Ultrafast Experiments Hangwen Guo Solid State II Department of Physics & Astronomy, The University of Tennessee

Outline Introduction Femtosecond pulse generation Pump-Probe technique Application Summary

Introduction Advantage of Laser: Long coherent length Narrow spectral bandwidth Intrinsically well-defined polarization High irradiance Produce extremely short pulses

Measurement of time intervalLonger than 1 ns Q-switching Nano-second pulses Mode locking Pico-second pulses Saturable dye absorber Ti-sapphire laser Femto-second pulses

Femto-second pulse generation Face to face configuration of two mirrors L=30cm Typical HeNe’s bandwidth of 1.5GHz 3 longitudinal modes ∆ν=0.5 GHz

Mode locking each of these modes oscillate independently with random phase each mode operates with a fixed phase locked to the same value----constructively interfere with each other----producing an intense pulse of light

∆ν G ---- Gain bandwidth---- determines the pulse length The gain profile of Ti-sapphire is the broadest yet discovered, ranging from 700 nm to 1000 nm and most efficient around 800 nm modes ---- generate pulses at order of femtosecond

The Pump-Probe Techniques Characteristic timescales of electronic and lattice dynamics are in the fs to ps region, thus an all-optical technique is needed for measurement

Since photo-induced changes are relatively small (10 −6 to 10 −3 ), therefore conventional lock-in detection techniques are often used to extract the small relative change of ΔR/R The detector output, being some time-average of the probe pulses, is sent to a lock-in amplifier, which only collects signal at exact frequency and phase of the pump beam modulator Only changes induced by pump are thus recorded

Further Application Except the wide use of laser pump with laser probe techniques, the technique using x-ray and electrons as probe is developed: X-rays have a wavelength approximately equal to the distances between atoms, and hence enable atomic movements to be visualized directly Electrons are less damaging to specimens The scattering length of electrons better matches the optical penetration depths “pumped” volume of most samples

Experimental Application Using pump-probe technique to measure the laser- induced melting of polycrystalline Al maxima in G(r) give the most probable inter-atomic distances present in the sample

Summary Ultrafast laser systems have already reached the performance, stability and ease of use to make them part of many optical laboratories around the globe. Many problems in physics, chemistry, biology and also information technology can greatly benefit from femtosecond time resolution being the “ultimate” time-scale of the dynamics observed. Many new and surprising phenomena are also being discovered along the way.

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