New lasers that fire terahertz beams could propel medical imaging and contraband detection

 

  A terahertz laser sits on top of a small square cooler. The laser could enable new portable medical       diagnostics and explosive detectors.

 

ALI KHALATPOUR/MASSACHUSETTS INSTITUTE OF TECHNOLOGY

https://www.sciencemag.org/news/2020/11/new-lasers-fire-terahertz-beams-could-propel-medical-imaging-and-contraband-detection

 Robert F. Service

Compact, chip-based lasers have conquered much of the electromagnetic spectrum, from ultraviolet to infrared, enabling technologies from digital communications and barcode readers to laser pointers and printers. But one key region of the spectrum remained untamed: the terahertz band, which lies between infrared light and microwaves. Engineers hankered for a ready source of terahertz radiation, which can penetrate opaque objects and probe chemical fingerprints inside. But compact terahertz lasers have only worked at ultralow temperatures, limiting them mostly to laboratory settings.

No longer. In today’s issue of Nature Photonics, researchers report creating a grain-of-rice–size terahertz laser on a chip that operates at 250 K, or –23°C, within reach of a plug-in cooler the size of a cracker.

“This is a great achievement,” says Miriam Vitiello, a condensed matter physicist at the Nanoscience Institute of Italy’s National Research Council. “It has been a long-term goal in the community to push up the temperature of terahertz lasers,” she adds. “There is now a plethora of applications that can be done,” from medical imaging to explosives detection at airports

Standard chip-based lasers generate their photons when electrons fall into electron vacancies within a semiconductor alloy, whose makeup determines the color. Gallium nitride, for example, emits blue light, whereas gallium arsenide emits red. However, no semiconductor alloys emit photons in the terahertz range. (“Terahertz” refers to the light’s frequency: trillions of cycles per second.) In 1994, researchers at AT&T Bell Labs created a new kind of laser in which the semiconductor’s structure, not just its chemistry, determined the wavelength. Called a quantum cascade laser (QCL), it contained hundreds of layers of semiconductors of precise thicknesses. Electrons injected into the structure cascade down hundreds of energy steps, shedding a photon at each one. Those photons were infrared in the first QCL, but in 2002 researchers in Italy and the United Kingdom created QCL lasers that emitted terahertz photons.

Those devices needed to be chilled to 50 K, but last year, researchers led by physicist Jérôme Faist at ETH Zurich unveiled a terahertz QCL made up of hundreds of alternating layers of gallium arsenide and aluminum gallium arsenide (AlGaAs) that works at 210 K. It still required bulky and expensive cryogenic coolers, however.

At higher temperature the electrons leap the barriers between layers rather than cascading through the structure one step at a time. “Over-the-barrier electron leakage was the killer,” says Qing Hu, an electrical engineer at the Massachusetts Institute of Technology. So Hu and his colleagues added more aluminum to the AlGaAs barriers in hopes of better confining the electrons. Hu’s team also had to prevent electrons from interacting in a way that caused them to leak through the AlGaAs barriers.

Now, Hu’s team has shown that by tailoring its layered structure even more precisely—some layers were just seven atoms thick—it could make electrons behave at temperatures warm enough to be reached with standard compact thermoelectric coolers. What’s more, Hu says, the same strategy should enable the team to eventually make room temperature terahertz lasers.

Room temperature terahertz sources could be paired with terahertz detectors that also work at room temperature, which Vitiello and other researchers are now developing. That marriage could lead to technologies such as terahertz imagers able to distinguish skin cancer from normal tissue without a biopsy or watch airline passengers and cargo for hidden explosives, illegal drugs, and even pharmaceutical fakes. Faist says: “We have hoped for this for a very long time. 

