New terahertz-band laser offers broad tunability

By UCLA Samueli Newsroom

A UCLA-led research team has developed a laser that generates precise wavelengths of light across a large part of the terahertz light spectrum

The technology could improve drug screening equipment by making them able to analyze more compounds, or the technology could be incorporated into future telescopes that study dusty regions of space where new stars are forming.

The innovative design could also be the basis for other types of lasers that can vary the wavelength of light they emit.

The study was published in Nature Photonics and led by Benjamin Williams, a professor of electrical and computer engineering at the UCLA Samueli School of Engineering.

The terahertz band of the electromagnetic spectrum lies between the infrared and microwave bands. Its particular advantage is it can reveal the chemical composition of many solids, gasses, and molecules by looking at what wavelengths of light are absorbed, reflected, and transmitted – a process analogous to looking at the color of an object using visible light.

“Different molecules absorb terahertz light at very particular wavelengths, and in ways that are specific to each molecule,” said Williams, who leads the Terahertz Devices and Intersubband Nanostructures Laboratory. “In other words, each molecule has a spectroscopic ‘fingerprint’ that can tell you what something is made of.”

For example, a drug screening tool using terahertz light could tell if something is an illegal compound or harmless sugar.  Another application would be in terahertz telescopes – such as NASA’s proposed Origins Space Telescope – to study star formation processes, histories of galactic evolution, and the signatures of life and planetary formation.

However, the maturity of technologies that use the terahertz band lag far behind their counterparts in other wavelengths – in some ways the terahertz band is the electromagnetic spectrum’s “last frontier.”

One promising terahertz tool, the quantum cascade laser or QCL, has been in development for nearly 20 years. It has the capability of generating the extremely precise wavelengths of terahertz light needed to make precise spectral measurements. The first terahertz QCLs were deployed a few years ago, on airborne and balloon observatories for astrophysical measurements.

One thing terahertz QCLs are missing for wider adoption is tunability – that is, the capability to change the wavelength of the light that the laser emits. Typical QCLs can only change their wavelength only by about 1% – tuning by a little more is possible, but comes at the expense of laser performance. That limits the type of molecules that a single laser can analyze – if more wavelengths are available, then more molecules are able to be identified.

The UCLA researchers’ new QCL design can emit terahertz light across nearly a fifth of that band, the broadest available. That’s equivalent to a laser pointer that changes its color from blue, to green, to yellow, and finally to orange.

“Our breakthrough not only achieves a record high fractional tuning of a terahertz laser frequency, at 19% of the terahertz spectrum, it also has a record high continuous output power, and excellent beam patterns,” Williams said. “These qualities are important for practical terahertz applications.”

Their design is called a metasurface “vertical-external-cavity surface-emitting laser,” or “VECSEL.” While VECSELs have been around for a while at much shorter wavelengths in the visible and near-infrared, it is the “metasurface” which is new and allows terahertz operation.

To operate, a laser usually needs a space where the light is reflected and amplified before being sent out. Called a laser cavity, it consists of two mirrors with a ‘gain medium’ in between that amplifies the light.

Ordinarily, the laser cavity is considerably longer than the light wavelength – this allows the light sufficient time to be amplified and sustain oscillation.

“In our new terahertz design, the metasurface acts as both a mirror and amplifier, allowing us to build laser cavities as short as a single wavelength,” said UCLA doctoral student Chris Curwen, the study’s lead author. “It is this short cavity that allows us to achieve very broad tuning of the laser frequency.”

The paper’s other author was John Reno, principal member of the technical staff at Sandia National Laboratory in New Mexico.

The study was funded by the National Science Foundation and NASA.

Benjamin Williams at UCLA presents talk" Next generation terahertz sources: quantum cascade lasers and active metamaterials"

Benjamin Williams

UCLA, Electrical Engineering Department

what
when	Sep 19, 2012 from 01:00 PM to 02:30 PM
where	Engr. IV Bldg., Shannon Room 54-134
contact name	Prof. Ben Williams

Abstract

The terahertz frequency range (roughly 0.3-10 THz, or wavelengths of 30-1000 μm) remains one of the least developed regions of electromagnetic spectrum. Compared to the neighboring microwave/mm-wave or infrared spectral ranges it remains challenging to manipulate, to detect, and particularly to generate THz radiation. However, the location of the terahertz range offers unique opportunities for hybrid devices, as photonic techniques for achieving gain (i.e. stimulated emission and lasing) can be combined with lower-frequency circuit and antenna techniques for waveguiding and radiation control. In the past decade, the terahertz quantum cascade (QC) laser has emerged as a promising and flexible source of THz continuous-wave radiation with milliwatt power in the 2-5 THz range. However challenges remain in issues of output power, beam quality, operating temperature, and frequency tunability.

