3,563 research outputs found
Optical frequency combs from high-order sideband generation
We report on the generation of frequency combs from the recently-discovered
phenomenon of high-order sideband generation (HSG). A near-band gap
continuous-wave (cw) laser with frequency was transmitted
through an epitaxial layer containing GaAs/AlGaAs quantum wells that were
driven by quasi-cw in-plane electric fields between 4 and 50
kV/cm oscillating at frequencies between 240 and 640 GHz.
Frequency combs with teeth at
( even) were produced, with maximum reported , corresponding to a
maximum comb span THz. Comb spectra with the identical product
were found to have similar spans and shapes
in most cases, as expected from the picture of HSG as a scattering-limited
electron-hole recollision phenomenon. The HSG combs were used to measure the
frequency and linewidth of our THz source as a demonstration of potential
applications
Applications of Graphene at Microwave Frequencies
In view to the epochal scenarios that nanotechnology discloses, nano-electronics has the potential to introduce a paradigm shift in electronic systems design similar to that of the transition from vacuum tubes to semiconductor devices. Since low dimensional (1D and 2D) nano-structured materials exhibit unprecedented electro-mechanical properties in a wide frequency range, including radio-frequencies (RF), microwave nano-electronics provides an enormous and yet widely undiscovered opportunity for the engineering community. Carbon nano-electronics is one of the main research routes of RF/microwave nano-electronics. In particular, graphene has shown proven results as an emblematic protagonist, and a real solution for a wide variety of microwave electronic devices and circuits. This paper introduces graphene properties in the microwave range, and presents a paradigm of novel graphene-based devices and applications in the microwave/RF frequency range
Diamond semiconductor technology for RF device applications
This paper presents a comprehensive review of diamond electronics from the RF perspective. Our aim was to find and present the potential, limitations and current status of diamond semiconductor devices as well as to investigate its suitability for RF device applications. While doing this, we briefly analysed the physics and chemistry of CVD diamond process for a better understanding of the reasons for the technological challenges of diamond material. This leads to Figure of Merit definitions which forms the basis for a technology choice in an RF device/system (such as transceiver or receiver) structure. Based on our literature survey, we concluded that, despite the technological challenges and few mentioned examples, diamond can seriously be considered as a base material for RF electronics, especially RF power circuits, where the important parameters are high speed, high power density, efficient thermal management and low signal loss in high power/frequencies. Simulation and experimental results are highly regarded for the surface acoustic wave (SAW) and field emission (FE) devices which already occupies space in the RF market and are likely to replace their conventional counterparts. Field effect transistors (FETs) are the most promising active devices and extremely high power densities are extracted (up to 30 W/mm). By the surface channel FET approach 81 GHz operation is developed. Bipolar devices are also promising if the deep doping problem can be solved for operation at room temperature. Pressure, thermal, chemical and acceleration sensors have already been demonstrated using micromachining/MEMS approach, but need more experimental results to better exploit thermal, physical/chemical and electronic properties of diamond
Sub-gap optical response across the structural phase transition in van der Waals layered \alpha-RuCl
We report magnetic, thermodynamic, thermal expansion, and on detailed optical
experiments on the layered compound -RuCl focusing on the THz and
sub-gap optical response across the structural phase transition from the
monoclinic high-temperature to the rhombohedral low-temperature structure,
where the stacking sequence of the molecular layers is changed. This type of
phase transition is characteristic for a variety of tri-halides crystallizing
in a layered honeycomb-type structure and so far is unique, as the
low-temperature phase exhibits the higher symmetry. One motivation is to
unravel the microscopic nature of spin-orbital excitations via a study of
temperature and symmetry-induced changes. We document a number of highly
unusual findings: A characteristic two-step hysteresis of the structural phase
transition, accompanied by a dramatic change of the reflectivity. An electronic
excitation, which appears in a narrow temperature range just across the
structural phase transition, and a complex dielectric loss spectrum in the THz
