quantum cascade laser based terahertz frequency combs … · (a) Octave-spanning spectrum from a dry-etched 2 mm x 50 µm laser operating in CW mode (9.7 V, 0.35 A, 350 A/cm 2 ) at

TERACOMBan FP7-project

q u a n t u m c a s c a d e l a s e r b a s e d t e r a h e r t z f r e q u e n c y c o m b s

Important milestones on the road to THz frequency combs

- Terahertz quantum cascade laser with gain bandwidth 1 THz - Terahertz amplification in bandwidth 500 GHz - Terahertz frequency comb with bandwidth > 500 GHz



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TERACOMB an FP7-project


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The Project

The Goals

TERACOMB is an ambitious project focused on pursuing the technology of quantum cascade lasers (QCL) to generate a frequency comb (FC) in the terahertz (THz) frequency region.To achieve the project goals, the main players in the QCL and FC technology have been brought on board. They are represented by senior scientists in their mid-career stage guaranteeing a long lasting dissemination/exploitati-on of the project achievements.The expertise of the TERACOMB project partners covers the physics and technology of quantum cascade lasers, the growth of advanced se-miconductor heterostructures, tera-hertz time-domain and time-resolved spectroscopy, microwave techniques, fibre laser technology, precise time and frequency measurement tech-niques, and the frequency comb technology. All that knowledge is ex-ploited towards success of the TERA-COMB project – the demonstration of a reliable terahertz frequency comb.

The project goal is to provide the enabling technology for realizing a compact, high power and cost effecti-ve way of THz wave synthesis using a THz frequency comb.

Important milestones on the road to THz frequency combs - Terahertz quantum cascade laser

with gain bandwidth 1 THz - Terahertz amplification in band-

width 500 GHz - Terahertz frequency comb with

bandwidth > 500 GHz

The THz FCs envisioned in this pro-ject will be based on THz quantum cascade lasers, a novel, compact and powerful THz semiconductor laser source. THz FCs will be generated by mode-locked THz QCLs, and/or by using THz QCLs as semiconductor am-plifiers. This will allow the production of FCs with average powers in excess of 10 mW, and a spectral bandwidth > 1 THz. Such high power THz FCs will be combined with highly sensitive co-herent detection techniques based on compact fs-fiber lasers that will be developed ad hoc in this project. The ultimate goal is the realization of an enabling THz technology, which may be adapted for a wide variety of appli-cations in fields such as physics, che-mistry, biology and medicine.

The recent development of frequen-cy combs has revolutionized the field of high-resolution spectrosco-py. These combs can be used as fre-quency domain ‘rulers,’ and can be realized from either a short-pulse mode-locked laser [1], or via nonline-ar processes [2,3]. The laser emission from a comb can be stabilized and frequency-locked to highly stable mi-crowave oscillators. The most com-mon—and most efficient—method to stabilize the offset frequency of a comb is based on a self-referencing approach [4], which requires laser emission spanning at least one octa-ve. It is therefore important to achie-ve 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) [8,9] regions of the electromagnetic spec-trum. The effective metrology and high-precision spectroscopy mea-surements that the combs enable [1,10–12] have many applications in several fundamental research and in-

dustrial environment contexts. Quan-tum cascade lasers (QCLs) [13] are based on intersubband transitions. They can be used as compact coher-ent sources that emit radiation across mid-IR and THz wavelengths [14]. QCLs constitute an ideal platform for broadband sources and non-linear 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 exploi-ting the quantum engineering poten-tial of intersubband transitions. We integrate different designs of a quan-tum cascade structure in the same laser ridge, but tailored for different frequencies. This heterogeneous cascade concept was first demons-trated 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 THz, 2.6 THz, and 2.3 THz, and stacked together to fill the core of a broadband, cutoff-free double metal resonator [20,21]. As illustrated in Fig. 1, THz lasers rely on

Octave-spanning semiconductor laser for frequency comb applications

A new terahertz quantum cascade laser, in continuous wave operation, with an emission that covers more than one frequency octave without gap.

