Abstract-High-speed modulation of a terahertz quantum cascade laser by coherent acoustic phonon pulses

• Aniela Dunn, 
• Caroline Poyser, 
• Paul Dean, 
• Aleksandar Demić, 
• Alexander Valavanis, 
• Dragan Indjin, 
• Mohammed Salih, 
• Iman Kundu, 
• Lianhe Li, 
• Andrey Akimov, 
• Alexander Giles Davies, 
• Edmund Linfield, 
• John Cunningham & 
• Anthony Kent 
• 
  

https://www.nature.com/articles/s41467-020-14662-w#Abs1

The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology. In the terahertz-frequency range, the quantum-cascade laser (QCL) is a high-power source with the potential for high-frequency modulation. However, conventional electronic modulation is limited fundamentally by parasitic device impedance, and so alternative physical processes must be exploited to modulate the QCL gain on ultrafast timescales. Here, we demonstrate an alternative mechanism to modulate the emission from a QCL device, whereby optically-generated acoustic phonon pulses are used to perturb the QCL bandstructure, enabling fast amplitude modulation that can be controlled using the QCL drive current or strain pulse amplitude, to a maximum modulation depth of 6% in our experiment. We show that this modulation can be explained using perturbation theory analysis. While the modulation rise-time was limited to ~800 ps by our measurement system, theoretical considerations suggest considerably faster modulation could be possible.

Abstract-Continuous frequency tuning with near constant output power in coupled Y-branched terahertz quantum cascade lasers with photonic lattice

Iman Kundu, Paul Dean, Alex Valavanis, Joshua R. Freeman, Mark C. Rosamond, Lianhe H. Li, Yingjun Han, Edmund H. Linfield, and Alexander Giles Davies

https://pubs.acs.org/doi/abs/10.1021/acsphotonics.8b00251?journalCode=apchd5

We demonstrate continuous frequency tuning in terahertz quantum cascade lasers with double metal waveguides using a Y-branched coupler. Two THz QCLs placed side-by-side couple by evanescent fields across the air gap between them. Each QCL waveguide comprises a 48-μm-wide coupler and S-bend section, which are connected to an 88-μm-wide Y-branch through an impedance matching tapered section. Photonic lattices are patterned on top of the coupler section in each QCL using focused ion-beam milling to control the spectral characteristics. The waveguide design used for individual QCL sections is optimized using finite element modelling and the spectral characteristics are modelled using a transfer matrix model. Continuous frequency tuning of ~19 GHz is demonstrated while maintaining an output power of ~4.2–4.8 mW and a heat sink temperature of 50 K. The tuning is controlled electrically through Stark shift and cavity pulling effects by driving both QCLs simultaneously and represents the widest electrically-controlled continuous tuning performance from a THz QCL without significant change in output power

Abstract-Two-Dimensional Multimode Terahertz Random Lasing with Metal Pillars

Yongquan Zeng, Guozhen Liang, Bo Qiang, Kedi Wu, Jin Tao, Xiaonan Hu, Lianhe H. Li, Alexander Giles Davies, Edmund H. Linfield, Hou Kun Liang, Ying Zhang, Yidong Chong,  Qi Jie Wang,

https://pubsdc3.acs.org/doi/10.1021/acsphotonics.8b00260

Random lasers employing multiple scattering and interference processes in highly disordered media have been studied for several decades. However, it remains a challenge to achieve broadband multimode random laser with high scattering efficiency, particularly at long wavelengths. Here, we develop a new class of strongly multimode random lasers in the terahertz (THz) frequency range in which optical feedback is provided by multiple scattering from metal pillars embedded in a quantum cascade (QC) gain medium. Compared with the dielectric pillars or air hole approaches used in previous random lasers, metal pillars provide high scattering efficiency over a broader range of frequencies and with low ohmic losses. Complex emission spectra are observed with over 25 emission peaks across a 0.4 THz frequency range, limited primarily by the gain bandwidth of the QC wafer employed. The experimental results are corroborated by numerical simulations which show the lasing modes are strongly localized.

Abstract-Terahertz emission from localized modes in one-dimensional disordered systems [Invited]

Yongquan Zeng, Guozhen Liang, Bo Qiang, Bo Meng, Hou Kun Liang, Shampy Mansha, Jianping Li, Zhaohui Li, Lianhe Li, Alexander Giles Davies, Edmund Harold Linfield, Ying Zhang, Yidong Chong, and Qi Jie Wang

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-6-2-117&origin=search

We demonstrate terahertz (THz) frequency laser emission around 3.2 THz from localized modes in one-dimensional disordered grating systems. The disordered structures are patterned on top of the double-metal waveguide of a THz quantum cascade laser. Multiple emission peaks are observed within a frequency range corresponding to the bandgap of a periodic counterpart with no disorder, indicating the presence of mode localization aided by Bragg scattering. Simulations and experimental measurements provide strong evidence for the spatial localization of the THz laser modes.

© 2018 Chinese Laser Press

Abstract-Designer Multimode Localized Random Lasing in Amorphous Lattices at Terahertz Frequencies

Yongquan Zeng†, Guozhen Liang†, Hou Kun Liang‡, Shampy Mansha§, Bo Meng†, Tao Liu†, Xiaonan Hu†, Jin Tao†, Lianhe Li∥, Alexander Giles Davies∥, Edmund Harold Linfield∥, Ying Zhang‡, Yidong Chong§, and Qi Jie Wang*†§ 

† Centre for OptoElectronics and Biophotonics, School of Electrical and Electronic Engineering & The Photonic Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

‡ Singapore Institute of Manufacturing Technology, 2 Fusionopolis Way, Singapore 138634, Singapore

§ School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore

∥ School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom

ACS Photonics, Article ASAP

DOI: 10.1021/acsphotonics.6b00711

Publication Date (Web): November 29, 2016

Copyright © 2016 American Chemical Society

*E-mail: qjwang@ntu.edu.sg.
http://pubs.acs.org/doi/abs/10.1021/acsphotonics.6b00711

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, benefiting from multiple lasing modes. However, achieving controlled localized multimode 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) multimode 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 predesigned randomness in the THz frequency range and may have potential applications such as speckle-free imaging.