Laser-induced band-gap collapse in GaAs

Strong nonlinear growth of energy coupling during laser irradiation of transparent dielectrics and its significance for laser induced damage

“…Previous work [9,10] also pointed out the importance of temperature dependent absorption in the damage initiation process. Early experimental studies of the temperature dependence of absorption [18] suggested at least a partial bandgap collapse with increasing temperature in fused silica, analogous to the complete bandgap collapse known for semiconductors [19]. An experimental demonstration of reduced damage threshold in fused silica with temperature was found in [14].…”

Ultrafast magnetic light

“…2B), respectively. Such dropping Re(ε) and rising Im(ε) magnitudes are consistent with the increasing intraband contribution at the monotonously rising EHP density, while the following rise represents characteristic red-shift changes of these quantities across absorption bands in strongly photo-excited semiconductors due to their prompt ρ ehdependent bandgap renormalization [35,41,43].Ultrafast photoexcitation of a silicon nanoparticle-The observed large, ultrafast changes of optical dielectric permittivity properties in silicon at fluences below its ablation threshold (for spallation under these experimental conditions, F eff ≈ 0.3 J/cm 2 [45]) can significantly alter optical response of a silicon nanoparticle, supporting a magnetic Mie-type resonance. Basing on the extracted dielectric permittivity values of photoexcited silicon, we study ultrafast dynamics of scattering properties (cross section and diagram) of a silicon nanoparticle near its magnetic resonance by means of full-wave numerical simulations carried out in CST Microwave Studio.…”

“…Such almost prompt, fs-laser induced optical tunability appears to be much broader for (semi)insulating materials with very minor initial carrier concentrations, extending in a sub-ablative regime in the visible range, e.g., in terms of dielectric permittivity (ε) from large positive (Re(ε) ≃ +10 for the initial undoped material) to deeply negative (Re(ε) ≃ −10 for its strongly-excited, metallized surface layer) values (the so-called insulator-metal transition [35]). In detail, such ultrafast transient modulation of optical dielectric permittivity in semiconductors and dielectrics is related to transient variation of free-carrier (electron-hole plasma, EHP) density (ρ eh ) through its basic intraband and interband contributions [35][36][37][38][39][40][41][42][43]. Specifically, an optically driven insulator-conductor transition occurs in such materials, when the fs-laser fluence-dependent EHP frequency passes through the probe frequency, which becomes evident as subsequent reflectivity increase for stronger ionized materials through their intraband electronic transitions [36][37][38].…”

“…Simultaneously, ultrafast transient optical modulation for silicon [41,43] and other materials [35] is additionally enhanced due to a strong prompt EHP-driven isotropic renormalization of their direct bandgap, resulting in drastic enhancement of interband transitions and corresponding red spectral shift of the optical dielectric permittivity [35].…”

“…Specifically, an optically driven insulator-conductor transition occurs in such materials, when the fs-laser fluence-dependent EHP frequency passes through the probe frequency, which becomes evident as subsequent reflectivity increase for stronger ionized materials through their intraband electronic transitions [36][37][38]. Simultaneously, ultrafast transient optical modulation for silicon [41,43] and other materials [35] is additionally enhanced due to a strong prompt EHP-driven isotropic renormalization of their direct bandgap, resulting in drastic enhancement of interband transitions and corresponding red spectral shift of the optical dielectric permittivity [35].In this work a realistic dependence of the dielectric permittivity for photo-excited silicon versus incident fslaser fluence at 800-nm laser wavelength was obtained through measurements of single-shot fs-laser pump selfreflectivity (R) from smooth Si surface at its s-(R s (45• )) and p-(R p (45 • )) polarizations at the 45• -incidence angle and variable effective (absorbed) laser fluences where the prompt ρ eh -dependent bandgap shrinkage effect on interband transitions is accounted by the spectral dielectric permittivity dependence with the effective photon frequency ω * = ω + Θρ eh /ρ bgr with the factor Θ, the characteristic renormalization EHP density (ρ bgr ), the characteristic band capacity (ρ bf ) of the specific photo-excited regions of the first Brillouine zone in the k-space, the EHP frequency ω pl and electronic damping time τ e at the pump frequency ω (for details of calculations, see Supplementary materials). The resulting model ρ eh -dependent pump reflectivities R s,p (45…”