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.

Building a bright future for lasers

Professor Benjamin Williams, at left, and 2016 Ph.D. graduate Benjamin Burnett at work in the Terahertz Devices and Intersubband Nanostructures Laboratory

Katherine Kornei
http://newsroom.ucla.edu/stories/one-of-the-last-frontiers-of-the-electromagnetic-spectrum-by 

Invisible to the human eye, terahertz electromagnetic waves can “see through" everything from fog and clouds to wood and masonry — an attribute that holds great promise for astrophysics research, detecting concealed explosives and many other applications.

Terahertz lasers can produce photons with frequencies of trillions of cycles per second — energies between those of infrared and microwave photons. These photons, however, are notoriously difficult to generate — and that’s where UCLA associate professor of electrical engineering Benjamin Williams comes in. He and his research group at the UCLA Henry Samueli School of Engineering and Applied Science are hard at work exploring “one of the last frontiers of the electromagnetic spectrum,” as Williams describes it.

Most optical and infrared lasers operate by electrons transitioning between two energy levels in a semiconductor crystal and emitting a photon. However, this process is not so easily extended to the terahertz range.

“If you want to make terahertz radiation, you need a very low-energy photon, so you need two energy levels that are very close together, and that’s hard to do with the semiconductors that nature gives us,” said Williams.

He and his collaborators at the Terahertz Devices and Intersubband Nanostructures Laboratory instead produce terahertz photons by engineering artificial materials that mimic the energy levels of atoms. These so-called “quantum cascade lasers” are made by arranging different semiconductors in layers — some only a few atoms thick — to form quantum wells. Quantum wells are like tiny “boxes” that confine electrons to certain energy levels chosen by design. As an electron transitions between different energy levels, it emits photons. A single electron can cascade between the many quantum wells in a quantum cascade laser and trigger the emission of multiple terahertz photons, thereby producing a powerful laser beam. Another advantage of quantum cascade lasers is that the frequency of the emitted photons can be modulated.

“Instead of being limited to the band gap that nature gives you, we can change the width of these quantum wells to choose the effective band gap [and change the photons’ frequency]. That’s a very powerful concept,” said Williams.

While quantum cascade lasers are both powerful and tunable in frequency, a significant disadvantage has been their low beam quality.

“Think of a laser pointer, which has a very nice beam, “Williams said. “The beam goes where you want it, and it looks like a nice spot. You’re not wasting the light.”

Terahertz lasers, on the other hand, often have beams that are highly divergent, meaning that the light beam spreads out and accordingly becomes less powerful. In some cases, the beam of a terahertz laser diverges so much that only 0.1 percent of it ends up where it was initially intended to go.

A major achievement of Williams’ lab has been creating a type of terahertz quantum cascade laser that possesses both an excellent beam pattern and high power.

“Our innovation was to make an artificial surface that’s made up of lots of little laser antennas [metal structures that each function like a quantum cascade amplifier]. The net effect is a mirror that reflects terahertz light as it amplifies and focuses it at the same time,” said Williams. “We believe that this ability will allow us to create lasers with control of nearly all of the properties of the light—its wavelength, amplitude, phase, and polarization.”

Williams and his team are also exploring how quantum cascade lasers can be designed to operate at room temperature. Currently, scientists must cool their lasers down to 77 Kelvin (-321°F), a step that limits the lasers’ use outside of a laboratory. Now, Williams is investigating building those lasers using quantum dots instead of quantum wells. While quantum wells confine electrons’ motion in only one dimension, quantum dots restrict their motion in all three dimensions. The extra confinement in quantum dots is predicted to drastically reduce how much the electrons scatter, which would allow these lasers to work at room temperature.

“We’re currently working with Diana Huffaker [professor of electrical engineering at UCLA], who grows quantum dots,” said Williams. “[Her work] would allow us to do the same kinds of quantum engineering with quantum dots that we presently do with quantum wells.”

Last May, Williams was honored by President Obama with a Presidential Early Career Award for Scientists and Engineers, a prize that is given to particularly promising young researchers whose achievements span both academics and community service. In 2012, he received a National Science Foundation CAREER award and, in 2008, a DARPA Young Faculty Award. 

This story was adapted from the original published in UCLA Engineer, the news magazine of the UCLA Henry Samueli School of Engineering and Applied Science.

Terahertz Sources: Terahertz QC metasurface VECSEL has near-Gaussian, low-divergence output

By Gail Overton 
Senior Editor

Since the demonstration of continuous-wave (CW) quantum-cascade (QC)-based terahertz lasers in the early 2000s, researchers have continued to increase power-output levels to the low milliwatt range, reduce divergence angles to around 10°, and improve beam quality.

Unfortunately, improving any one of these performance parameters is often at the expense of another, making simultaneous improvement of multiple parameters very challenging. For example, it is difficult to increase the output power for a semiconductor QC laser, as scaling up the waveguide-based gain cavity has the deleterious effect of increasing multiple-transverse-mode oscillations that lead to poor beam quality and modal instabilities. And because achieving terahertz wavelengths typically requires subwavelength metallic and/or plasmonic waveguide structures, the result is highly divergent beams with large side lobes unless special antenna-coupled distributed feedback (DFB) or photonic-crystal cavities are used.

