91 research outputs found
Analysis of terahertz generation using tilted-pulse-fronts
A 2-D spatio-temporal analysis of terahertz generation by optical
rectification of tilted-pulse-fronts is presented. Closed form expressions of
terahertz transients and spectra in two spatial dimensions are furnished in the
undepleted limit. Importantly, the analysis incorporates spatio-temporal
distortions of the optical pump pulse such as angular dispersion,
group-velocity dispersion due to angular dispersion, spatial and temporal chirp
as well as beam curvature. The importance of the radius of curvature to the
tilt-angle and group-velocity dispersion due to angular dispersion to terahertz
frequency, conversion efficiency and peak field is revealed.In particular, the
deterioration of terahertz frequency, efficiency and field at large pump
bandwidths and beam sizes is analytically shown
Raman Shifting induced by Cascaded Quadratic Nonlinearities for Terahertz Generation
We introduce a new regime of cascaded quadratic nonlinearities which result
in a continuous red shift of the optical pump, analogous to a Raman shifting
process rather than self-phase modulation. This is particularly relevant to
terahertz generation, where a continuous red shift of the pump can resolve
current issues such as dispersion management and laser-induced damage. We show
that in the absence of absorption or dispersion, the presented Raman shifting
method will result in optical-to-terahertz energy conversion efficiencies that
approach which is not possible with conventional cascaded
difference-frequency generation. Furthermore, we present designs of
aperiodically poled structures which result in energy conversion efficiencies
of even in the presence of dispersion and absorption
Terahertz-induced cascaded interactions between spectra offset by large frequencies
We explore the dynamics of a system where input spectra in the optical domain
with very disparate center frequencies are strongly coupled via highly
phase-matched, cascaded second-order nonlinear processes driven by terahertz
radiation. The only requirement is that one of the input spectra contain
sufficient bandwidth to generate the phase-matched terahertz-frequency driver.
The frequency separation between the input spectra can be more than ten times
larger than the phase-matched terahertz frequency. A practical application of
such a system where the cascading of a narrowband pump line centered at 1064 nm
induced by a group of weaker seed lines centered about 1030 nm and separated by
the phase-matched terahertz frequency is introduced. This approach is predicted
to generate terahertz radiation with percent-level conversion efficiencies and
millijoule-level pulse energies in cryogenically-cooled periodically poled
lithium niobate. A model that solves for the nonlinear coupled interaction of
terahertz and optical waves is employed. The calculations account for second
and third-order nonlinearities, dispersion in the optical and terahertz domains
as well as terahertz absorption. Ramifications of pulse formats on
laser-induced damage are estimated by tracking the generated free-electron
density. Strategies to mitigate laser-induced damage are also outlined
Limitations to THz generation by optical rectification using tilted pulse fronts
Terahertz (THz) generation by optical rectification (OR) using
tilted-pulse-fronts is studied. One-dimensional (1-D) and 2-D spatial models,
which simultaneously account for (i) the nonlinear coupled interaction of the
THz and optical radiation, (ii) angular and material dispersion, (iii)
absorption, iv) self-phase modulation and (v) stimulated Raman scattering are
presented. We numerically show that the large experimentally observed cascaded
frequency down-shift and spectral broadening (cascading effects) of the optical
pump pulse is a direct consequence of THz generation. In the presence of this
large spectral broadening, the phase mismatch due to angular dispersion is
greatly enhanced. Consequently, this cascading effect in conjunction with
angular dispersion is shown to be the strongest limitation to THz generation in
lithium niobate for pumping at 1 micron. It is seen that the exclusion of these
cascading effects in modeling OR, leads to a significant overestimation of the
optical-to-THz conversion efficiency. The simulation results are supported by
experiments
Terahertz generation by beamlet superposition
We analytically show how a superposition of beamlets produces terahertz
radiation with greater spatial homogeneity and efficiency compared to
tilted-pulse-fronts generated by diffraction gratings. The advantages are
particularly notable for large pump bandwiths and beam sizes, alluding to
better performance in the presence of cascading effects and higher energy
pumping. A theory of terahertz generation using a superposition of beamlets is
developed. It is shown how such an arrangement produces a distortion free
tilted-pulse-front. Closed form expressions for terahertz spectra and
transients in three spatial dimensions are derived. Conditions for obtaining
performance parity and bounds for optimal parameters are furnished
High Efficiency Terahertz Generation in a Multi-Stage System
We describe a robust system for laser-driven narrowband terahertz generation
with high conversion efficiency in periodically poled Lithium Niobate (PPLN).
