Quantum cascade lasers - Qi Jie Wang's Group

1. Topological quantum cascade laser Topological insulator is a new phase of matter that is conducting on the edges but insulating in the interiors. It led to the 2016 Nobel Prize in Physics for three of its pioneers. In these years, the concept has migrated from condensed matter physics into photonics and various structures such as gyromagnetic photonic crystals, helical waveguide arrays, and coupled ring resonators have been demonstrated to support topologically-protected edge states which is unidirectionally-propagating, backscattering-free and robust. The photonic topological edge state is intrinsically advantageous for laser emission as it is immune to fabrication defects or disorders. Recently, a new type of lasing utilising topologically-protected photonic edge states have been reported [1–3]. However, these demonstrations of topological lasers have required an external laser source for optical pumping, and operated in the conventional optical frequency regime [1–3]. Unlike previous topological lasers that relied on large-scale features to impart topological protection, in our work, we employ a compact valley photonic crystal and demonstrate the first electrically-pumped THz QCL based on topologically-protected valley edge states [4]. Lasing with regularly-spaced emission peaks occurs in a sharp-cornered triangular cavity, even with the introduction of perturbations, due to the existence of topologically-protected valley edge states that circulate around the cavity without experiencing localisation. The properties of the topological lasing modes are further probed by adding different outcouplers to the cavity. Looking ahead, it is promising to use the valley degree of freedom in other active photonic devices, and the realization of an electrically-pumped topological laser points the way towards incorporating topological protection into practical device applications.
Reference: [1] M. A. Bandres, S. Wittek, G. Harari, M. Parto, J. Ren, M. Segev, D. N. Christodoulides, M. Khajavikhan, Science (80-. ). 2018, 359, eaar4005. [2] G. Harari, M. A. Bandres, Y. Lumer, M. C. Rechtsman, Y. D. Chong, M. Khajavikhan, D. N. Christodoulides, M. Segev, Science (80-. ). 2018, 359, eaar4003. [3] B. Bahari, A. Ndao, F. Vallini, A. El Amili, Y. Fainman, B. Kanté, Science (80-. ). 2017, 358, 636. [4] Y. Zeng, U. Chattopadhyay, B. Zhu, B. Qiang, J. Li, Y. Jin, L. Li, A. G. Davies, E. H. Linfield, B. Zhang, Y. Chong, Q. J. Wang, Nature 2019, accepted.
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Figure 1. Fabrication and characterization of the topological THz QCL. *(a),* SEM image of the THz QCL, whose optical cavity consists of an in-plane triangular loop of a side length 21a. The shaded yellow area is pumped by electrical injection, while the other parts are passive. The green dashed line indicates the domain wall. The black rectangle indicates a defect (39 μm×33.5 μm) etched entirely through the active medium of the THz QCL. Inset: cross-sectional schematic and magnified figure of the domain wall. *(b),* Calculated quality (Q) factors of the structure’s eigenmodes, with realistic material absorption losses (~ 20 cm-1) within the passive region. The shaded area indicates the photonic bandgap of the valley Hall lattice. *(c),* Typical eigenmode electric field (\|Ez\|) profiles at around 3.23 THz, without an outcoupling defect, with a side defect, and a corner defect. *(d),* Emission spectra for the QCL without an outcoupling defect (top), with a side defect (middle), and a corner defect (bottom). Gray vertical lines indicate the peak frequencies of the defect-free QCL, which correspond closely to those of the QCL with a defect. The emission spectra are vertically offset with increasing pumping intensities for clarity.

Published papers: [1] *Zeng, Y. Q., Chattopadhyay, U., Zhu, B. F., et al. “Electrically Pumped Topological Laser with Valley Edge Modes”, Nature, 578, 246-250, 2020. [2] Zeng, Y. Q., Qiang, B. & Wang, Q. J. “Photonic Engineering Technology for the Development of Terahertz Quantum Cascade Lasers”, Advanced Optical Materials 1900573 (2019). [3] Kim, K., Bittner, S., Zeng, Y. Q., et al. “Electrically pumped semiconductor laser with low spatial coherence and directional emission", Applied Physics Letters, 115, 071101 (2019). [4] Cerjan, A., Bittner, S., Constantin, M., Guy, M., Zeng, Y. Q.*, et al. “Multimode lasing in wave-chaotic microcavity lasers”, accepted, Physical Review B (2019). [5] Li, N., Yuan, H., Xu, et al. “Radiation Enhancement by Graphene Oxide on Microelectromechanical System Emitters for Highly Selective Gas Sensing”, ACS sensors 4, 2746-2753 (2019).

