Friday, July 21, 2017

Abstract-Active KTaO3 hybrid terahertz metamaterial

The dielectric properties of an active KTaO3 hybrid metamaterial structure and its tunability under external electric fields are investigated at room temperature by means of terahertz time-domain spectroscopy. Application of the electric field leads to an appreciable tuning of the dielectric loss, which is up to 17%. Meanwhile, the refractive index also changes appreciably. These findings are attributed to the internal space charge field in the crystal caused by the excited free carriers.

Abstract-All-Dielectric Meta-lens Designed for Photoconductive Terahertz Antennas

 Qing Yu,  Jianqiang Gu, Quanlong Yang,  Ying Zhang,   Yanfeng Li, Zhen Tian, Chunmei Ouyang,   Jiaguang Han, John F. O'Hara, Weili Zhang

Impact Statement:
In this numerical study, we present a metasurface based lens directly integrated to a terahertz PCA transmitter which is rarely reported. Because its all-dielectric nature, the meta-lens not only offers an excellent collimation function, but also has a better transmittance efficiency than the traditional Si hyper-semispheric lens and most metal based terahertz meta-lenses. The meta-lens proposed here have promising applications in next-generation terahertz imaging and spectroscopy techniques.
We present an all-dielectric meta-lens designed to collimate terahertz waves emitted from a terahertz antenna. The meta-lens is not only thinner than a conventional bulk silicon lens, but also promises to eliminate the use of parabolic mirrors in a terahertz time-domain spectroscopy system. A systematic numerical study reveals that the meta-lens exhibits excellent performance in both the emitter and detector modules, converting between the spherical wave of the antennas and the collimated beam. The frequency and alignment dependences of the meta-lens are also investigated to comprehensively map its response characteristics. The all-dielectric meta-lens presented here may pave a way in developing high-performance integrated photoconductive terahertz antenna components.

Abstract-Controllable Continuous evolution of electronic states in a single quantum ring

Intense terahertz laser field is shown to have a profound effect on the electronic and optical properties of quantum rings, where the isotropic and anisotropic quantum rings can now be treated on equal footing. We have demonstrated that in isotropic quantum rings the laser field creates irregular AB oscillations that are usually expected in anisotropic rings. Further, we have shown for the first time that intense laser fields can restore the {\it isotropic} physical properties in anisotropic quantum rings. In principle, all types of anisotropies (structural, effective masses, defects, etc.) can evolve as in isotropic rings, in our present approach. Most importantly, we have found a continuous evolution of the energy spectra and intraband optical characteristics of structurally anisotropic quantum rings to those of isotropic rings, in a controlled manner, with the help of a laser field.

Thursday, July 20, 2017

New technique for manipulating polarization of terahertz radiation

Researchers have developed a new device that can split a beam of terahertz radiation by polarization state.
Credit: Mittleman Lab / Brown University