Abstract-Compact and ultra-efficient broadband plasmonic terahertz field detector

Yannick Salamin, Ileana-Cristina Benea-Chelmus, Yuriy Fedoryshyn, Wolfgang Heni, Delwin L. Elder, Larry R. Dalton, Jérôme Faist,  Juerg Leuthold

https://www.nature.com/articles/s41467-019-13490-x

Terahertz sources and detectors have enabled numerous new applications from medical to communications. Yet, most efficient terahertz detection schemes rely on complex free-space optics and typically require high-power lasers as local oscillators. Here, we demonstrate a fiber-coupled, monolithic plasmonic terahertz field detector on a silicon-photonics platform featuring a detection bandwidth of 2.5 THz with a 65 dB dynamical range. The terahertz wave is measured through its nonlinear mixing with an optical probe pulse with an average power of only 63 nW. The high efficiency of the scheme relies on the extreme confinement of the terahertz field to a small volume of 10−8(λTHz/2)3. Additionally, on-chip guided plasmonic probe beams sample the terahertz signal efficiently in this volume. The approach results in an extremely short interaction length of only 5 μm, which eliminates the need for phase matching and shows the highest conversion efficiency per unit length up to date.

Abstract-THz surface plasmon polariton modes coupled to complementary metasurfaces tuned by inter meta-atom distance

Janine Keller, Curdin Maissen,   Johannes Haase,  Gian Lorenzo Paravicini-Bagliani,  Federico Valmorra, José Palomo, Juliette Mangenev,  Jérôme Tignon,   Sukhdeep S. Dhillon,  Giacomo Scalari, Jérôme Faist

http://ieeexplore.ieee.org/document/8087672/

Tailoring the electro-magnetic response of materials beyond naturally occurring properties is possible with the concept of meta-materials [1]. Subwavelength elements which are usually closely spaced can influence the electro-magnetic response and form a fundamental building block of modern optics. The influence of the spacing of the meta-atoms has been investigated for direct meta-materials [2] but only little for complementary metamaterials [3], which are of interest e.g. in ultra-strong coupling experiments at THz frequencies [4]. The effective medium condition is changing due to the presence of a metal sheet in between the meta-atoms which has a very high refractive index in the THz.THz time domain spectroscopy was performed for 15 samples with a constant frequency of the complementary split ring resonator (cSRR) and varying inter meta-atom distances from 40 μm to 160 μm [6], a sample sketch is shown in Fig. 1 a). For spacings of the cSRR that are not strictly subwavelength anymore, a regime of resonant coupling to THz surface plasmon polaritons (SPPs) [5] is entered. We observe an anti-crossing (shown in Fig. 1 b)) of the cSRR LC-mode and the SPP-mode, leading to a strong coupling with a normalized coupling ratio of 3.5% at the resonance frequency of 1.07 THz. Finite element simulations with CST MWS show the characteristic field distribution of the two modes in the plane of the resonator and the intermixing of the LCmode with the SPP-mode very clearly as well as simulations of the mode extensions into the substrate (see Fig. 1 c)). Analytical modeling with a simple two oscillator model describes the coupling well and yields an effective relative permittivity of 11.6 for the coupled system. Measurements of the broader, dipole-like resonance of the cSRR in orthogonal polarization direction show a Fano-like lineshape when the SPPs tune across. Utilizing rectangular array configurations we show that the excitation direction lies along the polarization of the exciting THz pulse. Additionally, we measured the dependence of the incident angle on the frequency of the measured SPPs, where we see a splitting of the SPP. We demonstrate that the understanding of the SPP modes is fundamental for research and applications in which the metasurface has to be designed for special needs.

Abstract-Intensity autocorrelation measurements of frequency combs in the terahertz range

Ileana-Cristina Benea-Chelmus, Markus Rösch, Giacomo Scalari, Mattias Beck, and Jérôme Faist

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.96.033821

We report on direct measurements of the emission character of quantum cascade laser based frequency combs, using intensity autocorrelation. Our implementation is based on fast electro-optic sampling, with a detection spectral bandwidth matching the emission bandwidth of the comb laser, around 2.5 THz. We find the output of these frequency combs to be continuous even in the locked regime, but accompanied by a strong intensity modulation. Moreover, with our record temporal resolution of only few hundreds of femtoseconds, we can resolve correlated intensity modulation occurring on time scales as short as the gain recovery time, about 4 ps. By direct comparison with pulsed terahertz light originating from a photoconductive emitter, we demonstrate the peculiar emission pattern of these lasers. The measurement technique is self-referenced and ultrafast, and requires no reconstruction. It will be of significant importance in future measurements of ultrashort pulses from quantum cascade lasers.

