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-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.

Abstract-Integrated Terahertz Graphene Modulator with 100% Modulation Depth

Guozhen Liang†, Xiaonan Hu†, Xuechao Yu†, Youde Shen‡, Lianhe H. Li§, Alexander Giles Davies§, Edmund H. Linfield§, Hou Kun Liang∥, Ying Zhang∥, Siu Fung Yu⊥, and Qi Jie Wang*†‡

†OPTIMUS, School of Electrical and Electronic Engineering, and ‡CDPT, School of Physical and Mathematical Sciences, The Photonics Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

§ School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, U.K.

∥ Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore

⊥ Department of Applied Physics, Hong Kong Polytechnic University, Kowloon, Hong Kong

ACS Photonics, Article ASAP

DOI: 10.1021/acsphotonics.5b00317

Publication Date (Web): October 19, 2015

Copyright © 2015 American Chemical Society

*E-mail: qjwang@ntu.edu.sg.

http://pubs.acs.org/doi/10.1021/acsphotonics.5b00317

Terahertz (THz) frequency technology has many potential applications in nondestructive imaging, spectroscopic sensing, and high-bit-rate free-space communications, with an optical modulator being a key component. However, it has proved challenging to achieve high-speed modulation with a high modulation depth across a broad bandwidth of THz frequencies. Here, we demonstrate that a monolithically integrated graphene modulator can efficiently modulate the light intensity of the THz radiation from a THz quantum cascade laser with a 100% modulation depth for certain region of the pumping current, as a result of the strongly enhanced interaction between the laser field and the graphene enabled by this integration scheme. Moreover, the small area of the resulting device in comparison to existing THz modulators enables a faster modulation speed, greater than 100 MHz, which can be further improved through optimized designs of the laser cavity and modulator architectures. Furthermore, as the graphene absorption spectrum is broadband in nature, our integration scheme can be readily scaled to other wavelength regions, such as the mid-infrared, and applied to a broad range of other optoelectronic devices.