Second harmonic generation (also called frequency doubling or abbreviated SHG) is a nonlinear optical process, in which photons with the same frequency interacting with a nonlinear material are effectively “combined” to generate new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons. Second harmonic generation, as an even-order nonlinear optical effect, is only allowed in media without inversion symmetry. It is a special case of sum frequency generation and is the inverse of half-harmonic generation.
Second harmonic generation was first demonstrated by Peter Franken, A. E. Hill, C. W. Peters, and G. Weinreich at the University of Michigan, Ann Arbor, in 1961. The demonstration was made possible by the invention of the laser, which created the required high intensity coherent light. They focused a ruby laser with a wavelength of 694 nm into a quartz sample. They sent the output light through a spectrometer, recording the spectrum on photographic paper, which indicated the production of light at 347 nm. Famously, when published in the journal Physical Review Letters, the copy editor mistook the dim spot (at 347 nm) on the photographic paper as a speck of dirt and removed it from the publication. The formulation of SHG was initially described by N. Bloembergen and P. S. Pershan at Harvard in 1962. In their extensive evaluation of Maxwell’s equations at the planar interface between a linear and nonlinear medium, several rules for the interaction of light in non-linear mediums were elucidated.
Generating the second harmonic, often called frequency doubling, is also a process in radio communication; it was developed early in the 20th century, and has been used with frequencies in the megahertz range. It is a special case of frequency multiplication.
A-Star?BBO Crystal Second Harmonic Generation
When an intense electromagnetic wave of frequency ω impinges on an overcritical plasma, higher harmonics nω, n = 2, 3, … can be observed in the spectrum of the reflected radiation. Harmonic generation was investigated in a number of experiments on laser-produced plasmas /l — 1 5/ and at the interaction of strong microwaves with plasmas /1.6. 17/. Among them, there are time and space resolved measurements /10 — 12/ and the observation of backscattered har-monies up to 5ω /13/ and 11ω /12, 14/, respectively, at laser intensities with Iλ 2 of the order of 10 16 W /um 2 /cm 2. Recently published results describe observations up to the 46th harmonic at high power CO 2 laser facilities /18, 19/. Since the harmonic radiation is created mainly near the critical density, it is of importance for the diagnostics of processes taking place at the critical sheath /20–25/. So. it can help us understand the mechanisms involved in the absorption of laser radiation.
BBO Crystal Second Harmonic Generation