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Abstract-Waveguide Embedding of a Double-Metal 1.9-THz Quantum Cascade Laser: Design, Manufacturing, and Results

 Matthias Justen. Keita Otani,  Dana Turčinková, Fabrizio Castellano,  Mattias Beck, Urs U. Graf, Denis Büchel, Michael Schultz,  Jérôme Faist

http://ieeexplore.ieee.org/document/8025815/


We present the details of a coupling structure to embed a 1.9-THz double-metal quantum cascade lasers (QCLs) into a 120-μm-wide full-height rectangular waveguide. We describe the split-block manufacturing of the waveguide coupler together with a diagonal feed horn and present power and beam shape measurements of two different devices. The two devices differ in coupling factor, which can be chosen in a wide range by the mounting position of the QCL. Our waveguide embedding allows coupling of a large fraction of the laser power into the waveguide and the subsequent horn antenna emitting with a large Gaussian mode content, enabling efficient integration into a diffraction limited optics setup. This is illustrated by a self-mixing experiment and by using the embedded QCL as a local oscillator in a heterodyne receiver.

Abstract-Electrically controlled terahertz magneto-optical phenomena in continuous and patterned graphene

• Jean-Marie Poumirol
• , Peter Q. Liu
• , Tetiana M. Slipchenko
• , Alexey Y. Nikitin
• , Luis Martin-Moreno
• , Jérôme Faist
•  & Alexey B. Kuzmenko

http://www.nature.com/articles/ncomms14626

The magnetic circular dichroism and the Faraday rotation are the fundamental phenomena of great practical importance arising from the breaking of the time reversal symmetry by a magnetic field. In most materials, the strength and the sign of these effects can be only controlled by the field value and its orientation. Furthermore, the terahertz range is lacking materials having the ability to affect the polarization state of the light in a non-reciprocal manner. Here we demonstrate, using broadband terahertz magneto-electro-optical spectroscopy, that in graphene both the magnetic circular dichroism and the Faraday rotation can be modulated in intensity, tuned in frequency and, importantly, inverted using only electrostatic doping at a fixed magnetic field. In addition, we observe strong magneto-plasmonic resonances in a patterned array of graphene antidots, which potentially allows exploiting these magneto-optical phenomena in a broad THz range.

Abstract-Ultra-broadband quantum cascade laser operating from 1.88 to 3.82 THz

Markus Rösch, Mattias Beck, Martin J. Süess, Jérôme Faist, Giacomo Scalari

(Submitted on 22 Dec 2016)

https://arxiv.org/abs/1612.07594

We report on a heterogeneous active region design for terahertz quantum cascade laser based frequency combs. Dynamic range, spectral bandwidth as well as output power have been significantly improved with respect to previous designs. When operating individually the lasers act as a frequency comb up to a spectral bandwidth of 1.1 THz, while in a dispersed regime a bandwidth up to 1.94 THz at a center frequency of 3 THz can be reached. A self-detected dual-comb setup has been used to verify the frequency comb nature of the lasers.

Abstract-Strain-Compensated InGaAs Terahertz Quantum Cascade Lasers

Keita Ohtani, Mattias Beck*, and Jérôme Faist*

Institute for Quantum Electronics, ETH Zurich, Auguste-Piccard-Hof 1, 8093 Zurich, Switzerland

ACS Photonics, Article ASAP

DOI: 10.1021/acsphotonics.6b00376

Publication Date (Web): November 9, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.6b00376

Strain-compensated InGaAs/AlInGaAs terahertz quantum cascade lasers grown by molecular beam epitaxy are reported. A choice of a moderate amount of strain in the wells (−0.24%) and in the quaternary barriers (+1.07%) makes it possible to coherently grow an active region as thick as 10 μm. Lasers based on a four quantum well design emit at 3.3 THz with a maximum operation temperature of 149 K, which is among the highest temperatures of InGaAs-based THz quantum cascade lasers.

