Yong-Xin Yan

Impulsive stimulated scattering: General importance in femtosecond laser pulse interactions with matter, and spectroscopic applications

It is shown that ‘‘impulsive’’ stimulated Raman scattering (ISRS) should occur, with no laser intensity threshold, when a sufficiently short laser pulse passes through many types of matter. ISRS excitation of coherent optic phonons, molecular vibrations, and other excitations (including rotational, electronic, and spin) may play important roles in femtosecond pulse interactions with molecules, crystals, glasses (including optical fibers), semiconductors, and metals. Spectroscopic applications of ISRS, including time‐resolved spectroscopy of vibrationally distorted molecules and crystals, are discussed.

Impulsive stimulated light scattering. I. General theory

Theoretical analysis of the time‐resolved impulsive stimulated light scattering (ISS) method is presented. A general theoretical framework is developed to describe ISS experiments on any type of material mode which is active in light scattering and conforms to linear response theory. ISS experiments permit time‐resolved observation of material motion through the dielectric response functionG ee(q,t). In the simplest case of ideal time and wave vector resolution, ISS signal gives ‖G ee(q,t)‖2 directly. Various consequences of limited t and q resolution are discussed in detail. ISS experiments on acoustic and optic phonons, Debye relaxational modes, and some combinations of modes are treated explicitly. A detailed comparison of time‐domain impulsive stimulated scattering and frequency‐domain spontaneous light‐scattering spectroscopy is presented in the companion paper.

Impulsive stimulated light scattering. II. Comparison to frequency‐domain light‐scattering spectroscopy

A comparison between time‐domain impulsive stimulated light‐scattering (ISS) and frequency‐domain, spontaneous light‐scattering (LS) spectroscopies is carried out in theoretical and practical terms. In some cases, the two experiments probe different material responses. In many cases the information content of ISS and LS data is identical in principle. The results can be related to each other through the time‐ and frequency‐dependent response functions Gee(q,t) and Gee(q,ω), or through the time‐correlation function Cee(q,t). Simulated ISS and LS data from vibrational and Debye relaxational modes are compared in view of experimental considerations, including wave vector and time or frequency resolution and range, and sources of ‘‘noise.’’ In many cases, one or the other experimental approache offers significant advantages in practice. The complementary nature of the techniques is illustrated.

The temperature‐dependent distribution of relaxation times in glycerol: Time‐domain light scattering study of acoustic and Mountain‐mode behavior in the 20 MHz–3 GHz frequency range

A time‐domain light scattering study of acoustic and Mountain modes in glycerol is reported. By using light‐scattering angles between 0.89° and 88.9°, a wide range of acoustic frequencies is sampled. The data also yield information about time‐dependent density responses to stress and to heat (the latter is the time‐dependent thermal expansion). These responses are associated with the Mountain mode and provide additional information about structural relaxation dynamics. A theoretical framework is presented which can treat these experiments as well as ultrasonics and specific heat spectroscopy. The time or frequency dependences of the elastic modulus, heat capacity, and pressure response to temperature change are all accounted for and appear to be significant. The experimental results are fit best with a distribution of relaxation times which is somewhat less asymmetric than a Cole–Davidson distribution. The width of the distribution (on a logarithmic frequency scale) does not change significantly in the 20...

Picosecond impulsive stimulated brillouin scattering: Optical excitation of coherent transverse acoustic waves and application to time-domain investigations of structural phase transitions

Abstract A practical stimulated Brillouin-scattering method for spectroscopic characterization of low-frequency excitations is extended to permit optical generation and detection of phase-coherent transverse, longitudinal, and mixed polarization acoustic phonons, tunable over a wide frequency range, in solid media. Results are presented for BK-7 glass and the KH 2 PO 4 crystal near its structural phase transition (SPT). Time-domain observations of acoustic phonon-induced ordering near SPTs in KH 2 PO 4 and KD 2 PO 4 were carried out, and applications of the technique to measurements of SPT dynamics are discussed.

Ultrasonic and hypersonic properties of molten KNO3–Ca(NO3)2 mixture

A picosecond time‐domain light scattering technique is used to study the viscoelastic KNO3–Ca(NO3)2 60:40 glass‐forming liquid mixture. By using scattering angles between 1.92° and 85.67°, acoustic frequencies from 50 MHz to 4 GHz are sampled. Together with existing ultrasonic and Brillouin scattering data, a temperature‐dependent distribution of relaxation times is found to be well fit with a Cole–Cole distribution whose width changes from several decades in the glassy state to nearly single relaxation time in the high‐temperature liquid state. The characteristic relaxation time is found to obey the Vogel–Tammann–Fulcher law with T0=338 K.

Impulsive stimulated Brillouin scattering in KD 2 PO 4 near the structural phase transition

Impulsive stimulated Brillouin scattering is used to characterize anomalous and highly dispersive acoustic behavior, including heavily overdamped acoustic motion, in KD 2 PO 4 crystals near the structural phase transition temperature T c . Photoelastic and electrooptic effects are observed to diverge near T c , suggesting applications in efficient light modulation and switching.