Second-harmonic generation (Frequency Doubling)

Second-harmonic generation (SHG), also known as Frequency doubling, is a process where two photons of the same frequency interact within a nonlinear material to produce a new photon with double the energy, or equivalently, double the frequency and half the wavelength. This interaction is a fundamental nonlinear optical phenomenon that happens in different systems such as optical, radio, and atmospheric systems, and is key for applications like increasing laser frequencies. SHG maintains the coherence of the initial light, making it a notable example of how waves can interact and transform in non-linear environments.

The ability of a material to facilitate SHG is described by its second-order nonlinear susceptibility. However, SHG does not occur in materials with inversion symmetry, except under certain conditions like the Bloch–Siegert shift, which can induce SHG in systems with symmetric properties. Additionally, SHG is generally not observed in certain crystal structures, with a few exceptions under specific conditions.

Efficiency in converting light energy to the second-harmonic frequency can vary widely. In some scenarios, nearly all the light energy can be converted using intense laser pulses and large crystals with precise alignment for phase matching. In other applications, such as second-harmonic imaging microscopy, only a small fraction of light is converted to the second harmonic, yet it can still be detected with specialized optical filters.

1064nm Laser frequecy doubling to 532nm green laser Application Case

For practical applications of frequency doubling, you can click here to view specific case studies on the use of DPSS lasers for frequency doubling to produce 532nm green lasers.