Abstract-Short pulse generation and mode control of broadband terahertz quantum cascade lasers

Dominic Bachmann, Markus Rösch, Martin J. Süess, Mattias Beck, Karl Unterrainer, Juraj Darmo, Jérôme Faist, and Giacomo Scalari

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-3-10-1087

Ultra-short pulses are an attractive way of expanding today’s terahertz time-domain systems toward frequencies above 2 THz, and moreover mode control enables reliable generation of terahertz frequency combs based on quantum cascade lasers. We report on a waveguide engineering technique that enables the generation of a bandwidth up to ~ THz ~ 1 
 and an ultra-short pulse length of 2.5 ps in injection-seeded terahertz quantum cascade lasers. The reported technique is able to control and fully suppress higher order lateral modes in broadband terahertz quantum cascade lasers by introducing side-absorbers to metal–metal waveguides. The side-absorbers consist of a top metallization setback with respect to the laser ridge and an additional lossy metal layer. In continuous wave operation, the side-absorbers lead to octave-spanning laser emission, ranging from 1.63 to 3.37 THz, exhibiting a 725 GHz wide flat top within a 10 dB intensity range, as well as frequency comb operation with a bandwidth of 442 GHz. Numerical and experimental studies have been performed to optimize the impact of the side-absorbers on the emission properties and to determine the required increase of waveguide losses. Furthermore, these studies have led to a better understanding of the pulse formation dynamics of injection-seeded quantum cascade lasers.

© 2016 Optical Society of America

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Octave-spanning semiconductor laser for frequency comb applications

http://spie.org/x113054.xml?highlight=x2404&ArticleID=x113054

Giacomo Scalari, Markus Rösch, Mattias Beck and Jérôme Faist

A new terahertz quantum cascade laser, in continuous wave operation, has an emission that homogeneously covers more than one frequency octave.

3 April 2015, SPIE Newsroom. DOI: 10.1117/2.1201503.005783

The recent development of frequency combs has revolutionized the field of high-resolution spectroscopy. These combs can be used as frequency domain ‘rulers,’ and can be realized from either a short-pulse mode-locked laser1 or via nonlinear processes.2, 3 The laser emission from a comb can be stabilized and frequency-locked to highly stable microwave oscillators. The most common—and most efficient—method of stabilizing the offset frequency of a comb is based on a self-referencing approach,4 which requires laser emission that spans at least one octave. It is therefore important to achieve an octave-spanning spectrum with any broadband laser that is used for frequency comb generation.

Frequency combs have so far been demonstrated in the visible,5 mid-IR,3, 6,7 and terahertz (THz)8,9regions of the electromagnetic spectrum. The effective metrology and high-precision spectroscopy measurements that the combs enable1, 10–12 have many applications in several fundamental research and industrial environment contexts. Quantum cascade lasers (QCLs)13 are based on intersubband transitions between quantized electronic energy levels in the conduction band of semiconductor heterostructures. They can be used as compact coherent sources that emit radiation across mid-IR and THz wavelengths.14QCLs constitute an ideal platform for broadband sources and nonlinear optics as they exhibit an absence of reabsorption across the band gap.

We have developed new QCLs with ultra-broad gain bandwidths. We achieve these bandwidths by exploiting the quantum engineering potential of intersubband transitions. We integrate different designs of a quantum cascade structure in the same laser ridge, but tailored for different frequencies. This heterogeneous cascade concept was first demonstrated for mid-IR QCLs,15 and has also been successfully implemented in THz QCLs.16–19 In our work, we use three different active regions that are centered at 2.9, 2.6, and 2.3THz, and stacked together to fill the core of a broadband, cutoff-free double metal resonator.20, 21 As illustrated in Figure 1, THz lasers rely on metal-metal waveguides for optimal mode confinement. We have therefore taken special care in the design of the active regions and the resulting gain profile to obtain uniform power across the entire lasing region.

 

Figure 1. Scanning electron microscope image of a processed 50μm dry-etched laser (top). The inset shows the electric field intensity distribution of a metal-metal waveguide. Light-current and current-voltage characteristics for a 2mm ×150μm laser (thickness about 13μm) operated in continuous wave (CW) mode at different temperatures (bottom). The first power axis is normalized to a measurement made with a broad area terahertz (THz) absolute power meter (TK Instruments, aperture 55 ×40mm2). The second power axis shows measurements made with an Ophir THz absolute power meter with a smaller detector surface area (aperture diameter 12mm). A maximum power of 3.4mW in CW at 25K was achieved.

