50 research outputs found
Two-photon excitation spectroscopy of 1,5--Diphenyl-1,3,5-hexatriene using phase modulation
We have used two-photon Fourier transform spectroscopy to investigate the
first singlet excited state (S1) of a prototypical polyene molecule 1,5 --
Diphenyl-1,3,5-hexatriene. As the S1 state in the polyenes is a one-photon
forbidden transition, its structure of the vibrational levels cannot be studied
using resonant linear excitation. Although this level is accessible with
two-photon excitation, previous studies done by using wavelength tunable pulsed
lasers did not have enough resolution to investigate the details of the
vibrational levels. In Fourier transform spectroscopy, one uses a pair of laser
beams to excite the sample. The measurements are done by varying the time delay
between the pulses. The spectral resolution is given by the inverse of the
maximum time delay rather than the spectral width of the pulses. We have used
the method to investigate the vibrational levels of the S1 state. In our
implementation, we have used a phase modulation technique to carry out the
measurements in the rotating frame, which require fewer data points along the
time delay thereby significantly reducing the measurement time
Direct Measurement of Fast Transients by Using Boot-strapped Waveform Averaging
An approximation to coherent sampling, also known as boot-strapped waveform
averaging, is presented. The method uses digital cavities to determine the
condition for coherent sampling. It can be used to increase the effective
sampling rate of a repetitive signal and the signal to noise ratio
simultaneously. The method is demonstrated by using it to directly measure the
fluorescence lifetime from rhodamine 6G by digitizing the signal from a fast
avalanche photodiode. The obtained lifetime of 4.4+-0.1 ns is in agreement with
the known values
The nature of relaxation processes revealed by the action signals of phase modulated light fields
We introduce a generalized theoretical approach to study action signals
induced by the absorption of two-photons from two phase modulated laser beams
and subject it to experimental testing for two types of photoactive samples,
solution of rhodamine 6G and GaP photodiode. In our experiment, the phases of
the laser beams are modulated at the frequencies f1 and f2, respectively. The
action signals, such as photoluminescence and photocurrent, which result from
the absorption of two photons, are isolated at frequencies m f (f=|f1-f2|,
m=0,1,2...). We demonstrate that the ratio of the amplitudes of the secondary
(m=2) and the primary (m=1) signals is sensitive to the type of relaxation
process taken place in the system and thus can be used for its identification.
Such sensitivity originates from cumulative effects of non-equilibrated state
of the system between the light pulses. When the cumulative effects are small,
i.e. the relaxation time is much shorter then the laser repetition rate or the
laser intensity is high enough to dominate the system behavior, the ratio
achieves its reference value 1:4 (the signature of two-photon absorption). In
the intermediate regimes the ratio changes rapidly with the growth of intensity
from zero value in case of second order relaxation process, while it
demonstrates slow monotonic decrease for linear relaxation. In the article we
also determine the value of the recombination rate in a GaP photodiode by using
the above approach
Digital Cavities and Their Potential Applications
The concept of a digital cavity is presented. The functionality of a tunable
radio-frequency/microwave cavity with unrestricted Q-factor is implemented. The
theoretical aspects of the cavity and its potential applications in high
resolution spectroscopy and synchronization of clocks together with examples in
signal processing and data acquisition are discussed
Optimization Schemes for Efficient Multiple Exciton Generation and Extraction in Colloidal Quantum Dots
Multiple exciton generation is a process in which more than one electron hole
pair is generated per absorbed photon. It allows us to increase the efficiency
of solar energy harvesting. Experimental studies have shown the multiple
exciton generation yield of 1.2 in isolated colloidal quantum dots. However
real photoelectric devices require the extraction of electron hole pairs to
electric contacts. We provide a systematic study of the corresponding quantum
coherent processes including extraction and injection and show that a proper
design of extraction and injection rates enhances the yield significantly up to
values around 1.6.Comment: 5 pages, accepted by The Journal of Chemical Physic
Generalized lock-in amplifier for precision measurement of high frequency signals
We herein formulate the concept of a generalized lock-in amplifier for the
precision measurement of high frequency signals based on digital cavities.
Accurate measurement of signals higher than 200 MHz using the generalized
lock-in is demonstrated. The technique is compared with a traditional lock-in
and its advantages and limitations are discussed. We also briefly point out how
the generalized lock-in can be used for precision measurement of giga-hertz
signals by using parallel processing of the digitized signals
Probing Silicon Carbide with Phase-Modulated Femtosecond Laser Pulses: Insights into Multiphoton Photocurrent
Wide bandgap semiconductors are widely used in photonic technologies due to
their advantageous features, such as large optical bandgap, low losses, and
fast operational speeds. Silicon carbide is a prototypical wide bandgap
semiconductor with high optical nonlinearities, large electron transport, and a
high breakdown threshold. Integration of silicon carbide in nonlinear photonics
requires a systematic analysis of the multiphoton contribution to the device
functionality. Here, multiphoton photocurrent in a silicon carbide
photodetector is investigated using phase-modulated femtosecond pulses.
Multiphoton absorption is quantified using a 1030 nm phase-modulated pulsed
laser. Our measurements show that although the bandgap is less than the energy
of three photons, only four-photon absorption has a significant contribution to
the photocurrent. We interpret the four-photon absorption as a direct
transition from the valance to the conduction band at the {\Gamma} point. More
importantly, silicon carbide withstands higher excitation intensities compared
to other wide bandgap semiconductors making it an ideal system for high-power
nonlinear applications.Comment: 14 pages, 4 figure
Compressed Sensing for Reconstructing Coherent Multidimensional Spectra
We apply two sparse reconstruction techniques, the least absolute shrinkage
and selection operator (LASSO) and the sparse exponential mode analysis (SEMA),
to two-dimensional (2D) spectroscopy. The algorithms are first tested on model
data, showing that both are able to reconstruct the spectra using only a
fraction of the data required by the traditional Fourier-based estimator.
Through the analysis of a sparsely sampled experimental fluorescence detected
2D spectra of LH2 complexes, we conclude that both SEMA and LASSO can be used
to significantly reduce the required data, still allowing to reconstruct the
multidimensional spectra. Of the two techniques, it is shown that SEMA offers
preferable performance, providing more accurate estimation of the spectral line
widths and their positions. Furthermore, SEMA allows for off-grid components,
enabling the use of a much smaller dictionary than the LASSO, thereby improving
both the performance and lowering the computational complexity for
reconstructing coherent multidimensional spectra
Effects of impurity band on multiphoton photocurrent from InGaN and GaN photodetectors
Multiphoton absorption of wide band-gap semiconductors has shown great
prospects in many fundamental researches and practical applications. With
intensity-modulated femtosecond lasers by acousto-optic frequency shifters,
photocurrents and yellow luminescence induced by two-photon absorption of InGaN
and GaN photodetectors are investigated experimentally. Photocurrent from InGaN
detector shows nearly perfect quadratic dependence on excitation intensity,
while that in GaN detector shows cubic and higher order dependence. Yellow
luminescence from both detectors show sub-quadratic dependence on excitation
intensity. Highly nonlinear photocurrent from GaN is ascribed to absorption of
additional photons by long-lived electrons in traps and impurity bands. Our
investigation indicates that InGaN can serve as a superior detector for
multiphoton absorption, absent of linear and higher order process, while GaN,
which suffers from absorption by trapped electrons and impurity bands, must be
used with caution