Second-harmonic generation (SHG)

What is Second-Harmonic Generation (SHG)?

Second-harmonic generation (SHG) is a nonlinear optical phenomenon, where two photons with the same frequency interact with a medium, producing a photon with twice the frequency (half the wavelength) and twice the energy of the incident photons. SHG occurs only in materials that lack inversion symmetry, such as crystals, and is a useful tool for investigating the structure and properties of materials.

SHG has several advantages over linear optical techniques, such as fluorescence, Raman scattering, and absorption, as it has higher sensitivity, spatial resolution, and selectivity. Thus, SHG has found widespread applications in various fields, such as materials science, biology, medicine, and spectroscopy.

How Does SHG Work?

The SHG process involves the coherent superposition of two waves, where the electric field of one wave induces a polarization in the medium, which, in turn, generates a second wave with twice the frequency. The magnitude and direction of the second wave depend on the polarization and symmetry of the medium, and the incident angle and polarization of the input wave.

The SHG efficiency depends on several factors, such as the material properties, the wavelength and intensity of the input wave, the phase matching condition, and the confinement of the input wave. The phase matching condition refers to the requirement that the phase velocities of the input and output waves must be equal, which can be achieved by adjusting the angle and wavelength of the input wave or by using birefringent crystals.

The SHG signal can be detected by measuring the intensity, polarization, or spectrum of the output wave using various techniques, such as photodiodes, cameras, microscopes, and spectrometers.

Applications of SHG: Examples

SHG has numerous applications in various fields, such as:

  • Material characterization: SHG can probe the surface and bulk properties of materials, such as crystal structure, defects, interfaces, and orientation, with high sensitivity and resolution. SHG has been used to study semiconductors, metals, polymers, ceramics, and biological tissues.
  • Nonlinear optics: SHG is a fundamental process in nonlinear optics, which encompasses a wide range of phenomena, such as frequency conversion, optical parametric amplification, harmonic generation, and soliton dynamics. Nonlinear optics has important applications in telecommunications, spectroscopy, microscopy, and laser technology.
  • Biomedical imaging: SHG can image biological tissues without the need for labeling, as it can selectively image collagen, which is abundant in various tissues, such as skin, cornea, bone, and cartilage. SHG has been used for diagnosis and monitoring of diseases, such as cancer, fibrosis, and osteoporosis.
  • Quantum optics: SHG can generate entangled photon pairs, which are essential for quantum communication, cryptography, and computation. SHG has been used to demonstrate various quantum phenomena, such as quantum teleportation, entanglement swapping, and Bell inequality violation.

Current Research and Future Developments in SHG

SHG is an active area of research, with many recent advances and ongoing investigations. Some of the current research topics and future directions in SHG include:

  • Novel materials: Researchers are developing new materials with enhanced SHG properties, such as organic molecules, nanoparticles, and metasurfaces. These materials could lead to new applications in nanophotonics, plasmonics, and quantum optics.
  • Ultrafast dynamics: Researchers are studying the ultrafast dynamics of SHG, which can reveal the electronic and phononic properties of materials on femtosecond time scales. These studies could lead to new insights into the fundamental mechanisms of nonlinear optics and materials science.
  • Multimodal imaging: Researchers are combining SHG with other imaging techniques, such as two-photon fluorescence, third-harmonic generation, and coherence tomography, to achieve multimodal imaging with complementary contrast mechanisms. These techniques could improve the sensitivity, specificity, and accuracy of biomedical imaging and diagnosis.
  • Quantum applications: Researchers are exploring new applications of SHG in quantum information processing, such as quantum gates, quantum memories, and quantum repeaters. These applications could enable practical quantum communication and computation, which have important implications for cryptography, security, and simulation.