As has previously been shown,19 lasing spectra broaden gradually when their bias current increases. The broadest bandwidth is reached for a current density of 350A/cm2. A typical spectrum that we obtained at this operating point from a 2mm × 50μm dry-etched laser ridge is shown in Figure 2(a). In this spectrum, at temperatures up to 30K, the lasing region extends from 1.64 to 3.35THz (i.e., it covers more than one octave). The mode intensity is very well distributed, and we achieved a total of 84 modes above the lasing threshold. The broadband emission from this laser is present up to 40K, where the bandwidth is still 1.53Thz.

 

Figure 2. (a) Octave-spanning spectrum from a dry-etched 2mm ×50μm laser operating in CW mode (9.7V, 0.35A, 350A/cm2) at 18K. (b) Spectral emission for the maximum bandwidth of the comb regime, measured at 380mA. (c) Corresponding electrical beatnote, measured with an antenna. a.u.: Arbitrary units. BW: Bandwidth.

To characterize the spectral emission and coherence from our new laser, we performed beatnote (microwave signal caused by nonlinear mixing of laser modes inside the cavity) measurements at different points along the light-current curve. For our typical cavity lengths, the beatnote is in the 10–20GHz range. The presence of a beatnote, its intensity, and its linewidth are quantitative parameters that can be used to characterize the coherence of different lasing modes in a multimode laser. Beatnote analysis such as this is routinely used with frequency combs. A very narrow beatnote (less than 4kHz) is present up to an operating current of 20mA. When the bias current is increased further, however, the beatnote instantaneously broadens to have a linewidth of hundreds of MHz. It has been shown experimentally3, 9and theoretically22 that the beatnote collapse of a QCL is a clear indication that the laser is acting as a frequency comb.

With QCLs, comb operation does not correspond to the formation of short pulses in the time domain. This is in contrast to frequency combs that are obtained from mode-locked lasers. For QCL combs—as for Kerr combs that are based on micro-resonators2—the output power is about constant in time. As further evidence for comb operation, we have shown that filtering the laser signal at different frequencies does not affect the presence and linewidth of an optically measured beatnote.20 An electrical beatnote measurement, with corresponding spectral emission in the THz domain, is shown in Figure 2(b) and (c). The maximum bandwith—at which comb operation is observed—is 625GHz. This frequency corresponds to 23% bandwidth, with respect to the central frequency of 2.67THz.

We have created an octave-spanning semiconductor laser that emits in the THz range, with no spectral holes in either pulsed or continuous wave operation. We believe that this is the first octave-spanning semiconductor laser to have been developed. Our laser features a comb region with a sub-kHz beatnote, and corresponding spectral emission of more than 600GHz bandwidth.20 Although the operating conditions of our laser are limited to cryogenic temperatures, we believe that these lasers can constitute the building blocks for a compact, high-resolution spectroscopic system that is based on THz combs. We have also demonstrated that THz QCLs can be integrated easily into portable systems.23 Our future research will focus on self-stabilization of the frequency comb and the application of these devices in dual-comb-based spectrometers,24 to enable fast THz spectroscopy without moving parts.

We gratefully acknowledge funding for this work as part of the European Union's TERACOMB research project (FP7-ICT-2011-C, project 296500). We also thank G. Villares, A. Hugi, C. Bonzon, and S. Barbieri for helpful discussions.

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Giacomo Scalari, Markus Rösch, Mattias Beck, Jérôme Faist

Institute of Quantum Electronics
Swiss Federal Institute of Technology (ETHZ)

Zurich, Switzerland

Giacomo Scalari obtained his PhD in 2005 and is currently a senior staff scientist. His research interests range from THz QCLs to THz strong light-matter coupling. He received the 2006 Swiss Physical Society award for applied physics.

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2. P. Del'Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, T. J. Kippenberg, Optical frequency comb generation from a monolithic microresonator, Nature 450, p. 1214-1217, 2007.

