2.1.

Film Deposition

The thin-film deposition technique mainly includes the processing above the surface of the substrate by adding material to the substrate either in the form of structured layers or as spacers so that later it can be removed.^50,51 Depending on the substrate structure, operating temperature, and rate of deposition chiefly two types of deposition techniques are used namely chemical deposition technique and physical deposition technique. In chemical deposition, a chemical reaction takes place between the hot substrate and inert gases present in the gas chamber at low or atmospheric pressure and thus films are deposited on the substrate.⁵² The technique can be further classified into several categories, e.g. CVD, atomic layer deposition (ALD), spin coating, etc. In the case of physical deposition, the raw materials are released and moved physically onto the upper surface of the substrate in the form of a solid, liquid, or vapor. Some common examples of physical deposition are thermal evaporation, MBE, and sputtering. The film deposition process is followed by lithographic techniques to pattern the sample and etching to release the final structure.

2.2.

Pattern Defining

To define the desired pattern on the substrate, the lithographic technique is mostly used. Alloys Senefelder invented the lithography technique in 1796.^53,54 Several lithographic techniques used for PhC fabrication are summarized in Table 1 in chronological order.

Table 1

Lithographic techniques used for PhC fabrication.

2.2.1.

Electron-beam lithography (EBL)

Among the several lithographic techniques, the first lithographic technique for the fabrication of PhC was e-beam lithography, which was used by Cheng and Scherer.⁵⁵ Here the combination of wet and dry etching technologies was used to fabricate 3D PBG structures in the optical wavelength range. A hexagonal hole pattern was created on the surface of the sample using electron-beam lithography with a 30 kV incident beam energy. Later, also in 2003, the electron-beam lithography technique was used for structuring 3D self-assembled PhCs.⁵⁶ After that, several research papers have been published on PhC fabrication using e-beam lithography.^57–62

Fig. 3

Fabrication steps of PhC.

In EBL, a focused beam of electrons is used to pattern a resist layer on the substrate.^63,64 The resist layer is sensitive to electron beam (e-beam) and consists of organic polymer material. Resist can be classified into two types (positive resist and negative resist) depending on the chemical response to the electron exposure (Fig. 4).^65–67 In the case of positive resist, the resist becomes soluble to the resist developer when exposed to the e-beam and in the case of negative resist, the portion of the resist layer becomes insoluble to the resist developer when exposed to the e-beam. The sensitivity of the lithography depends on many factors, such as resist material, the thickness of the resist layer, the substrate material, and also the developer used in the process. E-beam lithography has a higher resolution than photolithography because it is not limited by diffraction and has a very high resolution of nm scale in complex patterns. It also allows multiple designs to be fabricated on a single wafer.

Fig. 4

Lithography technique with positive and negative resist.⁶³

A process of fabrication of defects such as waveguides and resonators embedded in a 3D PhC using EBL was proposed by Murakowski et al.⁶⁸ as shown in Fig. 5. Scanning electron micrograph of the 3D PhC is shown in Fig. 6.

Fig. 5

Waveguide patterned in a 3D PhC.⁶⁸

Fig. 6

Scanning electron micrograph of the 3D PhC.⁶⁸

2.2.2.

X-ray lithography

In the x-ray lithography technique, the pattern is defined using a special type of mask with different local x-ray absorption. After being placed on a substrate, this design is reproduced on an x-ray-sensitive substance known as a resist (usually a silicon wafer). Depending on the chemical nature of the resist, cross-linking (for negative resists), or bond breaking (for positive resists) may occur when the x-ray traveling through the pattern strikes the resist. Following exposure, the entire object is submerged in a certain solvent, which, depending on the nature of the solvent, either causes the exposed region to resist to dissolve and produce a pattern or the opposite. The remaining portion of the resistance will remain unaltered. This is how nano-patterns are produced on the substrate via x-ray lithography. The block diagram of the production of nano-patterns is shown in Fig. 7.

Fig. 7

Block diagram of production of nano-patterns using x-ray lithography.

In 1997, deep x-ray lithography was used by researchers to fabricate PhC-based devices.⁶⁹ Multiple tilted x-ray lithography exposures were used to construct structures with photonic band gaps in the infrared region. In another research paper, three consecutive exposures of polymethyl methacrylate (PMMA) resist were used to an x-ray beam through a triangular lattice of holes.⁷⁰ Later, various researchers used deep x-ray lithography to create 2D and 3D PhC.^71,72

2.3.

