In reflectance measurements, the broadband white light was directed to the sample with the objective, reflected, collected by the same objective, and detected by an InGaAs photodiode array (PyLoN-IR, Princeton Instruments). For the reflectance at the low-density limit, the spectrally filtered and collimated white light from a 3200 K halogen lamp (KLS EKE/AL) was used. Reflectance was also taken for the white light probe in the same geometry as transient reflectance to confirm that it is in the linear regime. A 150-nm gold film deposited by electron beam evaporation on the same Si/SiO2 substrate was used as a reflectance standard.

In transient reflectance measurements, femtosecond laser pulses from the Ti:sapphire regenerative amplifier (Coherent RegA 9050, 250 kHz, 800 nm, 100 fs) was split into two beams: One was used to pump the visible optical parametric amplifier (Coherent OPA 9450) to generate tunable pump light, and the other was focused onto a sapphire crystal to generate white light continuum probe light. The pump was then chirp compensated by a prism pair, delayed by a motorized translation stage, modulated by an optical chopper, combined with the probe, and directed collinearly to the sample by the objective. To achieve homogenous excitation, average over a sufficient area, and reduce nonlinear effect of probe, both beams were focused onto the back focal plane of the objective to obtain a large beam diameter at the sample plane, unless otherwise specified. The reflected probe light was then collected by the same objective, spectrally filtered to remove pump light, and recorded with the InGaAs photodiode array (PyLoN-IR, Princeton Instruments). This detector was synchronized with the optical chopper through a home-made frequency doubler. At each specific pump-probe delay, the reflected probe spectra with and without pump was recorded, and the transient reflectance (ΔR/R) was calculated. We determined the sign of the transient reflectance signal by recording the chopper output with a data acquisition board (National Instruments) triggered by the InGaAs detector. The chopper modulation frequency was selected to maximize the signal-to-noise ratio of transient reflectance signal.

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