3. A. Hugi, G. Villares, S. Blaser, H. C. Liu, J. Faist, Mid-infrared frequency comb based on a quantum cascade laser, Nature 492, p. 229-233, 2012. doi:10.1038/nature11620

4. S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, T. Udem, T. W. Hänsch, Direct link between microwave and optical frequencies with a 300THz femtosecond laser comb, Phys. Rev. Lett. 84, p. 5102-5105, 2000. doi:10.1103/PhysRevLett.84.5102

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7. F. Keilmann, C. Gohle, R. Holzwarth, Time-domain mid-infrared frequency-comb spectrometer, Opt. Lett. 29, p. 1542-1544, 2004.

8. T. Yasui, S. Yokoyama, H. Inaba, K. Minoshima, T. Nagatsuma, T. Araki, Terahertz frequency metrology based on frequency comb, IEEE J. Sel. Topics Quantum Electron.17, p. 191-201, 2011. doi:10.1109/JSTQE.2010.2047099

9. D. Burghoff, T.-Y. Kao, N. Han, C. W. I. Chan, X. Cai, Y. Yang, D. J. Hayton, J.-R. Gao, J. L. Reno, Q. Hu, Terahertz laser frequency combs, Nat. Photon. 8, p. 462-467, 2014. doi:10.1038/nphoton.2014.85

10. R. Holzwarth, T. Udem, T. W. Hänsch, J. C. Knight, W. J. Wadsworth, P. S. J. Russell, Optical frequency synthesizer for precision spectroscopy, Phys. Rev. Lett. 85, p. 2264-2267, 2000. doi:10.1103/PhysRevLett.85.2264

11. T. Yasui, Y. Kabetani, E. Saneyoshi, S. Yokoyama, T. Araki, Terahertz frequency comb by multifrequency-heterodyning photoconductive detection for high-accuracy, high-resolution terahertz spectroscopy, Appl. Phys. Lett. 88, p. 241104, 2006. doi:10.1063/1.2209718

12. B. Bernhardt, A. Ozawa, P. Jacquet, M. Jacquey, Y. Kobayashi, T. Udem, R. Holzwarth, G. Guelachvili, T. W. Hänsch, N. Picqué, Cavity-enhanced dual-comb spectroscopy, Nat. Photon. 4, p. 55-57, 2010. doi:10.1038/nphoton.2009.217

13. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, A. Y. Cho, Quantum cascade laser, Science 264, p. 553-556, 1994.

14. J. Faist, Quantum Cascade Lasers, p. 328, Oxford Univ. Press, 2013.

15. C. Gmachl, D. L. Sivco, R. Colombelli, F. Capasso, A. Y. Cho, Ultra-broadband semiconductor laser, Nature 415, p. 883-887, 2002. doi:10.1038/415883a

16. J. R. Freeman, O. P. Marshall, H. E. Beere, D. A. Ritchie, Electrically switchable emission in terahertz quantum cascade lasers, Opt. Express 16, p. 19830-19835, 2008. doi:10.1364/OE.16.019830

17. J. R. Freeman, J. Madéo, A. Brewer, S. Dhillon, O. P. Marshall, N. Jukam, D. Oustinov, J. Tignon, H. E. Beere, D. A. Ritchie, Dual wavelength emission from a terahertz quantum cascade laser, Appl. Phys. Lett. 96, p. 051120, 2010. doi:10.1063/1.3304783

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19. D. Turčinková, G. Scalari, F. Castellano, M. I. Amanti, M. Beck, J. Faist, Ultra-broadband heterogeneous quantum cascade laser emitting from 2.2 to 3.2 THz, Appl. Phys. Lett. 99, p. 191104, 2011. doi:10.1063/1.3658874

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Abstract-Electrically tunable terahertz quantum cascade lasers based on a two-sections interdigitated distributed feedback cavity

Dana Turčinková¹, Maria Ines Amanti^1,2, Giacomo Scalari¹, Mattias Beck¹ andJérôme Faist¹
http://scitation.aip.org/content/aip/journal/apl/106/13/10.1063/1.4916653
1 ETH Zurich, Institute for Quantum Electronics, Auguste-Piccard-Hof 1, 8093 Zurich, Switzerland
2 Univ. Paris Diderot, Lab. Matererk iaux et Phenomenes Quantiques, F-75205 Paris, France
Appl. Phys. Lett. 106, 131107 (2015); http://dx.doi.org/10.1063/1.4916653

The continuous electrical tuning of a single-mode terahertz quantum cascade laser operating at a frequency of 3 THz is demonstrated. The devices are based on a two-section interdigitated third-order distributed feedback cavity. The lasers can be tuned of about 4 GHz at a constant optical output power of 0.7 mW with a good far-field pattern.