Pattern Transfer

In the pattern transfer technique, the pattern drawn on the resist material through the lithography technique is transferred to the hard mask, usually by the reactive ion etching (RIE) technique, and finally, deep etching is performed using RIE, inductively coupled plasma (ICP) etching, etc.

Etching is a process in which a layer is chemically removed from the surface of a wafer during the manufacturing process. The wafer is immersed in an etching solution and then rinsed clean with hot deionized water. Precautions should be taken so that no etching reactant remains in the sample after the cleaning process as this may lead to a defective sample. Etching can be further classified into two broad categories: liquid phase or wet etching and plasma phase or dry etching. By monitoring the resistivity of the deionized water, the rinsing time can be determined. If the resistivity of the rinsing water is indifferent to its previous value, then the sample can be considered clean.

Etch rate mainly depends on two factors: saturation of the etchant bath and the chemical concentration. The etch rate can be affected by how long the etchant bath has been sitting, and/or how much material has been etched in the bath. Etch rate and other properties of the etchant depend very strongly on the ratios and concentrations of the chemicals in the etchant.

2.4.

Etchless Fabrication Method of PhC

Some researchers demonstrated etchless silicon slab fabrication.³³ The method is followed by two steps; in the first step, a suspended organic membrane is created using direct-write EBL and in the next step, the organic membrane is coated with silicon to create a high contrast of RI.

For the creation of the organic membrane, at first, the substrate material is coated with a thick layer of MMA/MAA copolymer by spinning the liquid solution of the copolymer at a specific speed for a specific time. Then the structure is heated at a specific temperature to evaporate the solvent. Then again, the silicon substrate is coated with a comparatively thin PMMA layer by spinning the PMMA solution and baking out the solvent at a particular temperature. Now the silicon substrate is coated with copolymer and has the PMMA layer on top. Then the sample is exposed to EBL to pattern the PhC. As both the MAA and PMMA layers are sensitive to e-beam, the electrons can easily perforate through the MAA layer, and the PMMA layer underneath the MAA layer is exposed to the e-beams and thus the polymer chains break due to the interaction with the electrons. So when the sample is immersed in the methyl isobutyl ketone (MIBK) solution, then the regions of the sample exposed to the e-beam dissolve in the solution. As the MAA layer has a higher cessation rate than PMMA, it dissolves faster than PMMA as soon as holes open up in the PMMA layer. As a consequence, an expansive region of MAA is vulnerable to the MIBK solution, The PMMA is undercut, and a suspended perforated membrane is created. Different steps associated with the fabrication technique are described in Fig. 8.

Fig. 8

Steps of etchless fabrication technique of PhC.

The RI of PMMA is not sufficient to create a sufficient PBG. So in the second step of this fabrication method, silicon is coated on the PMMA membrane by DC sputter coating.

2.5.

Suspended PhC Fabrication Using Bulk Wafers

In this process, at first, a layer of silicon oxide is grown-up thermally on the double-side polished single-crystal silicon (SCS) wafer using an oxidation furnace. Then, using photolithography and the lift-off technique, the layer of photoresist is patterned and placed on top of the silicon oxide layer. The subsequent etch in the oxide layer is caused by the photoresist layer. Thus, the required design is transplanted to the oxide layer and consequently on the silicon substrate by dry etching in an RIE system. The thickness of the required PhC slab is determined by the depth of the dry etching. After removing the photoresist layer, the sample is dry oxidized to create a silicon dioxide layer on the sample in order to avoid contamination with the oxidation furnace. After the dry etching is finished, the sample is immersed in a 33% wt KOH solution to release it from the substrate (Fig. 9).

Fig. 9

Steps included in suspended PhC fabrication.

2.6.

