英语翻译3.RESULTSANDDISCUSSIONSFigure2showsthedifferentialreflectivitytransients,ΔR/R,obtainedfromtheannealedZnOsamplefortwodifferentpumppowerof0.4I0,I0(1.0mW)byusingthereflectivepumpandprobetechniquewith110pssc

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英语翻译
3.RESULTS AND DISCUSSIONS
Figure 2 shows the differential reflectivity transients,ΔR / R ,obtained from the annealed ZnO
sample for two different pump power of 0.4I0,I0(1.0 mW) by using the reflective pump and probe
technique with 110 ps scans.Both traces are scaled to have the same maximum height for easier
comparison.The signals reveal an initial positive peak with a width very close to that of the pump
pulse used.Then the differential reflectivity turns negative,at about 5 ps after the pump pulse,it
reaches a maximum value,followed by the subsequent recovery back towards to its background
level.This positive change in the differential reflectivity signals is due to an instantaneous
contribution to Δn by band-gap renormalization(BGR),possibly complicated by plasma screening
of electron-hole interactions.Both effects,as widely discussed on GaAs-based materials,21 can be
expected to occur almost instantaneously after a pulsed photoexcitation,i.e.,before the carriers can
be described by a Fermi-Dirac distribution and certainly before most of carriers fall to the band edge
and cool to the lattice temperature.The small “dip” contained in ΔR / R of the annealed ZnO
sample showed amplitudes as large as several parts of 10 -3 The present results,including magnitude
and sign of the “dip” signals,are in good agreement with theoretical predictions of the
BGR-induced Δn and thus ΔR / R for the spectral regime much below band edge in bulk
semiconductors.22 On the other hand,this change over in sign of the pump-probe signal from
positive to negative is resulted from two types of contributions to the index and thus measured
reflectivity changes:(1) the interband contribution due to state and band filling(BF),and (2) the
intraband contribution due to free-carrier absorption(FCA).Both,in contrast to BGR,are expected
to generate substantial decrease in the refractive index (Δn) and thus contribute negatively to
ΔR / R as large as 10 -2 for below gap energies.The signal size almost linearly scaled with
increased carrier density N0 for the present cases.It is also interesting to mention that the negative
increase in the reflectivity in each case persists until about 5 ps after femtosecond excitation.This
buildup time for the negative response represents an overall time taken for the initial nonthermal
carrier population to relax towards the continuum extremes and then fill into the bandtail states
interrogated by the probe.Such a slower increase in ΔR / R is likely caused by blocking of
carrier(mostly electron) scattering with longitudinal optical(LO) phonon or trapping of
carriers/excitons at the bandtail states.

Answer 1

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