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-Patch array antenna coupling of THz source and detector

Lorenzo Bosco, Giacomo Scalari, Mattias Beck, and Jerome Faist

https://www.osapublishing.org/abstract.cfm?uri=cleo_si-2017-SM3J.5&origin=search

We study the performance of a Terahertz (THz) source-detector system coupled through equal patch-array antennas, using a single mode Quantum Cascade Laser and a Quantum Well Infrared Photodetector. The antenna allows surface emission and detection of light and use Benzocyclobutene as support.

© 2017 OSA

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-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-Measuring intensity correlations of a THz quantum cascade laser around its threshold at sub-cycle timescales

Ileana Cristina Benea Chelmus, Christopher Bonzon, Curdin Maissen, Giacomo Scalari, Mattias Beck, Jerome Faist

(Submitted on 29 Nov 2016)

https://arxiv.org/abs/1611.09562

The quantum nature of photonic systems is reflected in the photon statistics of the light they emit. Therefore, the development of quantum optics tools with single photon sensitivity and excellent temporal resolution is paramount to the development of exotic sources, and is particularly challenging in the THz range where photon energies approach kbT at T=300 K. Here, we report on the first room temperature measurement of field g1({\tau}) and intensity correlations g2({\tau}) in the THz range with sub-cycle temporal resolution (146 fs) over the bandwidth 0.3-3 THz, based on electro-optic sampling. With this system, we are able to measure the photon statistics at threshold of a THz Quantum Cascade Laser.

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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Abstract-Dynamics of ultra-broadband terahertz quantum cascade lasers for comb operation.

Hua Li, Pierre Laffaille, Djamal Gacemi, Marc Apfel, Carlo Sirtori, Jeremie Leonardon, Giorgio Santarelli, Markus Rösch, Giacomo Scalari, Mattias Beck, Jerome Faist, Wolfgang Hänsel, Ronald Holzwarth, Stefano Barbieri

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http://www.pubfacts.com/detail/26831993/Dynamics-of-ultra-broadband-terahertz-quantum-cascade-lasers-for-comb-operation

We present an experimental investigation of the multimode dynamics and the coherence of terahertz quantum cascade lasers emitting over a spectral bandwidth of ~1THz. The devices are studied in free-running and under direct RF modulation. Depending on the pump current we observe different regimes of operation, where RF spectra displaying single and multiple narrow beat-note signals alternate with spectra showing a single beat-note characterized by an intense phase-noise, extending over a bandwidth up to a few GHz. We investigate the relation between this phase-noise and the dynamics of the THz modes through the electro-optic sampling of the laser emission. We find that when the phase-noise is large, the laser operates in an unstable regime where the lasing modes are incoherent. Under RF modulation of the laser current such instability can be suppressed and the modes coherence recovered, while, simultaneously, generating a strong broadening of the THz emission spectrum.

Abstract-SUB-CYCLE MEASUREMENT OF INTENSITY CORRELATIONS IN THE TERAHERTZ RANGE

Ileana-Cristina Benea-Chelmus, Giacomo Scalari, Mattias Beck, Jerome Faist

http://www.mathpubs.com/detail/1512.02198v1/Sub-cycle-measurement-of-intensity-correlations-in-the-Terahertz-range

The Terahertz frequency range bears intriguing opportunities, beyond very advanced applications in spectroscopy and matter control. Peculiar quantum phenomena are predicted to lead to light emission by non-trivial mechanisms. Typically, such emission mechanisms are unraveled by temporal correlation measurements of photon arrival times, as demonstrated in their pioneering work by Hanbury Brown and Twiss. So far, the Terahertz range misses an experimental implementation of such technique with very good temporal properties and high sensitivity. In this paper, we propose a room-temperature scheme to measure photon correlations at THz frequencies based on electro-optic sampling. The temporal resolution of 146 fs is faster than one cycle of oscillation and the sensitivity is so far limited to ~1500 photons. With this technique, we measure the photon statistics of a THz quantum cascade laser. The proposed measurement scheme allows, in principle, the measurement of ultrahigh bandwidth photons and paves the way towards THz quantum optics.