PhC Fabrication Using SOI Wafers

In this process, an SOI wafer is used as a substrate.⁴¹ The SOI wafer has a silicon layer as a core layer and a thick silicon dioxide (${\mathrm{SiO}}_2$) layer as the lower cladding layer. It consists of a single crystal silicon layer placed at the top and separated from the bulk silicon layer by an insulating ${\mathrm{SiO}}_2$ layer (Fig. 10). At first, using the EBL technique and the ICP etching technique, the upper silicon layer is etched into the lattice of silicon rods. Then a polymer layer is formed among and over the rods, and the polymer has the same RI as the ${\mathrm{SiO}}_2$ layer. SOI wafers are advantageous compared to other techniques due to their low power consumption, as parasitic device capacitance is reduced due to their isolation from the bulk silicon substrate, and also for their higher speed of operation. Scanning electron microscopic images of the PhC ring resonator are shown in Fig. 11.³⁶ Prather et al.³³ fabricated a 2D PhC device on an SOI wafer. The top-view, perspective view, and zoom-in view of the fabricated device are shown in Fig. 12.

Fig. 10

Schematic diagram of SOI.

Fig. 11

Scanning electron microscopic images of a PhC ring resonator.³⁶

Fig. 12

(a) Top-view, (b) perspective view, and (c) zoom-in view of the fabricated PhC device on SOI wafers.³³

3.1.

PhC-Based Logic Gates

PhC-based logic gates are gaining more attention among researchers compared to conventional logic gates due to their compactness, high switching speed, low power consumption, and outstanding integration capabilities into photonic integrated circuits.^91–97 Several logic functions can be accomplished using optical logic gates. The basic requirements for an optical logic gate to be used in integrated circuits are a low delay time, a high contrast ratio, and a small dimension.^98–105

Optical logic gates are a hot area of research as they are the main building block for optical processors. The use of PhC in the optical logic gates has miniaturized the size and enhanced the probability to make integrated optical circuits. PhC-based optical logic gates are advantageous compared to conventional logic gates in terms of their low power consumption (in the order of microwatts) and high switching speed (in the order of femtoseconds) (Table 2).

AND logic gates have found applications in different aspects of the communication field, such as for address recognition, data integrity checking purposes, in the optical oscilloscope, and also for sampling gates. For the assessment of data patterns for address identification, parity checking, data encryption, and decryption purposes, and also for generating pseudorandom patterns, XOR gate is utilized. The main usage of the NOT gate is as an inverter, and the XNOR gate is used for threshold detection purposes.

Several structures have been designed by researchers on PhC to perform different logic operations; such as PhC-based waveguides,^{92–94,106,107} microring resonator structures^{105,108–110} Mach–Zehnder interferometer structure,^111,112 coupled waveguide-based structure,^91,113 PhC based semiconductor optical amplifier (SOA) structure^114,115 and many others.

Based on the interference effect logic gates can be designed using a liner PhC ring resonator (RR).¹⁰⁵ To design a NOT gate a single RR structure [Fig. 13(a)] and to analyze the NOR gate a cascaded RR structure [Fig. 13(b)] is used in the article. A continuous-wave optical bias signal having an optical power of $P_a$ and wavelength of 1573 nm is used as the reference signal and is continuously given to the structure. If the power received at the output is comparable with $P_a$ then it is considered as “logic 1” and if the output power is zero or very low compared with $P_a$ then the output is considered as“logic 0.”

Fig. 13

(a) Single RR structure to attain the NOT gate and (b) cascaded RR structure to attain the NOR gate.¹⁰⁵

Considering the first structure of PhC [Fig. 13(a)], when no input signal is given to the circuit, then due to the bias signal the optical power received at the output port is $0.85P_a$ which can be considered as “logic 1.” If the input signal is given in the circuit, then the input signal destructively interferes with the bias signal at the upper waveguide of the structure and as a result, very low optical power ($0.05P_a$) is received at the output port which is treated as “logic 0.” Thus the single RR structure acts as the NOT gate.

Two NOT gates are connected in series to form the NOR gate, as described in Ref. 105 In the cascaded structure, there are two logic input ports “input 1” and “input 2” and one bias signal input. The bias signal has the power of $P_a$. When both the input signals are absent, then due to the bias signal the output power is $0.6P_a$ which is considered as “logic 1.” When either of the two input signals of power $P_a$ is given, then due to destructive interference with the bias input light can not pass to propagate to the output port (output power $0.05P_a$) and remains trapped in the RR. When both the inputs are present, destructive interference occurs in the structure’s upper waveguide, and obtained output power is $0.04P_a$ which can be considered as “logic 0.”

