basics_of_femtosecond_laser___spectroscopy(文件)
【正文】 eraction, I – laser light intensity, A – geometrical factor (usually unknown) TPA PL technique requires a reference measurement 22r e fr e f P L r e fr e fPLcI BcI?bb???22refnBn?for collimated beams Good reference materials: laser dyes (Fluorescein, Rhodamin, Coumarin) C. Xu and W. W. Webb, J. of Am. Opt. Soc. 13, 481 (1996) TPA Measurements in NonFluorescent Materials ZScan Technique z Sample Lens Transmission of the sample Intensity of light Open aperture Zscan, TPA measurements ZScan Measurements of Kerr’s NonLinearity Sample Aperture Z Z DT Closed aperture Zscan 22on n n I??? Kerr lens focuses or defocuses light clipped by the aperture thus modulating its transmission Summary on Zscan Cons: ? Zscan works if the thickness of the sample is much smaller than the beam’s waist length. ? Data processing apparatus relies on the Gaussian profile of the beam. Very accurate characterization of the pump beam is required. ? Requires high energy pump pulses as well as high concentration of TPA absorber in order to achieve reasonable accuracy of the data. ? Artifacts are possible due to longliving excited state absorption. Pros: ? Works with nonfluorescent materials ? Allows one to measure real part of highordrer susceptabilities M. SheikBahae et al, IEEE J. of Quantum Electronics, 26(4), p. 760 (1990) TPA Applications ? 3D optical memory ? 3D holographic gratings and photonic structures ? Remote sensing and hires imaging TPA Microfabrication . Cumpston et al., Nature 398, p. 51 (1999) a. Photonic crystal b. Magnified view of (a) c. Tapered waveguide d. Array of cantilevers TPA Imaging ~l Single photon imaging lTwo photon imaging (works even under the surface!) Image from Heidelberg University website TimeResolved Emission Spectroscopy 1ps 1ns 1ms 1ms PL upconversion Timecorrelated photon counting techniques Singleshot measurements Dt SingleShot PL Decay Measurement Beamsplitter Sample Spectrometer Digital Oscilloscope Emission Pickup Optics And Filters From a pulsed laser (YAG:Nd, Ti:Sapphire) PMT Trigger Fast Photodiode Signal PC ? Temporal resolution is limited by the detector(~20 ns) ? Works the best on amplified laser systems. ? Can collect the data in 1 shot of the laser. (In macroscopic systems) TimeResolved Luminescence Experiments TimeCorrelated Single Photon Counting (TCSPC) Spectrometer Correlator PC “Stop” “Start” Filter Sample Beamsplitter MCP PMT Fast PD 360470 nm 100fs Dt “Start” “Stop” Laser Luminescence Dt N Histogram Erdman R., “Time Correlated Single Photon Counting amp。Dt ≈ 2p Dw ≈ 2p/Dt Large spectral bandwidth for short pulses Dl ≈ l2 /(cDt) Dl ≈ 21 nm for 100 fs pulses with l0 = 800 nm Wavelength fs pulse ns pulse Large bandwidth limits the choice of the laser active medium (broadband materials only,
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