Upon pulsed laser irradiation of the sample, free excess charge carriers, i.e. electrons and holes, are created. Free excess charge carriers induce a relative change of reflected microwave power (ΔP(t)/P). Irradiation of sample by laser induces sample conductivity perturbation σ(t), which directly influences the relative change of initial reflected microwave power. The relation between conductivity and microwave absorption is given by the following expression:

ΔP(t)/P = A Δσ(t) = A e Σ_i Δn_i(t) μ_i

where Δn_i  corresponds to the number of excess charge carriers i at time t, μ_i to the mobility of charge carriers, and A to the time-independent sensitivity factor.

The TRMC signal is described by two parameters: the maximum value I_max and the time decay I(t). The maximum signal intensity determines the density of excess free charge carriers created during irradiation, while the decay I(t) corresponds to the trapping, recombination of charge carriers, or electron/proton transfer over time ttt, spanning from the nanosecond to the microsecond timescale.

TRMC signals obtained at (a) 360 nm (a) and (b) 420 nm laser excitation [Xu2022]

Laser source: OPO pulsed laser (EKSPLA, NT342B) tunable from 225 to 2000 nm. It delivers 8 ns FWMH pulses at a repetition rate of 10 Hz.

Microwave source: The incident microwaves are generated by a Gunn diode of the Kα band at 30 GHz.

Sample: solid