Sharifi et al.⁹³ proposed a triangular lattice structure of circular holes in a GaAs substrate to design PhC logic gates using the threshold logic concept.

To analyze the properties of a comparator circuit, a PhC structure was designed and simulated using the hexagonal-type lattice structure of GaAs rods.⁹⁴ The proposed optical comparator circuit is very small in size and can be used for very high-speed logic circuits due to a high bit rate of 3.33 Tb/s.

Parandin et al.¹⁰⁶ proposed a simple PhC structure that used line defects and point defects to design three logical gates (NOT, XOR, and NOR). The small dimension, high contrast ratio (30 dB), and low power transfer delay (0.1 ps) of the proposed structure have made it suitable for integrated circuits and high-speed logic circuits.¹⁰⁶

The nonlinear Kerr effect is also used for designing PhC-based logic circuits. Destructive interference of the input signals and the phase difference caused by different traveling paths are the main concept of the operation of such types of logic circuits.⁹² In an article published in 2015, Mohebbi et al. proposed a PhC structure⁹² of Si rods along with three rods of the nonlinear Kerr material silicon carbide at the bottom of the bent waveguide. A high contrast ratio of up to 20 dB can be achieved with the proposed structure, which makes it suitable for optical communication and data processing.

PhC plays a crucial role in enhancing the performance and reducing the size of an SOA as PhC has the great confining ability of light. Low absorption loss, less power consumption, and the suppression of undesirable nonlinear effects are the main reasons to prefer the PhC-based SOA over the conventional SOA. By the incorporation of PhC-based SOA in an MZI, the logic operation of OR and AND gates can be implemented.¹¹⁵ InP (RI = 3.17) is used to make the cladding layers and GaInAsP (RI = 3.47) is used to make the active (core) layer (Fig. 14). The schematic representation of the MZI structure to realize the AND operation is depicted in Fig. 15(a). The data signals used at the input to perform the desired logic operations are Gaussian-shaped and termed as A and B. A data signal (A) and its delayed version are injected in the upper and lower arms of the MZI at a wavelength of ${\lambda }_A$. Another signal B (wavelength of ${\lambda }_B$) given to the middle arm through an optical coupler (OC) is split into two parts with equal intensity and phase and combined with the data signal A and with its delayed version using wavelength selective couplers (WSC) in the upper and lower arms of the MZI and passes through SOA1 and SOA2, respectively. Data signal A and its delayed version create a phase difference and allow B to constructively interfere in the second OC. In the absence of data signal A, no phase difference has occurred and thus the output becomes zero. Only when both the data signals are at logic 1 state, we get the output signal. For the realization of OR gate, A and B and the continuous wave (CW) probe are given to the SOA combined [Fig. 15(b)]. By cross-phase modulation nonlinear effect, a phase difference has occurred in the CW beam in which A and B play a significant role. The CW signal splits into two parts by an OC and is given to the two arms of a delayed interferometer (DI). When both the input signals are absent, then no phase difference is rendered in the CW signal, and as a result, the output is “0” from the DI. Additionally, if any of the data signals A and B or both are present, then the CW acquires the phase change and results in “1” at the DI output. The operation of MZI structures was analyzed at 160 Gb/s.

Fig. 14

PhC-based SOA structure.

Fig. 15

Schematic representation of MZI structure based on PhC-based SOA to design (a) AND logic gate and (b) OR logic gate.¹¹⁵

3.2.

PhC-Based Sensors

The growing demand for optical sensing applications has raised the significance of PhC-based optical technologies. Due to the PBG property and the light confinement capability of the PhC-based structures, PhCs and their applications in healthcare,^116–120 RI sensing,^121–124 quality control of food,^125–128 and in many other fields^129–132 is now the sphere of interest for researchers in the present era (Table 3).

Table 2

Comparative study of different logic gates proposed in recent years.