G. Scalari, M. Roesch, M. Beck and J. FaistInstitute for Quantum Electronics, ETH Zürich, Zürich, Switzerland



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

metal-metal waveguides for optimal mode confinement. We have there-fore taken special care in the design of the active regions and the resulting gain profile to obtain uniform power across the entire lasing region.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 350 A/cm2. A typical spectrum that we obtained at this operating point from a 2 mm x 150 µm wet-etched laser ridge is shown in Fig. 2(a). In this spectrum, at temperatures up to 30 K, the la-sing region extends from 1.64 THz to 3.35 THz (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 40 K, where the bandwidth is still 1.53 THz.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) mea-surements at different points along the light-current curve. For our typi-cal cavity lengths, the beatnote is in the 10–20 GHz range. The presence of a beatnote, its intensity, and its li-newidth are quantitative parameters that can be used to characterize the coherence of different lasing mo-des in a multimode laser. Beatnote analysis such as this are routinely used with frequency combs. A very narrow beatnote (less than 4 kHz) is present up to an operating current of 420 mA. When the bias current is in-creased further, however, the beat-

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

note instantaneously broadens to have a linewidth of hundreds of MHz. It has been shown experimentally [3,9], and theoretically [22] 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—like for Kerr combs that are based on micro-resonators [2]—the output power is about con-stant in time. As further evidence for comb operation, we have shown that filtering the laser signal at different

frequencies does not affect the pre-sence and linewidth of an optically-measured beatnote [20]. An elec-trical beatnote measurement, with corresponding spectral emission in the THz domain, is shown in Fig. 2(b) and Fig. 2(c). The maximum band-with—at which comb operation is ob-served—is 625 GHz. This frequency corresponds to 23% bandwidth, with respect to the central frequency of 2.6 THz.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



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References[1] T. Udem et al., “Optical frequency metrology“, Nature, 416, pp. 233–237, 2002.

[2] P. Del’Haye et al., “Optical frequency comb ge-neration from a monolithic microresonator“, Nature 450, pp. 1214–1217, 2007.

[3] A. Hugi et al., “Mid-infrared frequency comb based on a quantum cascade laser“, Nature 492, pp. 229–233, 2012.

[4] S. A. Diddams et al., “Direct link between mi-crowave and optical frequencies with a 300 THz femtosecond laser comb“, Phys. Rev. Lett. 84, pp. 5102–5105, 2000.

[5] S. A. Diddams, “The evolving optical frequency comb“, J. Opt. Soc. Am. B 27, pp. B51– B62, 2010.

[6] A. Schliesser et al., “Mid-infrared frequency combs“, Nat. Photon. 6 (7), pp. 440–449, 2012.

[7] F. Keilmann et al., “Time-domain mid-infrared frequency- comb spectrometer“, Opt. Lett. 29, pp. 1542–1544, 2004.

[8] T. Yasui et al., “Terahertz frequency metrology based on frequency comb“, IEEE J. Sel. Topics Quan-tum Electron. 17, pp. 191–201, 2011.

[9] D. Burghoff et al., “Terahertz laser frequency combs“, Nat. Photon. 8, pp. 462– 467, 2014.

[10] R. Holzwarth et al., “Optical frequency synthe-

sizer for precision spectroscopy“, Phys. Rev. Lett. 85, pp. 2264–2267, 2000.

[11] T. Yasui et al., “Terahertz frequency comb by multifrequency-heterodyning photoconductive de-tection for high- accuracy, high-resolution terahertz spectroscopy“, Appl. Phys. Lett. 88, p. 241104, 2006.

[12] B. Bernhardt et al., “Cavity-enhanced dual-comb spectroscopy“, Nat. Photon. 4, pp. 55–57, 2010.

[13] J. Faist et al., “Quantum cascade laser“, Science 264, pp. 553–556, 1994.

[14] J. Faist, “Quantum Cascade Lasers“, p. 328, Ox-ford Univ. Press, 2013.

[15] C. Gmachl et al., “Ultra-broadband semicon-ductor laser“, Nature 415, pp. 883–8877, 2002.

[16] J. R. Freeman et al., “Electrically switchable emission in terahertz quantum cascade lasers“, Opt. Express 16, pp. 19830–19835, 2008.

[17] J. R. Freeman et al., “Dual wavelength emissi-on from a terahertz quantum cascade laser“, Appl. Phys. Lett. 96, p. 051120, 2010.

[18] S. P. Khanna et al., “Electrically tunable tera-hertz quantum-cascade laser with a heterogeneous active region“, Appl. Phys. Lett. 95, p. 181101, 2009.

[19] D. Turčinkova et al., “Ultra-broadband hetero-geneous quantum cascade laser emitting from 2.2 to 3.2 THz“, Appl. Phys. Lett. 99, p. 191104, 2011.