regime, which could indicate remnants of Kitaev physics. Despite significant
symmetry changes across the monoclinic to rhombohedral phase transition, phonon
eigenfrequencies and the majority of spin-orbital excitations are not strongly
influenced. Obviously, the symmetry of the single molecular layers determine
the eigenfrequencies of most of these excitations. Finally, from this combined
terahertz, far- and mid-infrared study we try to shed some light on the so far
unsolved low energy (< 1eV) electronic structure of the ruthenium
electrons in -RuCl.Comment: 22 pages, 9 figure
A tunable cavity-locked diode laser source for terahertz photomixing
An all solid-state approach to the precise frequency synthesis and control of widely tunable terahertz radiation by differencing continuous-wave diode lasers at 850 nm is reported in this paper. The difference frequency is synthesized by three fiber-coupled external-cavity laser diodes. Two of the lasers are Pound-Drever-Hall locked to different orders of a Fabry-Perot (FP) cavity, and the third is offset-frequency locked to the second of the cavity-locked lasers using a tunable microwave oscillator. The first cavity-locked laser and the offset-locked laser produce the difference frequency, whose value is accurately determined by the sum of an integer multiple of the free spectral range of the FP cavity and the offset frequency. The dual-frequency 850-nm output of the three laser system is amplified to 500 mW through two-frequency injection seeding of a single semiconductor tapered optical amplifier. As proof of precision frequency synthesis and control of tunability, the difference frequency is converted into a terahertz wave by optical-heterodyne photomixing in low-temperature-grown GaAs and used for the spectroscopy of simple molecules. The 3-dB spectral power bandwidth of the terahertz radiation is routinely observed to be ≾1 MHz. A simple, but highly accurate, method of obtaining an absolute frequency calibration is proposed and an absolute calibration of 10^(-7) demonstrated using the known frequencies of carbon monoxide lines between 0.23-1.27 THz
Photonic chip based optical frequency comb using soliton induced Cherenkov radiation
By continuous wave pumping of a dispersion engineered, planar silicon nitride
microresonator, continuously circulating, sub-30fs short temporal dissipative
solitons are generated, that correspond to pulses of 6 optical cycles and
constitute a coherent optical frequency comb in the spectral domain. Emission
of soliton induced Cherenkov radiation caused by higher order dispersion
broadens the spectral bandwidth to 2/3 of an octave, sufficient for self
referencing, in excellent agreement with recent theoretical predictions and the
broadest coherent microresonator frequency comb generated to date. In a further
step, this frequency comb is fully phase stabilized. The ability to preserve
coherence over a broad spectral bandwidth using soliton induced Cherenkov
radiation marks a critical milestone in the development of planar optical
frequency combs, enabling on one hand application in e.g. coherent
communications, broadband dual comb spectroscopy and Raman spectral imaging,
while on the other hand significantly relaxing dispersion requirements for
broadband microresonator frequency combs and providing a path for their
generation in the visible and UV. Our results underscore the utility and
effectiveness of planar microresonator frequency comb technology, that offers
the potential to make frequency metrology accessible beyond specialized
laboratories.Comment: Changes: - Added data (new Fig.4) on the first full phase
stabilization of a dissipative Kerr soliton (or dissipative cavity soliton)
in a microresonator - Extended Fig. 8 in the SI - Introduced nomenclature of
dissipative Kerr solitons - Minor other change
Pulsed THz radiation due to phonon-polariton effect in [110] ZnTe crystal
Pulsed terahertz (THz) radiation, generated through optical rectification
(OR) by exciting [110] ZnTe crystal with ultrafast optical pulses, typically
consists of only a few cycles of electromagnetic field oscillations with a
duration about a couple of picoseconds. However, it is possible, under
appropriate conditions, to generate a long damped oscillation tail (LDOT)
following the main cycles. The LDOT can last tens of picoseconds and its
Fourier transform shows a higher and narrower frequency peak than that of the
main pulse. We have demonstrated that the generation of the LDOT depends on
both the duration of the optical pulse and its central wavelength. Furthermore,
we have also performed theoretical calculations based upon the OR effect
coupled with the phonon-polariton mode of ZnTe and obtained theoretical THz
waveforms in good agreement with our experimental observation.Comment: 9 pages, 5 figure
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