Numerous photonic structural designs on various platforms have been extensively researched and used in sensing applications. PhCs provide strong optical confinement to a very tiny volume, enabling the identification of chemical species classified according to their nanoscale size. PhC waveguide components are now being investigated for use in microfluidic and biochemical sensing. Owing to the requirement of minimal sample preparation and high sensitivity, PhC-based RI sensors are now extensively investigated by researchers. In the 2D PhC structure, based on the different RI of the analyte samples or owing to changes in the different environmental parameters, the shift in the resonance wavelength takes place and verifying different parameters such as sensitivity, quality factor, the figure of merit, etc. the sensing performance is evaluated. PhC is a nanostructure, which affects the motion of photons within it. The variation in the structural parameters or variation in the analyte samples alters the light-matter interaction within the crystal. As PhCs are periodically modulated structures and create a PBG by forbidding a range of wavelengths to propagate through the crystal, therefore, the localized modes present in the PhC surface can be used for sensing purposes. The RI of the biochemical sample depends on the concentration of the sample and hence the analyte sample can be detected without any modifications by analyzing the light-matter interactions. As the change in the RI of the analyte sample, affects the light confinement properties of the crystal, so by analyzing the confining property of light within the PhC cavity and by optical properties of the PhC structure biochemical sensor can be designed.

A 1D PhC-based sensor was proposed in 2021.¹³³ Blood plasma and cancer cells can be detected by the introduction of a defect layer sandwiched between two identical 1D PhC structures of silica and titania. The sensitivity of the proposed sensor depends on the thickness of the sample layer. The increase in the sample layer thickness increases the sensitivity of the proposed sensor. By adding an extra layer of ${\mathrm{SiO}}_2$, on both stacks of PhCs, the sensitivity can be increased without altering the sample layer. By the addition of extra layers of silica, sample thickness can be reduced by 14%. The obtained sensitivity for blood plasma and cancer cells are 72 and 73 nm/RIU, respectively.

A PhC-based biosensor was also investigated which consists of two waveguides critically coupled to a RR.¹³⁴ The proposed biosensor is employed for the detection of glycated hemoglobin in human blood. The RR has two single-mode guided waveguides that are critically linked. The bulk sensor is examined in two ways: first, only the silica inside the bore is etched and prepared to be filled with the analyte, and second, the silica from the hub and the bore is etched and prepared to be filled with biomaterial. Here only the background material (silica) is etched, leaving the RR rods in place as shown in Fig. 16. The local RI changes as a result of the binding mechanism between the biomaterial used as the analyte and the cavity or the bio-recognition molecules used in the cavity. As a result, the resonance wavelength gets shifted, and employing the FDTD algorithm, the shift in the resonance wavelength and also the quality factors, the figure of merit, changes in the amplitude of the output transmission are analyzed, and consequently, the concentration of glycated hemoglobin is detected.

Fig. 16

PhC structure to detect concentration of glycated hemoglobin.

Another novel technique was proposed by Panda and Devi¹³⁵ where the differentiation between a normal cell and a cancerous cell is done using a square lattice structure of silicon rods with a central defect. Analyzing the reflected light from the central defect the nature of the cell is detected. The reflected wavelength from normal cells falls under orange-colored wavelength and reflected wavelengths fall under yellow-colored wavelengths. The sensitivity, resolution, and quality factor of the reported sensor were noted as 2360.12 nm/RIU, $1.78\times {10}^{-6}$, and 99.765, respectively. It is noted from the investigation that, the cancerous cell has a comparatively higher RI compared with the normal cells due to the fast splitting of cells and because in the cancerous cell more protein particles are absorbed in the cellular organelles.

A 2D hole-type PhC microcavity biosensor was proposed in 2007.²¹ The internal surface of the air holes of the sensor is coated with different sizes of protein molecules. In this reported article, the binding of glutaraldehyde and protein molecules creates a monolayer on the surface of the sensor (Fig. 17). Based on the different sizes of protein molecules different amount of resonance redshift occurs, and thus the device performance is analyzed.

Fig. 17

Creation of monolayer in the internal surface of the sensor.²¹

In recent years PhC structures are adequately used by researchers for various virus detection purposes.^136–138 Recently, a 1D PhC was proposed by researchers for the detection of the dengue virus. To identify the dengue virus in human, a biosensor has been developed in the recent year.¹³⁶ This has been accomplished by using a single defect layer of blood samples (plasma, platelets, and hemoglobin) in a 1D PhC. The defect mode is shifted within a bandgap by the change in RI due to the change of concentration of the blood sample, and this reflects the sensitivity in the event of dengue virus detection.¹³⁶ For on-chip and real-time coronavirus detection, a surface plasmon resonance biosensor was investigated by Aliee and Mozafari.¹³⁷ In the miniature design, a tiny gold layer is used on the outer layer of an air hole in a photonic quasicrystal fiber with a 12-fold symmetry, allowing the surface plasmon resonance mode on the gold to be excited by leakage of the fiber core mode. The suggested biosensor has a sensitivity of 1172 nm/RIU in the detection of coronaviruses in saliva, according to 3D finite-difference time-domain simulations.

