Abstract-Metasurface terahertz laser with electronically-controlled polarization

 Daguan Chen,  Luyao Xu, Christopher A. Curwen,  Mohammad Memarian, John L. Reno, Tatsuo Itoh,  Benjamin S. Williams,

http://ieeexplore.ieee.org/document/8083356/

We report a terahertz metasurface quantum-cascade VECSEL laser without moving parts that can electronically switch between near-orthogonal linearly polarized output. It exhibits excellent beam pattern, single-mode operation, and power up to 93 mW at 77 K.

US Patent-Quantum cascade external cavity laser with metasurfaces

United States Patent 9728930
Inventors:
Williams, Benjamin Stanford (Los Angeles, CA, US) 
Xu, Luyao (Los Angeles, CA, US) 
Chen, Daguan (Los Angeles, CA, US)

http://www.freepatentsonline.com/9728930.html

A metasurface reflector for quantum cascade lasing is disclosed. The metasurface reflector uses an array of subcavities disposed on a substrate and spaced with a sub-wavelength period. Each of the subcavities includes a layer of quantum-cascade-laser-active material sandwiched between two metallic layers. The array of subcavities reflect an incident light of a resonant frequency with amplification. When used with an output coupler, a quantum cascade laser beam can be generated.

UCLA-led team develops technique to control laser polarization

Artist’s depiction of the laser polarization metasurface that can tune the laser’s polarization state purely electronically, without any moving parts

The approach operates purely electronically, without any moving parts

Matthew Chin

http://newsroom.ucla.edu/releases/ucla-engineers-develop-technique-to-control-laser-polarization

A research team led by UCLA electrical engineers has developed a new technique to control the polarization state of a laser that could lead to a new class of powerful, high-quality lasers for use in medical imaging, chemical sensing and detection, or fundamental science research.

Think of polarized sunglasses, which help people see more clearly in intense light. Polarizing works by filtering visible light waves to allow only waves that have their electric field pointing in one specific direction to pass through, which reduces brightness and glare.

Like brightness and color, polarization is a fundamental property of light that emerges from a laser. The traditional way to control the polarization of a laser was to use a separate component like a polarizer or a waveplate. To change its polarization, the polarizer or waveplate must be physically rotated, a slow process that results in a physically larger laser system.

The team from the UCLA Henry Samueli School of Engineering and Applied Science developed a specialized artificial material, a type of “metasurface,” that can tune the laser’s polarization state purely electronically, without any moving parts. The research was published in Optica. The breakthrough advance was applied to a class of lasers in the terahertz range of frequencies on the electromagnetic spectrum, which lies between microwaves and infrared waves.

“While there are a few ways to quickly switch polarization in the visible spectrum, in the terahertz range there is currently a lack of good options,” said Benjamin Williams, associate professor of electrical engineering and the principal investigator of the research. “In our approach, the polarization control is built right into the laser itself. This allows a more compact and integrated setup, as well as the possibility for very fast electronic switching of the polarization. Also, our laser efficiently generates the light into the desired polarization state — no laser power is wasted generating light in the wrong polarization.”

Terahertz radiation penetrates many materials, such as dielectric coatings, paints, foams, plastics, packaging materials, and more without damaging them, Williams said.

“So some applications include non-destructive evaluation in industrial settings, or revealing hidden features in the study of art and antiquities,” said Williams, who directs the Terahertz Devices and Intersubband Nanostructures Laboratory. “For example, our laser could be used for terahertz imaging, where the addition of polarization contrast may help to uncover additional information in artwork, such as improved edge detection for hidden defects or structures.”

The work is based on the group’s recent development of the world’s first vertical-external-cavity surface-emitting laser, or VECSEL, that operates in the terahertz range.

Their new metasurface covers an area of 2 square millimeters and has a distinct zigzag pattern of wire antennas running across its surface. An electric current runs through the wires, selectively energizing particular segments of the laser material, which allows a user to change and customize the polarization state as needed.

The lead authors of the research are electrical engineering graduate student Luyao Xu and electrical engineering undergraduate student Daguan Chen. Other authors include electrical engineering graduate student Christopher Curwen; Mohammad Memarian, a postdoctoral scholar in UCLA’s microwave electronics lab; John Reno of Sandia National Laboratories; and UCLA electrical engineering professor Tatsuo Itoh, who holds the Northrop Grumman Chair in Engineering.

The research was supported by the National Science Foundation and NASA.

Abstract-Terahertz quantum cascade VECSEL

Luyao Xu, Christopher A. Curwen, Philip W. C. Hon, Tatsuo Itoh, Benjamin S. Williams

Univ. of California, Los Angeles (United States)

Proc. SPIE 9734, Vertical External Cavity Surface Emitting Lasers (VECSELs) VI, 97340G (March 10, 2016); doi:10.1117/12.2213230

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2503446

Vertical-external-cavity surface-emitting lasers (VECSELs) have been successfully used in the visible and near-infrared to achieve high output power with excellent Gaussian beam quality. However, the concept of VECSEL has been impossible to implement for quantum-cascade (QC) lasers due to the "intersubband selection rule". We have recently demonstrated the first VECSEL in the terahertz range. The enabling component for the QC-VECSEL is an amplifying metasurface reflector composed of a sparse array of metallic sub-cavities, which allows the normally incident radiation to interact with the electrically pumped QC gain medium. In this work, we presented multiple design variations based on the first demonstrated THz QC-VECSEL, regarding the lasing frequencies, the output coupler and the intra-cavity aperture. Our work on THz QC-VECSEL initiates a new approach towards achieving scalable output power in combination with a diffraction-limited beam pattern for THz QC-lasers. The design variations presented in this work further demonstrate the practicality and potential of VECSEL approach to make ideal terahertz QC-laser sources.
 © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

First Semiconductor THz Laser Eyed for Sensing Applications

http://www.photonics.com/Article.aspx?AID=58109

LOS ANGELES, Dec. 21, 2015 — A semiconductor laser that emits at terahertz frequencies could drive development of a new class of lasers for materials analysis and threat detection. 

