Efficiency of optical frequency conversion in quadratic media critically depends on spatial modulation of the nonlinear optical response of the materials. This modulation ensures, via the quasi phase matching (QPM), an efficient energy exchange between optical waves at different frequencies. The QPM structures, also known as the nonlinear photonic crystals, offer a variety of novel properties and functionalities that cannot be obtained in uniform nonlinear crystals. In particular the 3-dimensional modulation of nonlinearity allows one to realize the so-called nonlinear volume holograms which extends the concept of volume holography to nonlinear optics. Nonlinear volume hologram represents the 3- dimensional distribution of the nonlinear polarization which arises from interaction between input fundamental beam and generated wave with complex wave front, in medium exhibiting quadratic nonlinearity (χ(2)). When illuminated by fundamental beam, the nonlinear hologram gives rise to a wave at different frequency (e.g. second harmonic of fundamental wave) having complex intensity distribution following the transverse structure of the hologram itself. In this way one can combine generation of waves at new frequencies with simultaneous shaping of their transverse and longitudinal intensity profiles. In this work we present formation of various types of nonlinear volume holograms in ferroelectric crystals by using unique all-optical domain inversion technique and demonstrate their application in optical wave-shaping.
We present an indirect and non-destructive optical method for domain statistic characterization in disordered nonlinear crystals, having a spatially random distribution of ferroelectric domains with homogeneous refractive index. This method, based on a combination of numerical simulations and experimental measurements, analyses the wavelengthdependent second harmonic spatial distribution. We apply this technique to the characterization of two different random media, with drastically different statistical distributions of ferroelectric domains.
We have developed a high-aspect ratio optical pipeline aiming to produce a highly collimated stream of micron-size
particles in either gaseous or vacuum environments. A hollow, first-order quasi-Bessel beam with variable divergence
was generated with a phase-only spatial light modulator (SLM), by superimposing the quadratic phase of a lens and an
axicon with a 0.5° base angle. The beam was further re-imaged to form a centimetre-long funnel beam with ~5μm
diameter and up to 2000 length-to-diameter aspect ratio. The divergence of the central core of the Bessel beam was
controlled by varying the effective lens in the hologram. The SLM-based optical beam was compared to a similar
beam composed using a physical axicon. The experimental tests were conducted with 2-μm size polystyrene spherical
particles to evaluate the optical force. We present estimated optical forces exerted on the particles in the transverse
plane, both depending on the particle size, laser power, and background-gas pressure.
We demonstrate that a speckle pattern in the spatially coherent laser field transmitted by a diffuser forms a multitude of
three-dimensional bottle-shaped micro-traps. These multiple traps serve as a means for an effective trapping of large
number of air-born absorbing particles. Confinement of up to a few thousand particles in air with a single beam has been
achieved. The ability to capture light-absorbing particles suspended in gases by optical means opens up rich and diverse
practical opportunities, including development of photonic shielding/fencing for environmental protection in
nanotechnology industry and new methods of touch-free air transport of particles and small containers, which may hold
dangerous substances, or viruses and living cells.
We investigate nonlinear diffraction (NLD) of laser radiation in circularly poled nonlinear quadratic crystal for the case
of single and two fundamental pump beams. We show that single pump beam excitation (10 ps @ 1.053 μm) along Z
axis of circularly poled structure (with period 7.5 μm) leads to the second harmonic signal being emitted in a form of
multiple low order cones (rings) and one strong external SH cone (ring) defined by the longitudinal phase matching
conditions. We study a dependence of the NLD pattern as a function of the incidence angle of the pump. We demonstrate
that two noncollinear pump beams intersecting exactly in the center of the structure results in a new type nonlinear
diffraction, which does not have an analogue in linear optics. It features a set of nonlinearly diffracted beams originating
from each individual pump accompanied by the set of additional diffraction rings which originate from photons coming
from both pumps. The corresponding phase matching conditions responsible for the observed NLD effects are discussed.
The observed effects represent nonlinear generalization of optical diffraction in linear media and we believe can find
possible applications in second harmonic optical microscopy.
Singular optical beams have been studied for many years after the pioneering work where the wave function of the laser radiation is presented as a steady-state solution of the wave equation for a harmonic oscillator. A major step in understanding the nature of singular beams has been made by introducing the concept of the angular momentum of light and analyzing local energy transfer in a vortex beam. It is now well accepted that the orbital angular momentum of light is an intrinsic feature of the optical vortex. However, the orbital angular momentum was always analyzed for travelling modes and the important issue of the orbital angular momentum associated with standing waves still remains open. The main motivation of our work is to reveal the structure of the orbital angular momentum in a standing wave formed by the counter-propagating optical vortices and study its suitability for an optical trapping and guiding. In this work we show that a superposition of two (or more) vortex beams generates a field structure which has a form of a standing wave in both the radial and longitudinal directions, but it is rotating simultaneously along the tangential direction. We demonstrate that then field of this optical vortex structure could be used as an optical trap and simultaneously transfer the angular momentum of the electromagnetic wave to an object inside the area of vortex localisation. We believe this study provides a basis for developing a novel concept of three-dimensional optical traps where vortices could be created in a local volume by a direct transfer of the angular orbital momentum of the electromagnetic wave to trapped objects.
We generate conical second-harmonic waves through the parametric frequency conversion in a two-dimensional annular, periodically poled nonlinear photonic structure under the transverse excitation with a fundamental Gaussian beam. We explain the effects observed experimentally by applying the concept of nonlinear Bragg diffraction to the case of the conical frequency generation. We study the polarization properties of the conical emission at the second-harmonic
frequency and demonstrate that each of the parametrically generated waves represents a superposition of the Bessel beams.
