We report what is believed to be the first experimental demonstration of the azimuthal self-breaking of intense beams containing a vortex phase dislocation into sets of optical spatial solitons in a quadratic nonlinear material. The observations were performed in a KTP crystal.
We report experimental evidence of spatial filtering of light beams by three-dimensional, low-refraction-indexcontrast photonic crystals. The photonic crystals were fabricated in a glass bulk, where the refraction index has been periodically modulated using tightly focused femtosecond laser pulses. We observe filtered areas in the angular distributions of the transmitted radiation, and we interpret the observations by theoretical and numerical study of light propagation in index-modulated material in paraxial model.
We demonstrate infrared femtosecond laser-induced inversion of ferroelectric domains. This process can be realised solely by using tightly focused laser pulses without application of any electric field prior to, in conjunction with, or subsequent to the laser irradiation. As most ferroelectric crystals like LiNbO3, LiTaO3, and KTiOPO4 are transparent in the infrared, this optical poling method allows one to form ferroelectric domain patterns much deeper inside a ferroelectric crystal than by using ultraviolet light and hence can be used to fabricate practical devices. We also propose in situ diagnostics of the ferroelectric domain inversion process by monitoring the Čerenkov second harmonic signal, which is sensitive to the appearance of ferroelectric domain walls.
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 The concept of flat PhC lensing is based on the transformation of the phases of the angular field components. The convex-curved phase shifts of field components accumulated during propagation inside the PhC can be compensated by the usual concave-curved phase shifts during propagation in a homogeneous material, both in front of and behind the PhC, resulting in focusing behind the PhC. The distance between the object and the PhC, l 1 , and between the PhC and the image, l 2 , [ Fig. 1(a)] obey the relation l 1 l 2 f , where f is the focal distance of the flat PhC lens. This is in contrast to the usual focusing by conventional or by Fresnel lenses, where the well-known relation 1∕l 1 1∕l 2 1∕f holds.In this Letter we experimentally demonstrate full two-dimensional (2D) focusing by a polymer-based threedimensional (3D) woodpile PhC. Full 2D flat lens focusing has been experimentally shown for microwaves [7] and for sound waves [8]. Moreover, even 1D focusing/imaging by PhC slabs has thus far been experimentally demonstrated only in the near-IR frequency range [9].PhC lensing is usually considered for modulation periods of the order of wavelength. Flat lensing occurs due to the convex-curved spatial dispersion (or isofrequency) lines in the first, or at most in the second, propagation band. In particular, for PhCs of square symmetry, the corner of the Brillouin zone (BZ) is positioned at λ d 0n (λ is the wavelength,n is the effective refractive index of the PhC, and d 0 is the lattice period). The self-collimation (SC) (the flattening of the spatial dispersion lines) occurs at frequencies below the corner of the BZ, i.e., at λ > d 0n . The flat lensing, which is based on anomalously curved spatial dispersion lines, generally occurs between the frequencies of SC and of the edge of the BZ. The experimental demonstration of flat lensing in the visible range is therefore a difficult task, due to technological limitations of PhC fabrication techniques at this scale. We use an alternative approach based on PhCs with relatively large modulation periods, but searching the flat lensing effects in higher order bands. This, on one hand, simplifies the fabrication of the samples, but on the other hand makes the observation and interpretation of the
We predict and experimentally observe the enhancement by three orders of magnitude of phase mismatched second and third harmonic generation in a GaAs cavity at 650 and 433 nm, respectively, well above the absorption edge. Phase locking between the pump and the harmonics changes the effective dispersion of the medium and inhibits absorption. Despite hostile conditions the harmonics resonate inside the cavity and become amplified leading to relatively large conversion efficiencies. Field localization thus plays a pivotal role despite the presence of absorption, and ushers in a new class of semiconductor-based devices in the visible and uv ranges.Since it was discovered by Franken in the 1960s, second harmonic ͑SH͒ generation has been one of the most studied phenomena in nonlinear optics ͓1͔. To date most efforts have been directed at improving the efficiency of the process by developing new materials with high effective nonlinear coefficients, accompanied by phase and group velocity matching ͓2-10͔. Consequently, most studies have been concerned with maximizing conversion efficiencies, generally achievable at or very near phase matching ͑PM͒ conditions, ensuring maximum energy transfer from the fundamental beam to the harmonics. A special effort was focused toward engineering new artificial materials capable of compensating material dispersion, for example, using quasiphase matching techniques ͓11,12͔ or structured materials ͓13͔. Outside of PM conditions, which generally coincide with low conversion efficiencies ͓3͔, the only relevant processes that have been investigated are cascaded parametric processes that can produce phase-modulation of the fundamental beam ͓14͔, pulse breaking ͓15͔ or nonlinear diffraction ͓16͔. This has caused other possible working conditions to remain largely unexplored. A relevant feature is that in all these previous studies the nonlinear material was assumed to be transparent for both fundamental and harmonics beams, since conventional wisdom holds that an absorptive material will reabsorb any generated harmonic signal.More recently, an effort was initiated to systematically study the behavior of SH and third harmonic ͑TH͒ fields in transparent and opaque materials under conditions of phase mismatch ͓17-19͔. Briefly, when a pump pulse crosses an interface between a linear and a nonlinear medium there are always three generated SH ͑and/or TH͒ components. One component is generated backward into the free space, due the presence of the interface, and the remaining components are forward moving. These components may be understood on the basis of the mathematical solution of the homogeneous and inhomogeneous wave equations at the SH frequency ͓4͔. Continuity of the tangential components of all the fields at the boundary leads to generation of the two forwardpropagating components that interfere in the vicinity of the entry surface and give rise to Maker fringes ͓2,20͔ and to energy exchange between the fundamentals and SH and/or TH beams. It turns out that while the homogeneous component tr...
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