Brown University researchers have developed a new method of manipulating the polarization of light at terahertz frequencies.
The technique uses stacks of carefully spaced metal plates to make a polarizing beamsplitter, a device that splits a beam of light by its differing polarization states, sending vertically polarized light in one direction and horizontally polarized light in another. Such a beamsplitter could be useful in a wide variety of systems that make use of terahertz radiation, from imaging systems to future communications networks.
In the imaging world, the ability to deliver and detect radiation at different polarizations could be useful in terahertz microscopy and material characterization. In communications, polarized beams can enable multiple data streams to be sent down the same medium without interference.
"This stack-of-plates idea has advantages over traditional methods of manipulating polarization in the terahertz region," said Dan Mittleman, a professor in Brown's School of Engineering and senior author of a research paper describing the work in the journal Scientific Reports. "It's cheaper and physically more robust than other methods, and it's more versatile in what it allows us to do."
Rajind Mendis, a research assistant professor at Brown, led the work along with Mittleman, Brown graduate student Wei Zhang and Masaya Nagai, an associate professor at Osaka University in Japan.
The terahertz range is the swath of the electromagnetic spectrum between microwave and infrared frequencies. Use of terahertz waves in technological applications such as spectroscopy, sensing, imaging and ultra-high-bandwidth communications is growing, and researchers are working to develop the hardware components necessary to build these advanced terahertz systems.
Polarization refers to the orientation of an electromagnetic wave's peaks and valleys as the wave propagates. If a wave is propagating toward you, the peaks and valleys can be oriented vertically, horizontally or anywhere in between.
"Polarization is one of the key properties of any electromagnetic wave," Mittleman said. "Being able to manipulate polarization -- to measure it or to change it -- is one of the important capabilities you need in any electromagnetic system."
In the visible light realm, for example, manipulating polarization is used to create modern 3-D movies and to make sunglasses that reduce the glare of reflected light. Polarizing sunglasses are made by arranging polymer strands horizontally within lenses like bars on a jail cell. Those strands allow light that's polarized vertically to pass through, while blocking horizontally polarized light, which is the dominant polarization state of light reflected off shiny surfaces like cars and water.
Existing methods of manipulating polarization in the terahertz range are very similar to the technique used in polarizing sunglasses, albeit scaled to the much longer wavelengths of terahertz light compared to visible light. Polarizing filters for terahertz are generally an array of metal wires a few microns in diameter and spaced several microns apart.
The new technique the Brown and Osaka team developed replaces the wires with a stack of closely-spaced steel plates. Each pair of plates forms what's known as a parallel-plate waveguide. When terahertz light is shined on the stack at a 45-degree angle, it splits the beam by exciting two waveguide modes. One beam of vertically polarized light passes straight through the device, while another beam of horizontally polarized light is reflected in a 90-degree angle from the original beam axis.
The technique has a number of advantages over traditional wire filters, the researchers say. The stack-of-plates architecture, which is knows as an "artificial dielectric," is easy to make, and the materials are inexpensive. The plates are also much less fragile than wires.
"The artificial-dielectric concept also makes the device more versatile," Mendis said. "The device can be easily tuned for use at different terahertz frequencies simply by changing the size of the spacers separating the plates or by changing the illuminating angle."
Another advantage is that with the addition of a second similar artificial-dielectric structure, the researchers were able to build a device called an isolator. Isolators are used on high-powered lasers to prevent light from being reflected back into a laser emitter, which could destabilize or even damage it. A terahertz isolator could be an important component for future high-powered terahertz devices.
The Brown and Osaka team is in the process of patenting the new artificial-dielectric devices, and the researchers are hopeful that these devices will enable the development of new terahertz systems with far better capabilities.
"In anything you might want to do with an optical system, it's useful to be able to manipulate polarization," Mittleman said. "This is a simple, efficient, effective and versatile way to do that."

US Patent- Terahertz quantum cascade laser implementing a {hacek over (C)}erenkov difference-frequency generation scheme

United States Patent 9711948
Belkin, Mikhail (Austin, TX, US
Adams, Robert (Austin, TX, US)
Amann, Markus Christian (Garching, DE)
Vizbaras, Augustinas (Garching, DE)

A terahertz source implementing a {hacek over (C)}erenkov difference-frequency generation scheme in a quantum cascade laser. The laser includes an undoped or semi-insulating InP substrate with an exit facet that is polished at an angle between 10° to 40°. The laser further includes a first waveguide cladding layer(s) in contact with an active layer (arranged as a multiple quantum well structure) and a current extraction layer on top of the substrate. Furthermore, the laser includes a second waveguide cladding layer(s) on top of the active layer, where the first and second waveguide cladding layers are disposed to form a waveguide structure by which terahertz radiation generated in the active layer is guided inside the laser. The terahertz radiation is emitted into the substrate at a {hacek over (C)}erenkov angle relative to a direction of the nonlinear polarization wave in the active layer, and once in the substrate, propagates towards the exit facet.