The first vertical-external-cavity surface-emitting laser (VECSEL) capable of operating in this range, the device achieves output power >5 mW. For amplification it uses a metasurface mirror composed of subwavelength antenna-coupled quantum-cascade subcavities.

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A metasurface used to amplify a vertical-external-cavity surface-emitting laser that works in the terahertz range. Images courtesy UCLA Engineering .

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VECSELs that use visible light have been used extensively to generate high-powered beams, but the technique has not previously been adapted for terahertz frequencies. 

"This is the first time a metasurface and a laser have been combined," said professor Benjamin Williams, whose team led the development. "The VECSEL approach provides a route to have higher output powers simultaneously with excellent beam quality in the terahertz range. The metasurface approach further allows one to engineer the beam to have the desired polarization, shape and spectral properties."

Creating a beam that is symmetrical and straight over large distances and changing thermal conditions is a challenge for many semiconductor lasers, but particularly for terahertz quantum cascade lasers, which usually use metal laser cavities with dimensions much smaller than the wavelength. 

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A schematic of the metasurface and polarizer used to tune the laser.

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"By using this amplifying metasurface as part of the external cavity, not only can we improve the beam pattern, but we can also introduce new functionality to this laser with different cavity designs," said graduate student Luyao Xu. "For example, by using a freestanding wire-grid polarizer, or filter, as a second mirror, we could optimize the lasers' output power and efficiency simply by rotating the polarizer." 

The terahertz range of frequencies occupies the space on the electromagnetic spectrum between the IR region and microwaves. Terahertz waves can be used to analyze plastics, clothing, semiconductors and works of art without damaging the materials being examined. They are also useful for chemical sensing and identification, and to investigate the formation of stars and the compositions of planetary atmospheres. 

Funding came from the National Science Foundation. The research was published in Applied Physics Letters (doi: 10.1063/1.4936887 [open access]).

UCLA researchers develop ‘metasurface’ laser for terahertz range

Advance could create new class of lasers for possible use in aerospace and law enforcemen

Bill Kisliuk

http://newsroom.ucla.edu/releases/ucla-researchers-develop-metasurface-laser-for-terahertz-range

UCLA Engineering

Unlike a simple mirror, the metasurface developed in Benjamin Williams’ lab amplifies terahertz waves as well as reflecting them.

​Researchers at the UCLA Henry Samueli School of Engineering and Applied Science have identified a new way to make a semiconductor laser that operates at terahertz frequencies. The breakthrough could lead to development of a new class of high-quality, powerful lasers for use in space exploration, military and law enforcement efforts and other applications.

The terahertz range of frequencies occupies the space on the electromagnetic spectrum between microwave and infrared. Terahertz waves can be used to analyze plastics, clothing, semiconductors and works of art without damaging the materials being examined; for chemical sensing and identification; and to investigate the formation of stars and composition of planetary atmospheres.

Researchers led by Benjamin Williams, a UCLA associate professor of electrical engineering, have created the first vertical-external-cavity surface-emitting laser, or VECSEL, that operates in the terahertz range. VECSELs that use visible light have been used extensively to generate high-powered beams, but the technique has not previously been adapted for terahertz frequencies.

To make it possible to build an external cavity laser with a high-quality beam, the UCLA researchers created a VECSEL with a “reflectarray metasurface mirror.” The device is so named because it is made up of an array of many small antenna-coupled laser cavities such that when a terahertz wave hits the array, it doesn’t “see” the cavities, but rather is reflected as if it were being reflected from a simple, flat mirror. Unlike a simple mirror however, the mirror amplifies terahertz waves as well as reflecting them.

“This is the first time a metasurface and a laser have been combined,” Williams said. “The VECSEL approach provides a route to have higher output powers simultaneously with excellent beam quality in the terahertz range. The metasurface approach further allows one to engineer the beam to have the desired polarization, shape and spectral properties.”

The research was published this month in Applied Physics Letters.

UCLA Engineering

Luyao Xu

Creating a beam that is symmetrical and straight over large distances and changing thermal conditions is a challenge for many semiconductor lasers, but particularly for terahertz quantum cascade lasers, which usually use metal laser cavities with dimensions much smaller than the wavelength.

Luyao Xu, a graduate researcher in Williams’ lab and lead author of the study, said, “By using this amplifying metasurface as part of the external cavity, not only can we improve the beam pattern, but we can also introduce new functionality to this laser with different cavity designs. For example, by using a freestanding wire-grid polarizer, or filter, as a second mirror, we could optimize the lasers’ output power and efficiency simply by rotating the polarizer.”

Xu said the researchers already are working on several new designs to further advance the technology.

Other authors of the study were Christopher Curwen, a UCLA graduate researcher in Williams’ lab; Philip W.C. Hon, who recently earned his doctorate from UCLA; Qi-Sheng Chen, then an engineer at Northrop Grumman Aerospace Systems; and Tatsuo Itoh, who holds UCLA’s Northrop Grumman Chair in Electrical Engineering.

The research was funded by the National Science Foundation. Researchers used the UCLA Nanoelectronics Research Facility to make the device.