Beta Barium Borate (BBO – β-BaB2O4):
- Properties: Wide transparency range from the ultraviolet (about 190 nm) to the infrared (about 3500 nm), high damage threshold, and excellent nonlinear optical coefficients.
- Applications: Frequency doubling, OPOs, and other applications requiring high power and UV generation.
Lithium Niobate (LiNbO3):
- Properties: Strong electro-optic and nonlinear optical effects, wide transparency range, but has a relatively low damage threshold.
- Applications: Widely used in integrated optics as modulators and switches, as well as in terahertz wave generation.
Potassium Titanyl Phosphate (KTP – KTiOPO4):
- Properties: Efficient at room temperature, high optical damage threshold, and effective phase matching properties.
- Applications: Frequency doubling of Nd:YAG lasers, OPOs, and in medical and dental lasers.
Lithium Triborate (LBO – LiB3O5):
- Properties: Broad transparency range, high damage threshold, and suitable for high-power applications.
- Applications: Frequency doubling, especially for high power lasers, and for generating UV light through SHG.
Potassium Dihydrogen Phosphate (KDP – KH2PO4) and its isomorph Potassium Dideuterium Phosphate (KD*P – KD2PO4):
- Properties: High electro-optic coefficients and good UV transmission but lower damage thresholds.
- Applications: Frequency doubling, especially for high-energy lasers like those used in inertial confinement fusion.

Considerations in Choosing Nonlinear Crystals

Phase Matching Conditions: To maximize efficiency, the phase velocity of the interacting waves must be matched, which can be controlled by the crystal’s temperature, angle, or wavelength.
Damage Threshold: The maximum intensity the crystal can withstand without being damaged is critical, especially in high-power laser applications.
Nonlinear Coefficients: Higher coefficients allow for more efficient nonlinear interactions and lower power requirements.
Transparency Range: Determines the spectral range over which the crystal is effective without absorbing too much light.
Hygroscopicity: Some crystals absorb moisture easily, which can degrade optical properties unless properly sealed or treated.

Advancements and Future Prospects

Technological advances continue to develop new synthetic methods and doping techniques to enhance the properties of NLO crystals. These improvements aim to achieve better efficiency, higher damage thresholds, and more robust phase matching capabilities. Emerging applications in quantum computing, secure communications, and ultrafast laser systems are driving research into discovering new materials and optimizing existing ones.

Nonlinear crystals are foundational to numerous photonics applications, allowing for the manipulation of light in ways that are not possible with linear optical materials, expanding the capabilities of modern optical technologies significantly.