Abstract-Broadband terahertz amplification in a heterogeneous quantum cascade laser

Broadband terahertz amplification in a heterogeneous quantum cascade laser Dominic Bachmann, Norbert Leder, Markus Rösch, Giacomo Scalari, Mattias Beck, Holger Arthaber, Jérôme Faist, Karl Unterrainer, and Juraj Darmo  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-3-3117

Optics Express, Vol. 23, Issue 3, pp. 3117-3125 (2015)
http://dx.doi.org/10.1364/OE.23.003117

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We demonstrate a broadband terahertz amplifier based on ultrafast gain switching in a quantum cascade laser. A heterogeneous active region is processed into a coupled cavity metal-metal waveguide device and provides broadband terahertz gain that allows achieving an amplification bandwidth of more than 500 GHz. The temporal and spectral evolution of a terahertz seed pulse, which is generated in an integrated emitter section, is presented and an amplification factor of 21 dB is reached. Furthermore, the quantum cascade amplifier emission spectrum of the emerging sub-nanosecond terahertz pulse train is measured by time-domain spectroscopy and reveals discrete modes between 2.14 and 2.68 THz.

© 2015 Optical Society of America

Abstract-How to tune terahertz quantum cascade lasers

Keita Ohtani, Mattias Beck and Jérôme Faist http://spie.org/x108049.xml?highlight=x2404&ArticleID=x108049 A transistor structure integrated into a laser waveguide electrically controls the emission frequency. 28 April 2014, SPIE Newsroom. DOI: 10.1117/2.1201404.005402 Laser-based spectroscopic sensing is now widely used in a variety of field applications such as environmental monitoring. The most suitable light source is quantum cascade lasers (QCLs), which emit a high-output optical power with a very narrow linewidth in the spectral range from mid-IR (MIR) to terahertz (THz). For spectroscopic sensing applications, the laser emission frequency must be tuned to the specific frequency at which target molecules and ions absorb light. MIR QCLs are easy to tune by controlling the device temperature, which changes the effective refractive index. In contrast, temperature has little effect on the refractive index of THz QCLs, which cannot be tuned in the same way.1 We have developed a novel technique to tune THz QCLs.2 Our device is based on a three-terminal configuration consisting of two layered sections: a THz QCL based on gallium arsenide (target frequency: 4.7THz) and a pseudomorphic high-electron-mobility transistor (PHEMT) with an indium gallium arsenide (InGaAs) single quantum well (QW) electron channel, where the carrier density in the electron channel is modulated by applying voltage to the buried contact and the PHEMT electron channel (see Figure1).3 The buried contact is integrated into the laser waveguide (through which the emitted laser light travels). This varies the effective refractive index of the single surface plasmon waveguide,4 shifting the laser emission frequency.   Figure 1. Schematic of a three-terminal terahertz (THz) quan- tum cascade laser (QCL) with a single surface plasmon waveguide. It consists of two sections: a gallium arsenide/aluminum gallium arsenide (GaAs/Al0.15Ga0.85As) THz QCL active region and an indium gallium arsenide/aluminum gallium arsenide (In0.1Ga0.9As/Al0.3Ga0.7As) pseudomorphic high-electron-mobility transistor with a single quantum well electron channel. For QCL operation, a pulsed voltage was applied between the top metal contact on the active region and the middle contact on the buried n-GaAs contact layer, while the bottom contact on the electron channel was used to control electron density in the channel by electrical gating with applied voltage Vg.   