R. A. Lewis

A review of terahertz sources

Bibliometric data set the scene by illustrating the growth of terahertz work and the present interest in terahertz science and technology. After locating terahertz sources within the broader context of terahertz systems, an overview is given of the range of available sources, emphasizing recent developments. The focus then narrows to terahertz sources that rely on surface phenomena. Three are highlighted. Optical rectification, usually thought of as a bulk process, may in addition exhibit a surface contribution, which, in some cases, predominates. Transient surface currents, for convenience often separated into drift and diffusion currents, are well understood according to Monte Carlo modelling. Finally, terahertz surface emission by mechanical means—in the absence of photoexcitation—is described.

Stacked-and-drawn metamaterials with magnetic resonances in the terahertz range

We present a novel method for producing drawn metamaterials containing slotted metallic cylinder resonators, possessing strong magnetic resonances in the terahertz range. The resulting structures are either spooled to produce a 2-dimensional metamaterial monolayer, or stacked to produce three-dimensional multi-layered metamaterials. We experimentally investigate the effects of the resonator size and number of metamaterial layers on transmittance, observing magnetic resonances between 0.1 and 0.4 THz, in good agreement with simulations. Such fibers promise future applications in mass-produced stacked or woven metamaterials.

Far-infrared reflection and transmission of La1-xCaxMnO3

Abstract Colossal magnetoresistance compounds of composition La1−xCaxMnO3 have been investigated by far-infrared spectroscopy. In contrast to earlier work, which has concentrated on reflection data for La2/3Ca1/3MnO3, the present work includes transmission data and covers the Ca substitution range 0≤x≤0.5. The spectra are found to be sensitive to processing variables such as precursor composition, processing route, and annealing temperature. The (La,Ca)–(MnO3) vibration (∼200 cm−1) decreases in absorption and increases in energy with x. The O–Mn–O bending mode (∼400 cm−1) behaves likewise. The Mn–O stretching mode (∼600 cm−1) decreases in absorption with increasing Ca, but, in contrast to the other modes, initially moves to lower energies as the Ca content increases.

Electronic and thermal transport in hot carrier solar cells with low-dimensional contacts

Hot carrier solar cells are a third generation solar cell device where electrons and holes, heated by solar radiation, are removed from the absorber via low-dimensional energy selective contacts before they can thermalise to the band edge. Here, a new model is presented for calculating the performance of these devices, which takes into account the energy spectrum of the contacts. It is shown that efficiency is maximised with a certain ideal number of contacts and that the energy spectra of these should be narrow.

Strong terahertz emission from (100) p-type InAs

Terahertz emission has been observed from (100) Zn-acceptor-doped InAs under illumination by fs pulses of near-infrared radiation. Turning the crystal about the surface normal produces two maxima per rotation, whether the angle of incidence is 45° or 75°, in contrast to (111) p-InAs, where three maxima per rotation have been reported. The emitted terahertz power has a quadratic variation with the pump power and decreases with increasing temperature in the range 20–300K. This behavior is consistent with a photocurrent surge being the dominant terahertz generating mechanism at low excitation fluences. The p-type InAs generates about two orders of magnitude more power than the standard unbiased terahertz emitter, 1mm thick ZnTe.

In situ micro-Raman studies of laser-induced bismuth oxidation reveals metastability of beta-Bi2O3 microislands

We report the laser irradiation-induced oxidation of bismuth metal investigated in situ by micro-Raman spectroscopy as a function of irradiation power and time. The purely optical synthesis and characterization of β-Bi2O3 oxide microislands on metallic Bi surfaces is shown to be stable over time, even at room-temperature. By closely examining possible reactions on simple Bi morphologies it is revealed for the first time that the ensuing oxide phase is critically dependent on the final oxide volume and follows a fixed kinetic transformation sequence: 32O2(g)+2Bi(l)→β–Bi2O3(s)→α–Bi2O3(s). These findings are unusual within the framework of traditional Bi2O3 thermal transformation relations. An electrostatic mechanism involving a changing Bi2O3 surface-to-volume ratio is proposed to explain the room-temperature metastability of small β-Bi2O3 volumes and the subsequent transformation sequence, as well as unifying the results of previous studies.

Fiber metamaterials with negative magnetic permeability in the terahertz

We present a novel method for producing metamaterials with a terahertz magnetic response via fiber drawing, which can be inexpensively scaled up to mass production. We draw a centimeter preform to fiber, spool it, and partially sputter it with metal to produce extended slotted resonators. We characterize metamaterial fiber arrays with different orientations via terahertz time domain spectroscopy, observing distinct magnetic resonances between 0.3 and 0.4 THz, in excellent agreement with simulations. Numerical parameters retrieval techniques confirm that such metamaterials possess negative magnetic permeability. Combined with fiber-based negative permittivity materials, this will enable the development of the first woven negative index materials, as well as the fabrication of magnetic surface plasmon waveguides and subwavelength waveguides.

High-field magnetotransport in a two-dimensional electron gas in quantizing magnetic fields and intense terahertz laser fields

We present a combined experimental and theoretical study of interactions between two-dimensional electron gases (2DEGs) and terahertz (THz) free-electron lasers in the presence of quantizing magnetic fields. It is found both experimentally and theoretically that when an intense THz field and a quantizing magnetic field are applied simultaneously to a GaAs-based 2DEG in the Faraday geometry, a strong cyclotron resonance (CR) effect on top of the magnetophonon resonances can be observed by transport measurements at relatively high temperatures. With increasing radiation intensity and/or decreasing temperature, the peaks of the CR are broadened and split due to magnetophoton–phonon scattering.

Solid-state thermionics and thermoelectrics in the ballistic transport regime

It is shown that equations for electrical current in solid-state thermionic and thermoelectric devices converge for devices with a width equal to the mean free path of electrons, yielding a common expression for intensive electronic efficiency in the two types of devices. This result is used to demonstrate that the materials parameters for thermionic and thermoelectric devices are equal, rather than differing by a multiplicative factor as previously thought.It is shown that the equations for electrical current in solid-state thermionic and thermoelectric devices converge for devices with a width equal to the mean free path of electrons, yielding a common expression for the intensive electronic efficiency in the two types of devices. This result is used to demonstrate that the material parameters for thermionic and thermoelectric refrigerators are equal, rather than differing by a multiplicative factor as previously thought.

Extremely broadband characterization of a Schottky diode based THz detector

A Schottky diode based module for direct detection of THz power provides fast measurement capabilities at room temperature. This paper presents the broadband spectral characterization of a Schottky diode in comparison to a Golay cell in the frequency range from 0.1 THz up to 2 THz.