arXiv subjects

Bikash Kumar Das

Publications and source records attributed to Bikash Kumar Das.

5 recordsLinked to original sources

arXiv2026

Carrier-Envelope Phase Control of Orbital Angular Momentum in Solid-State High-Harmonic Generation

High-order harmonic generation (HHG) driven by optical vortices is a powerful route to produce structured light in different spectral regions. The nonlinear process transfers orbital angular momentum (OAM) from the driving field to the emitted harmonics according to the scaling law $l_q = q\times l$, a consequence of the rotational invariance and angular momentum conservation. Here, we show that, in the regime of few-cycle pulses, the topological charge (TC) of the harmonic radiation detected within a finite spectral window is no longer fixed by this scaling law alone, but is governed by the interplay between broken crystal inversion symmetry and carrier-envelope phase (CEP)-sensitive sub-cycle electron dynamics. By driving HHG in a ZnO crystal with few-cycle ($\approx 1.5$ cycles) vortex beams centered at 3.2~$\mu$m, we observed that the measured TC becomes strongly CEP-dependent, switching between adjacent integer values, but only when the inversion symmetry is broken and the harmonic emission is CEP-sensitive. The TC switching vanishes when either condition is removed. Numerical analysis reveals that the TC switching originates from a CEP-controlled redistribution of spectral weight among spectrally overlapping harmonic orders, which changes the dominant OAM channel within the detection window. These results identify the CEP as a degree of freedom for tailoring the topological structure of high-harmonic radiation, pointing toward waveform-controlled structured attosecond light sources.

physics.optics↗

arXiv2026

Field-driven helicity in solid-state high-harmonic generation

The polarization state of light plays a central role in strong-field light--matter interactions and is widely used to probe electronic structure in solids via high-order harmonic generation (HHG). In particular, helicity-resolved HHG has been interpreted as a fingerprint of crystal symmetry and topology. Here, we demonstrate deterministic and continuous control of harmonic helicity in solids using polarization-crafted beams, formed by two orthogonally polarized pulses with a controlled time delay. By tuning this delay, the polarization state of individual harmonics can be driven from linear to circular, independent of the material under investigation. We show that this behavior is robust across systems with distinct symmetry and topology, and originates from the sub-cycle modulation of the light--matter interaction mediated by the dipole coupling. Furthermore, the orthogonal configuration allows to break the dynamical symmetry of the light-matter interaction which is manifested in the generation of otherwise forbidden harmonics under standard selection rules.. These results establish harmonic helicity as a field-controlled observable rather than a direct material fingerprint.

physics.optics↗

arXiv2026

Twisting harmonics: Transfer of orbital angular momentum in solid-state high-harmonic generation

Although solid-state platforms underpin modern electronics, little is known about how intense ultrashort light pulses carrying orbital angular momentum (OAM) interact with solids. This gap persists even though, for more conventional light-matter interactions, the complex underlying electron dynamics can often be confined to a single Brillouin zone and described well within the dipole approximation. Previous studies were restricted to nonlinear, perturbative regimes, largely because the generation of intense ultrashort vortex pulses, particularly in the mid-infrared spectral regime, has remained a long-standing challenge. Consequently, the role of structured light in driving nonlinear, non-perturbative processes in solids, and the associated transfer of angular momentum during these interactions, has not been systematically explored. Here, we investigate solid-state high-harmonic generation (HHG) driven by intense ultrashort structured light using a versatile experimental approach applicable to different materials and geometries. We demonstrate that the OAM of the driving field is coherently transferred to the emitted harmonics. In particular, we show that the OAM is conserved independently of the crystal symmetry, the range of electronic interactions, and the presence of strong spin-orbit coupling. These results establish OAM-resolved HHG as a robust framework for characterizing and controlling angular momentum transfer in solid-state HHG and open new avenues for structured-light-driven quantum technologies and topological materials investigations.

physics.optics↗

arXiv2025

Sculpting ultrafast mid-infrared light for solid-state high harmonic generation

The ability to sculpt light in space, time, and polarization has revolutionized studies of light-matter interaction and enabled breakthroughs in optical communication, imaging, and ultrafast science. Among the many degrees of freedom of light, orbital angular momentum (OAM) further expands these capabilities by unlocking new regimes of control in information encoding, particle trapping and manipulation, and symmetry-driven selection rules. However, exploiting OAM to drive nonlinear, non-perturbative effects in solids remains challenging, especially in the mid-infrared (MIR) spectral regime-a key region for accessing these effects in ambient air, where spatial light modulators do not operate. Here, we circumvent this limitation by generating femtosecond, few-cycle MIR Bessel-Gauss vortex (BGV) and perfect optical vortices (POVs), using a robust, static spatial-shaping strategy. By utilizing these beams to drive nonlinear optical processes such as second-harmonic generation (SHG) and high-harmonic generation (HHG) in various solid-state materials, we show that the resulting harmonic beams faithfully inherit the structural characteristics of the drivers: the constant-intensity ring of the POVs is preserved across harmonic orders, while the BGV harmonic beams retain their intrinsic topological charge-dependent intensity profiles. Furthermore, by verifying the linear OAM up-scaling law, we confirm the conservation of OAM during SHG and HHG in solids. These results establish strong-field HHG in solids as a robust platform for synthesizing ultrafast structured harmonic light with controllable, high-value OAM.

physics.optics↗

arXiv2024

High-Order Harmonic Generation Driven by Perfect Optical Vortex Beams: Exploring the Orbital Angular Momentum Upscaling Law

Orbital angular momentum (OAM) light beams for high-order harmonic generation (HHG) provide an additional degree of freedom to study the light-matter interaction at ultrafast timescales. A more sophisticated configuration is a perfect optical vortex (POV) beam, a light beam with a helical wavefront characterized by a phase singularity at its center and an azimuthal phase variation. POV beams are characterized by a radial profile which is independent of the OAM. Here we study the non-perturbative process of gas-phase HHG using a linearly polarized POV beam. We observe that the harmonics are emitted with similar divergence due the perfectness of the POV-driven harmonics. Furthermore, the topological charge upscaling is rigorously followed. We show that a POV beam is more advantageous than that of the Laguerre-Gaussian beam for cases where a large topological charge with a small core size is required. Our research establishes a pathway for producing bright structured extreme ultraviolet (XUV) coherent radiation sources--a pivotal tool with multifaceted applications across various technological domains.

physics.optics↗