Absence of phase-dependent noise in time-domain reflectivity studies of impulsively excited phonons

There have been several reports of phase-dependent noise in time-domain reflectivity studies of optical phonons excited by femtosecond laser pulses in semiconductors, semimetals, and superconductors. It was suggested that such behavior is associated with the creation of squeezed phonon states although there is no theoretical model that directly supports such a proposal. We have experimentally re-examined the studies of phonons in bismuth and gallium arsenide, and find no evidence of any phase-dependent noise signature associated with the phonons. We place an upper limit on any such noise at least 40–50 dB lower than previously reported.

Figure 1

REOS signals as a function of pump-probe delay for (a) Bi and (b) GaAs. Inset in (a) shows spectrum of $\Delta I\left(t\right)$.

Figure 2

(a) Terahertz emission from GaSe detected by electro-optic sampling. (b) Signal and noise measurements performed over a two cycle portion of (a). In (b) the noise is measured with a 10-ms lock-in time constant and an interval of 10 s between consecutive data points. The horizontal dashed line indicates the noise floor in the absence of the pump beam.

Figure 3

Fractional noise in electro-optic sampling measurements of terahertz emission from GaSe as a function of lock-in amplifier time constant with an interval of 1 s between points. The noise signal is averaged over two cycles of the terahertz signal. The horizontal dashed line indicates the noise floor in the absence of the pump beam.

Figure 4

Fractional REOS signal, $\Delta I/I_{\text{o}}$, and fractional noise in $I$, $\Delta I_{\text{n}}/I_{\text{o}}$, for (a) Bi and (b) GaAs. The dotted curve at the bottom of each panel shows the estimated contribution to the noise from timing jitter and the horizontal dashed lines indicate the noise floor in the absence of the pump beam.