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

5. S. A. Diddams, The evolving optical frequency comb, J. Opt. Soc. Am. B 27, p. B51-B62, 2010.

6. A. Schliesser, N. Picqué, T. W. Hänsch, Mid-infrared frequency combs, Nat. Photon.6(7), p. 440-449, 2012.

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

18. S. P. Khanna, M. Salih, P. Dean, A. G. Davies, E. H. Linfield, Electrically tunable terahertz quantum-cascade laser with a heterogeneous active region, Appl. Phys. Lett.95, p. 181101, 2009. doi:10.1063/1.3253714

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

20. M. Rösch, G. Scalari, M. Beck, J. Faist, Octave-spanning semiconductor laser, Nat. Photon. 9, p. 42-47, 2015. doi:10.1038/nphoton.2014.279

21. G. Scalari, C. Walther, M. Fischer, R. Terazzi, H. E. Beere, D. A. Ritchie, J. Faist, THz and sub-THz quantum cascade lasers, Laser Photon. Rev. 3, p. 45-66, 2009.

22. J. Khurgin, Y. Dikmelik, A. Hugi, J. Faist, Coherent frequency combs produced by self frequency modulation in quantum cascade lasers, Appl. Phys. Lett. 104, p. 081118, 2014. doi:10.1063/1.4866868

23. M. I. Amanti, G. Scalari, M. Beck, J. Faist, Stand-alone system for high-resolution, real-time terahertz imaging, Opt. Express 20, p. 2772-2778, 2012.

24. G. Villares, A. Hugi, S. Blaser, J. Faist, Dual-comb spectroscopy based on quantum-cascade-laser frequency combs, Nat. Commun. 5, p. 5192, 2014. doi:10.1038/ncomms6192

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-Low-Bias Active Control of Terahertz Waves by Coupling Large-Area CVD Graphene to a Terahertz Metamaterial

Federico Valmorra *†, Giacomo Scalari *†, Curdin Maissen †, Wangyang Fu ‡, Christian Schönenberger‡, Jong Won Choi §, Hyung Gyu Park §, Mattias Beck†, and Jérôme Faist †

http://pubs.acs.org/doi/abs/10.1021/nl4012547

We propose an hybrid graphene/metamaterial device based on terahertz electronic split-ring resonators directly evaporated on top of a large-area single-layer CVD graphene. Room temperature time-domain spectroscopy measurements in the frequency range from 250 GHz to 2.75 THz show that the presence of the graphene strongly changes the THz metamaterial transmittance on the whole frequency range. The graphene gating allows active control of such interaction, showing a modulation depth of 11.5% with an applied bias of 10.6 V. Analytical modeling of the device provides a very good qualitative and quantitative agreement with the measured device behavior. The presented system shows potential as a THz modulator and can be relevant for strong light–matter coupling experiments.

Abstract-Room temperature terahertz polariton emitter

http://apl.aip.org/resource/1/applab/v101/i14/p141118_s1

Markus Geiser, Giacomo Scalari, Fabrizio Castellano, Mattias Beck, and Jérôme Faist

Institute for Quantum Electronics, ETH Zurich, Wolfgang-Pauli-Strasse 16, 8093 Zurich, Switzerland 

Terahertz (THz) range electroluminescence from intersubband polariton states is observed in the ultra strong coupling regime, where the interaction energy between the collective excitation of a dense electron gas and a photonic mode is a significant portion of the uncoupled excitation energy. The polariton's increased emission efficiency along with a parabolic electron confinement potential allows operation up to room temperature in a nonresonant pumping scheme. This observation of room temperature electroluminescence of an intersubband device in the THz range is a promising proof of concept for more powerful THz sources.

© 2012 American Institute of Physics