A repository & source of cutting edge news about emerging terahertz technology, it's commercialization & innovations in THz devices, quality & process control, medical diagnostics, security, astronomy, communications, applications in graphene, metamaterials, CMOS, compressive sensing, 3d printing, and the Internet of Nanothings. NOTHING POSTED IS INVESTMENT ADVICE! REPOSTED COPYRIGHT IS FOR EDUCATIONAL USE.
Prashanth Gopalan, Ashish Chanana, Sriram Krishnamoorthy, Ajay Nahata, Michael A. Scarpulla, and Berardi Sensale-Rodriguez
Fig. 1 (a) XRD scans indicating highly crystalline WSe2 films showing only the (002) family of planes. (b) Raman mode at 251.2 cm−1, which agrees with reports in [24] for multilayer films. (c) UV-vis absorption measurements of WSe2 thin films indicating a strong excitonic absorption peak at ~740 nm. Based on this, optical excitation for OPTP and TRTS were chosen at 800 nm and 400 nm. (d) SEM image of WSe2 thin film shows grain size of ~1 µm.
Tungsten diselenide (WSe2) thin films exhibit ultrafast carrier recombination lifetimes, which makes them promising candidates for high speed modulators. With pulsed optical excitation, they could be used to realize all-optical, frequency agile, terahertz devices. Looking into the potential of this material for such applications, time-resolved terahertz spectroscopy can provide significant insight into its free carrier and exciton dynamics such as recombination lifetimes, photo-induced conductivity and decay pathways. In this study, we measure transient terahertz conductivity and photo-generated carrier lifetimes in custom-grown large-area WSe2thin-films. We discuss its dependence on grain size and number of layers. By analyzing the tradeoffs between carrier-lifetimes, photo-generated conductivity, grain size, and the number of layers, we show that the response of these films can be tailored by controlling the growth parameters. Customizing the film terahertz response can enable large modulation without the need for integration with bulk semiconductors, as widely reported in the literature, thereby achieve high terahertz photoconductivity and high-speed operation. Across samples, our measurements show carrier decay timescale on the order ~10 to 100 ps and a transient conductivity that shows non-Drude behavior. This deviation from a Drude response is dominant within the first few picoseconds (<10 ps) before changing into a Drude like free-carrier response at longer delays. Based on our grown films, we experimentally demonstrate a metamaterial terahertz modulator with WSe2 as the only active element, attaining ~40% modulation depth.
Three-dimensional (3D) semimetals have been predicted and demonstrated to have a wide variety of interesting properties associated with its linear energy dispersion. In analogy to two-dimensional (2D) Dirac semimetals, such as graphene, Cd3As2, a 3D semimetal, has shown ultra-high mobility, large Fermi velocity, and has been hypothesized to support plasmons at terahertz frequencies. In this work, we demonstrate synthesis of high-quality large-area Cd3As2 thin-films through thermal evaporation as well as the experimental realization of plasmonic structures consisting of periodic arrays of Cd3As2 stripes. These arrays exhibit sharp resonances at terahertz frequencies with associated quality-factors (Q) as high as ~ 3.7. Such spectrally-narrow resonances can be understood on the basis of a large kinetic-inductance, resulting from a long momentum scattering time, which in our films can approach ~1 ps at room-temperature. Moreover, we demonstrate an ultrafast tunable response through excitation of photo-induced carriers in optical pump / terahertz probe experiments. Our results evidence that the intrinsic 3D nature of Cd3As2 provides for a very robust platform for terahertz plasmonic applications. Overall, our observations pave a way for the development of myriad terahertz (opto) electronic devices based on Cd3As2 and other 3D Dirac semimetals, benefiting from strong coupling of terahertz radiation, ultrafast transient response, magneto-plasmon properties, etc. Moreover, the long momentum scattering time, thus large kinetic inductance in Cd3As2, also holds enormous potential for the re-design of passive elements such as inductors and hence can have a profound impact in the field of RF integrated circuits
Ajay Nahata, Ashish Chanana, Berardi Sensale-Rodriguez, Debdeep Jena, Hugo O. Condori Quispe, Huili Grace Xing, Jimy Encomendero, Mingda Zhu, Nicole Trometer, https://aip.scitation.org/doi/abs/10.1063/1.5032102 We report experimental studies on the excitation of synchronized plasmon resonances in AlGaN/GaN High Electron Mobility Transistor (HEMT) arrays. In contrast to the commonly employed grating-gate configurations, the analyzed structure contains periodically patterned ohmic contacts to the two-dimensional electron gas, which are laid-out parallel to the gate fingers. In this structure, the terahertz to plasmon coupling mechanism is fundamentally different from that in grating-gate configurations. Whereas the grating-gate configuration constitutes a coupled resonant system in which the resonance frequency depends on the grating periodicity, when periodical ohmic contacts are incorporated, the system behaves as a synchronized resonant system in which each unit cell is effectively independent. As a result, in a HEMT-array, the resonance is no longer set by the periodicity but rather by the gate and the ungated region length. Experimental results of fabricated samples compare well with numerical simulations and theoretical expectations. Our work demonstrates that the proposed approach allows: (i) more efficient excitation of high order plasmon modes and (ii) superior overall terahertz to plasmon coupling, even in configurations having less number of devices per unit area. From this perspective, our results reveal a simple way to enhance the terahertz to plasmon coupling and thus improve the performance of electron plasma wave-based devices; this effect can be exploited, for example, to improve the response of HEMT-based terahertz detectors.