An alternative design engineered by researchers at the University of California, Los Angeles (UCLA) to combat many of these performance tradeoffs is a vertical-external-cavity surface-emitting laser (VECSEL) operating at terahertz frequencies that consists of an amplifying metasurface reflector and a flat output coupler.¹

The metasurface—a two-dimensional (2D) metamaterial surface—is essentially an array of antenna-coupled QC cavities, while the output coupler consists of an off-the-shelf wire-grid polarizer. The coupler provides a partially transmitting mirror to create the laser cavity and couple out a certain amount of power.

A terahertz quantum-cascade (QC) vertical-external-cavity surface-emitting laser (VECSEL) consists of an active metasurface reflector and output coupler (a); the metasurface is patterned with a metal-metal waveguide subcavity array (b). A scanning-electron microscope (SEM) image shows the metasurface reflector and its 1.5 × 1.5 mm2 active area (c).(Courtesy of UCLA)

Beam engineering

The active metasurface reflector consists of metal-metal ridge waveguides of width w that sandwich the gallium arsenide (GaAs)/aluminum GaAs (AlGaAs) QC laser medium above and below the metal strips. Each of these 10-μm-thick sub-cavity waveguides are separated by the period Λ, which is less than the free-space wavelength λ₀, to prevent diffractive losses or surface-mode excitation from the normally incident radiation.

These engineered subcavities confine the modes neatly to the active material within the waveguide on the GaAs substrate; however, they are designed with tapered absorbing regions at each end to suppress feedback and prevent lasing in the conventional confined waveguide modes. Rather, each subcavity is essentially an antenna that couples incident terahertz radiation to the QC-laser gain medium, amplifies it via stimulated emission, and reradiates it normal to the plane of the metasurface.

Taken collectively, an array of these subcavities acts as a "metasurface reflectarray" mirror that amplifies incident terahertz waves. Such an amplifying mirror can then be used to build an external cavity, which makes it straightforward to generate a high-quality beam. Because the metasurface response is polarization-specific, rotating the wire-grid polarizer used as an output coupler varies the transmission from the cavity for on-the-fly laser output coupling optimization.

Experimental results for 1.5 × 1.5 mm² gain cavities (about 17 subcavity ridges) designed for nominal 2.9 THz operation confirm lasing from 2.8 to 2.9 THz under different bias conditions, and with ridge widths varying from 11.5 to 13.5 μm and a period of 90 μm. Of these variations, maximum lasing power occurred for a ridge width of 12.5 μm, which most likely has the best overlap of the gain spectrum with the metasurface resonance.

In an external-cavity configuration with a wire-grid polarizer coupler, the metasurface laser reaches 5.5 mW of output power with a 16.7 mW/A slope efficiency. The far-field beam pattern is near-Gaussian in shape with an approximately 5° divergence angle. Multiple methods for improving the CW laser output are envisioned for future research.

"This is the first time a reflectarray metasurface and a laser have been combined. This combination makes the VECSEL approach possible for terahertz wavelengths and provides a route to higher output powers simultaneously with excellent beam quality," says Benjamin Williams, associate professor of Electrical Engineering at UCLA. "The metasurface approach may in the future allow one to engineer the beam to have arbitrary polarization, shape, phase front, and spectral properties."

REFERENCE

1. L. Xu et al., Appl. Phys. Lett., 107, 22, 221105 (2015).

Novel semiconductor lasers can be used for space exploration

http://www.goodchinabrand.com/49062400070en.html

According to foreign media reports, the University of California researchers using a new method of manufacturing a semiconductor laser work at terahertz frequencies. The semiconductor laser can be used to analyze the formation of stars and planets atmospheric composition.

In the electromagnetic spectrum, terahertz frequency range lies between microwaves and infrared. Terahertz wave can be detected without damaging the material to be analyzed under the premise of plastic, clothing, semiconductors and art materials, etc., it can also be used to analyze the stars the formation and composition of the planet's atmosphere.

Currently using visible vertical external cavity surface emitting laser (VECSEL) has been widely used to generate high-energy beam, but this technique is not applicable to previously terahertz frequency range. Associate Professor of the University of California, Los Angeles, led by electrical engineering Benjamin Williams ` Development of the first team in the terahertz frequency range of VECSEL. findings recently published in the (Applied Physics Letters) on.

In order to emit high-energy beam VECSEL terahertz frequency range, developed with the Williams team called a 'super-reflective front surface mirror' means VECSEL. This device is so named because it contains a large number of tiny by the array antenna coupled laser cavity consisting of, so that when the terahertz wave passes this array will 'see' not see the laser cavity, it will be reflected back, like being an ordinary mirror reflecting back the same.

'Taking over the material surface and the laser combine for the first time.' Williams said, this method only allows the laser in the THz frequency range of the output more power, but also a high-quality laser beam, and metamaterials It allows researchers to use laser beams to further designed to generate the desired polarization, shape and frequency.

A member of one of the research, first author Xu Luyao (phonetic) further explained: 'by super material surface as part of the outer chamber, we can not only improve the shape of the laser beam, but also through different external cavity design to a laser band to new features such as you can to maximize output power and efficiency of the laser. By using the stand-alone wire polarizer or filter as a second reflecting surface, we simply rotating polarizer. '