Ultrashort Laser Pulse Phenomena, 2nd Edition

520 Measurement Techniques of Femtosecond Spectroscopy

state |2 by a succession of optical excitations via the dipole allowed transitions

|1→| and |→|2, involving photons of frequencies ω

1

and ω

2

.

Because of the broad bandwidth of the fs pulse, stimulated Raman scattering

can occur through the mixing of various spectral components of the ultrashort

optical pulse [Fig. 10.20(b)]. For example, a molecular vibration is initiated

by the sudden impulse exerted by the electric field of the pulse. The sample

selects a pair of frequency components whose difference is in resonance with

the eigenfrequencies of a Raman transition. This type of Raman scattering is

called “impulsive stimulated Raman scattering” [26]. The fs excitation makes

it possible to excite in phase a macroscopic ensemble of vibrating molecules.

In solids, it is a coherent excitation of lattice vibrations that is achieved.

It is generally not possible to achieve a complete excitation of the Raman

transition with a single fs pulse. Many Raman active modes can sometimes be

accessed by the same fs pulse. However, if the process of impulsive stimulated

Raman scattering is repeated at each cycle 2π/ω

ν

of a Raman transition, the

excitation will be enhanced. The selectivity of the process is also increased by

the periodic excitation [27].

Impulsive stimulated Raman scattering can be used to analyze vibrational

motions—for instance to determine their decay through pump–probe techniques.

Synchronous excitation by a train of pulses can lead to substantially larger ampli-

tudes of motion. This excitation process can generate high frequency vibration.

A train of pulses spaced by a picosecond can generate THz LO phonons in

semiconductors, which have a wavelength in the 100 Å range, and can therefore

be used for high resolution imaging in solids.

10.10.2. Detection

The change in matter properties associated with the Raman excitation can be

probed in a variety of ways. One can, for instance, probe an induced birefrin-

gence, in which case the rotation of the probe polarization will be measured,

as detailed in Section 10.5. In parallel polarization (probe polarization parallel

to that of the pump), the attenuation of the probe will be modulated with delay,

because the probe pulse can also induce Raman transitions. The phase of the oscil-

lations of attenuation versus delay of the probe depends on the particular spectral

component that is being probed. In the example reproduced in Figure 10.21,

the pump and probe have the same polarization and are sent nearly collinearly

through a sample of liquid CH

2

Br

2

[26]. The transmitted probe is dispersed by a

monochromator. Two frequency components (609 nm and 620 nm) are displayed

as a function of delay in Fig. 10.21.

Both spectral components are seen to oscillate with delay at the molecular

vibration frequency, but with opposite phase. A simple explanation is that at a

Impulsive Stimulated Raman Scattering 521

Probe beam transmission

0.0 0.40 0.80

Time in picoseconds

Forward ISRS in CH

2

Br

2

609 nm

620 nm

1.20 1.60 2.00

Figure 10.21 Transmission versus delay for two spectral components of the probe signal, for a

sample of CH

2

Br

2

pumped by a 65 fs pulse of a few µJ energy at 615 nm. The excitation and probe

pulses are focused to a 200-µm spot size, at an angle of 5

◦

, into a 2-mm sample cuvette (Adapted

from Ruhman et al. [26]).

particular delay, the position of the vibrating coordinates is such that the 609 nm

radiation is absorbed, and the 620 nm reinforced by the Raman transition. For a

delay corresponding to half a vibration cycle later, the 609 nm transition will be

reinforced and the 620 nm attenuated.

So far we have assumed a single pump pulse to induce the Raman signal.

A standing wave pattern can also be generated for the impulsive stimulated Raman

signal, either through a periodic configuration of the sample, or through the use

of two intersecting pump pulses.

An example of sample periodicity is an MQW structure, of which the spacing

between wells is made to match the wavelength of the phonon to be generated.

The phonon can be generated by a train of fs pulses spaced by the phonon period,

tuned to the intraband absorption in the quantum wells. The periodic spatial

structure that is excited is responsible for the spatial coherence of the phonon [28].

The excitation by a periodic pulse sequence ensures temporal coherence of the

created phonons.

It is also possible to create a standing wave Raman excitation with two inter-

secting pump pulses of the same frequency [26]. The temporal evolution of the