Quantum dot laser

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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 In0.1Ga0.9As/Al0.4Ga0.6As 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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A novel distributed feedback quantum cascade laser (DFB-QCL) modulated by travelling surface acoustic wave (SAW) is proposed and theoretically studied. The device is based on a highly piezoelectric Zinc Oxide (ZnO) thin film applied directly on top of the QCL to enhance the SAW modulation of the device. To increase the coupling efficiency between the optical lasing mode and the SAW-induced DFB grating, air-waveguide and surface plasmon waveguide structures with two-section active regions are proposed. Simulation results show that a moderate coupling coefficient of ~2.5 cm-1 can be achieved for the structures, assuming high quality piezoelectric material and device fabrication can be achieved. The proposed scheme may provide a potentially alternative approach to achieve single-mode, tunable QCLs.

Random lasers are a special class of laser in which light is confined through multiple scattering and interference process in a disordered medium, without a traditional optical cavity. They have been widely studied to investigate fundamental phenomena such as Anderson localization, and for applications such as speckle-free imaging, benefitting from multiple lasing modes. However, achieving controlled localized multi-mode random lasing at long wavelengths, such as in the terahertz (THz) frequency regime, remains a challenge. Here, we study devices consisting of randomly-distributed pillars fabricated from a quantum cascade gain medium, and show that such structures can achieve transverse-magnetic polarized (TM) multi-mode random lasing, with strongly localized modes at THz frequencies. The weak short-range order induced by the pillar distribution is sufficient to ensure high quality-factor modes that have a large overlap with the active material. Furthermore, the emission spectrum can be easily tuned by tailoring the scatterer size and filling fraction. These “designer” random lasers, realized using standard photolithography techniques, provide a promising platform for investigating disordered photonics with pre-designed randomness in the THz frequency range, and may have potential applications such as speckle-free imaging.

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[1] M. A. Bandres, S. Wittek, G. Harari, M. Parto, J. Ren, M. Segev, D. N. Christodoulides, M. Khajavikhan, Science (80-. ). 2018, 359, eaar4005.

Quantum cascade laser

[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).

B. Meng and Q. J. Wang*, “Broadly tunable single-mode mid-infrared quantum cascade lasers”, Journal of Optics 17, 023001, 2015. Invited review.

[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.

[1] G. Liang, Y. Zeng, X. Hu, H. Yu, H. Liang, Y. Zhang, L. Li, A. G. Davies, E. H. Linfield, and Q. J. Wang*, "Monolithic Semiconductor Lasers with Dynamically Tunable Linear-to-Circular Polarization",  ACS Photonics  4, 517-524, 2017.

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Qclscientific

[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).

The ability to control the polarization state of emission from semiconductor lasers is essential for many applications in spectroscopy, imaging, and communications, inter alia, with monolithic integration approaches being extremely beneficial. Although manipulating the output polarization of radiation from a laser can be achieved through a number of approaches, obtaining continuous dynamic control, e.g., from linear to circular, remains extremely challenging. In this paper, we demonstrate that the polarization of terahertz (THz) frequency radiation can be continuously tuned electronically from linear to circular polarization by monolithically integrating in-plane metasurfaces with two phase-locked semiconductor-based THz quantum cascade lasers. Moreover, the metasurfaces—metal antenna arrays in this case—also act as efficient beam collimators, yielding a collimated beam divergence of~10° ×10°. Our results, however, have broad applicability to a wide range of semiconductor lasers operating from the visible to THz regions of the electromagnetic spectrum.

B. Meng, M. Yamanishi, C. Pflugel, Kazuue Fujita, F. Capasso, and Q. J. Wang*, “Investigation of tunable single-mode quantum cascade lasers via surface-acoustic wave modulation”, IEEE Journal of Quantum Electronics 49, 1053-1061, 2013.

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量子级联激光器

[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).

[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).

Quantum well

The travelling SAW is generated by applying an RF signal on the integrated interdigital transducer (IDT) and propagates along the x’[110] crystallographic direction. The inset shows the general waveguide structure and the conduction band (CB) electron density modulation of QCLs caused by the travelling SAW. A Zinc Oxide (ZnO) thin film is deposited on the top of the waveguide structure to increase the coupling efficiency.

[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.

Semiconductor 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.

[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.

High-speed room temperature quantum well infrared photodetectors (QWIPs) at λ ~ 4.9 µm have been realised in a strain compensated In0.1Ga0.9As/Al0.4Ga0.6As 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.

[1] Rodriguez, E., et al. “Al0.25Ga0.75As/GaAs Quantum well infrared photodetectors on Si substrate using Ge buffer.”, submitted, IEEE Photonics Technology Letters (2019).

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.

B. Meng, J. Tao, X. H. Li, Y. Q. Zeng, S. We, and Q. J. Wang*, “Tunable single-mode slot waveguide quantum cascade lasers”, Applied Physics Letters 104, 201106, 2014.

2.3 Al0.25Ga0.75As/GaAs Quantum well infrared photodetectors on Si substrate using Ge buffer Al0.25Ga0.75As/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 Al0.25Ga0.75As/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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QCLlaser

Femtosecond laser spectroscopic techniques (1 fs = 10-15 s, or a millionth of a billionth of a second) allow us to study ultrafast physical phenomena.

Al0.25Ga0.75As/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 Al0.25Ga0.75As/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.

[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).

We report experimental demonstration of tunable, monolithic, single-mode quantum cascade lasers (QCLs) at ~10 μm with a two-section etched slot structure. A single-mode tuning range of 77 cm-1 (785 nm), corresponding to ~7.8% of the relative tuning range, was realized with a ~20 dB side mode suppression ratio within the whole tuning range. Compared with integrated distributed feedback QCLs, our devices have the advantages of easy fabrication and a broader tuning range. Further theoretical analyses and numerical simulations show that it is possible to achieve a broad continuous tuning range by optimizing the slot structures. The proposed slot-waveguide design could provide an alternative but simple approach to the existing tuning schemes for realizing broadly continuous tunable single-mode QCLs.

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