Abstract-Photonic Engineering Technology for the Development of Terahertz Quantum Cascade Lasers

Yongquan Zeng, Bo Qiang, Qi Jie Wang,

https://onlinelibrary.wiley.com/doi/10.1002/adom.201900573

A terahertz (THz) quantum cascade laser (QCL) is an electrically pumped semiconductor laser based on the inter‐subband electron transitions in a multiple‐quantum‐well heterostructure. Comparing with many other THz wave generation methods, THz QCL is mostly developed with compact fingerprint, high power, and high efficiency in the demanding frequency range of 0.8–5.4 THz with the aid of advanced electronic engineering technology of the active materials. This triggers various important applications including nonlinear optics, astronomy, imaging, sensing, and spectroscopy. However, the applications of THz QCL require good output characteristics in terms of the emission spectrum, beam quality, power efficiency, and polarization control, which are beyond the reach of the electronic engineering techniques. The main focus here is placed on photonic engineering of the THz QCLs with attention to the significant improvement of THz QCL output characteristics. Various photonic solutions to manipulate the laser output are thoroughly reviewed. Some innovative photonic designs with impressive achievements are highlighted. Nonconventional cavities with exotic physics and special functionalities are also discussed in the end, which may be exploited for potential applications in the future.

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.