Coherent field transients below 15 THz from phase-matched difference frequency generation in 4H-SiC

Refractiveindex.info database of optical constants

We introduce the refractiveindex.info database, a comprehensive open-source repository containing optical constants for a wide array of materials, and describe in detail the underlying dataset. This collection, derived from a meticulous compilation of data sourced from peer-reviewed publications, manufacturers' datasheets, and authoritative texts, aims to advance research in optics and photonics. The data is stored using a YAML-based format, ensuring integrity, consistency, and ease of access. Each record is accompanied by detailed metadata, facilitating a comprehensive understanding and efficient utilization of the data. In this descriptor, we outline the data curation protocols and the file format used for data records, and briefly demonstrate how the data can be organized in a user-friendly fashion akin to the books in a traditional library.

Investigation of the electro-optic effect in high-Q 4H-SiC microresonators

Silicon carbide (SiC) recently emerged as a promising photonic and quantum material owing to its unique material properties. In this work, we carried out an exploratory investigation of the Pockels effect in high-quality-factor (high-Q) 4H-SiC microresonators and demonstrated gigahertz-level electro-optic modulation for the first time. The extracted Pockels coefficients show certain variations among 4H-SiC wafers from different manufacturers, with the magnitudes of r ₁₃ and r ₃₃ estimated to be in the range of (0.3-0.7) pm/V and (0-0.03) pm/V, respectively.

Combined effects of polarization and secondary ablation on precision machining of microgrooves by laser-induced microjet-assisted ablation

In this study, we propose a polarization-modulated laser-induced microjet-assisted ablation method for machining microgrooves with controllable cross-sections. A novel mathematical model is presented to accurately predict the cross-sections by considering the combined effects of polarization and secondary ablation. The simulation and experimental results reveal that the effect of secondary ablation becomes more obvious when steeper grooves are ablated with higher repetition frequency and larger pulse energy. The polarization effect and secondary ablation of target material result in asymmetric ablation of linearly polarized laser beam. To avoid the asymmetric ablation, we present the cylindrical vector beams to achieve scanning-path independent laser micromachining with various cross-sections. Based on the prediction model, the cross-sectional characteristics are precisely designed and fabricated by tuning laser processing parameters. Our work provides a reliable approach for the controllable fabrication of microgrooves at the scale of tens of micrometers.

Multibranch pulse synthesis and electro-optic detection of subcycle multi-terahertz electric fields

We present a robust, compact pulse synthesis scheme generating intense phase-locked subcycle multi-terahertz waveforms. The ultrabroadband laser fundamental is split into two parallel branches driving optical rectification in crystals of GaSe and LiGaS₂, each operated at the group velocity matching point. The coherent combination of the resulting pulses yields a continuous multi-terahertz spectrum covering 1.5 optical octaves. The corresponding 0.8-cycle electric field waveform is directly mapped out by electro-optic sampling, revealing peak fields of 15 kV/cm at a repetition rate of 0.4 MHz. The multiplexable and power scalable scheme opens the door to strong-field custom-tailored waveforms driving nonlinear optics and light wave electronics.

Coordinated multi-band angle insensitive selection absorber based on graphene metamaterials

In this paper, we propose a tunable, multi-band, selective absorber composed of multiple layers. Each layer consisted of SiO₂/graphene/SiC, and a layer of silver was used as the ground plane of the entire structure. Simulation results show that we can passively and actively coordinate the resonant frequency of the perfect absorption peak by changing the geometric parameters of the array and the Fermi level of the graphene. The absorber is not sensitive to the angle of incidence and the direction of polarization. We propose a theoretical basis for the formation of multiple absorption peaks. The theoretical calculations are in good agreement with the simulation results. In addition, we simulated the three- and four-layer structures. The results show that in the terahertz (THz) band, composite structures of three and four layers can obtain three and four perfect absorption peaks, respectively. Our results provide new insights into the THz band of harmonizable multi-band absorbers that can be applied to THz imaging to coordinate sensors and other optoelectronic devices.

Terahertz time-domain spectroscopy of zone-folded acoustic phonons in 4H and 6H silicon carbide

We investigate the dielectric properties of the 4H and 6H polytypes of silicon carbide in the 0.1-19 THz range, below the fundamental transverse-optical phonons. Folding of the Brillouin zone due to the specific superlattice structure of the two polytypes leads to activation of acoustic phonon modes. We use a combination of ultrabroadband terahertz time-domain spectroscopy and simulations based on density-functional perturbation theory to observe and characterize these modes, including band splitting due to the dissimilar carbon and silicon sublattices of the structures, and an indirect measurement of the anisotropic sound velocities in the two polytypes.