[20] M. Rösch et al., “Octave-spanning semiconduc-tor laser“, Nat. Photon. 9, pp. 42, 2015.

[21] G. Scalari et al., “THz and sub-THz quantum cascade lasers“, Laser Photon. Rev. 3, pp. 45–66, 2009.

[22] J. Khurgin et al., “Coherent frequency combs produced by self frequency modulation in quantum cascade lasers“, Appl. Phys. Lett. 104, p. 081118, 2014.

[23] M. I. Amanti et al., “Stand-alone system for high- resolution, real-time terahertz imaging“, Opt. Express 20, pp. 2772–2778, 2012.

laser been developed. Our laser fea-tures a comb region with a sub-kHz beatnote, and corresponding spectral emission of more than 600 GHz band-width [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 sys-tems [23].

In Figure 1 we report an example of the effect of the RF modulation of the QCL current on the emission spectra of an ultra-broadband THz QCL [2]. In particular, we observe that under RF modulation the THz emission is signi-ficantly broadened. This work is presently under review [1] and contains a complete investi-gation of the QCL coherence proper-ties using three techniques: (i) THz FTIR spectroscopy, (ii) RF beatnote spectroscopy [3] and (iii) electro-op-

tic sampling of the THz QCL emission using a fs-laser [4]. In particular, the presented results are the fruit of se-veral technical improvements obtai-ned in collaboration with the other partners of the TERACOMB project. The two major improvements are (i) a widening of the RF modulation band-width of the QCLs, and (ii) an increase of 15 dB of the SNR of the electro-optic sampling setup.The widening of the RF modulation bandwidth was obtained through a

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

Mode dynamics and coherence of terahertz quantum cascade laser emission over a spectral bandwidth of 1 THz are presented.

Figure 1. THz emission (in normalized arbitrary units) vs driving current in intensity color scale. (a) QCL opera-ted in free running. (b) QCL modulated at 13.22 GHz and +23 dBm of RF power.

Hua Li, Pierre Laffaille, Djamal Gacemi, Marc Apfel, Carlo Sirtori and Stefano BarbieriLaboratoire Matériaux et Phénomènes Quantiques, Université Paris 7 and CNRS UMR 7162, Paris, France

Jeremie Leonardon and Giorgio Santarelli Laboratoire Photonique, Numérique et Nanosciences (LP2N), IOGS - CNRS - Universités de Bordeaux 1, Talence, France

Markus Rösch, Giacomo Scalari, Mattias Beck and Jerome FaistETH Zurich, Institute of Quantum Electronics, Zurich, Switzerland

Wolfgang Hänsel and Ronald Holzwarth Menlo Systems GmbH, Martinsried, Germany

QCL in free running QCL RF modulated (13.22 GHZ; +23dBm)



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Figure 2. Improved low loss cryogenic RF injection system based on a broadband RF launcher. Also shown in the picture are a metal-metal THz QCL and a 50 Ω stripline bridging between the launcher and the QCL.

re-design of the cryostat cold-finger in order to accommodate a broad-band RF launcher (see Fig. 2). Mo-reover, we fabricated a quarter-wave microstrip transmission line impe-dance adapter based on cyclic olefin

co-polymer (COC) [5]. The validation of the impedance adapter using the quarter-wave adaptation is shown in Fig. 3, where it is compared to diffe-rent RF injection methods, namely (i) a standard 50 Ω transmission line with coaxial feed, and (ii) a standard 50 Ω transmission line with RF end launcher feed. The spectra of Fig. 3 are obtained using the microwave rectification technique described in Ref. [6]. They clearly show that the impedance adaptation reduces signi-ficantly the microwave attenuation.The improvement of the SNR of the electro-optic sampling setup was the fruit of three main steps: (i) The deve-lopment of a custom made 1550 nm fs-laser with 1 GHz repetition rate, (ii) the development of a custom-made

Figure 3. 2 mm-long metal-metal waveguide THz QCL. Comparison of the microwave rectification curves mea-sured under lasing operation with a COC quarter-wave adapter and a broadband launcher (red), a 50 Ω strip-line with broadband launcher (black), and a 50 Ω stripline with a coaxial feed (blue).

