We use a spatially non-local model of semiconductor Bloch equations to simulate ultrashort laser-induced plasma dynamics in a direct gap semiconductor nanowire. After laser exposure, the generated electron-hole plasma briefly undergoes plasma oscillations even in the near absence of Coulomb screening in 1D material. The cause of these oscillations is shown to be the dynamics of the propagating laser field coupled with the spatially non-local material model, resulting in momentum transfer from the laser field to the carriers during absorption and the appearance of non-zero off-diagonal density matrix elements. We further show how this plasma responds to probing THz radiation and examine the role of Coulomb and phonon scattering during the plasma oscillations.
Control of the time duration of a laser pulse as it focuses spatially in a material provides a means for delaying the onset of nonlinear effects during propagation. We investigate simultaneous space-time focusing (SSTF) of femtosecond radially-chirped annular pulses in Kerr dielectrics. The energy and temporal chirp of pulses incident upon a grating-grating-lens system are varied in simulations that solve the unidirectional pulse propagation equation. This system is modeled by inserting transformations that act on the electric field obtained from propagation from one component to the next. The propagation is coupled to the time evolution of the free charge density as a function of space. The resulting “ionization tracks” are taken as a metric for predicting material modification and/or damage in bulk fused silica. As expected from linear-optical considerations, the temporal pre-chirp determines the overall pulse duration as the focusing annulus closes. We find in addition that, for a given pulse energy, the temporal pre-chirp also determines the on-axis intensity distribution as energy collapses onto the propagation axis. This effect determines how the local ionization-induced decrease in refractive index shifts energy in time relative to energy arriving on-axis from the spatially collapsing beam. The magnitude of the pre-chirp can thus control the spatial structure of ionization that may lead to material modification and/or damage.
In calculations of ultrafast laser-induced ionization the treatment of fundamental mechanisms such as photoionization and the Kerr effect are treated in isolation using monochromatic perturbative approaches. Such approaches are often questionable for pulses of ultrashort duration and multi-chromatic spectra. In this work we address this issue by solving the quantum optical Bloch equations in a 3D quasi-momentum space and show how to couple this model to ultrashort pulse propagation in dielectrics. This approach self-consistently couples a quantum calculation of the photoionization yield, the photoionization current, and the current from free-carriers with the traditional Kerr effect (self-focusing and self phase modulation) without resort to a perturbative treatment. The material band structure is taken in the tight binding limit and is periodic in the crystal momentum space. As this model makes no assumption about the pulse spectrum, we examine the laser-material interaction of strongly chirped pulses and multi-color multi-pulse schemes of laser-induced material modification. These results are compared to those predicted by standard treatments, such as the Keldysh model of photoionization, for pulses of ultrashort duration.
The use of femtosecond lasers in industrial, biomedical, and defense related applications during the last 15 years has necessitated a detailed understanding of pulse propagation coupled with ultrafast laser-material interactions. Current models of ultrashort pulse propagation in solids describe the pulse evolution of fields with broad spectra and are typically coupled to models of ionization and laser-plasma interaction that assume monochromatic laser fields. In this work we address some of the errors introduced by combining these inconsistent descriptions. In particular, we show that recently published experiments and simulations demonstrate how this contradiction can produce order-of-magnitude errors in calculating the ionization yield, and that this effect leads to altered dimensions and severity of optical breakdown and laser-induced modifications to dielectric solids. We introduce a comprehensive treatment of multi-chromatic non-equilibrium laser-material interaction in condensed matter and successfully couple this model to a unidirectional (frequency-resolved) pulse propagation equation for the field evolution. This approach, while more computationally intensive than the traditional single rate equation for the free electron density, reduces the number of adjustable phenomenological parameters typically used in current models. Our simulation results suggest that intentionally multi-chromatic fields (i.e. strongly chirped pulses or co-propagating pulses of different frequencies) can be arranged to control ionization yields and hence ultrafast laser induced material modifications.
