plasmonic contact electrode gratings

d a

0

h

s

TEM mode

TM mode

TM mode

ETMHTM

kTM

Several guided modes exist atwavelengths much larger than theslit periodicity:

The first guided mode is at thewavelength range much larger thanthe slit height (l >> h), and the rest ofguided modes are at the resonantwavelengths of the subwavelengthslab waveguide formed by thesubwavelength metallic slits

0 scutoff d


First generation plasmonic source

Quantum efficiency enhancement by using plasmonic electrodes No shadowing by the contact electrodes Photocarrier concentration enhancement near the contact electrode

Plasmonic Photoconductive source

ConventionalPhotoconductive source


First generation plasmonic source

Epump

E pum

p

Berry et al., Nature Communications 4 (2013)

50 times enhancement in optical‐to‐terahertz conversion efficiencies offered by the plasmonic photoconductive terahertz source

Two conventional and plasmonic photoconductiveemitters are integrated with identical bowtieantennas and fabricated side‐by‐side on the sameLT‐GaAs substrate and tested under the sameconditions.

Laser beam focus and polarization was optimizedfor each emitter by maximizing radiated power.


Terahertz detection sensitivity enhancement by utilizing plasmonic contact electrodes

Conventional photoconductive detector Plasmonic photoconductive detector


Demonstration of terahertz detection responsivity enhancement

30 times higher responsivity levels are offered by the plasmonic photoconductive detector compared to the conventional detector over the 0.1-1.5 THz frequency range



Noise analysis of plasmonic and conventional photoconductive terahertz detectors

The output noise level of the photoconductivedetectors is estimated by measuring the time-domain output photocurrent of eachphotoconductive detector while blocking theincident terahertz radiation, and calculating thefrequency components of the measured time-domain output photocurrents, subsequently.

Conventional and plasmonic photoconductivedetector prototypes have similar output noisecurrent levels at 50 mW optical pump power.This is due to the fact that the dominant noisesource in both photoconductive detectorprototypes is the Johnson–Nyquist noise ratherthan the photoconductor Shot noise.

Because of the same output noise levels, the 30 times responsivity enhancement offered by the plasmonic photoconductive detector over the 0.1-1.5 THz frequency

range, implied 30 times detection sensitivity enhancement over this frequency range.



Large area plasmonic photoconductive emitters

Extending radiation bandwidth Suppressing the carrier screening

effect and thermal breakdown at highpump power levels

Large area plasmonic photoconductive emitters

Yardimci et al., Trans. THz Sci. Technol. 5(2015)


Second generation plasmonic source

15

Photoconductor cross section where optical pump is incident

Electron

Hole

A high‐aspect ratio plasmonic grating is employed to allow efficient optical pump transmission into the nanoscale photoconductor active regions, localizing the majority of the photocarriers in close proximity to the contact electrodes.

A logarithmic spiral antenna is employed as the terahertz radiating element to offer broadband radiation properties required for pulsed terahertz generation



Quantum efficiency enhancement by using 3D plasmonic electrodes Photocarriers generated away from the surface of the substrate do not efficiently

contribute to terahertz generation when using 2D plasmonic contacts Photocarriers generated deep in the substrate can efficiently contribute to

terahertz generation when using 3D plasmonic contacts

Photoconductive source with 3D plasmonic contact electrodes

Photoconductive source with 2D plasmonic contact electrodes


Design of 3D plasmonic gratings

17

Multiple guided modes supported by subwavelength gratings

d a

0

h

s

TEM mode

TM mode

TM mode

ETMHTM

kTM

0 scutoff d

Several guided modes exist atwavelengths much larger than theslit periodicity:

The first guided mode is at thewavelength range much larger thanthe slit height ( >> h), and the restof guided modes are at the resonantwavelengths of the subwavelengthslab waveguide formed by thesubwavelength metallic slits


Design of 3D plasmonic gratings

18

The high‐aspect ratio grating is designed to excite the 4th order TEM guided mode of the subwavelength slab waveguides formed by the metallic gratings in response to a TM‐polarized optical pump.

• Optical pump transmission through the high‐aspect ratio gratings is greatly dependent on the grating height.

Yang et al., Opt. Lett. 38 (2013)


Fabrication of 3D plasmonic gratings

19

Depositing a 200nm thick SiO2film on top of GaAs substrate

Electron beam lithography, Ni (80nm) deposition and liftoff to

pattern a hard mask grating layer.

Etching 200nm-hight SiO2 grating followed by etching (ICP-RIE) 400nm

high-aspect ratio GaAs grating

Sputtering Ti/Au (20Å/ 200Å) on the SiO2/GaAs grating to

form ohmic contact

Wet Etching SiO2 grating and liftoff Au above the SiO2/GaAs grating

interface.

Depositing anti-reflection coating (2000Å) to reduce Fresnel reflection loss.

