145 research outputs found
Pure Even Harmonic Generation from Oriented CO in Linearly Polarized Laser Fields
The first high harmonic spectrum, containing only the odd orders, was
observed in experiments 30 years ago (1987). However, a spectrum containing
pure even harmonics has never been observed. We investigate the generation of
pure even harmonics from oriented CO molecules in linearly polarized laser
fields employing the time-dependent density-functional theory. We find that the
even harmonics, with no odd orders, are generated with the polarization
perpendicular to the laser polarization, when the molecular axis of CO is
perpendicular to the laser polarization. Generation of pure even harmonics
reveals a type of dipole acceleration originating from the permanent dipole
moment. This phenomenon exists in all system with permanent dipole moments,
including bulk crystal and polyatomic molecules
Initial carrier-envelope phase of few-cycle pulses determined by THz emission from air plasma
The evolution of THz waveform generated in air plasma provides a sensitive
probe to the variation of the carrier envelope phase (CEP) of propagating
intense few-cycle pulses. Our experimental observation and calculation reveal
that the number and positions of the inversion of THz waveform are dependent on
the initial CEP, which is near 0.5{\pi} constantly under varied input pulse
energies when two inversions of THz waveform in air plasma become one. This
provides a method of measuring the initial CEP in an accuracy that is only
limited by the stability of the driving few-cycle pulses.Comment: 13 pages, 4 figure
A multistep pulse compressor for 10s to 100s PW lasers
High-energy tens (10s) to hundreds (100s) petawatt (PW) lasers are key tools
for exploring frontier fundamental researches such as strong-field quantum
electrodynamics (QED), and the generation of positron-electron pair from
vacuum. Recently, pulse compressor became the main obstacle on achieving higher
peak power due to the limitation of damage threshold and size of diffraction
gratings. Here, we propose a feasible multistep pulse compressor (MPC) to
increase the maximum bearable input and output pulse energies through modifying
their spatiotemporal properties. Typically, the new MPC including a prism pair
for pre-compression, a four-grating compressor (FGC) for main compression, and
a spatiotemporal focusing based self-compressor for post-compression. The prism
pair can induce spatial dispersion to smooth and enlarge the laser beam, which
increase the maximum input and output pulse energies. As a result, as high as
100 PW laser with single beam or more than 150 PW through combining two beams
can be obtained by using MPC and current available optics. This new optical
design will simplify the compressor, improve the stability, and save expensive
gratings/optics simultaneously. Together with the multi-beam tiled-aperture
combining method, the tiled-grating method, larger gratings, or negative chirp
pulse based self-compression method, several 100s PW laser beam is expected to
be obtained by using this MPC method in the future, which will further extend
the ultra-intense laser physics research fields.Comment: 18 pages, 10 figure
Interpretable and Efficient Beamforming-Based Deep Learning for Single Snapshot DOA Estimation
We introduce an interpretable deep learning approach for direction of arrival
(DOA) estimation with a single snapshot. Classical subspace-based methods like
MUSIC and ESPRIT use spatial smoothing on uniform linear arrays for single
snapshot DOA estimation but face drawbacks in reduced array aperture and
inapplicability to sparse arrays. Single-snapshot methods such as compressive
sensing and iterative adaptation approach (IAA) encounter challenges with high
computational costs and slow convergence, hampering real-time use. Recent deep
learning DOA methods offer promising accuracy and speed. However, the practical
deployment of deep networks is hindered by their black-box nature. To address
this, we propose a deep-MPDR network translating minimum power distortionless
response (MPDR)-type beamformer into deep learning, enhancing generalization
and efficiency. Comprehensive experiments conducted using both simulated and
real-world datasets substantiate its dominance in terms of inference time and
accuracy in comparison to conventional methods. Moreover, it excels in terms of
efficiency, generalizability, and interpretability when contrasted with other
deep learning DOA estimation networks.Comment: 10 pages, 10 figure
Waveform-Controlled Terahertz Radiation from the Air Filament Produced by Few-Cycle Laser Pulses
Waveform-controlled Terahertz (THz) radiation is of great importance due to
its potential application in THz sensing and coherent control of quantum
systems. We demonstrated a novel scheme to generate waveform-controlled THz
radiation from air plasma produced when carrier-envelope-phase (CEP) stabilized
few-cycle laser pulses undergo filamentation in ambient air. We launched
CEP-stabilized 10 fs-long (~ 1.7 optical cycles) laser pulses at 1.8 {\mu}m
into air and found that the generated THz waveform can be controlled by varying
the filament length and the CEP of driving laser pulses. Calculations using the
photocurrent model and including the propagation effects well reproduce the
experimental results, and the origins of various phase shifts in the filament
are elucidated.Comment: 5pages, 5 figure
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