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.

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.

Novel laser produces random mid-infrared light for improved imaging applications

Random lasers remove speckling while maintaining brightness and could be used for applications where imaging quality is important, such as checking mail or airport security. Credit: Tomasz Wyszołmirski/iStock/Thinkstock

http://phys.org/news/2014-02-laser-random-mid-infrared-imaging-applications.html#jCp

Most lasers produce coherent light, meaning that the light waves are perfectly synchronized with each other. Spatially coherent waves, however, can interfere with one another and produce speckles in an image. With this in mind, scientists are turning to so-called random lasers, which not only show promise for applications such as biological and environmental imaging, but also mimic natural, disordered scattering from objects such as clouds.

Hou Kun Liang and co-workers at the A*STAR Singapore Institute of Manufacturing Technology and Nanyang Technological University, Singapore, have now developed a random laser that emits light in the mid-infrared range1. Moreover, the random laser is driven by electricity, making it more suitable for practical applications.

"Most random lasers are driven by optical pumping—this requires another laser to excite the random media," says Liang. "With electrical pumping we can make the laser smaller, less complex and cheaper."

The researchers modified a design known as a quantum cascade laser that contains several thin layers of compound semiconductors. When an external voltage is applied, electrons are driven across the layers and emit photons at every step. The frequency of the emitted light can be controlled by adjusting the thickness of the layers.

"A quantum cascade laser is like an electron reservoir," says Liang. "After an electron relaxes to a lower energy level, instead of becoming inactive, it flows to the subsequent active region where it is 're-used'. This is important for our laser, because loss in the mid-infrared region is high, and so we need a high gain to compensate for it."

Crucially, Liang and co-workers used plasma etching to create a random pattern of small holes—each only three micrometers in diameter—on the top surface of their laser. This design causes the laser light to be randomly scattered before it is emitted through the holes.

Currently, the random laser must be cooled to very low temperatures using liquid nitrogen to maximize the gain, but Liang and co-workers anticipate that their design can be improved to reduce the loss of mid-infrared radiation at room temperature. Liang also points out that their design gives them great freedom to explore other laser frequencies.

"For example, terahertz lasers can penetrate thick plastics and papers and, unlike X-rays, are harmless to humans. These lasers could be used for applications, such as checking mail or airport security, where imaging quality is important—a random laser would remove speckling while maintaining brightness."

 Explore further: Harnessing randomness to improve lasers

More information: Liang, H. K., Meng, B., Liang, G., Tao, J., Chong, Y. et al. "Electrically pumped mid-infrared random lasers." Advanced Materials 25, 6859–6863 (2013). DOI: 10.1002/adma.201303122

Journal reference: Advanced Materials