Second-harmonic generation (SHG) is a nonlinear optical process where two photons of the same frequency interact with a nonlinear material and combine to form a single photon with twice the frequency (and half the wavelength).
How It Works:
- Requires high-intensity light, typically from a laser.
- Occurs in materials with non-centrosymmetric crystal structures (like BBO or KDP).
- The input photons are usually in the infrared or visible range, and the output is shifted toward the blue/UV.
Example:
- A 1064 nm laser (infrared) passing through a nonlinear crystal can produce 532 nm green light — commonly used in green laser pointers.
Applications:
- Laser frequency conversion.
- Biomedical imaging (e.g., SHG microscopy).
- Optical data storage and telecommunications.
- Quantum optics experiments.
SHG is one of the simplest and most widely used nonlinear optical effects, enabling frequency control and enhancement in many light-based technologies.