Kestutis Staliunas

Spatial walk-off limits second-harmonic generation in birefringent crystals by separating the fundamental and second-harmonic beams, particularly for tightly focused fields. We show that wavelength-scale periodic modulation of the refractive index can suppress this limitation by reshaping the angular dispersion near an existing birefringent phase-matching point. The modulation couples two second-harmonic propagation directions so that their transverse energy flows cancel, eliminating the leading walk-off contribution while phase matching is recovered on the modified dispersion branch. We derive simple design conditions for this regime and confirm the mechanism with spatially resolved simulations that include pump depletion and grating-induced modification of both frequencies. For representative type-I second-harmonic generation in beta barium borate, the modulated structure can maintain spatial overlap and enhance the interaction at a millimeter scale. The approach is therefore most relevant to compact, walk-off-limited frequency converters and tightly focused beams.

Beam filamentation limits the beam quality of broad-area semiconductor lasers (BASLs), and effective mitigation strategies remain an open issue. We investigate the impact of spatial modulation of the injected current using a spatio-temporal traveling-wave model that describes the coupled longitudinal and transverse dynamics of the optical field and carrier density. The modulation is implemented as a two-dimensional current pattern, producing simultaneous variations of gain and refractive index compatible with structured electrodes. Transverse modulation alone stabilizes the near-field profile but introduces far-field side lobes associated with out-of-phase modes. The addition of longitudinal modulation modifies this behavior and can suppress these side lobes for modulation periods near Talbot-type resonance conditions. These effects persist, although reduced, in the presence of thermal refractive index variations. The results provide a realistic assessment of spatially structured current injection for beam shaping in BASLs and clarify its limitations and potential for practical implementations.

We propose and experimentally demonstrate an integrated approach to suppress Kerr self-focusing by engineering the spatial dispersion of a medium using photonic crystals of Laue geometry (Laue-type PhC). The PhC, featuring longitudinally chirped and constant-period sections, was designed using a beam-propagation model and fabricated within a UV-fused silica substrate via femtosecond direct laser writing with a Bessel beam. We characterized the structure by performing nonlinear transmission measurements and analyzing the output beam profile evolution as a function of input pulse energy. Compared to a non-resonant reference structure and the bulk material, the dispersion-engineered PhC successfully counteracts self-focusing, resulting in a 12% increase in the nonlinear absorption threshold. This work presents the first practical demonstration of nonlinearity management through spatial dispersion control in a monolithic PhC, offering a new pathway to bypass the power-scaling limitations of nonlinear processes in solid-state materials.

There is no simple and universal analytical description of various micro‐optical systems related with Fano resonances. This especially concerns modulated thin films, which, when coupled to external fields, show Fano resonances. Usually, such micro‐optic schemes are simulated numerically, frequently by the use of commercial software. The study fills this gap of the lack of universal analytical description by introducing and exploring a simple mechanical equivalent, the oscillator chain, which mimics such schemes involving Fano resonances. The model does not necessarily provide the rigorous description of complicated micro‐optical schemes, however does capture the main properties of such Fano‐related micro‐optical systems. The model captures different modifications of the thin film arrangement as well: thin film with amplification, non‐Hermitical thin films, and others. It also covers the case of multiple Fano resonances in a thin film. The model is validated by comparing with the rigorously calculated wave propagation in the thin films.

We present a novel, to the best of our knowledge, design of a polarizing Meta-Mirror and implement it in combination with polarized pumping in an Nd:YAG microchip laser to enhance the laser’s polarization contrast ratio. Our experimental results show that the polarizing Meta-Mirror not only significantly improves polarization contrast, especially at high pump powers, but also shows low sensitivity to variations of pump polarization direction. Additionally, this setup improves beam quality, output power, and overall brightness of the emitted beam.