138 research outputs found
Quantum-path analysis and phase matching of high-order harmonic generation and high-order frequency mixing processes in strong laser fields
We study phase-matching conditions for high-order harmonic generation as well as high-order sum- and difference-frequency mixing processes in strong laser fields, using a graphical approach described in Balcou et al (1997 Phys. Rev. A 55 3204-10). This method is based on the analysis of the different quantum paths that contribute, with different phase properties, to the single-atom response. We propose a simple numerical method to disentangle the quantum paths contributing to the generation process. We present graphical maps of the phase matching around the laser focus, which allow one to predict the geometries that optimize the conversion efficiency of the process considered. The method is applied to the study of sum- and difference-frequency mixing processes. The qualitative predictions of the graphical phase-matching approach are confirmed by numerical propagation calculations
Which group velocity of light in a dispersive medium?
The interaction between a light pulse, traveling in air, and a generic
linear, non-absorbing and dispersive structure is analyzed. It is shown that
energy conservation imposes a constraint between the group velocities of the
transmitted and reflected light pulses. It follows that the two fields
propagate with group velocities depending on the dispersive properties of the
environment (air) and on the transmission properties of the optical structure,
and are one faster and the other slower than the incident field. In other
words, the group velocity of a light pulse in a dispersive medium is
reminiscent of previous interactions. One example is discussed in detail.Comment: To be submitted on PR
High-order Harmonic-generation In Rare-gases With An Intense Short-pulse Laser
We present experimental studies of high-order harmonic generation in the rare gases performed with a short-pulse titanium sapphire laser operating at 794 nm in the 10(14)-10(15) W/cm2 range. The harmonic yields generated in neon and in argon are studied for all orders as a function of the laser intensity. They vary first rather steeply, in the cutoff region, then much more slowly in the plateau region, and, finally, they saturate when the medium gets ionized. The dependence of the high-order harmonic cutoff with the laser intensity in neon and argon is found to be lower than that predicted in single-atom theories. We observe high-order harmonics in argon and xenon (up to the 65th and 45th, respectively) at 10(15) W/cm2, which we attribute to harmonic generation from ions. We also show how the harmonic and fundamental spectra get blueshifted when the medium becomes ionized
On a universal photonic tunnelling time
We consider photonic tunnelling through evanescent regions and obtain general
analytic expressions for the transit (phase) time (in the opaque barrier
limit) in order to study the recently proposed ``universality'' property
according to which is given by the reciprocal of the photon frequency.
We consider different physical phenomena (corresponding to performed
experiments) and show that such a property is only an approximation. In
particular we find that the ``correction'' factor is a constant term for total
internal reflection and quarter-wave photonic bandgap, while it is
frequency-dependent in the case of undersized waveguide and distributed Bragg
reflector. The comparison of our predictions with the experimental results
shows quite a good agreement with observations and reveals the range of
applicability of the approximated ``universality'' property.Comment: RevTeX, 8 pages, 4 figures, 1 table; subsection added with a new
experiment analyzed, some other minor change
Controlling phase matching of high-order harmonic generation by manipulating the fundamental field
Measurement of Superluminal optical tunneling times in double-barrier photonic bandgaps
Tunneling of optical pulses at 1.5 micron wavelength through double-barrier
periodic fiber Bragg gratings is experimentally investigated. Tunneling time
measurements as a function of barrier distance show that, far from the
resonances of the structure, the transit time is paradoxically short, implying
Superluminal propagation, and almost independent of the distance between the
barriers. These results are in agreement with theoretical predictions based on
phase time analysis and also provide an experimental evidence, in the optical
context, of the analogous phenomenon expected in Quantum Mechanics for
non-resonant superluminal tunneling of particles across two successive
potential barriers. [Attention is called, in particular, to our last Figure].
PACS nos.: 42.50.Wm, 03.65.Xp, 42.70.Qs, 03.50.De, 03.65.-w, 73.40.GkComment: LaTeX file (8 pages), plus 5 figure
Plasma-Induced Frequency Chirp of Intense Femtosecond Lasers and Its Role in Shaping High-Order Harmonic Spectral Lines
We investigate the self-phase modulation of intense femtosecond laser pulses
propagating in an ionizing gas and its effects on collective properties of
high-order harmonics generated in the medium. Plasmas produced in the medium
are shown to induce a positive frequency chirp on the leading edge of the
propagating laser pulse, which subsequently drives high harmonics to become
positively chirped. In certain parameter regimes, the plasma-induced positive
chirp can help to generate sharply peaked high harmonics, by compensating for
the dynamically-induced negative chirp that is caused by the steep intensity
profile of intense short laser pulses.Comment: 5 pages, 5 figure
Negative group delay for Dirac particles traveling through a potential well
The properties of group delay for Dirac particles traveling through a
potential well are investigated. A necessary condition is put forward for the
group delay to be negative. It is shown that this negative group delay is
closely related to its anomalous dependence on the width of the potential well.
In order to demonstrate the validity of stationary-phase approach, numerical
simulations are made for Gaussian-shaped temporal wave packets. A restriction
to the potential-well's width is obtained that is necessary for the wave packet
to remain distortionless in the travelling. Numerical comparison shows that the
relativistic group delay is larger than its corresponding non-relativistic one.Comment: 10 pages, 5 figure
Possibility of the tunneling time determination
We show that it is impossible to determine the time a tunneling particle
spends under the barrier. However, it is possible to determine the asymptotic
time, i.e., the time the particle spends in a large area including the barrier.
We propose a model of time measurements. The model provides a procedure for
calculation of the asymptotic tunneling and reflection times. The model also
demonstrates the impossibility of determination of the time the tunneling
particle spends under the barrier. Examples for delta-form and rectangular
barrier illustrate the obtained results.Comment: 8 figure
Polarization state of the optical near-field
The polarization state of the optical electromagnetic field lying several
nanometers above complex dielectric structures reveals the intricate
light-matter interaction that occurs in this near-field zone. This information
can only be extracted from an analysis of the polarization state of the
detected light in the near-field. These polarization states can be calculated
by different numerical methods well-suited to near--field optics. In this
paper, we apply two different techniques (Localized Green Function Method and
Differential Theory of Gratings) to separate each polarisation component
associated with both electric and magnetic optical near-fields produced by
nanometer sized objects. The analysis is carried out in two stages: in the
first stage, we use a simple dipolar model to achieve insight into the physical
origin of the near-field polarization state. In the second stage, we calculate
accurate numerical field maps, simulating experimental near-field light
detection, to supplement the data produced by analytical models. We conclude
this study by demonstrating the role played by the near-field polarization in
the formation of the local density of states.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
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