Editorial Feature

What to Know About Nonlinear Optical Materials

Since the emergence of nonlinear optics in the 1960s, significant progress has been made in photonic technologies and their applications. This has led to the development of several materials that have nonlinear interactions with matter. This article delves into nonlinear optical materials by examining their properties and applications.

Nonlinear Optical Materials, Nonlinear Optics

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What is Nonlinear Optics and Where is it Applied?

In linear optics, light directly influences the medium, leading to phenomena such as reflection, refraction, and transmission. Alternatively, nonlinear optics involves nonlinear interactions between the light and the medium. In other words, the response of the medium depends on the wavelength, intensity, direction, and polarization of the light.

Nonlinear optics have found their application in the following sectors:

  1. Telecommunications: Nonlinear optics had limited data transmission capability in the early days of fiber-optic telecommunications, especially in underwater applications. However, in recent years, nonlinear optics has been recognized to play a crucial role in creating advanced fiber-optic systems. 
  1. Lasers and Coherent Light Diversification: New laser types and applications can be developed by incorporating components that support nonlinear optics. The incorporation of this component inside the laser cavity enables fine tuning of the timing and color of the laser light. On the other hand, its incorporation outside the laser cavity allows for alterations in the coherence of emitted light. 
  1. Optical Imaging: Nonlinear optics is used to study pharmaceuticals, offering several benefits, including non-invasive chemical analysis and providing high-quality images in terms of time and space.  It also facilitates its use in biological and water-based environments without the need for special labels or compatibility considerations.

Nonlinear Optical Materials 

Nonlinear optical materials can alter the wavelength and frequency of laser light, making it possible to access the unused electromagnetic spectrum. Understanding the unique properties of these materials, such as their ability to change light intensity, durability, and adaptability to various technological needs, has enhanced their significance in the field of optics.

The nonlinear optical materials provide access to the infrared  (IR) region, creating opportunities for laser-guided targeting systems, IR sensors, vehicle protection systems (VPS), and others.

The nonlinear optical materials are characterized by their nonlinear coefficients, such as optical switching, frequency conversion, and harmonic generation. These coefficients determine the efficiency of the nonlinear optical process of the material.

Based on the types of cohesive forces that bind the charges and polarization together, the composition of nonlinear optical materials can be classified into three categories:

  • Ionic crystals consisting of oxygen-polyhedron-based solids
  • Covalent crystals found in semiconductors.
  • Molecular crystals are related to organic materials, including disordered and shapeless solids, such as glasses and polymers.

How do Nonlinear Optical Materials Work?

Nonlinear optical materials commonly used are optical crystals, wherein the electric field of light interacts with the crystal lattice in a nonlinear manner, which occurs only under very intense irradiation, such as from a laser. This can be used to shift the laser wavelength to the spectral range for practical applications.

For efficient conversion, the crystal must:

  • Have a nonlinear d-coefficient that is sufficiently large to generate tunable wavelengths over a broad range.
  • Have a nonzero nonlinearity.
  • Match different phases.
  • Be highly transparent at the required input and output wavelengths.

An "ideal" nonlinear optical material does not exist. The nonlinear process that is involved determines the applicability of the material. For nonlinear optical materials with favorable features such as large nonlinearity, high damage threshold, and appropriate crystal growth habits, its application depends on its ability to utilize the crystal efficiently.

Organic Nonlinear Optical Materials

Conjugated donor-acceptor (DA)-substituted organic molecules exhibit significant nonlinear optical and electro-optical effects, indicating their potential application in telecommunication systems and optical computing. Ionic crystal-based nonlinear optical materials modify the frequency of the laser light, facilitating fast data processing tasks, including optical data storage and information processing.

Furthermore, photorefractive polymers are cost-effective alternatives to inorganic crystals, which facilitate the creation of dynamic holograms through the interaction of light-induced charges and second-order optical nonlinearity.

Organic compounds with delocalized electrons exhibit excellent nonlinear optical properties and quick responses. Organometallic and coordination complex materials exhibit nonlinear optical behaviors suitable for photonic applications.

Inorganic Nonlinear Optical Materials

Nonlinearity in inorganic optical materials arises from the non-association of electrons with the nuclei, as found in metals and semiconductors. The nonlinearity of these materials can be analyzed by determining the electronic characteristics of the bulk medium.