The performance of the optical biosensor is evaluated by testing different parameters such as sensitivity, figure of merit, quality factor, limit of detection, and many others. The most commonly evaluated parameter in the biosensor is sensitivity, which is defined as the smallest change in the resonance wavelength with the change in the RI of the analyte sample and is articulated in nm/RIU. The quality factor is another essential performance parameter for a biosensor and is defined as the ratio of the resonance wavelength to the full-width half maxima of the resonance peak. The high figure of merit of a sensor indicates the high performance of the sensor and is defined as the ratio of the sensitivity to the full-width half maxima. The lowest concentration of analyte sample that can be detected is defined as the limit of detection and a high limit of detection usually enhances the sensitivity.

As the PhC cavities have strong confinement of field and have a long photon lifetime, the resonance wavelength is highly sensitive to perturbation of RI of the infiltrated air holes or the RI of the rods. A PhC nanocavity-based RI sensor was proposed¹²² in 2020, coupled with two waveguides, where the resonant peak shifts with the change in the RI.

Another highly sensitive PhC-based RI sensor was proposed¹³⁹ incorporating a ring-shaped GaAs cavity. The structure can detect RI from 1.33 to 1.5 and also possesses an excellent figure of merit of 737/RIU.

PhC structures work with only a certain frequency range of light. The light having frequencies that do not fall within PBG cannot pass through the crystal. For variations in temperature, the RI of the material will change which will change the resonance wavelength of the structure. With the increase in temperature for the materials in 2007 having a positive temperature coefficient, RI will increase and will reduce for the materials having a negative temperature coefficient and vice versa. For a temperature sensor, the resonance wavelength gets red-shifted with the increase in material RI and gets blue-shifted with the decrease in material RI.^140,141

Several PhC-based gas sensors were proposed by different researchers in the earlier years to sense several toxic gases present in the environment such as carbon-di-oxide, methane, carbon-mono-oxide, and many others. Recently, an ${\mathrm{H}}_2\mathrm{S}$ gas sensor having high sensitivity and detection limit of $1.2\times {10}^4\mathrm{nm}/\mathrm{RIU}$ and $1.87\times {10}^{-6}\mathrm{RIU}$, respectively, is reported by researchers.¹⁴² In the proposed sensor air holes are placed on an SOI substrate in a triangular lattice structure. On the inner wall of the cavity, a layer of grapheme is formed so that the gas can be absorbed. When the concentration of the ${\mathrm{H}}_2\mathrm{S}$ gas varies, then the RI of the graphene layer also changes ultimately affecting the resonance wavelength of the sensor. The change in the concentration of ${\mathrm{H}}_2\mathrm{S}$ gas affects the electrical conductivity of the graphene layer and consequently, the alteration in the RI of the graphene layer takes place which leads to the shift in the resonance wavelength. The resonance wavelength shifts by almost 3.6 nm for a gas concentration change of 1 ppm.

A 2D PhC-based gas sensor was analyzed numerically with three different types of line defects:¹⁴³ by removing a row of holes, changing the diameter of a row of holes, and a design with an air channel. The proposed gas sensor works on the principle of transmitting light having a single wavelength from the air holes filled with acetylene gas.

A biperiodic gas sensor was also proposed by researchers¹⁴⁴ for any fluid or gas sensing purpose. The PhC structure consists of a triangular array of air holes with a biperodic structure of waveguide in silicon. Through the biperiodic waveguide, only the resonance wavelengths are allowed to pass; all other wavelengths are reflected back from the crystal. The sensing principle mainly lies in the resonance wavelength shift when the super cavities are filled with some fluid or gas. The proposed sensor is investigated using FDTD analysis. When the cavities are filled with gases, the sensitivity of the proposed sensor is obtained as 610 nm/RIU and the minimum detection limit is 0.0001 RIU. When the structure is used as a fluid sensor, the obtained sensitivity is 300 nm/RIU with an RI detection range of 1.0 to 1.5, and the obtained minimum detection limit is 0.001.

