Photonic crystal devices are uniquely engineered structures that can help control light propagation. These devices are widely applied in various cutting-edge technologies, such as logic gates, lasers, and sensors, to create, manipulate, and detect light. This article delves into the intricacies of the design and fabrication of photonic crystal devices and their potential use in commercial applications.
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What are Photonic Crystals?
Before delving into the intricacies of the design and fabrication of photonic crystal devices, it is crucial to understand the foundational concept of photonic crystals. The concept of photonic crystals was first conceived by the renowned scientist Eli Yablonovitch in 1987.
He suggested that similar to a lattice of atoms that can give forbidden and allowed electronic energies, lattices with low and high refractive indices (RI) can give forbidden and allowed electromagnetic radiation.
The photonic crystals are synthetically composed of macroporous materials with unique optical properties. They have periodic nanostructures, where the wavelength of the period compliments that of visible light (in the order 300–500 nm) and forms a photonic band gap structure. Thus, light of certain wavelengths is prevented from propagating in single or multiple polarization directions within the material.
Photonic Crystal Devices - Working Principle
Photonic crystal devices are applied in light-emitting diodes, laser diodes, solar and photovoltaic cells, displays, optical amplifiers, and other devices for separating light waves of different wavelengths.
The working principle of photonic crystals is similar to that of semiconductors, which aid in the construction of electronic devices. The crystals have a band gap that permits the propagation of light of only a specific wavelength, thus controlling the behavior of light.
Photonic crystals form a diffraction grating, where the scattered light is in the form of Bragg scattering with no shift in frequency, and this scattered light propagates in a direction that is different from the initial direction.
A waveguide layer of finite thickness with embedded photonic crystal structures interacts with a guided light beam, generating both guided light Bragg scattering and out-of-plane Bragg scattered light.
Light media with varying electromagnetic properties result in the formation of a band structure, where the velocity of light changes with frequency, depending on the properties of the medium. Light waves propagated in a periodic medium are termed Bloch waves, and their periodicity matches that of the crystal.
Fabrication of Photonic Crystals Devices and Their Applications
Single and multidimensional photonic crystals have a wide range of applications in which the control of light propagation is crucial. For instance, one-dimensional (1D) crystals, fabricated by spin coating, etching for porous silicon crystals, sol-gel methods, and soft lithography approaches, help regulate reflection and color in paints. These photonic crystals are used in dielectric multilayers for applications such as vertical surface-emitting lasers.
Two-dimensional (2D) crystals integrated via photolithography or hole drilling on substrate materials are used as photonic crystal fibers with characteristics that are different from those of conventional optical fibers. The arrangement of crystals on a material provides great control over the propagation of light and is applied in high-speed communications, fiber lasers, and power transmissions.
Three-dimensional (3D) crystal-integrated materials are prepared via angled drilling, laser writing, layer stacking, or sphere self-assembly. However, the challenges in fabricating these complex structures hinder their commercial application.
Precise design and robust fabrication methods are crucial for realizing such structures. Additionally, fine-tuning the physical properties of the substrate material can lead to the invention of new applications in sensor technology.
An article published in Nature Communications reported the fabrication of a photonic crystal device to amplify nonlinear optomechanical measurement of mechanical motion, which is often weak, using linked optical and mechanical modes in the photonic crystal device.
The same study also demonstrated optomechanical measurements under reduced power input and explained their correlation. This approach realized robust mechanical coupling by harnessing the elasticity of the material without altering the elasticity of the device. The researchers of the study foresee the application of the present method in two-phonon heralding, multimode phonon lasing, and in nonlinear quantum optomechanics.
Another article published in Advanced Materials reported the design and fabrication of a rectangular 3D composite photonic-crystal-based optoelectronic device to detect ultratrace analytes. In this study, a droplet evaporation platform was developed on a robust superhydrophobic mesh that was placed in a silicone oil bath.
The 3D photonic crystals could increase in size by transitioning from spherical to ellipsoidal shapes upon increasing the volume of droplets containing silica nanoparticle dispersions. The signal-to-noise ratio of the designed photonic-crystal-based device increased from 30-40 dB to approximately 60-70 dB. The device exhibited the advantages of compact, low cost, and high reliability, demonstrating its applicability in portable optoelectronic devices.
A recently accepted article which is set to be published in Measurement reported the construction of a 2D photonic crystal-based biosensor to detect different types of cancer cells, including HeLa, PC12, MDA, MCF, and Jurkat. The sensor was designed using a silicon-on-insulator (SOI) substrate with a triangular lattice.
The designed 2D photonic crystal-based biosensor was integrated with machine learning techniques to enable intelligent and precise investigation of optical data, thus facilitating accurate and precise detection of cancer cells.
Scope for Future
With progress in nanoprocessing technology over the next decade, researchers in the field anticipate a substantial improvement in the designing of devices with enhanced accuracy and precision. They foresee that 2D photonic crystals have immense potential to contribute to the enhancement of silicon-based systems for their integration into electronic circuits.
Furthermore, combining optical and electrical circuits can contribute to the optical switching, tuning, and delay capabilities in the designed devices. Undoubtedly, these robust devices will be compact, powerful, and cost-effective alternatives to conventional devices based only on electric circuits.
Overall, photonic crystal devices are uniquely engineered structures used to control the propagation of light. These devices are prepared by integrating photonic crystals on a dielectric material and by leveraging the unique optical properties of photonic crystals. Modulation of the refractive index in photonic crystals influences the manipulation of light propagation.
Based on the modulation dimension of the substrate material, 1D, 2D, and 3D photonic crystals can be fabricated and utilized for the intended application. Although photonic crystals have proven to be promising candidates for applications in sensor and optical technologies, their intricate fabrication and precision challenges impede the realization of their full potential.
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References and Further Reading
Gangwar, R. K., Pathak, A. K., Kumar, S. (2023). Recent Progress in Photonic Crystal Devices and Their Applications: A Review. In Photonics, 10 (11), 1199). https://doi.org/10.3390/photonics10111199
Burgwal, R., Verhagen, E. (2023). Enhanced nonlinear optomechanics in a coupled-mode photonic crystal device. Nature Communications, 14(1), 1526. https://doi.org/10.1038/s41467-023-37138-z
Hou, Y., Yuan, S., Zhu, G., You, B., Xu, Y., Jiang, W., Wang, L. (2023). Photonic Crystal‐Integrated Optoelectronic Devices with Naked‐Eye Visualization and Digital Readout for High‐Resolution Detection of Ultratrace Analytes. Advanced Materials, 35(7). https://doi.org/10.1002/adma.202209004
Balaji, V. R., Jahan, M. I., Sridarshini, T., Geerthana, S., Thirumurugan, A., Hegde, G., Dhanabalan, S. S. (2023). Machine Learning enabled 2D Photonic Crystal biosensor for Early Cancer Detection. Measurement, 113858. https://doi.org/10.1016/j.measurement.2023.113858