A terahertz (THz or T-rays) photomixer consisting of a meander type antenna with integrated nanoelectrodes on a low temperature grown GaAs (LT-GaAs) is demonstrated. The antenna was designed for molecular fingerprinting and sensing applications within a spectral range of 0.3-0.4 THz. A combination of electron beam lithography (EBL) and focused ion beam (FIB) milling was used to fabricate the T-ray emitter. Antenna and nanoelectrodes were fabricated by standard EBL and lift-off steps. Then a 40-nm-wide gap in an active photomixer area separating the nanoelectrodes was milled by a FIB. The integrated nano-contacts with nano-gaps enhance the illuminated light and THz electric fields as well as contribute to a better collection of photo-generated electrons. T-ray emission power from the fabricated photomixer chips were few hundreds of nanowatts at around 0.15 THz and tens of nanowatts in the 0.3-0.4 THz range.
Liquid crystals are candidate materials for optical devices operating in the Terahertz (THz) frequency region of the
electromagnetic spectrum. Proposed devices include THz phase shifters and THz quarter wave plates. To assist in
designing for these applications, the fundamental properties of the materials should be determined. Fundamental optical
properties to be determined over the frequency range of interest are the refractive index n and the absorption coefficient
α. According to the orientation of the liquid crystals relative to the polarisation of the light field, ordinary and
extraordinary values for the refractive index may be distinguished. In early work, employing time-domain spectroscopy,
a rise in both no and ne with optical frequency in the THz region was reported. Later work, employing two-colour
generation of THz radiation, indicated the values of no and ne were both relatively constant in the THz region. We have
now made measurements of the two common nematic liquid crystals K15 and E7 using time-domain THz spectroscopy
and confirm that no and ne show little change over the spectral region 0.15 to 1 THz.
Over the last 15 years, many groups have analyzed terahertz generation by optical rectification and
subsequently many different expressions are present in the literature. The theory has been developed
for the (100), (110), (111) and more recently the (112) crystal faces and compared to experimental
results. A recent paper by Hargreaves, Radhanpura and Lewis (HRL) deals with optical rectification
in zinc blende crystals for arbitrary excitation conditions. The current paper analyzes expressions
from the literature to reconcile any differences. In most cases, we have found that the generalized
theory reproduces the results published in previous papers with some phase shift in azimuthal angle.
However, these phase shifts not only differ between papers but also, within the one paper, between
different crystal orientations. As notations tend to differ between papers, the need for a generalized
and agreed definition of co-ordinates and angles becomes apparent. Identifying where these
corrections originate is made more difficult with some of the papers missing explicit definitions of
co-ordinate systems and azimuthal angles. It has been found that the differences originate from the
definition of the azimuthal angle and direction of rotation. With these differences reconciled, the
general theory is able to reproduce the azimuthal angle dependence of terahertz generation by optical
rectification.
There is a demand for more efficient sources of electromagnetic radiation in the terahertz (THz, 1012 Hz)
frequency region. One common method of generating THz-frequency radiation is to direct fs pulses of near-infrared
laser radiation onto a material which then re-radiates. This approach permits coherent pulses of THz
radiation to be produced which, for example, may be used for time-domain spectroscopy (TDS). There are three
principal mechanisms by which THz radiation is generated under the stimulus of ultra-short pulses: optical
rectification (OR) in electro-optic materials, photoconductivity (PC) from materials with suitable electrodes,
and surface-field (SF) effects in other cases. The III-V compound semiconductor GaAs doped with the acceptor
impurity Be produces relatively small amounts of THz radiation via the OR and SF mechanisms, but relatively
large amounts via the PC mechanism. We have studied the PC emission of THz radiation from layers of GaAs(Be)
grown epitaxially on GaAs substrates. The THz power generated depends on the bias applied to the electrodes
approximately quadratically. This is typical of the PC mechanism. The dependence of the THz power on the
power of the pump beam is approximately linear. In general, the THz generated tends to decrease as the doping
level increases. If the doping level exceeds the Mott limit and the material becomes highly conductive then the
photoconductivity and consequently the THz production are quenched.
KEYWORDS: Resistance, Electron transport, Scattering, Phonons, Solid state electronics, Modulation, Superlattices, Energy efficiency, Physics, Interfaces
We consider the effect that the barrier shape has on the electron energy spectrum and lattice thermal conductivity, and together the effect of these coefficient of performance of thermionic refrigerators. Whilst it is shown that wide barriers are also desirable to enhance the electron energy spectrum, the primary motivation to increase barrier width to the maximum allowable value with ballistic transport is to reduce thermal conductivity. It is shown that the barriers which produce the highest electronic coefficient of performance do not necessarily give the highest coefficient of performance when thermal conductivity is considered if electronic heat current is reduced. While mean free path length multibarrier geometries may offer reduced thermal conductivity due to the possibility of interface scattering and phonon miniband formation, this effect needs to be significant to achieve coefficient of performance comparable with a single barrier device. Finally, we show that maximum refrigerator coefficient of performance is achieved by transmitting electrons over a tuned energy range only, which may be approximated by the transmission probability associated with a Gaussian modulated superlattice.
Conventional solid-state and vacuum thermionic devices restrict the flow of electrons between the hot and cold reservoirs according to the magnitude of their momentum in the direction of transport only. Recently it has been suggested that devices may be developed where the filtering of transmitted electrons occurs according to their total momentum. We compare the performance of these two different methods of electron momentum filtering in single barrier and resonant tunneling thermionic refrigeration devices. It is shown that total momentum filtered single barrier refrigerators always outperform conventional single barrier refrigerators due to their larger heat current which is particularly important when the thermal conductivity of the system is significant. We show that whilst conventionally filtered resonant tunneling thermionic refrigerators are outperformed by total momentum resonant tunneling thermionic refrigerators in many conditions, their performance is superior at (1) high temperatures or (2) when the transmission energy is very close to the Fermi energy.
We enunciate the general principles that govern the transport of charge and heat in a thermionic device. We illustrate the application of these principles to the subject of domestic refrigeration. A complementary application is power generation. We distinguish Class 2 devices, in which the potential barrier on the hot side plays a role, from Class 1 devices, in which this barrier is irrelevant. We show that the effect of heat backflow is to drastically reduce the efficiency of thermionic devices in both GaAs and InSb representative semiconductor systems. We conclude that practical devices are not likely with bulk, single-barrier devices.
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