Figure 4. 3 mm-long metal-metal waveguide THz QCL driven at 448 mA and 30 K heat sink tempe-rature. (a) Down-converted THz spectrum in the RF range measured with an electro-optic sampling setup based on a 1 GHz repetition rate fiber laser and a broadband balanced detector (see text). The resolution bandwidth of the spectrum analyzer is 1 MHz [4]. (b) Corresponding spectrum in the THz range, collected with the FTIR spectrometer (spec-tral resolution 7.5 GHz). The spectrum is displayed in logarithmic scale. The number of modes within 15 dB from the peak of the most intense one (appro-ximately 14 modes) corresponds to the number of down-converted lines in (a). Note that there is not a one to one correspondence between the down-converted THz modes in (a) and the actual THz mo-des in (b). Indeed, the QCL roundtrip frequency of 13.225 GHz is 225 MHz larger that the 13th harmo-nic of the 1 GHz fs-laser repetition, which leads to a folding of the THz down-converted modes (see Ref. [4] for a detailed explanation of this process).

References[1] H. Li et al., “Dynamics of ultra-broadband quan-tum cascade lasers for comb operation”, submitted to Opt. Expr. (August 2015).

[2] M. Rösch et al., “Octave spanning semiconduc-tor laser”, Nature Photon. 9, 42, 2015.

[3] P. Gellie et al., “Injection-locking of THz quantum cascade lasers up to 35 GHz using RF amplitude mo-dulation”, Opt. Expr. 18, 20799, 2010.

[4] S. Barbieri et al., “Coherent sampling of active mode−locked terahertz quantum cascade lasers and frequency synthesis”, Nature Photon. 5, 306, 2011.

[5] In collaboration with J-F. Lampin and E. Peytavit, Laboratoire IEMN (Lille, France).

[6] S. Barbieri et al., “13 GHz direct modulation of THz quantum cascade lasers”, Appl. Phys. Lett., 91, 143510, 2007.

[7] A. W. M. Lee et al. “High-power and high-tem-perature THz quantum cascade lasers based on lens-coupled metal-metal waveguides”, Opt. Lett. 39, 2840, 2007.

[8] H. Li et al., “Phase locking of metal-metal THz QCL to a fs-fiber comb”, Manuscript in preparation.

balanced detection exploiting a pair of broadband fibered InGaAs pho-todiodes with a bandwidth of 500 MHz, and (iii) an improvement of the detection optics, including the use of a hyper-hemispherical silicon lens positioned on the QC laser facet [7]. Thanks to these new developments we can now sample the emission of ultra-broadband THz QCLs based on metal-metal resonators with a SNR of 15 dB (1 MHz resolution bandwidth) for the most intense modes (see an example of a sampled spectrum in Fig. 4) [8].



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An important alternative to the di-rect generation of powerful terahertz frequency combs (THz FC) using mo-de-locked quantum cascade lasers (QCLs) [1,2] is the realization of THz FCs derived from near-infrared (NIR) FCs, which are generated by femtose-cond solid-state lasers. Due to the low efficiency (10-6) of this frequency conversion process, the power of the produced THz FCs is rather low. The demonstrated THz amplifier [3] could provide the necessary power boost for such combs. We have shown a proof-of-principle THz amplifier de-vice by harvesting the development of heterogeneous THz QCLs with

Broadband amplification of terahertz frequency combs

A broadband terahertz amplifier based on a quantum cascade heterost-ructure is demonstrated.

Dominic Bachmann, Karl Unterrainer and Juraj DarmoPhotonics Institute, TU Wien, Vienna, Austria

Markus Rösch, Giacomo Scalari, Mattias Beck and Jerome FaistInstitute for Quantum Electronics, ETH Zurich, Zurich, Switzerland

Figure 1. Broadband QCL based THz amplifier. (a) Schematic of the setup for amplification of a THz FC comb. (b) Sketch of the coupled cavity metal-metal QCL device with attached silicon lens.

spectrally broad optical gain [4,5].The system used for amplification of a THz FC is based on a standard time-domain spectroscopy setup (THz-TDS) [6] and is shown in Fig. 1(a). A Ti:Sapphire laser is used to generate phase-locked THz seed pulses in the emitter section of a coupled cavity QC device [7] and to coherently de-tect the THz electric field emerging from the QC amplifier section. Syn-chronized sub-nanosecond RF pulses, derived from the fs laser beam are injected into the QC amplifier section to achieve efficient gain switching [8].The THz amplifier device consists of two sections, a THz emitter serving

as an electro-optic convertor of NIR FCs to THz FCs and the actual QC am-plifier section, which is formed by a Fabry-Pérot amplifier. In order to en-sure a robust optical system, the THz emitter and the QC amplifier sections are monolithically integrated and as-sembled into a compact unit together with THz outcoupling optics.Figure 2 shows the time-domain THz electric field output of the QC ampli-fier for two operation modes – when the amplifier section is switched off (OFF mode) and when the amplifier section is synchronously gain-swit-ched with the injection of the THz seed pulses (AMP mode). While the output in the OFF mode represents the THz FC as generated from the