We present experimental and theoretical study of refractive index modification induced by femtosecond laser
pulses in photorefractive crystals. The single pulses with central wavelength of 800 nm, pulse duration of 150 fs,
and energy in the range of 6-130 nJ, tightly focused into the bulk of Fe-doped LiNbO3 and stoichiometric LiTaO3
crystals induce refractive index change of up to about 10-3 within the volume of about (2.0 x 2.0 x 8.0) μm3.
The photomodification is independent of the polarization orientation with respect to the crystalline c-axis. The
recorded region can be erased optically by a defocused low-intensity single pulse of the same laser. Recording
and erasure can be repeated at the same position many times without loss of quality. These findings demonstrate
the basic functionality of the ultrafast three-dimensional all-optical rewritable memory. Theoretically they are
interpreted by taking into account electron photogeneration and recombination as well as formation of a space-charge
field. The presented analysis indicates dominant role of photovoltaic effect for our experimental conditions,
and suggests methods for controlling various parameters of the photomodified regions.
Infiltrated photonic crystal fibres (PCFs) offer a new way of studying nonlinearity in periodic systems. A wide
range of available structures and the ease of infiltration opens up a large range of new experimental opportunities
in bio-physics, nonlinear optics, and the study of long range interactions in nonlinear media. Devices relying
on these effects have many applications, from bio-sensors, to all optical switches. To further understand these
nonlinear interactions and realise their potential applications, the effects of nonlinearity need to be studied on
the short time scale. In this work we study the temporal dynamics of thermally induced spatial nonlinearity
in liquid-filled photonic crystal fibres. Light is injected into a single hole of an infiltrated PCF cladding, and
the subsequent response is measured at a few milliseconds time scale. We experimentally demonstrate the short
time scale behavior of such systems, and characterise the effects of this thermal nonlinearity.
We study the second-harmonic generation via transversely-matched interaction of two counter-propagating ultra-short
pulses in χ(2) photonic structures with either random ferroelectric domains or annular periodic poling. The
profile of the transverse second-harmonic signal is given by the cross-correlation of the pulses and can be used
to characterise the temporal structure of the pulses.
We study soliton compression in bulk quadratic nonlinear materials at 800 nm, where group-velocity mismatch dominates. We develop a nonlocal theory showing that efficient compression depends strongly on characteristic nonlocal time scales related to pulse dispersion.
We review our experimental development in the field of optical lattices, emphasizing their unique properties for
control of linear and nonlinear propagation of light. We draw some important links between optical lattices and
photonic crystals, pointing towards practical applications in the fields of optical communications and computing,
beam shaping, and bio-sensing.
We present a novel experimental technique for retrieving the spatial profile of a nonlocal response function in a medium
with thermal nonlinearity. Our method is based on the quantitative measurement of the light-induced nonlinear phase
distribution by holographic interferometry combined with an iterative process of two-dimensional deconvolution using
the known pump beam intensity distribution.
We consider the femtosecond phase-matched noncollinear second-harmonic generation (SHG) in Strontium Barium Niobate (SBN) crystals with random ferroelectnc domains. We study both planar and radial second-harmonic (SH) radiation for the average input power Pf up to 600 mW. We show that the effect of thermal self-focusing of the fundamental wave occurring at Pf > 250 mW results in novel effects including the spatial localization of SHG, a change of the SH efficiency slope, and significant spectral broadening of both fundamental and SH beams
We study theoretically arid generate experimentally two-dimensional nonlinear optically-induced photonic lattices with periodic phase modulation of different geometries in a photorefractive medium, including the periodicnonlinear waves with an internal energy flow or vortex lattices. We demonstrate that the light-induced periodically modulated nonlinear refractive index is highly anisotropic and nonlocal, and it depends on the orientation of a two-dimensional lattice relative to the crystal axis. We discuss stability of such optically-induced photonic two-dimensional structures and demonstrate experimentally their waveguiding properties.
We report on the first experimental observation of a large spatial lateral shift in the interaction of obliquely oriented spatial-dark soliton stripes. We demonstrate by numerical simulations that this new effect can be attributed to the specific features of optical media with nonlocal nonlinear response.
We predict theoretically and observe experimentally tunable refraction of beams in optically-induced lattices. By selective excitation of diferent Bloch modes in a tilted lattice, we observe positive and negative refraction for beams associated with the first and the second spectral band, respectively. We demonstrate tunability of the output beam position by dynamically adjusting the lattice depth. At higher laser intensities, the beam broadening due to difraction can be suppressed through nonlinear self-focusing while preserving the general steering properties.
A nonlinear nonlocal model for beam propagation in a liquid
crystal is analyzed. Using a perturbative approach interactions
between in-phase and out-of-phase solitons is described
analytically. Attraction of both in-phase and out-of-phase
solitons is predicted. The perturbative approach also predicts the
existence of a stable bound state of two out-of-phase solitons.
The analytical results are verified by direct numerical
We review our recent works on spatial solitons in nonlocal nonlinear media. In particular, we discuss stabilization of two dimensional bright solitons and vortex ring solitons as well as interaction of dark solitons.
We discuss properties of multicomponent spatial formed by simultaneous co-propagation of few mutually incoherent beams in a slow nonlinear medium. W present experimental results on formation and interaction of these solitons in photorefractive nonlinear crystal.