Hugo O. Condori Quispe, Ashish Chanana, Jimy Encomendero,Mingda Zhu, Nicole Trometer, Ajay Nahata, Debdeep Jena, Huili Grace Xing,Berardi Sensale-Rodriguez,
https://aip.scitation.org/doi/10.1063/1.5032102 We report experimental studies on the excitation of synchronized plasmon resonances in AlGaN/GaN High Electron Mobility Transistor (HEMT) arrays. In contrast to the commonly employed grating-gate configurations, the analyzed structure contains periodically patterned ohmic contacts to the two-dimensional electron gas, which are laid-out parallel to the gate fingers. In this structure, the terahertz to plasmon coupling mechanism is fundamentally different from that in grating-gate configurations. Whereas the grating-gate configuration constitutes a coupled resonant system in which the resonance frequency depends on the grating periodicity, when periodical ohmic contacts are incorporated, the system behaves as a synchronized resonant system in which each unit cell is effectively independent. As a result, in a HEMT-array, the resonance is no longer set by the periodicity but rather by the gate and the ungated region length. Experimental results of fabricated samples compare well with numerical simulations and theoretical expectations. Our work demonstrates that the proposed approach allows: (i) more efficient excitation of high order plasmon modes and (ii) superior overall terahertz to plasmon coupling, even in configurations having less number of devices per unit area. From this perspective, our results reveal a simple way to enhance the terahertz to plasmon coupling and thus improve the performance of electron plasma wave-based devices; this effect can be exploited, for example, to improve the response of HEMT-based terahertz detectors.
We experimentally demonstrate thin, error tolerant, and aberration corrected 3D printed diffractive 1D and 2D THz lenses for focusing THz beams with NA > 0.25 and an average efficiency greater than 75% (for all cases). Our work has relevance to THz lens design, and in general, THz imaging systems.
We demonstrate high-efficiency (>70%) all-dielectric Terahertz metasurfaces operating in the transmission mode. Example devices include high numerical-aperture (NA≈0.9) meta-lenses as well as cylindrical vector beam generator.
We report on terahertz characterization of La-doped BaSnO3 (BSO) thin-films. BSO is a transparent complex oxide material, which has attracted substantial interest due to its large electrical conductivity and wide bandgap. The complex refractive index of these films is extracted in the 0.3 to 1.5 THz frequency range, which shows a metal-like response across this broad frequency window. The large optical conductivity found in these films at terahertz wavelengths makes this material an interesting platform for developing electromagnetic structures having a strong response at terahertz wavelengths, i.e. terahertz-functional, while being transparent at visible and near-IR wavelengths. As an example of such application, we demonstrate a visible-transparent terahertz polarizer.
Controlling and modulating terahertz signals is of fundamental importance to allow systems level applications. We demonstrate an innovative approach for controlling the propagation properties of terahertz (THz) radiation, through use of both the excitation optical wavelength (colour) and intensity. We accomplish this using two-dimensional (2D) layered hybrid trihalide perovskites that are deposited onto silicon substrates. The absorption properties of these materials in the visible range can be tuned by changing the number of inorganic atomic layers in between the organic cation layers. Optical absorption in 2D perovskites occurs over a broad spectral range above the bandgap, resulting in free carrier generation, as well as over a narrow spectral range near the bandedge due to exciton formation. We find that only the latter contribution gives rise to photo-induced THz absorption. By patterning multiple 2D perovskites with different optical absorption properties onto a single device, we demonstrate both colour selective modulation and focusing of THz radiation. These findings open new directions for creating active THz devices.
A mineral discovered in Russia in the 1830s known as a
perovskite holds a key to the next step in ultra-high-speed communications and
computing.