In this talk, I will discuss our efforts to address these challenges for THz sources using novel material and electromagnetic approaches. Using the THz QC-laser gain material as a foundation, we have introduced the concept of active composite right/left handed (CRLH) THz metamaterial waveguides. By loading sub-wavelength transmission-line laser waveguides with additional inductive and capacitive circuit elements, we can engineer the waveguide dispersion to enable new functionality, such as laser antennas for beam-steering and shaping and efficient out-coupling of THz radiation. We have demonstrated proof-of-concept devices including passive CRLH metamaterial waveguides, and active QC-laser metamaterial leaky-wave antennas. I will also discuss the prospects for novel THz laser devices based upon metamaterial waveguides, including widely tunable THz sources, and integrated THz laser phased arrays.

Biography

Benjamin Williams is an Assistant Professor of Electrical Engineering at the University of California, Los Angeles. He received the Ph.D. degree from the Massachusetts Institute of Technology, Cambridge, Massachusetts in 2003 in Electrical Engineering and Computer Science, and was a Postdoctoral Associate at the Research Laboratory of Electronics at MIT from 2003-2006. His research interests include quantum cascade lasers, intersubband and intersublevel devices in semiconductor nanostructures, and terahertz metamaterials and sub-wavelength plasmonics. He is the recipient of the DARPA Young Faculty Award (2008) and the NSF CAREER award (2012).

A breakthrough for terahertz semiconductor lasers

http://thznetwork.net/index.php/archives/1158



Potential applications, says an engineering professor, include disease diagnosis and detection of concealed explosives.

Light is nothing short of awesome — it inspires painters and guides a midnight trip to the bathroom. But visible light occupies just one portion of the electromagnetic spectrum. Farther along the spectrum, radio waves enable the world to talk wirelessly. X-rays make medical imaging possible. Each region of the spectrum promises new technologies, if it can be harnessed.

A team led by Sushil Kumar, assistant professor of electrical and computer engineering, is helping to develop a largely unexploited region of the electromagnetic spectrum.

Working with researchers at MIT and Sandia National Laboratories, Kumar has made a semiconductor laser, also called a quantum-cascade laser (QCL), that emits terahertz (THz) radiation at higher operating temperatures than ever before. He reported his achievement recently in Nature Physics.

The breakthrough moves the technology closer to applications in disease diagnosis; quality control in drug manufacturing; detection of concealed weapons, drugs and explosives; the remote sensing of the earth’s atmosphere; and the study of star and galaxy formation.
It also erases doubts that there is a maximum temperature at which coherent THz radiation can be generated from semiconductor chips.

“Terahertz QCLs are required to be cryogenically cooled and improvement of their temperature performance is the single most important research goal in the field,” the researchers wrote in Nature Physics.

Progress toward a room-temperature THz laser
“Thus far, their maximum operating temperature has been empirically limited, [which] has bred speculation that a room-temperature terahertz QCL may not be possible in materials used at present.”

QCLs are attractive because of their size. Traditionally high-power THz radiation was produced by bulky, expensive lasers fueled by a molecular gas such as methane. Advances in semiconductors have made QCLs as tiny as the diode in a laser pointer, but the lasers require temperatures almost 200 degrees below zero to emit terahertz radiation.
His team has raised the QCL’s operating temperature, says Kumar, by exploiting its “tunability.”

The frequency of light generated in any material is naturally fixed and is determined by the spacing of energy levels at the molecular level. But the spacing of the QCL’s energy levels can be tuned, allowing the laser to emit THz radiation. QCLs are made of alternating layers of different semiconductors (such as gallium arsenide and aluminum gallium arsenide) because the thickness of each layer determines the spacing between the energy levels.
Proper tuning, says Kumar, is achieved by injecting electrons into the correct energy level of the semiconductor layers. The process is analogous to fuel injection in an automobile. Electrons (the fuel) hop from one energy level to another in the layered semiconductor to generate power in the form of THz photons.

But the THz photon energy, says Kumar, is much smaller than the thermal energy of electrons at room temperature.

“This makes it very difficult to selectively put electrons in the required energy levels for them to emit THz photons.”

Fuel injection — using electrons
To raise QCLs’ operating temperature, Kumar’s group has harnessed the “relaxation process.” Electrons tend to dissipate their energy in the form of lattice vibrations at higher temperatures, called “non-radiative relaxation,” which is typically detrimental to laser operation.

Kumar’s group used this natural phenomenon in a controlled manner to inject electrons into the correct energy levels. This scattering-assisted injection technique is less sensitive to the thermal energy of electrons and remains efficient at high temperatures as well.
“This tremendous achievement is very promising for the future of THz laser technologies,” says Alessandro Tredicucci, research director at the National Research Council of Italy and inventor of the first THz QCL. “It shows that the power of quantum design has yet to be fully tapped and encourages people to look for new materials and structures whose relaxation times can be slowed down.”

“It is remarkable how the science of QCLs has progressed hand-in-hand with advancements in crystal growth technology to make such an incredibly complex semiconductor device possible,” says John Reno of Sandia’s Center for Integrated Nanotechnologies, who coauthored the Nature Physics article.

More information: http://www.nature. … hys1846.html
Source: PhysOrg.com.