Ultrashort Laser Pulse Phenomena, 2nd Edition

Ultrafast Processes in Solid-State Materials 549

The dependence of the modulation amplitude on ϑ results from the electro-optic

effect, which here is responsible for the phonon-induced reflectivity change.

11.4.5. Laser-Induced Surface Disordering

In a strongly absorbing material, the energy deposited in a small surface layer

by a short light pulse can locally raise the temperature beyond the melting point.

What is the response of matter to a δ-function impulse of energy, sufficient to

cause melting? “How fast does melting occur?” is a question of fundamental

interest, which involves changes in order and structure. In addition to the possi-

bility of observing melting through “femtosecond photography” [35], nonlinear

techniques sensitive to the material symmetry can be applied [36]. Results of an

experiment to monitor changes in symmetry in GaAs during melting are shown

in Figure 11.11. In GaAs, melting can be considered as a transition from a

noncentrosymmetric material to an isotropic liquid. The second-order nonlinear

susceptibility χ

(2)

is therefore expected to change from a relatively large value

(for the crystal) to (almost) zero (for the liquid) during the phase transition. This

second-order susceptibility can be monitored by measuring the second harmonic

in reflection generated by a delayed probe [38–40]. The reflectivity for the fun-

damental increases from the solid reflectance value to that of liquid GaAs with

a characteristic time of about 1 ps. On the other hand, the SH signal drops

substantially on a time scale of about 100 fs. These data suggest an intermediate

state between the non-centrosymmetric crystal structure and the molten material.

Note that a transition to a centrosymmetric crystal would only require a small

Probe

Pump

BS

0510

Delay, ps

GaAs

Fundamental

I

R

Second

harmonic

0.4

0.2

0

0.5

I

SH

/I

SH

I

R

/I

R

I

SH

(a) (b)

Figure 11.11 (a) Experimental setup to monitor ultrafast phase changes on a GaAs surface. A strong

pump pulse induces the phase change. (b) The upper curve shows the reflectivity of a delayed probe.

The lower curve is a plot of the second harmonic (in reflection) of the probe signal, which is a

measure of the change in symmetry associated with the phase transition. (Adapted from Govorkov

et al. [37].)