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2. High speed quantum cascade laser and quntum well infrared photodetector 2.1 Tunability of the Free-Spectral Range by microwave injection into a Mid-Infrared Quantum Cascade Laser The Free-Spectral Range (FSR) of Quantum Cascade Lasers (QCLs) emitting at 4.7 µm can be tuned through direct microwave modulation. The intrinsic short carrier lifetime of intersubband transitions, along with an impedance matched electrical packaging, allow a high-speed modulation up to more than 30 GHz. A significant broadening and flattening of the lasing spectrum are observed under Radio-Frequency (RF) injection with frequencies close to the round-trip cavity. An accurate analysis of the high resolution spectra of the laser shows a comb-like regime for both free-running and RF modulated QCLs, if the modulation frequency is within the locking-range of the device. One of the main advantages of collecting high-resolution mid-infrared spectra, over the plain investigation of the beatnote in the microwave region, is the access to all the longitudinal modes and thus the accurate measure of the FSR over the whole optical spectrum. The use of high-resolution spectroscopy provides an in-depth and comprehensive analysis of lasing spectra under microwave modulations.

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Figure 1. *(a),* Artistic representation of the QCL lasing with the RF injection. *(b),* Illustration of the measurement M1 without and M2, M3, M4 and M5 with RF injection at different frequencies in and out of the locking-range represented in grey. *(c),* Representation of the variation Δω of the peak position between the free-running spectrum (grey) and injected spectrum (red). *(d),* Analysis of the variation of the FSR over the spectra of a QCL emitting at 4.7 µm driven by a current of 400 mA with RF injection at the extremity of the locking-range. *(e),* The right axis represents the injection frequency as a function of the average FSR measured over a stable area of the spectrum and the left axis is the histograms of the FSR repartition for each measurement. The yellow area represents the locking-range of the device. *(f),* Evolution of the spectrum for several power injections for a 4 mm QCL at 4.7 µm with a current of 300 mA corresponding to 1.6 J_th. *(g),* The variation of the peak position between the spectra injected (M2, M3, M4 or M5) and the spectrum non-injected taken as reference (M1) with the expected slope in dashed line (ΔM2, ΔM3, ΔM4, ΔM5).