Step 1: Step 3:Step 2:

Step 4: Step 6:Step 5:



Characterization of 3D plasmonic gratings

20

H = 400 nm

70% optical pump transmission through the high‐aspect ratio plasmonic gratings at 800 nm Strong dependence of the optical pump transmission on the grating height



Yang et al., Trans. THz Sci. Technol.. 4 (2014)

The photoconductive source with 3D plasmonic contact electrodes generates an order of magnitude higher terahertz power in the 0.1‐2 THz frequency range, exhibiting a record‐high optical‐to‐terahertz conversion efficiency of 7.5%

Two plasmonic photoconductive emitters with 2Dand 3D plasmonic electrodes integrated withidentical log‐spiral antennas and fabricated side‐by‐side on the same LT‐GaAs substrate and testedunder the same conditions.

Laser beam focus and polarization was optimizedfor each emitter by maximizing radiated power.


Photoconductive emitter with 3D plasmonic electrodes 2D plasmonic electrodes

0 10 20 30 40 50 60

125

100

75

50

25

0

Bias voltage (V)

Terahe

rtz po

wer (

W)


First generation plasmonic photomixers

Duty cycle = 2% Duty cycle = 2%

Berry et al., Appl. Phys. Lett. 105 (2014)


Tradeoff between radiation power and linewidth

Pump power = 100 mW

Mod

ulation

0.1 MHz



Optical pump

Microlensarray

500 nmTerahertz radiation

500 µm

Pushing toward higher radiation powers

Employing a 2D array of terahertz emitters Further reduction of the carrier screening effect

and thermal breakdown

Employing Spiral terahertz antennas Radiation efficiency-bandwidth optimization

Terahertz power (3x3 array)

25 50 75 1000

0.5

1.0

1.5

2.0

Voltage (V)

Tera

hertz

pow

er (m

W)

320 mW240 mW160 mW

Optical pump



Pushing toward fully integrated terahertz systemstwo section digital distributed feedback

laser diode

Yang et al., Opt. Express. 23 (2015)


Pushing toward fully integrated terahertz systems

f = 1.62 THz

Yang et al., Opt. Express. 23 (2015)


Application of plasmonic terahertz emitters for communication

The required data rate is projected to be 100 Gbit/s by 2020.

The speed of today's wireless communication systems is limited by carrier frequencies less than 300 GHz.

T. Nagatsuma et. Al., IMS Digest (2014)


Heterodyne terahertz detectors with optical pump eliminate the need for terahertz local oscillators and, therefore, offer significantly high spectrometry bandwidths.

Combination of plasmonic photomixers with widely tunable optical sources with high spectral purity and narrow linewidths offer broadband terahertz spectrometry with high spectral resolution and unprecedented sensitivity levels.

Heterodyne terahertz spectrometry using plasmonic photoconductors

Photoconductor cross section where optical pump is incident


Heterodyne terahertz spectrometry using plasmonic photoconductors

-20-30-40-50-60-70-80-90

-100-110

-20-30-40-50-60-70-80-90

-100

-1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0

IF frequency (GHz)

IF in

tens

ity (d

Bm

)

~ 300K DSB noise temperature@ 100 GHz

Popt = 100 mW, 1% duty cycle


Summary

Demonstration of efficiency and output power enhancement by using plasmonic photoconductive terahertz sources

Demonstration of detection sensitivity and bandwidth enhancement by using plasmonic photoconductive terahertz detectors and spectrometers

Demonstration of multi-spectral metallic gratings enabling optical-pump terahertz-probe measurements at nan-scale

The significant performance enhancements offered by plasmonic terahertz optoelectronics have a transformational impact on future terahertz imaging and sensing systems.

30


AcknowledgementsPast/Present Members of Terahertz Electronics Lab Christopher W. Berry Ning Wang Shang Hua Yang Nezih Tolga Yardimici Ping-Keng Lu

Collaborators Prof. Arthur Gossard, University of California Santa Barbara Prof. Chee Wei Wong, University of California Los Angeles Dr. Hamid Javadi, Jet Propulsion Laboratory Prof. Seth Bank, University of Texas Austin Prof. Sascha Preu, Technical University of Darmstadt Prof. Tal Carmon, Technion Prof. Liam Barry, Dublin City University Prof. Stéphane Blin, Université Montpellier

Sponsors

Mohammed R. Hashemi Mehmet Unlu Shenglin Li Sung-Ho Park Lei Hao Wei

31

Deniz Turan Soroosh Hemmati Sabareesh Nikhil Chinta Semih Cakmakyapan

Documents

Efficient Optical to Terahertz Wave Conversion through ... · [7] M. Jarrahi, "Advanced Photoconductive Terahertz Optoelectronics based on Nano- Antennas and NanoPlasmonic Light Concentrators",