31 research outputs found
Full 3D modelling of pulse propagation enables efficient nonlinear frequency conversion with low energy laser pulses in a single-element tripler
Although new optical materials continue to open up access to more and more wavelength bands where femtosecond laser pulses can be generated, light frequency conversion techniques are still indispensable in filling the gaps on the ultrafast spectral scale. With high repetition rate, low pulse energy laser sources (oscillators) tight focusing is necessary for a robust wave mixing and the efficiency of broadband nonlinear conversion is limited by diffraction as well as spatial and temporal walk-off. Here we demonstrate a miniature third harmonic generator (tripler) with conversion efficiency exceeding 30%, producing 246 fs UV pulses via cascaded second order processes within a single laser beam focus. Designing this highly efficient and ultra compact frequency converter was made possible by full 3-dimentional modelling of propagation of tightly focused, broadband light fields in nonlinear and birefringent media
Free-form Light Actuators - Fabrication and Control of Actuation in Microscopic Scale
Liquid crystalline elastomers (LCEs) are smart materials capable of reversible shape-change in response to external stimuli, and have attracted researchers' attention in many fields. Most of the studies focused on macroscopic LCE structures (films, fibers) and their miniaturization is still in its infancy. Recently developed lithography techniques, e.g., mask exposure and replica molding, only allow for creating 2D structures on LCE thin films. Direct laser writing (DLW) opens access to truly 3D fabrication in the microscopic scale. However, controlling the actuation topology and dynamics at the same length scale remains a challenge.
In this paper we report on a method to control the liquid crystal (LC) molecular alignment in the LCE microstructures of arbitrary three-dimensional shape. This was made possible by a combination of direct laser writing for both the LCE structures as well as for micrograting patterns inducing local LC alignment. Several types of grating patterns were used to introduce different LC alignments, which can be subsequently patterned into the LCE structures. This protocol allows one to obtain LCE microstructures with engineered alignments able to perform multiple opto-mechanical actuation, thus being capable of multiple functionalities. Applications can be foreseen in the fields of tunable photonics, micro-robotics, lab-on-chip technology and others
Optimization of broadband semiconductor chirped mirrors with genetic algorithm
Genetic algorithm was applied for optimization
of dispersion properties in semiconductor Bragg reflectors
for applications in femtosecond lasers. Broadband,
large negative group-delay dispersion was achieved in the
optimized design: The group-delay dispersion (GDD) as
large as −3500 fs2
was theoretically obtained over a 10-nm
bandwidth. The designed structure was manufactured and
tested, providing GDD −3320 fs2
over a 7-nm bandwidth.
The mirror performance was verified in semiconductor
structures grown with molecular beam epitaxy. The mirror
was tested in a passively mode-locked Yb:KYW laser
Time-frequency Domain Analogues of Phase Space Sub-Planck Structures
We present experimental data of the frequency resolved optical gating (FROG)
measurements of light pulses revealing interference features corresponding to
sub-Planck structures in phase space. For superpositions of pulses a small,
sub-Fourier shift in the carrier frequency leads to a state orthogonal to the
initial one, although in the representation of standard time-frequency
distributions these states seem to have a nonvanishing overlap.Comment: New title, minor change
Encoding a qubit into multilevel subspaces
We present a formalism for encoding the logical basis of a qubit into
subspaces of multiple physical levels. The need for this multilevel encoding
arises naturally in situations where the speed of quantum operations exceeds
the limits imposed by the addressability of individual energy levels of the
qubit physical system. A basic feature of the multilevel encoding formalism is
the logical equivalence of different physical states and correspondingly, of
different physical transformations. This logical equivalence is a source of a
significant flexibility in designing logical operations, while the multilevel
structure inherently accommodates fast and intense broadband controls thereby
facilitating faster quantum operations. Another important practical advantage
of multilevel encoding is the ability to maintain full quantum-computational
fidelity in the presence of mixing and decoherence within encoding subspaces.
The formalism is developed in detail for single-qubit operations and
generalized for multiple qubits. As an illustrative example, we perform a
simulation of closed-loop optimal control of single-qubit operations for a
model multilevel system, and subsequently apply these operations at finite
temperatures to investigate the effect of decoherence on operational fidelity.Comment: IOPart LaTeX, 2 figures, 31 pages; addition of a numerical simulatio
Nonlinear Effects with Ultrashort Laser Pulses
Propagation of an intense femtosecond laser pulse through a transparent nonlinear medium such as dielectric leads to a number of phenomena. In our experiment we observed complex spatial, spectral, and temporal structures appearing in the initially smooth femtosecond laser pulse when the pulse power is comparable to or higher than the critical power for self-focusing. We have also developed a complete, 3-dimensional theoretical model to describe the observed phenomena