Figure 2. (a) QC amplifier THz electric field output for the two operating modes OFF and AMP. The amplifier is operated at a heat-sink temperature of 10 K. (b) Temporal shape of the RF gain switching pulse with the relative timing to the THz pulse train.

NIR FC, the AMP mode output de-monstrates the emergence of a train of amplified pulses derived from the NIR FC. The length of the pulse train is limited by the duration of the RF pulse and the energy that is carried by this pulse train exceeds the ener-gy of the original seeded FC by orders of magnitude (total amplification factor >50).The frequency content of the pulse trains emerging from the device is shown in Fig. 3. The frequency reso-lution of these TDS spectra is 2.5 GHz and is therefore similar to the resolu-tion of commonly used commercial Fourier transform infrared spectrome-ters. They show discrete modes that are spaced by the free spectral range



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Figure 3. High-resolution (2.5 GHz) intensity spectra, obtained from a 400 ps long time-domain output of the THz amplifier driven in the OFF and AMP mode. The individual modes are spaced by the free spectral range of the THz amplifier waveguide (21 GHz).

of the QC amplifier cavity (21 GHz). The achieved amplification factors are more than 10 dB within a band-width of 540 GHz (localized between 2.14 and 2.68 THz) and 30 dB for the central 150 GHz wide frequency range.In the future, an optical fiber based laser system is going to be used for seeding and detecting the THz fre-quency comb without any moving parts. Such a system modification to-gether with a LN2 operated THz QCL amplifier will provide a platform for versatile and table-top sources of THz FCs usable for molecular spectrosco-py such as gas tracing in the human breath [9].

References[1] D. Burghoff et al., “Terahertz laser frequency combs”, Nat. Photon. 8, 462, 2014.

[2] M. Rösch et al., “Octave-spanning semiconduc-tor laser”, Nat. Photon. 9, 42, 2015.

[3] D. Bachmann et al., “Broadband terahertz am-plification in a heterogeneous quantum cascade laser”, Opt. Express 23, 3117, 2015.

[4] D. Turčinková, et al., “Ultra-broadband heteroge-neous quantum cascade laser emitting from 2.2 to 3.2 THz”, Appl. Phys. Lett. 99, 191104, 2011.

[5] D. Bachmann et al. “Spectral gain profile of a multi-stack terahertz quantum cascade laser”, Appl. Phys. Lett. 105, 181118, 2014.

[6] D. Grischkowsky et al., “Far-infrared time-do-main spectroscopy with terahertz beams of diel-ectrics and semiconductors”, J. Opt. Soc. Am. B 7, 2006, 1990.

[7] M. Martl et al., “Gain and losses in THz quantum cascade laser with metal-metal waveguide”, Opt. Express 19, 733, 2011.

[8] N. Jukam et al., “Terahertz amplifier based on gain switching in a quantum cascade laser”, Nat. Photon. 3, 715, 2009.

[9] A. M. Fosnigth et al., “Chemical analysis of exhaled human breath using a terahertz spectro-scopic approach”, Appl. Phys. Lett. 103, 133703, 2013.

TU WienPhotonics InstituteVienna, Austria

Université Paris VII - CNRSLaboratoire MPQParis, France

ETH ZurichInstitute of Quantum ElectronicsZurich, Switzerland

Menlo Systems GmbHMartinsried, Germany

Bordeaux UniversityLaboratoire Photonique, Numérique et Nanosciences (LP2N)Talence, France

Cambridge UniversityCavendish LaboratoryCambridge, United Kingdom

Project Partners

ImprintScientific Contact

Administrative Contact

Dr. Juraj DarmoTU [email protected]

Verena Kopper, BScTU [email protected]

© TU Wien, Photonics Institute, 2015 www.teracomb.eu

Documents

quantum cascade laser based terahertz frequency combs … · (a) Octave-spanning spectrum from a dry-etched 2 mm x 50 µm laser operating in CW mode (9.7 V, 0.35 A, 350 A/cm 2 ) at