The element aluminum (Al) with electronic configuration  [Ne]3s23p1 has an empty d orbital and can form sp3, sp3d, and sp3d2 hybrid orbitals upon bonding with oxygen and fluorine atoms, leading to the formation of [AlOmFn] units.

Ultraviolet (UV) and deep-UV nonlinear optical materials are crucial for converting a laser’s wavelength to produce UV or deep-UV coherent light in the laser industry. In this regard, many aluminoborates and fluoroaluminoborates are used as next-generation UV/DUV nonlinear optical materials.

Recent Studies

An article published in Materials Today Physics reported the structural characteristics and nonlinear properties of defective diamond-like MGa2Te4 (M = Zn, Cd). Researchers have reported that these compounds crystallized in the non-centrosymmetric I-4 space group with a three-dimensional (3D) structure. These crystals demonstrate their potential application in mid- and far-infrared (MFIR) nonlinear optical fields.

Another article published in the journal Small reported the fabrication of Zn2HgP2S8 (ZHPS) with a diamond-like structure. This compound was reported to have a wide bandgap, exhibited an IR nonlinear optical response, and showed a good balance between bandgap and nonlinear optical effect for an excellent IR nonlinear material.

A recent study published in Angewandte Chemie reported a robust strategy for unbiased high-throughput screening of more than 140, 000 materials to explore IR nonlinear materials with wide bandgaps and high thermal conductivities that are crucial to the intrinsic laser damage threshold.

This strategy screened 106 compounds with nonlinear optical properties, desired thermal conductivities, and band gaps. The report mentioned the high thermal conductivity of nitrides over chalcogenides.

Conclusion

Overall, nonlinear optical materials are crucial in the field of photonics and optics and enable their application in various cutting-edge technologies. Their distinct properties enable these materials to manipulate light and enhance the functionality and efficiency of the optical devices.

Hence, understanding the properties, functionality, and applicability of nonlinear optical materials will help in the innovation and development of various next-generation optical technologies in the fields of medical diagnostics, telecommunications, and laser technologies.     

More from AZoOptics: Applications of Ultrafast Lasers in Materials Science

References and Further Reading 

Arivuoli, D. (2001). Fundamentals of nonlinear optical materials. Pramana, 57, 871-883.

What are Non-Linear Optical Materials? Accessed on 29 October. 

Jiao, J., Zhang, M., & Pan, S. (2023). Aluminoborates as nonlinear optical materials. Angewandte Chemie, 135(7).  https://doi.org/10.1002/ange.202217037  

Sun, M., Li, C., Shi, J., Lee, M. H., & Yao, J. (2023). Defect diamond-like tellurides as infrared nonlinear optical materials with a giant second-harmonic generation tensor. Materials Today Physics, 36, 101166. https://doi.org/10.1016/j.mtphys.2023.101166

Zhou, W., Wu, J., Liu, W., & Guo, S. P. (2023). Ag-based chalcogenides and derivatives as promising infrared nonlinear optical materials. Coordination Chemistry Reviews, 477, 214950. https://doi.org/10.1016/j.ccr.2022.214950   

Chu, D., Huang, Y et al. (2023). Unbiased Screening of Novel Infrared Nonlinear Optical Materials with High Thermal Conductivity: Long‐neglected Nitrides and Popular Chalcogenides. Angewandte Chemie, 135(16), e202300581. https://doi.org/10.1002/ange.202300581

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Bhavna Kaveti

Written by

Bhavna Kaveti

Bhavna Kaveti is a science writer based in Hyderabad, India. She has a Masters in Pharmaceutical Chemistry from Vellore Institute of Technology, India, and a Ph.D. in Organic and Medicinal Chemistry from Universidad de Guanajuato, Mexico. Her research work involved designing and synthesizing heterocycle-based bioactive molecules, where she had exposure to both multistep and multicomponent synthesis. During her doctoral studies, she worked on synthesizing various linked and fused heterocycle-based peptidomimetic molecules that are anticipated to have a bioactive potential for further functionalization. While working on her thesis and research papers, she explored her passion for scientific writing and communications.

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