30 research outputs found
Amplification of High Harmonics Using Weak Perturbative High Frequency Radiation
The mechanism underlying the substantial amplification of the high-order
harmonics q \pm 2K (K integer) upon the addition of a weak seed XUV field of
harmonic frequency q\omega to a strong IR field of frequency \omega is analyzed
in the framework of the quantum-mechanical Floquet formalism and the
semiclassical re-collision model. According to the Floquet analysis, the
high-frequency field induces transitions between several Floquet states and
leads to the appearance of new dipole cross terms. The semiclassical
re-collision model suggests that the origin of the enhancement lies in the
time-dependent modulation of the ground electronic state induced by the XUV
field.Comment: 8 pages, 2 figure
Controlling the Bandwidth of High Harmonic Emission Peaks with the Spectral Polarization of the Driver
We demonstrate a High-Harmonic-Generation scheme which offers control over
the bandwidth of the spectral peaks. The scheme uses a vectorial two-color
driver with close central frequencies, generated by spectrally splitting a
linearly-polarized input femtosecond-duration laser pulse and subsequent
recombining the two halves after their polarizations are made cross-elliptical
and counter-rotating. This results in the generation of new emission channels
which coalesce into broad odd-integer HHG peaks, the bandwidth of each being
proportional to the frequency difference between the two colors, to the
harmonic order and inversely-proportional to the driver fields' ellipticities.
Peak-broadening to the extent that a supercontinuum is formed is also
demonstrated. This source will find use in HHG applications benefiting from
high-flux broadband extreme ultra-violet radiation, such as attosecond
transient absorption spectroscopy.Comment: 8 pages, 4 figure
Adiabatic theorem for non-hermitian time-dependent open systems
In the conventional quantum mechanics (i.e., hermitian QM) the adia- batic
theorem for systems subjected to time periodic fields holds only for bound
systems and not for open ones (where ionization and dissociation take place)
[D. W. Hone, R. Ketzmerik, and W. Kohn, Phys. Rev. A 56, 4045 (1997)]. Here
with the help of the (t,t') formalism combined with the complex scaling method
we derive an adiabatic theorem for open systems and provide an analytical
criteria for the validity of the adiabatic limit. The use of the complex
scaling transformation plays a key role in our derivation. As a numerical
example we apply the adiabatic theorem we derived to a 1D model Hamiltonian of
Xe atom which interacts with strong, monochromatic sine-square laser pulses. We
show that the gener- ation of odd-order harmonics and the absence of
hyper-Raman lines, even when the pulses are extremely short, can be explained
with the help of the adiabatic theorem we derived
High Harmonic Generation without Tunnel-Ionization
A new High Harmonic Generation (HHG) scheme, which doesn't rely on
Tunnel-Ionization as the ionization mechanism but rather on Single-Photon
Ionization, is theoretically proposed and numerically demonstrated. The scheme
uses two driver fields: an extreme-ultraviolet driver which induces the
ionization, and a circularly-polarized, co-rotating, two-color infrared driver
carried at a fundamental frequency and its second harmonic which induces the
recollision. Using Classical and time-dependent Schr\"odinger equation
simulations of a model Argon atom, we show that in this scheme ionization is
essentially decoupled from recollision. Releasing the process from being
Tunneling-dependent reduces its degree of nonlinearity, which offers new
capabilities in attosecond science, such as generation of High Harmonics from
highly-charged ions, or from specific deep core electronic levels. It is shown
that the emitted high harmonics involve the absorption of photons of one color
of the infrared driver, and the emission of photons of the second color. This
calls for future examination of the possible correlations between the emitted
high harmonics
New photonic conservation laws in parametric nonlinear optics
Conservation laws are one of the most generic and useful concepts in physics.
In nonlinear optical parametric processes, conservation of photonic energy,
momenta and parity often lead to selection rules, restricting the allowed
polarization and frequencies of the emitted radiation. Here we present a new
scheme to derive conservation laws in optical parametric processes in which
many photons are annihilated and a single new photon is emitted. We then
utilize it to derive two new such conservation laws. Conservation of
reflection-parity (RP) arises from a generalized reflection symmetry of the
polarization in a superspace, analogous to the superspace employed in the study
of quasicrystals. Conservation of space-time-parity (STP) similarly arises from
space-time reversal symmetry in superspace. We explore these new conservation
laws numerically in the context of high harmonic generation and outline
experimental set-ups where they can be tested
Generation of phase-matched circularly-polarized extreme ultraviolet high harmonics for magnetic circular dichroism spectroscopy
Circularly-polarized extreme UV and X-ray radiation provides valuable access
to the structural, electronic and magnetic properties of materials. To date,
this capability was available only at large-scale X-ray facilities such as
synchrotrons. Here we demonstrate the first bright, phase-matched, extreme UV
circularly-polarized high harmonics and use this new light source for magnetic
circular dichroism measurements at the M-shell absorption edges of Co. We show
that phase matching of circularly-polarized harmonics is unique and robust,
producing a photon flux comparable to the linearly polarized high harmonic
sources that have been used very successfully for ultrafast element-selective
magneto-optic experiments. This work thus represents a critical advance that
makes possible element-specific imaging and spectroscopy of multiple elements
simultaneously in magnetic and other chiral media with very high spatial and
temporal resolution, using tabletop-scale setups.Comment: 12 text pages, 4 figures, supplamentary information 6 page