Figure 2. (a) Current-voltage-light output power (I-V-L) characteristics of the GaAs THz QCL section (78μm-wide and 1.4mm-long ridge) as a function of Vg. Current pulses of 100ns width at a duty cycle of 1% were used. (b) Waveguide loss changes (Δαw) as a function of Vg. We observed a significant increase of Jth (28%) and Δαw when Vg was increased from −3 to 2V. Figure 2(a) shows laser operation current-voltage-light output power (I-V-L) characteristics as a function of gate voltage (Vg). The threshold current density (Jth) increased with Vg from −3 to 2V although the I-V curves almost remained unchanged. We interpret the increase of Jth with Vg as indicating increased absorption in the waveguide due to electron accumulation in the InGaAs channel by Vg. The filled squares in Figure 2(b) represent the computed waveguide loss changes (Δαw) as a function of Vg. Δαw was estimated using an experimentally obtained QCL gain coefficient and a computed loss by facet reflection (mirror loss). We found that the change in waveguide loss Δαw is modulated by more than 20cm−1 as a function of Vg.   Figure 3. (a) Vg dependence of one dominating laser cavity mode. We observed a similar Vg dependence even though the laser operation condition is changed. The maximum frequency shift was about 2GHz. (b) Converted refractive index change Δneff as a function of Vg. The observed change in neff by Vg is qualitatively interpreted by the large dispersion of the intersub-band transition in the electron channel. Figure 3(a) shows the evolution of one dominating laser cavity mode at the heat sink temperature of 10K as a function of gate voltage Vg. The laser cavity mode first shows a blue shift (shift to a higher wavenumber) as Vgincreases from −2.5to 0V, whereas a red shift (i.e., to a lower wavenumber) is observed in a positive gate bias Vg from 0 to 1.5V. In this range of Vg, the maximum frequency shift is about 2GHz. The same qualitative gate dependence of the spectrum is observed at the different laser operation voltages. Figure 3(b) depicts the converted refractive index change (Δneff) from the observed frequency shift. Δneff first decreases when Vg increases from −2.25 to 0V and then starts to increase with positive Vg. Analysis showed that the Δneff dependence on n results from a refractive index change induced by the/an intersub-band optical transition in the electron channel. Both the energy shift of the intersub-band transition energy by the electric field and the electron accumulation/depletion in the electron channel play a crucial role in determining Δneff. An interesting feature of this device is the ability to perform frequency modulation for communication with a higher signal-to-noise ratio compared with amplitude modulation. We achieved the same output power with different laser emission frequencies at certain laser operating conditions by simultaneously varying Vg and the drive current J. As an example, the laser emission frequency is linearly reduced by ≈2GHz when Vg increases from 0.5 to 1.5V and the same output power is delivered if J is modulated. Thus, in principle, it would be possible to produce pure frequency modulation by modulating J and Vg simultaneously. In summary, electrical tuning of laser frequency is a promising technique that is easy to integrate into practical device structures. In addition, PHEMT detects internal light, which is useful for stabilizing optical output power. Our next step is to apply this scheme to a double metal waveguide exhibiting better temperature performance.5 Its large confinement factor is expected to provide a larger tuning range. Keita Ohtani, Mattias Beck, Jérôme Faist Institute for Quantum Electronics Swiss Federal Institute of Technology Zurich, Switzerland References: 1. M. S. Vitiello, A. Tredicucci, Tunable emission in terahertz quantum cascade lasers, IEEE Trans. THz Sci. Technol. 1, p. 76, 2011. doi:10.1109/TTHZ.2011.2159543 2. K. Ohtani, M. Beck, J. Faist, Electrical laser frequency tuning by three terminal terahertz quantum cascade lasers, Appl. Phys. Lett. 104, p. 011107, 2014. doi:10.1063/1.4861122 3. J. Teissier, S. Laurent, C. Manquest, C. Sirtori, A. Bousseksou, J. R. Coudevylle, R. Colombelli, G. Beaudoin, I. Sagnes, Electrical modulation of the complex refractive index in mid-infrared quantum cascade lasers, Opt. Express 20, p. 1172, 2012.doi:10.1364/OE.20.001172 4. R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, F. Rossi, Terahertz semiconductor-heterostructure laser, Nature 417, p. 156, 2002. doi:10.1038/417156a 5. K. Unterrainer, R. Colombelli, C. Gmachl, F. Capasso, H. Y. Hwang, A. M. Sergent, D. L. Sivco, A. Y. Cho, Quantum cascade lasers with double metal-semiconductor waveguide resonators, Appl. Phys. Lett. 80, p. 3060, 2002. doi:10.1063/1.1469657