3,405 research outputs found
Modulational-instability-free pulse compression in anti-resonant hollow-core photonic crystal fiber
Gas-filled hollow-core photonic crystal fiber (PCF) is used for efficient
nonlinear temporal compression of femtosecond laser pulses, two main schemes
being direct soliton-effect self-compression, and spectral broadening followed
by phase compensation. To obtain stable compressed pulses, it is crucial to
avoid decoherence through modulational instability (MI) during spectral
broadening. Here we show that changes in dispersion due to spectral
anti-crossings between the fundamental core mode and core wall resonances in
anti-resonant-guiding hollow-core PCF can strongly alter the MI gain spectrum,
enabling MI-free pulse compression for optimized fiber designs. In addition,
higher-order dispersion can introduce MI even when the pump pulses lie in the
normal dispersion region
Dominance of backward stimulated Raman scattering in gas-filled hollow-core photonic crystal fibers
Backward stimulated Raman scattering in gases provides a promising route to
compression and amplification of a Stokes seed-pulse by counter-propagating
against a pump-pulse, as has been already demonstrated in various platforms,
mainly in free-space. However, the dynamics governing this process when seeded
by noise has not yet been investigated in a fully controllable collinear
environment. Here we report the first unambiguous observation of efficient
noise-seeded backward stimulated Raman scattering in a hydrogen-filled
hollow-core photonic crystal fiber. At high gas pressures, when the backward
Raman gain is comparable with, but lower than, the forward gain, we report
quantum conversion efficiencies exceeding 40% to the backward Stokes at 683 nm
from a narrowband 532-nm-pump. The efficiency increases to 65% when the
backward process is seeded by a small amount of back-reflected
forward-generated Stokes light. At high pump powers the backward Stokes signal,
emitted in a clean fundamental mode and spectrally pure, is unexpectedly always
stronger than its forward-propagating counterpart. We attribute this striking
observation to the unique temporal dynamics of the interacting fields, which
cause the Raman coherence (which takes the form of a moving fine-period Bragg
grating) to grow in strength towards the input end of the fiber. A good
understanding of this process, together with the rapid development of novel
anti-resonant-guiding hollow-core fibers, may lead to improved designs of
efficient gas-based Raman lasers and amplifiers operating at wavelengths from
the ultraviolet to the mid-infrared.Comment: 6 pages and 8 figures in the main section. 4 pages and 5 figures in
the supplementary sectio
Linearons: highly non-instantaneous solitons in liquid-core photonic crystal fibers
The nonlinear propagation of light pulses in liquid-filled photonic crystal
fibers is considered. Due to the slow reorientational nonlinearity of some
molecular liquids, the nonlinear modes propagating inside such structures can
be approximated, for pulse durations much shorter than the molecular relaxation
time, by temporally highly-nonlocal solitons, analytical solutions of a linear
Schroedinger equation. The physical relevance of these novel solitary
structures, which may have a broad range of applications, is discussed and
supported by detailed numerical simulations.Comment: 4 pages, 3 figure
Near-ionization-threshold emission in atomic gases driven by intense sub-cycle pulses
We study theoretically the dipole radiation of a hydrogen atom driven by an
intense sub-cycle pulse. The time-dependent Schr\"odinger equation for the
system is solved by ab initio calculation to obtain the dipole response.
Remarkably, a narrowband emission lasting longer than the driving pulse appears
at a frequency just above the ionization threshold. An additional calculation
using the strong field approximation also recovers this emission, which
suggests that it corresponds to the oscillation of nearly-bound electrons that
behave similarly to Rydberg electrons. The predicted phenomenon is unique to
ultrashort driving pulses but not specific to any particular atomic structure.Comment: 8 pages, 2 figure
Continuously wavelength-tunable high harmonic generation via soliton dynamics
We report generation of high harmonics in a gas-jet pumped by pulses
self-compressed in a He-filled hollow-core photonic crystal fiber through the
soliton effect. The gas-jet is placed directly at the fiber output. As the
energy increases the ionization-induced soliton blue-shift is transferred to
the high harmonics, leading to a emission bands that are continuously tunable
from 17 to 45 eV
Transformation Optics with Photonic Band Gap Media
We introduce a class of optical media based on adiabatically modulated,
dielectric-only, and potentially extremely low-loss, photonic crystals. The
media we describe represent a generalization of the eikonal limit of
transformation optics (TO). The foundation of the concept is the possibility to
fit frequency isosurfaces in the k-space of photonic crystals with elliptic
surfaces, allowing them to mimic the dispersion relation of light in
anisotropic effective media. Photonic crystal cloaks and other TO devices
operating at visible wavelengths can be constructed from optically transparent
substances like glasses, whose attenuation coefficient can be as small as 10
dB/km, suggesting the TO design methodology can be applied to the development
of optical devices not limited by the losses inherent to metal-based, passive
metamaterials.Comment: 4 pages, 4 figure
Polarization-Tailored Raman Frequency Conversion in Chiral Gas-Filled Hollow Core Photonic Crystal Fibers
Broadband-tunable sources of circularly-polarized light are crucial in fields
such as laser science, biomedicine and spectroscopy. Conventional sources rely
on nonlinear wavelength conversion and polarization control using standard
optical components, and are limited by the availability of suitably transparent
crystals and glasses. Although gas-filled hollow-core photonic crystal fiber
provides pressure-tunable dispersion, long well-controlled optical
path-lengths, and high Raman conversion efficiency, it is unable to preserve
circular polarization state, typically exhibiting weak linear birefringence.
Here we report a revolutionary approach based on helically-twisted hollow-core
photonic crystal fiber, which displays circular birefringence, thus robustly
maintaining circular polarization state against external perturbations. This
makes it possible to generate pure circularly-polarized Stokes and anti-Stokes
signals by rotational Raman scattering in hydrogen. The polarization state of
the frequency-shifted Raman bands can be continuously varied by tuning the gas
pressure in the vicinity of the gain suppression point. The results pave the
way to a new generation of compact and efficient fiber-based sources of
broadband light with fully-controllable polarization state.Comment: 5 pages, 4 figure
- …