Published papers: [1] *Rodriguez, E.*, et al. “Tunability of the Free-Spectral Range by microwave injection into a Mid-Infrared Quantum Cascade Lasers.”, submitted, Laser & Photonics Reviews (2019). TOP
2.2 Room temperature, wide-band Quantum Well Infrared Photodetector for microwave optical links at 4.9 µm wavelength High-speed room temperature quantum well infrared photodetectors (QWIPs) at λ ~ 4.9 µm have been realised in a strain compensated In_0.1Ga_0.9As/Al_0.4Ga_0.6As heterostructure grown on a GaAs substrate. The high-speed properties at room temperature have been optimised by using a specifically designed air-bridge structure which greatly reduces the time constant of the effective RC circuit, thus allowing transmission and detection of high-frequency signals. By modulating a high-speed quantum cascade laser (QCL) centred at λ ~ 4.7 µm we were able to record a modulation of the photocurrent up to ~26 GHz, which is limited by our setup. At 300 K and under a bias voltage of –5 V our device shows high responsivity and detectivity of 100 mA/W and 1 x 10⁷ Jones, respectively. The developed high-performance QWIPs at this wavelength are highly promising for optical heterodyne measurement, high-speed free space communications in microwave optical links and frequency comb QCLs characterisations.
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Figure 1. *(a),* Band-structure of the developed qauntum well infrared photodetector (QWIP) with 3 quantum wells (QWs) active regions at a bias voltage of 2 V. The most relavant physical parameters of the sructure are indicated in the figure,. *(b),* SEM picture of the top view of the QWIP (The inset: zoom-in of the active region with the air bridge). *(c),* Spectra of the QWIP at several different temperatures up to 300 K. *(d),* The schematic illustration of the setup used to investigate the high-speed modulation of the QWIP. A high-speed direct-modulated quantum cascade laser (QCL) is used in the experiment. *(e),* Photoresponse of the QWIP with the QCL modulated at –10 dBm at 300 K (red line + symbol) at a bias voltage of –4 V. The rectification measurements of QWIP using the cryostat setup (blue line + symbol) and using the coplanar tip configuration (green line + symbol) with in dashed grey line the resonances due to parasitic effects of its electrical circuit. The black dashed line represents the value at -3 dB. *(f),* The first harmonic of the beatnote, corresponding to the 3 mm laser, is collected on the QWIP at 300 K. The inset: the second harmonic of the beatnote at 23.15 GHz; injection-locking observed on the QWIP at 300 K by *(g),* tuning the injection frequency on a 3 mm laser.
Published papers: [1] *Rodriguez, E., et al. “Room temperature, wide-band Quantum Well Infrared Photodetector for microwave optical links at 4.9 μm wavelength.”, ACS Photonics*, 2018, 5 (9), pp 3689–3694 DOI: 10.1021/acsphotonics.8b00704. TOP
2.3 Al_0.25Ga_0.75As/GaAs Quantum well infrared photodetectors on Si substrate using Ge buffer Al_0.25Ga_0.75As/GaAs Quantum Well Infrared Photodetectors (QWIPs) were grown by molecular beam epitaxy on Si substrate. A Ge buffer layer was introduced to reduce threading dislocation density in GaAs caused by the lattice mismatch between Al_0.25Ga_0.75As/GaAs and Silicon. Transmission electron microscope images showed good growth quality while preserving good control of the thickness of quantum wells and barriers. QWIPs grown on both Silicon and GaAs substrates were compared which show similar performances on the background current, the responsivity and the detectivity. A slightly higher background current observed in the QWIPs on Si substrate is due to the low-density threading dislocation. The development of high-quality QWIPs on Si substrate is highly promising for CMOS compatible wide-tunability mid-infrared detection, which would lead to a significant reduction in the cost and an enhanced production capacity.
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Figure 1. *(a),* The Transmission Electron Microscope (TEM) image of Quantum Well Infrared Photodetector (QWIP) grown on Si substrate showing threading dislocations. Only one dislocation was observed in the active region in this image. *(b),* The TEM image of the active region showing the thickness of GaAs and Al_0.25Ga_0.75As layers. *(c),* Band-structure of a single quantum well of the active regions at a bias voltage of 0 V. The most relevant physical parameters of the structure are indicated in the figure. *(d),* Current-voltage (IV) characteristics in background condition at 80 and 300 K. The solid lines and the short dash dots represent respectively the GaAs substrate and the Si substrate. *(e),* responsivity spectra as a function of the wavenumber of the QWIP on Si substrate (solid red line) and on GaAs substrate (short dash-dot black line) at a bias voltage of 1 Volt and at 80 K. inset: Responsivity as a function of the voltage of the QWIP on Si substrate (solid red line) and on GaAs substrate (short dash-dot black line) at 80 K.
Published papers: [1] *Rodriguez, E.*, et al. “Al_0.25Ga_0.75As/GaAs Quantum well infrared photodetectors on Si substrate using Ge buffer.”, submitted, IEEE Photonics Technology Letters (2019).


	Quantum cascade lasers (QCLs) are semiconductor lasers based on intersubband transitions and resonant tunneling in multiple quantum wells typically formed in the conduction band of semiconductor heterostructures. QCLs can be designed to emit radiation at various wavelengths over a significant portion the mid-infrared spectrum (~3 – 23 um) as well as the far-infrared spectrum (~60 – 300 um). These wavelength spectra are important for pollutant and toxic gas sensing, industrial process monitoring, free space communication, and military applications. Due to their efficiency in generating tens of photons per electron through a cascade scheme, and their flexibility in achieving different emission wavelengths by selecting particular design parameters of the quantum wells, QCLs are becoming important high performance and highly efficient light sources in this wavelength range.