In the multi-stage terahertz generation system, the pump pulse is recycled
after each PPLN stage for further terahertz generation. By out-coupling the
terahertz radiation generated in each stage, extra absorption is circumvented
and effective interaction length is increased. The separation of the terahertz
and optical pulses at each stage is accomplished by an appropriately designed
out-coupler. To evaluate the proposed architecture, the governing 2-D coupled
wave equations in a cylindrically symmetric geometry are numerically solved
using the finite difference method. Compared to the 1-D calculation which
cannot capture the self-focusing and diffraction effects, our 2-D numerical
method captures the effects of difference frequency generation, self-phase
modulation, self-focusing, beam diffraction, dispersion, and terahertz
absorption. We found that the terahertz generation efficiency can be greatly
enhanced by compensating the dispersion of the pump pulse after each stage.
With a two-stage system, we predict the generation of a mJ terahertz
pulse with total conversion efficiency at THz
using a 1.1 J pump laser with a two-line spectrum centered at 1 m. The
generation efficiency of each stage is above with the out-coupling
efficiencies above
Cascaded Parametric Amplification for Highly Efficient Terahertz Generation
A highly efficient, practical approach to high-energy terahertz (THz)
generation based on spectrally cascaded optical parametric amplification
(THz-COPA) is introduced. The THz wave initially generated by difference
frequency generation between a strong narrowband optical pump and optical seed
(0.1-10% of pump energy) kick-starts a repeated or cascaded energy
down-conversion of pump photons. This helps to greatly surpass the
quantum-defect efficiency and results in exponential growth of THz energy over
crystal length. In cryogenically cooled periodically poled lithium niobate,
energy conversion efficiencies >8% for 100 ps pulses are predicted. The
calculations account for cascading effects, absorption, dispersion and
laser-induced damage. Due to the coupled nonlinear interaction of multiple
triplets of waves, THz-COPA exhibits physics distinct from conventional
three-wave mixing parametric amplifiers. This in turn governs optimal
phase-matching conditions, evolution of optical spectra as well as limitations
of the nonlinear process.Comment: 5 pages, double colum
Terahertz-driven linear electron acceleration
The cost, size and availability of electron accelerators is dominated by the
achievable accelerating gradient. Conventional high-brightness radio-frequency
(RF) accelerating structures operate with 30-50 MeV/m gradients. Electron
accelerators driven with optical or infrared sources have demonstrated
accelerating gradients orders of magnitude above that achievable with
conventional RF structures. However, laser-driven wakefield accelerators
require intense femtosecond sources and direct laser-driven accelerators and
suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing
requirements due to the short wavelength of operation. Here, we demonstrate the
first linear acceleration of electrons with keV energy gain using
optically-generated terahertz (THz) pulses. THz-driven accelerating structures
enable high-gradient electron or proton accelerators with simple accelerating
structures, high repetition rates and significant charge per bunch. Increasing
the operational frequency of accelerators into the THz band allows for greatly
increased accelerating gradients due to reduced complications with respect to
breakdown and pulsed heating. Electric fields in the GV/m range have been
achieved in the THz frequency band using all optical methods. With recent
advances in the generation of THz pulses via optical rectification of slightly
sub-picosecond pulses, in particular improvements in conversion efficiency and
multi-cycle pulses, increasing accelerating gradients by two orders of
magnitude over conventional linear accelerators (LINACs) has become a
possibility. These ultra-compact THz accelerators with extremely short electron
bunches hold great potential to have a transformative impact for free electron
lasers, future linear particle colliders, ultra-fast electron diffraction,
x-ray science, and medical therapy with x-rays and electron beams
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