Transcript Ultrafast Experiments - University of Tennessee

Ultrafast Experiments
Hangwen Guo
Solid State II
Department of Physics & Astronomy, The
University of Tennessee
Outline
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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
Q-switching
Measurement of time interval
Longer than 1 ns
Mode locking
Nano-second pulses
Saturable dye absorber
Pico-second pulses
Femto-second pulses
Ti-sapphire laser
Femto-second pulse generation
Face to face configuration of two mirrors
L=30cm
Typical HeNe’s bandwidth of 1.5GHz
∆ν=0.5 GHz
3 longitudinal modes
Mode locking
• each of these modes oscillate independently with random
phase
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• 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 laserinduced 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!