Kestutis Staliunas

Theoretical prediction of the nondiffractive propagation of sonic waves through periodic acoustic media

We predict theoretically the nondiffractive propagation of sonic waves in periodic acoustic media (sonic crystals), by expansion into a set of plane waves (Bloch mode expansion), and by finite difference time domain calculations of finite beams. We also give analytical evaluations of the parameters for nondiffractive propagation, as well as the minimum size of the nondiffractively propagating acoustic beams.

Flat lensing in the visible frequency range by woodpile photonic crystals

We experimentally demonstrate full two-dimensional focalization of light beams at visible frequencies by a three-dimensional woodpile photonic crystal. The focalization (the flat lensing) with focal distances of the order of 50-70 μm is experimentally demonstrated. Experimental results are compared with numerical calculations and interpreted by harmonic expansion studies.

Second-harmonic parametric scattering in ferroelectric crystals with disordered nonlinear domain structures.

We study the second-harmonic (SH) parametric processes in unpoled crystals of Strontium Barium Niobate (SBN) with disordered structures of ferroelectric domains. Such crystals allow for the simultaneous phase matching of several second-order nonlinear processes. We analyze the polarization properties of these parametric processes using two types of generation schemes: quasi-collinear SH generation and transverse SH generation. From our experimental data we determine the ratio of d(32) and d(33) components of the second order susceptibility tensor and also the statistical properties of the random structure of the SBN crystal.

Spatial filtering of light by chirped photonic crystals

We propose an efficient method for spatial filtering of light beams by propagating them through two-dimensional (also three dimensional) chirped photonic crystals, i.e., through the photonic structures with fixed transverse lattice period and with the longitudinal lattice period varying along the direction of the beam propagation. We prove the proposed idea by numerically solving the paraxial propagation equation in refraction-index-modulated media and we evaluate the efficiency of the process by harmonic-expansion analysis. The technique can be also applied for filtering (for cleaning) of the packages of atomic waves (Bose condensates), also to improve the directionality of acoustic and mechanical waves.

Beam shaping in spatially modulated broad-area semiconductor amplifiers

Summary form only given. Broad area semiconductor (BAS) laser are relevant high conversion light sources despite that the spatial and temporal quality of the emitted beam is relatively low due to the absence of a natural transverse mode selection mechanism [1]. To overcome this serious drawback different solutions have been incorporated in the design, such as external gratings, external injection spatially modulated injection or distributed feedback. In addition, in BAS lasers a modulation instability (or Bespalov-Talanov instability) can occur due to nonlinear focussing, leading to filamentation effects and deteriorating the spatial quality of the laser emission. In absence of cavity mirrors, planar semiconductor structures can act as light amplifiers undergoing however analogue disadvantages.In the present work, we study a simple and effective new mechanism to improve the spatial beam quality of a planar semiconductor amplifier configuration. We consider a two-dimensional modulation, which can feasibly be realized by a periodical grid of electrodes for the electrical pump of the semiconductor, as illustrated in Fig. 1.a. The main result obtained is that such a micro-modulation of the spatial pump profile on a spatial scale of the order of several wavelengths, can indeed lead to a substantial improvement of the spatial quality of the amplified beam on a large spatial scale, see Fig. 1.b. The quantitative analysis of the spatial filtering is performed by numerical integration of a paraxial propagation model derived from [2,3], and on analytical estimations. Previous studies of wave propagation in media with spatially modulated Gain/Loss (GL) profiles show that a periodic modulation of GL on a wavelength scale can lead to particular beam propagation effects, such as self-collimation, spatial (angular) filtering, or beam focalisation [4]. In those works a purely GL modulation has been considered; however, in semiconductor media due to the linewidth enhancement factor, hfactor, a periodical spatial pump distribution causes a combined Gain and refraction Index Modulation (GIM). Hence, the aim of the present paper, performed under realistic parameters and conditions, is to demonstrate how the angular spectrum of the radiation from a spatially modulated GIM BAS amplifier becomes narrower while propagating and being amplified. The study predicts that the normalized beam quality factor - M 2 factor- can reduce almost to unity indicating that the BAS amplifier output becomes perfectly Gaussian for even strongly random initial input beam profiles, for a single-pass propagation length on the order of millimeters, see Fig. 1.c. Beyond the present report, this new technique could be implemented to improve the spatial quality of emission of BAS lasers.

Enhancement of sound in chirped sonic crystals

We propose and experimentally demonstrate a mechanism of sound wave concentration based on soft reflections in chirped sonic crystals. The reported controlled field enhancement occurs at around particular (bright) planes in the crystal and is related to a progressive slowing down of the sound wave as it propagates along the material. At these bright planes, a substantial concentration of the energy (with a local increase up to 20 times) was obtained for a linear chirp and for frequencies around the first band gap. A simple couple mode theory is proposed that interprets and estimates the observed effects. Wave concentration energy can be applied to increase the efficiency of detectors and absorbers.

Self collimation of ultrasound in a three-dimensional sonic crystal

We present the experimental demonstration of self-collimation (subdiffractive propagation) of an ultrasonic beam inside a three-dimensional (3D) sonic crystal. The crystal is formed by two crossed steel cylinders structures in a woodpilelike geometry disposed in water. Measurements of the 3D field distribution show that a narrow beam, which diffractively spreads in the absence of the sonic crystal, is strongly collimated in propagation inside the crystal, demonstrating the 3D self-collimation effect.

Planar second-harmonic generation with noncollinear pumps in disordered media

We study experimentally the process of the second harmonic generation by two noncollinear beams in quadratic nonlinear crystals with a disordered structure of ferroelectric domains. We show that the second-harmonic radiation is emitted in the form of two cones as well as in a plane representing the cross-correlation of the two fundamental pulses. We demonstrate the implementation of this parametric process for characterisation of femtosecond pulses, enabling the estimation of pulse width, chirp, and front tilt. This is achieved through monitoring the evolution of the autocorrelation trace inside the nonlinear crystal.

Spatial filtering with photonic crystals

Photonic crystals are well known for their celebrated photonic band-gaps—the forbidden frequency ranges, for which the light waves cannot propagate through the structure. The frequency (or chromatic) band-gaps of photonic crystals can be utilized for frequency filtering. In analogy to the chromatic band-gaps and the frequency filtering, the angular band-gaps and the angular (spatial) filtering are also possible in photonic crystals. In this article, we review the recent advances of the spatial filtering using the photonic crystals in different propagation regimes and for different geometries. We review the most evident configuration of filtering in Bragg regime (with the back-reflection—i.e., in the configuration with band-gaps) as well as in Laue regime (with forward deflection—i.e., in the configuration without band-gaps). We explore the spatial filtering in crystals with different symmetries, including axisymmetric crystals; we discuss the role of chirping, i.e., the dependence of the longitudinal period along the structure. We also review the experimental techniques to fabricate the photonic crystals and numerical techniques to explore the spatial filtering. Finally, we discuss several implementations of such filters for intracavity spatial filtering.

Third-harmonic generation via broadband cascading in disordered quadratic nonlinear media

We study parametric frequency conversion in quadratic nonlinear media with disordered ferroelectric domains. We demonstrate that disorder allows realizing broadband third-harmonic generation via cascading of two second-order quasi-phase matched nonlinear processes. We analyze both spatial and polarization properties of the emitted radiation and find the results in agreement with our theoretical predictions.