arXiv subjects

Khadga Jung Karki

Publications and source records attributed to Khadga Jung Karki.

11 recordsLinked to original sources

arXiv2026

Characterization of Nonlinear Dynamics in Semiconductors in Frequency Domain using Modulated Photoexcitation

Carrier dynamics in semiconductors is inherently complex owing to the coexistence of different excited species, such as free carriers and excitons, and their interactions among themselves and with traps and phonons. It is usual to use time-resolved responses to identify the processes that contribute to the dynamics. However, the responses often are non-exponential, leading to ambiguity in the interpretation. Here, we propose a frequency domain method to characterize nonlinear dynamics in semiconductors using CdSe as the test system. We show that by analyzing the frequency components in the total photoluminescence induced by a pair of phase-modulated beams, the parameters, such as rates of different types of recombination of free carriers and excitons, can be evaluated. The method can be used as a simple diagnostic tool to characterize ultrafast processes that are relevant to the functionality of semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2025

2D MoS$_2$/Au interfaces for enhanced opto-electronic response with sub-bandgap photons

Monolayer MoS$_2$ is a direct band gap semiconductor with potential applications in optoelectronics and photonics. MoS$_2$ also has a large optical nonlinearity. However, the atomic thickness of the monolayer limits the strength of the measured functional signals, such as the photocurrent or photoluminescence, in optoelectronic devices. Here, we show that photocurrent in monolayer MoS$_2$ can be induced by sub-band gap photons by depositing Au nanoparticles on it. In this system, the nonlinear light-matter interaction in Au nanoparticles enhanced by the localized surface plasmons results in the generation of supercontinuum, which is reabsorbed by MoS$_2$ due to efficient resonant energy transfer. Au nanoparticle assisted photocurrent is more than an order of magnitude larger than two-photon photocurrent in monolayer MoS$_2$. Optimization of the shape, size and composition of the nanoparticle has the potential to enhance the photocurrent significantly with the prospect of applications in the detection of NIR photons, and related technologies including optical telecommunication.

physics.optics↗

arXiv2025

ND1 centers in diamond for long-term data storage in extreme conditions

Practically feasible long-term data storage under extreme conditions is an unsolved problem in modern data storage systems. This study introduces a novel approach using ND1 centers in diamonds for high-density, three-dimensional optical data storage. By employing near-infrared femtosecond laser pulses, we demonstrate the creation of sub-micron ND1 defect sites with precise spatial control, enabling efficient data encoding as luminescent ''pits." The ND1 centers exhibit robust photoluminescence in the UV spectrum, driven by three-photon absorption, which intrinsically provides a 3D reading of the data. Remarkably, these centers remain stable under extreme electric and magnetic fields, temperatures ranging from 4 K to 500 K, and corrosive chemical environments, with no degradation observed over extended periods. A reading speed of 500 MBits/s, limited by the lifetime of the photoluminescence, surpasses conventional Blu-ray technology while maintaining compatibility with existing optical data storage infrastructure. Our findings highlight diamond-based ND1 centers as a promising medium for durable, high-capacity data storage, capable of preserving critical information for millions of years, even under harsh conditions.

physics.optics↗

arXiv2023

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.

physics.optics↗

arXiv2023

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 Γ 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.

physics.optics↗

arXiv2019

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.

eess.SP↗

arXiv2018

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.

physics.chem-ph↗

arXiv2017

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.

physics.data-an↗

arXiv2016

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.

physics.optics↗

arXiv2015

Isolation of coherent and incoherent nonlinear spectroscopic signals by phase modulation

We investigate the effect of phase modulation of laser beams on the coherent and incoherent non-linear signals arising from the interaction of femtosecond pulses with matter. We observe that two collinear beams, whose phases are modulated by frequencies $ϕ_1$ and $ϕ_2$, produce two second harmonic signals from non-linear crystals whose intensities at the detector are modulated at the frequencies $ϕ_2-ϕ_1$ and $2(ϕ_2-ϕ_1)$. We also observe that an incoherent action signal, such as fluorescence and photocurrent, which arises from the absorption of two photons, is modulated at the same frequencies as in the case of second harmonic generation. We present a theoretical analysis to explain our observations. These results are important to understand how phase modulation techniques can be used to isolate different field-matter interaction pathways in a non-linear process. Because the method uses modulation of the signal intensity rather than wave-vector matching to isolate different signals, it could be useful to perform multi-photon absorption studies on single molecules or nanoparticles.

physics.optics↗

arXiv2015

Phase modulated multiphoton microscopy

We show that the modulation of the phases of the laser beams of ultra-short pulses leads to modulation of the two photon fluorescence intensity. The phase modulation technique when used in multi-photon microscopy can improve the signal to noise ratio. The technique can also be used in multiplexing the signals in the frequency domain in multi-focal raster scanning microscopy. As the technique avoids the use of array detectors as well as elaborate spatiotemporal multiplexing schemes it provides a convenient means to multi-focal scanning in axial direction. We show examples of such uses. Similar methodology can be used in other non-linear scanning microscopies, such as second or third harmonic generation microscopy.

physics.optics↗