ABSTRACT
We study structural and morphological transformations caused by multipulse femtosecond-laser exposure of Bridgman-grown ϵ-phase GaSe crystals, a van der Waals semiconductor promising for nonlinear optics and optoelectronics. We unveil, for the first time, the laser-driven self-organization regimes in GaSe allowing the formation of regular laser-induced periodic surface structures (LIPSSs) that originate from interference of the incident radiation and interface surface plasmon waves. LIPSSs formation causes transformation of the near-surface layer to amorphous Ga2Se3 at negligible oxidation levels, evidenced from comprehensive structural characterization. LIPSSs imprinted on both output crystal facets provide a 1.2-fold increase of the near-IR transmittance, while the ability to control local periodicity by processing parameters enables multilevel structural color marking of the crystal surface. Our studies highlight direct fs-laser patterning as a multipurpose application-ready technology for precise nanostructuring of promising van der Waals semiconductors, whose layered structure restricts application of common nanofabrication approaches.
ABSTRACT
Ba5(BO3)3F single crystals of high optical quality and up to 1.5 cm in diameter were grown. Its transparency range is 0.23 to 6.6 µm (on 10% level). Direct allowed electronic transitions at the Γ-point give band gap values of 5.31 and 5.40 eV at 300 and 80 K, respectively. Luminescence is excited in the near-edge absorption bands near 265 and 365 nm. X-ray irradiation induces an additional absorption in dominant 252, 317 and 710 nm bands. Combined electron spin-resonance spectroscopy and theoretical analysis allow one to associate the three absorption peaks with O(5-), O(1-) and e6(-) (fluorine vacancy), respectively. The original transparency is restored after heating the crystal to 400 K and charge carrier release from traps with ET = 0.87 eV and s = 10(12) s(-1). Dispersion curves for the refractive indices were calculated and Sellmeier equations were built. Theoretical analysis shows strong localization of the Ba 5s and F 2s orbitals, strong ionicity of the Ba cations and strong covalency of the B-O bond. The optical properties of Ba5(BO3)3F are dominantly determined by electron transitions within the (BO3)(3-) groups, despite the transition between barium and oxygen also having a little contribution.
ABSTRACT
Phase-matched second-harmonic generation is obtained in various LiInS(2) crystals by use of the tunable picosecond output of the free-electron laser for infrared experiments (FELIX) as the pump source in the mid-IR range from 2.75 to 6.0 microm. Deviations from the phase-matching curve calculated from Boyd's refractive-index data are observed. Furthermore, the optical damage threshold of the crystals has been measured to be 1.1. J/cm(2)(>6 GW/cm(2)) at the 5-microm wavelength. LiInS(2) holds great promise for parametric interaction in the 1-13-microm range.
ABSTRACT
We report on the improvement of a tunable, high resolution, diode laser-based, difference-frequency spectrometer using an AgGaS(2) nonlinear crystal. We use a type-II cut crystal as a part of the improvement compared with a type-I cut, which was used in our preliminary setup. The two tunable laser-diodes are operating near lambda(3)=778 nm (pump) and lambda(2)=842 nm (signal) with a sub-100-kHz linewidth. The high resolution spectrometer is being developed as an alternative to CO(2) laser spectrometers in the 9- to Il-mum range. Using a dual-arm cavity to enhance the two radiation powers, and with 35 mW in front of the 778-nm arm and 100 mW in front of the 842 nm arm, about 70 nW of the tunable 10-mum radiation are generated. This power level is enough to investigate the linear absorption spectroscopy of SF(6). Doppler-limited spectra over 2 GHz, are recorded, showing the wide continuous tunability of the spectrometer.
ABSTRACT
Frequency divide-by-two (2:1) and divide-by-three (3:1) optical parametric oscillators (OPOs) are basic devices for the implementation of future accurate optical frequency division chains. We report our latest development toward a phase-locked 3:1 frequency division of a radiation at lambda(p) approximately 843 nm (355.9 THz), using doubly resonant oscillators (DROs) based on silver gallium sulfide (AgGaS(2 ) or AGS) and multigrating periodically poled lithium niobate (PPLN). Although stable single-mode pair operation is achievable without any active cavity length servo with the AGS-DRO, because of a strong passive thermal feedback servo, the PPLN-DRO requires an active intensity side-of-fringe locking servo to maintain long-term, single-mode pair operation. To overcome the limited idler output power (<1 mW) due to the weak mirror transmissivity at 2.53 mum, a CaF (2) Brewster-oriented plate was inserted in a longer-cavity PPLN-DRO as a variable output coupler. About 3 mW of idler wave is thus coupled outside the cavity, yielding 15 nW of doubled-idler. We obtained a 30 dB signal-to-noise ratio beatnote in a 100 kHz resolution bandwidth of a spectrum analyser. This beat signal will be used to phase-lock the divider.
