超快光学-第08章-非线性二阶效应.ppt

* * * * * NOPA specs Crystals for far-IR generation With unusual crystals, such as AgGaS2, AgGaSe2 or GaSe, one can obtain radiation to wavelengths as long as 20?mm. These long wavelengths are useful for vibrational spectroscopy. Gavin D. Reid, University of Leeds, and Klaas Wynne, University of Strathclyde 10 mm 1 mm Wavelength Elsaesser, et al., Opt. Lett., 23, 861 (1998) Difference-frequency generation in GaSe Angle-tuned wavelength Another 2nd-order process: Electro-optics Applying a voltage to a crystal changes its refractive indices and introduces birefringence. In a sense, this is sum-frequency generation with a beam of zero frequency (but not zero field!). A few kV can turn a crystal into a half- or quarter-wave plate. V If V = 0, the pulse polarization doesn’t change. If V = Vp , the pulse polarization switches to its orthogonal state. Abruptly switching a Pockels cell allows us to switch a pulse into or out of a laser. “Pockels cell” (voltage may be transverse or longitudinal) Polarizer * * Figure borrowed from Margaret Murnane’s HHG talk. * * * * * * * * * * * * * * * * * * * Drawing courtesy of George Stegeman of CREOL. * * * * * * * Second-order nonlinear-optical effects Symmetry issues Phase-matching in SHG Phase-matching bandwidth Group-velocity mismatch Nonlinear-optical crystals Practical numbers for SHG Electro-optics Difference-frequency generation and optical parametric generation First demonstration of second-harmonic generation P.A. Franken, et al, Physical Review Letters 7, p. 118 (1961) The second-harmonic beam was very weak because the process was not phase-matched. First demonstration of SHG: the data The actual published results… Input beam The second harmonic Note that the very weak spot due to the second harmonic is missing. It was removed by an overzealous Physical Review Letters editor, who thought it was a speck of dirt and didn’t ask the authors first. Symmetry in second-harmonic generation For this to hold, c(2) must be zero f