Numerous studies have investigated the prerequisite role of photoionization in ultrafast laser-induced damage (LID) of bulk dielectrics. This study examines the role of spectral width and instantaneous laser frequency in LID using a frequency dependent multiphoton ionization (MPI) model and numerical simulation of initially 800 nm laser pulses propagating through fused silica. Assuming a band gap of 9 eV, MPI by an 800 nm field is a six-photon process, but when the instantaneous wavelength is greater than 827 nm an additional photon is required for photoionization, reducing the probability of the event by many orders of magnitude. Simulation results suggest that this frequency dependence can significantly impact the onset of LID and ultrashort pulse filamentation in solids.
In the past decade it was demonstrated experimentally that negatively-chirped laser pulses can lower the surface
LIDT for wide band-gap materials by decreasing the number of photons required for photoionization on the
leading edge of the pulse. Similarly, simulations have shown that positively-chirped pulses resulting from selffocusing
and self-phase modulation in bulk dielectrics can alter the onset of laser-induced material modifications
by increasing the number of photons required for photoionization on the leading edge of the pulse. However, the
role of multi-chromatic effects in free-carrier absorption and avalanching has yet to be addressed. In this work a
frequency-selective model of free-carrier dynamics is presented, based on a recently extended multi-rate equation
for the distribution of electrons in the conduction band. In this model free-carriers gain energy from the field by
single-photon absorption at the instantaneous frequency, which varies as a function of space and time. For cases
of super-continuum generation it is shown that a Drude-type absorption can vary from 50% to over 200% the
absorption rate as evaluated at the central pulse frequency only. Simulations applying this model to ultrafast
laser-plasma interactions in fused silica explore how pulse chirps may be used as a distinguishing parameter for
LID resulting from otherwise identical pulses.
Numerous studies have investigated the role of photoionization in ultrafast laser-induced damage of bulk dielectrics.
This study examines the role of spectral width and instantaneous laser frequency in laser-induced
damage using a frequency dependent multiphoton ionization model and numerical simulation of an 800 nm
laser pulse propagating through fused silica. When the individual photon wavelengths are greater than 827 nm,
an additional photon is required for photoionization, reducing the probability of the event by many orders of
magnitude. Simulation results suggest that this frequency dependence may significantly affect the processes of
laser-induced damage and filamentation.
Recent experiments on optical damage by ultrashort laser pulses have demonstrated that the temporal pulse shape can dramatically influence plasma generation in fused silica. We use a modified 3+1D nonlinear Schrödinger equation for the pulse propagation coupled to a rate equation for the plasma density in the dielectric material to simulate pulse propagation and plasma formation in fused silica. We use these simulations to analyze the influence of pulse shape and beam geometry on the formation of the electron plasma and hence modification in the bulk material. In particular, we simulate the effect of pulses reconstructed from experimental data. It is expected that a better understanding of the dynamics of laser-induced plasma generation will enable the accurate simulation of optical damage in a variety of dielectrics, ultimately leading to an enhanced control of optical damage to real materials and optical devices.
Recent experiments on optical damage by ultrashort laser pulses have demonstrated that the temporal pulseshape
can dramatically influence plasma generation in fused silica and sapphire. In this work a modified 3+1D
nonlinear Schroedinger equation for the pulse propagation coupled to a rate equation for the plasma density
in the dielectric material is used to simulate pulse propagation and plasma formation in a range of dielectric
materials. We use these simulations to analyze the influence of
pulse-width, pulse-shape and beam geometry on
the formation of the electron plasma and hence damage in the bulk material. In particular, when possible, we
simulate the effect of pulses reconstructed from experimental data. It is expected that a better understanding
of the dynamics of laser-induced plasma generation will enable the accurate simulation of optical damage in a
variety of dielectrics, ultimately leading to an enhanced control of optical damage to real materials and optical
devices.
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