Ultrafast Experiments

Hangwen GuoSolid 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 interval Longer than 1 nsQ-switching

Nano-second pulses

Mode lockingPico-second pulses

Saturable dye absorber

Ti-sapphire laserFemto-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

• ∆νG---- 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---- 250000 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.

Thank you for your attention!