12.2 Types and fabrication of photonic crystals

Photonic crystals are materials with periodic structures that control light propagation. They come in 1D, 2D, and 3D forms, each offering unique ways to manipulate light. The crystal's structure creates a photonic bandgap, preventing certain wavelengths from traveling through.

Fabricating photonic crystals involves two main approaches: self-assembly and top-down methods. Self-assembly uses natural forces to create structures, while top-down techniques like lithography offer precise control. Each method has its own strengths and limitations in creating these light-controlling materials.

Types of Photonic Crystals

Dimensionality and Structures

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• 1D photonic crystals consist of alternating layers of materials with different refractive indices, creating a periodic structure in one dimension
• 2D photonic crystals have a periodic structure in two dimensions, typically formed by arranging dielectric rods or holes in a dielectric material
• 3D photonic crystals exhibit a periodic structure in all three dimensions, offering the most complete control over light propagation
• Opal structures are a type of 3D photonic crystal inspired by natural opal gemstones, consisting of close-packed arrays of dielectric spheres (silica, polystyrene)
• Inverse opal structures are created by infiltrating an opal structure with a high-refractive-index material and then removing the original spheres, leaving a connected network of air voids

Bandgap and Light Manipulation

• Photonic crystals are characterized by their photonic bandgap, a range of frequencies in which light propagation is prohibited
• The photonic bandgap arises from the periodic modulation of the refractive index, leading to destructive interference of certain wavelengths
• By carefully designing the photonic crystal structure, the bandgap can be engineered to control and manipulate light propagation
• Photonic crystals can be used to create waveguides that confine and guide light along specific paths (line defects in 2D crystals, channels in 3D crystals)
• Photonic crystal fibers, made by arranging air holes in a periodic pattern within a dielectric material, can guide light with low loss and unique dispersion properties

Fabrication Techniques

Self-Assembly Methods

• Self-assembly techniques rely on the spontaneous organization of building blocks into ordered structures driven by intermolecular forces and surface interactions
• Colloidal self-assembly involves the organization of colloidal particles (silica, polystyrene) into close-packed arrays, forming opal structures
• Langmuir-Blodgett deposition allows the creation of multilayer films by transferring a monolayer of amphiphilic molecules from a liquid surface onto a solid substrate
• Block copolymer self-assembly utilizes the microphase separation of immiscible polymer blocks to form periodic nanostructures (lamellar, cylindrical, or spherical domains)
• Self-assembly methods offer scalability and cost-effectiveness but may have limitations in terms of structural control and defect density

Top-Down Fabrication

• Lithography techniques, such as photolithography and electron beam lithography, involve patterning a resist layer and transferring the pattern to the underlying material
• Photolithography uses light and a photomask to selectively expose a photoresist, enabling the creation of 2D photonic crystal patterns
• Electron beam lithography offers higher resolution than photolithography by using a focused electron beam to directly write patterns in an electron-sensitive resist
• Etching processes, such as reactive ion etching (RIE) and focused ion beam (FIB) milling, are used to transfer the lithographically defined patterns into the desired material
• Layer-by-layer fabrication, such as chemical vapor deposition (CVD) and atomic layer deposition (ALD), enables the precise control of layer thickness and composition for creating 1D photonic crystals
• Top-down fabrication methods provide high precision and control over the photonic crystal structure but may be limited in terms of throughput and cost