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Ultrafast Experiments

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

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Ultrafast Experiments

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  1. Ultrafast Experiments HangwenGuo Solid State II Department of Physics & Astronomy, The University of Tennessee

  2. Outline • Introduction • Femtosecond pulse generation • Pump-Probe technique • Application • Summary

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

  4. Q-switching Measurement of time interval Longer than 1 ns Nano-second pulses Mode locking Saturable dye absorber Pico-second pulses Femto-second pulses Ti-sapphire laser H2 half-oscillation time—7.6fs

  5. 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

  6. 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

  7. ∆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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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.

  13. Thank you for your attention!

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