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The potential of nonlinear optical processes for expanding the spectral range of lasers to new wavelengths was recognized nearly 50 years ago, soon after the invention of the laser.
When exposed to an intense laser beam, a suitable nonlinear optical crystal can generate a new set of wavelengths above or below the input wavelength. This wavelength conversion process is a result of nonlinear dipole oscillations in the crystal induced by the intense laser beam, the same way as nonlinear oscillations of a mechanical spring when subjected to excessive force.
Second Harmonic Generation (SHG) is one of a number of possible nonlinear wavelength conversion processes, where the incident laser wavelength is converted to a shorter wavelength at half the input value. In a simplified photon picture, the process can be viewed as the coming together of two input photons to generate a single output photon at twice the energy (or frequency) or half the wavelength, with the process mediated by the nonlinear dipole oscillations in the crystal. Because of the presence of coherent input photons, SHG is the most efficient of all frequency conversion processes. However, nonlinear optical effects are intrinsically weak compared to laser amplification, and this places stringent demands on the properties on the nonlinear crystal as well as the input laser.
With the availability of new nonlinear crystals and advances in laser technology, the development of novel SHG devices offering exceptional power, efficiency, spectral and spatial beam quality, from the steady-state to ultrafast femtosecond time-scales, has now become a reality, opening up new opportunities for applications of these devices in spectroscopy, quantum information, frequency metrology, optical microscopy, biophotonics and nanoscience.
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