242,787 research outputs found
Full characterization of vibrational coherence in a porphyrin chromophore by two-dimensional electronic spectroscopy
In this work we present experimental and calculated two-dimensional electronic spectra for a 5,15-bisalkynyl porphyrin chromophore. The lowest energy electronic Qy transition couples mainly to a single 380 cmâ1 vibrational mode. The two-dimensional electronic spectra reveal diagonal and cross peaks which oscillate as a function of population time. We analyze both the amplitude and phase distribution of this main vibronic transition as a function of excitation and detection frequencies. Even though Feynman diagrams provide a good indication of where the amplitude of the oscillating components are located in the excitation-detection plane, other factors also affect this distribution. Specifically, the oscillation corresponding to each Feynman diagram is expected to have a phase that is a function of excitation and detection frequencies. Therefore, the overall phase of the experimentally observed oscillation will reflect this phase dependence. Another consequence is that the overall oscillation amplitude can show interference patterns resulting from overlapping contributions from neighboring Feynman diagrams. These observations are consistently reproduced through simulations based on third order perturbation theory coupled to a spectral density described by a Brownian oscillator model
Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications
Resonant frequencies of the two-dimensional plasma in FETs increase with the
reduction of the channel dimensions and can reach the THz range for sub-micron
gate lengths. Nonlinear properties of the electron plasma in the transistor
channel can be used for the detection and mixing of THz frequencies. At
cryogenic temperatures resonant and gate voltage tunable detection related to
plasma waves resonances, is observed. At room temperature, when plasma
oscillations are overdamped, the FET can operate as an efficient broadband THz
detector. We present the main theoretical and experimental results on THz
detection by FETs in the context of their possible application for THz imaging.Comment: 22 pages, 12 figures, review pape
On the development of QPOs in Bondi-Hoyle accretion flows
The numerical investigation of Bondi-Hoyle accretion onto a moving black hole
has a long history, both in Newtonian and in general-relativistic physics. By
performing new two-dimensional and general-relativistic simulations onto a
rotating black hole, we point out a novel feature, namely, that quasi-periodic
oscillations (QPOs) are naturally produced in the shock cone that develops in
the downstream part of the flow. Because the shock cone in the downstream part
of the flow acts as a cavity trapping pressure perturbations, modes with
frequencies in the integer ratios 2:1 and 3:1 are easily produced. The
frequencies of these modes depend on the black-hole spin and on the properties
of the flow, and scale linearly with the inverse of the black-hole mass. Our
results may be relevant for explaining the detection of QPOs in Sagittarius A*,
once such detection is confirmed by further observations. Finally, we report on
the development of the flip-flop instability, which can affect the shock cone
under suitable conditions; such an instability has been discussed before in
Newtonian simulations but was never found in a relativistic regime.Comment: 11 pages, 7 figure
Plasma mechanisms of resonant terahertz detection in two-dimensional electron channel with split gates
We analyze the operation of a resonant detector of terahertz (THz) radiation
based on a two-dimensional electron gas (2DEG) channel with split gates. The
side gates are used for the excitation of plasma oscillations by incoming THz
radiation and control of the resonant plasma frequencies. The central gate
provides the potential barrier separating the source and drain portions of the
2DEG channel. Two possible mechanisms of the detection are considered: (1)
modulation of the ac potential drop across the barrier and (2) heating of the
2DEG due to the resonant plasma-assisted absorption of THz radiation followed
by an increase in thermionic dc current through the barrier. Using the device
model we calculate the frequency and temperature dependences of the detector
responsivity associated with both dynamic and heating (bolometric) mechanisms.
It is shown that the dynamic mechanisms dominates at elevated temperatures,
whereas the heating mechanism provides larger contribution at low temperatures,
T=35-40 K.Comment: 7 pages, 4 figure
The propagating speed of relic gravitational waves and their refractive index during inflation
If the refractive index of the tensor modes increases during a conventional
inflationary stage of expansion the relic graviton spectrum is tilted towards
high frequencies. Two apparently diverse parametrizations of this effect are
shown to be related by a rescaling of the four-dimensional metric through a
conformal factor that involves the refractive index itself. Non-monotonic
spectra with a maximum in the MHz region correspond to a limited variation of
the refractive index terminating well before the end of inflation. After
exploring a general approach encompassing the ones proposed so far, we estimate
the required sensitivity for the direct detection of the predicted
gravitational radiation and demonstrate that the allowed regions of the
parameter space are within reach for some of the planned detectors operating
either in the audio band (like Ligo/Virgo and Kagra) or in the mHz band (like
Lisa, Bbo and Decigo).Comment: 32 pages, 12 figures; corrected typos and comments adde
Cavity-enhanced optical Hall effect in two-dimensional free charge carrier gases detected at terahertz frequencies
The effect of a tunable, externally coupled Fabry-P\'{e}rot cavity to
resonantly enhance the optical Hall effect signatures at terahertz frequencies
produced by a traditional Drude-like two-dimensional electron gas is shown and
discussed in this communication. As a result, the detection of optical Hall
effect signatures at conveniently obtainable magnetic fields, for example by
neodymium permanent magnets, is demonstrated. An AlInN/GaN-based high electron
mobility transistor structure grown on a sapphire substrate is used for the
experiment. The optical Hall effect signatures and their dispersions, which are
governed by the frequency and the reflectance minima and maxima of the
externally coupled Fabry-P\'{e}rot cavity, are presented and discussed. Tuning
the externally coupled Fabry-P\'{e}rot cavity strongly modifies the optical
Hall effect signatures, which provides a new degree of freedom for optical Hall
effect experiments in addition to frequency, angle of incidence and magnetic
field direction and strength
Simultaneous projection and detection system of four different frequencies for microwave imaging reflectometry in Large Helical Device
A simultaneous projection/detection system of four different frequencies for microwave imaging reflectometry (MIR) was developed for three-dimensional observation of electron density fluctuations in the Large Helical Device (LHD). The microwave with four frequency components at 60.410, 61.808, 63.008, and 64.610 GHz is projected in a continuous-wave mode to illuminate the target LHD plasma. A two-dimensional horn-antenna mixer array (2D HMA) receives the reflected wave from the plasma as well as the wave from the local oscillator operating at 55.800 GHz. The first intermediate frequency (IF) signals at 4.610, 6.008, 7.208, and 8.810 GHz were confirmed to be obtained by downconversion of these microwaves using the 2D HMA. Each of these first IF components is filtered from each other and downconverted again for the superheterodyne detection. It was confirmed that both the amplitudes and the phases of the detected signals reflect the fluctuations in LHD plasmas
- âŠ