Nonlinear optics Totally Explained

Everything about Nonlinear Optics totally explained

Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light. This nonlinearity is typically only observed at very high light intensities such as those provided by pulsed lasers. Nonlinear optics gives rise to a host of optical phenomena:

Frequency mixing processes

Second harmonic generation (SHG), or frequency doubling, generation of light with a doubled frequency (half the wavelength);
Sum frequency generation (SFG), generation of light with a frequency that's the sum of two other frequencies (SHG is a special case of this);
Third harmonic generation (THG), generation of light with a tripled frequency (one-third the wavelength) (usually done in two steps: SHG followed by SFG of original and frequency-doubled waves);
Difference frequency generation (DFG), generation of light with a frequency that's the difference between two other frequencies;
Parametric amplification, amplification of a signal input in the presence of a higher-frequency pump wave, at the same time generating an idler wave (can be considered as DFG);
Parametric oscillation, generation of a signal and idler wave using a parametric amplifier in a resonator (with no signal input);
Parametric generation, like parametric oscillation but without a resonator, using a very high gain instead;
Spontaneous parametric down conversion (SPDC), the amplification of the vacuum fluctuations in the low gain regime;
Optical rectification, generation of quasi-static electric fields.
Four-wave mixing (FWM), can also arise from other nonlinearities.

Other nonlinear processes

Optical Kerr effect, intensity dependent refractive index;

Self-focusing;
Kerr-lens modelocking (KLM).
Self-phase modulation (SPM), a $chi^)$ ;

that is, the generated beam amplitude is the complex conjugate of the signal beam amplitude. Since the imaginary part of the amplitude contains the phase of the beam, this results in the reversal of phase property of the effect.
Note that the constant of proportionality between the signal and conjugate beams can be greater than 1. This is effectively a mirror with a reflection coefficient greater than 100%, producing an amplified reflection. The power for this comes from the two pump beams, which are depleted by the process.
The frequency of the conjugate wave can be different from that of the signal wave. If the pump waves are of frequency ω₁ = ω₂ = ω, and the signal wave higher in frequency such that ω₃ = ω + Δω, then the conjugate wave is of frequency ω₄ = ω - Δω. This is known as frequency flipping.
Common SHG materials

806 nm light : lithium iodate (LiIO₃)

860 nm light : potassium niobate (KNbO₃)

980 nm light : KNbO₃

1064 nm light : monopotassium phosphate (KH₂PO₄, KDP), lithium triborate (LBO) and β-barium borate (BBO).

1300 nm light : GaSe

1319 nm light : KNbO₃, BBO, KDP, potassium titanyl phosphate (KTP), lithium niobate (LiNbO₃), LiIO₃, and Ammonium Dihydrogen Phosphate (ADP)Further Information

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Everything about Nonlinear Optics totally explained

Frequency mixing processes

Other nonlinear processes

Common SHG materials