Ultrafast x-ray measurement of laser heating in semiconductors: Parameters determining the melting threshold

Abstract

The pulse-width dependence of thermal melting and ablation thresholds in germanium and gallium arsenide is correlated to direct, ultrafast x-ray measurements of laser-heated depths. The heating dynamics, determined by the interplay of nonlinear optical absorption, delayed Auger heating, and high-density carrier diffusion, explain the scaling laws of thermal melting thresholds in different semiconductors.

XRD studies on the femtosecond laser ablated single-crystal germanium in air

“…For lower laser fluence (F a o10:2 J=cm 2 ), the ablation depth L can be described by the expression, L ffi d lnðF a =F d th Þ; where d is the optical penetration depth and F d th is the threshold laser fluence of ablation [13,14]. A fit to the experimental data gives F d th ffi 0:58 J=cm 2 and d ffi 113 nm: The ablation threshold estimated from the current work is in fairly good agreement with that of Linde [15]. Meanwhile, the observed optical penetration depth is found to be lower than that derived from the absorption coefficient of Ge at 800 nm (d ¼ 173 nm) [16].…”

A femtosecond X-ray/optical cross-correlator

“…The pulse energies reached up to 16µJ, as measured through the facility gas monitor detector 29 and independently by an intensity monitor within the PG2 beamline. With a spot size of (395 ± 23)µm x (274 ± 14)µm 30 the fluence stays well below the optical damage threshold of the GaAs surface (50 mJ/cm 2 for 30 fs pulse length at 800 nm) 31 .…”

“…At 800 nm probe wavelength, where in equilibrium only optical transitions across the direct band gap close to the Γ point are probed 35 Within 10 nm of the GaAs sample 10% of the incident photons are absorbed 38 (Fig. 3b), staying below the damage threshold of the GaAs surface (50 mJ/cm 2 ) 31 . Thus an excitation fluence of 10 mJ/cm 2 (equivalent to 1.6·10 15 photons at hv=39.5 eV) creates within the 10 nm thick surface layer a Ga 3d excitation density of approx.…”

Reconstructing longitudinal strain pulses using time-resolved x-ray diffraction

“…Understanding the dynamics driven by intense optical radiation is of fundamental importance to a diverse set of fields, ranging from structural phase transitions [1][2][3][4] to biophysics 5,6 . In solid state systems, for example, upon ultrafast optical excitation, the resultant impulsive surface heating can produce a transient longitudinal acoustic pulse.…”