Researchers
from the University of Utah’s departments of electrical and computer
engineering and physics and astronomy have discovered that a special kind of
perovskite, a combination of an organic and inorganic compound that has the
same structure as the original mineral, can be layered on a silicon wafer to
create a vital component for the communications system of the future. That
system would use the terahertz spectrum, the next generation of communications
bandwidth that uses light instead of electricity to shuttle data, allowing
cellphone and internet users to transfer information a thousand times faster
than today.
The new
research, led by University of Utah electrical and computer engineering
professor Ajay Nahata and physics and astronomy Distinguished Professor Valy
Vardeny, was published Monday, Nov. 6 in the latest edition of Nature
Communications.
PHOTO CREDIT: Dan
Hixon/University of UtahCollege of Engineering
U physics and astronomy Distinguished
Professor Valy Vardeny, left, and U electrical and computer engineering
professor Ajay Nahata have discovered that a special kind of perovskite, a
combination of an organic and inorganic compound.
The
terahertz range is a band between infrared light and radio waves and utilizes
frequencies that cover the range from 100 gigahertz to 10,000 gigahertz (a
typical cellphone operates at just 2.4 gigahertz). Scientists are studying how
to use these light frequencies to transmit data because of its tremendous
potential for boosting the speeds of devices such as internet modems or cell
phones.
Nahata
and Vardeny uncovered an important piece of that puzzle: By depositing a
special form of multilayer perovskite onto a silicon wafer, they can modulate
terahertz waves passing through it using a simple halogen lamp. Modulating the
amplitude of terahertz radiation is important because it is how data in such a
communications system would be transmitted.
Previous
attempts to do this have usually required the use of an expensive, high-power
laser. What makes this demonstration different is that it is not only the lamp
power that allows for this modulation but also the specific color of the light.
Consequently, they can put different perovskites on the same silicon substrate,
where each region could be controlled by different colors from the lamp. This
is not easily possible when using conventional semiconductors like silicon.
“Think
of it as the difference between something that is binary versus something that
has 10 steps,” Nahata explains about what this new structure can do. “Silicon
responds only to the power in the optical beam but not to the color. It gives
you more capabilities to actually do something, say for information processing
or whatever the case may be.”
Not
only does this open the door to turning terahertz technologies into a reality —
resulting in next-generation communications systems and computing that is a
thousand times faster — but the process of layering perovskites on silicon is
simple and inexpensive by using a method called “spin casting,” in which the
material is deposited on the silicon wafer by spinning the wafer and allowing
centrifugal force to spread the perovskite evenly.
Vardeny
says what’s unique about the type of perovskite they are using is that it is
both an inorganic material like rock but also organic like a plastic, making it
easy to deposit on silicon while also having the optical properties necessary
to make this process possible.
“It’s a
mismatch,” he said. “What we call a ‘hybrid.’”
Nahata
says it’s probably at least another 10 years before terahertz technology for
communications and computing is used in commercial products, but this new
research is a significant milestone to getting there.
“This
basic capability is an important step towards getting a full-fledged
communications system,” Nahata says. “If you want to go from what you’re doing
today using a modem and standard wireless communications, and then go to a
thousand times faster, you’re going to have to change the technology
dramatically.”
The
paper was co-authored by students, Ashish Chanana, Yaxin Zhai, Sangita Baniya
and Chuang Zhang.
Our work presents a novel technique to encode information onto terahertz metasurfaces comprised of geometrically identical unit cell arrays. Previous demonstrations on metasurfaces or frequency-selective surfaces have shown interesting concepts to engineer electromagnetic radiation, but such designs often require a spatial arrangement of geometrically varying unit cells, either by shape, size, orientation, etc. In some cases, the output response can be mapped by examining the arrangement of atoms. Here, we show that by fabricating an array of resonant structures that are nominally identical visually, but where individual structures can have different conductivities, we can hide image information that is revealed when imaged using the appropriate terahertz frequency and polarization. This is achieved because changes in the structure’s conductivity correspond to changes in the depth of the resonant absorption observed in transmission. Using the simplest unit cell consisting of a single dipole, we create images that have up to 9 different discernible gray levels when interrogated at a single frequency. When a slightly more complex cross structure is used in the unit cell, 36 discernible levels are encoded in the image using two different polarizations. Finally, when the unit cell consists of multiple dipoles designed for multiple frequencies, we observe 64 unique colors in an encoded image. We believe our results present a unique approach for hiding information that could be applied to security-related applications.
In this work we analyze metamaterial terahertz modulators consisting of 2D-material/metal hybrid-structures. Influence of the near-field enhancement in the modulator performance is discussed. Devices are fabricated and tested using graphene and MoS2 as active materials.