PhC structures can also be realized for mechanical sensing purposes. The PhC structure used for mechanical sensing purposes mainly relies on the photoelastic, piezoelectric, and electro-optic effects of the materials. Due to applied mechanical action, the RI of the materials changes, and also the deformation or deflection of the structure takes place. And as a consequence, the resonance wavelength also shifts to a different wavelength, and by exploiting the shift the applied mechanical actions are realized.

In 2021, PhC based MZI structure was proposed¹⁴⁵ by researchers for pressure sensing purposes. The MZI was integrated onto a silicon diaphragm. The PhC-based MZI consists of two parts: one part is flexible and the other one is a fixed part. As there is no material beneath the flexible portion when pressure is applied to the diaphragm, the flexible part moves downward; RI changes in the flexible arm and thus optical path length is increased in that arm. So light requires comparatively more time to travel in the longer arm compared to the shorter arm due to optical path difference. So the shift in the resonance wavelength takes place whenever pressure is applied to it. Using the finite-element method (FEM) analysis, the applied stress is analyzed, and by using the finite difference time domain (FDTD) method the change in the electromagnetic properties of the interferometer is realized. The pressure sensor has high sensitivity and shows a wavelength resolution of 2.25 nm for 1 MPa applied pressure and possesses a detection limit of the pressure of $873\mu \mathrm{Pa}$.

Another force sensor was realized¹⁴⁶ by designing a PhC GaAs/AlGaAs microcavity. In the proposed structure, a microcavity is coupled to a waveguide of the PhC structure which has a triangular lattice of air holes. By removing an air hole and changing the diameters of the surrounding holes, microcavity was created. The structure is feasible in designing PhC actuators where the emission properties of the active PhC can be modulated by the applied strain.

A 2D hexagonal lattice structured PhC design was proposed for pressure sensing purposes in 2019.¹⁴⁷ The structure has the capability of sensing a broad range of pressures, from 20 MPa to 10 GPa. To increase the sensitivity of the structure hexagonal-shaped ring with a hexagonal lattice structure is used. Two silicon cantilever tips are utilized to analyze the applied pressure. The cantilever tips work as the defect in the crystal and with the applied pressure moves downwards and thus the radius of the defect is changed.

3.3.

PhC-Based Solar Cells

Thin-film solar cells have attracted many researchers for the low processing temperature and low material consumption. But insufficient absorbance of light in the absorber layer has limited the performance of thin-film solar cells. Also in the case of a silicon wafer, due to an indirect bandgap sufficient light cannot be trapped in the absorber layer. The use of a wavelength-selective medium at the backside of the solar cell can resolve the problem. The absorption is enhanced in the absorber layer of a specially patterned solar cell by increasing the path length of light traveling in the absorber layer. Due to the periodic nanostructure of PhC, they have excellent properties of light trapping, so PhC can be a good choice for thin-film solar cells.^148–153 For light management in a solar cell, 1D PhC structures [distributed Bragg reflector (DBR)], 2D PhC structure [flattened light-scattering substrate (FLiSS)], and the micropyramid structure are used by researchers at the backside of the solar cell.¹⁵⁴ The performance of the solar cell using three types of structures is compared by analyzing the reflectance-transmittance properties using the FDTD simulation method. In the case of DBR, for different layer thicknesses and periods, the performance of the solar cell is analyzed. Different reflectance properties are compared for the different oxide materials in the FliSS structure which has a structurally flat but optically rough surface. In the case of the micro pyramid, structure comparison is done for different heights and with the planar structure. In the PhC structures, enhancement of absorption occurs due to the diffraction effect as the feature size is in the order of the wavelength of light. But in the micropyramid structure, the feature size is greater than the order of the effective wavelength and so enhancement in absorption occurs due to reflection. The reflectance of around 56% is obtained for AZO combined with a hydrogenated amorphous silicon FLiSS structure having a reflectance window ranging from 300 to 720 nm. The micropyramid structure has comparatively less peak reflectance of 40% with the spectral range of reflectance window 300 to 550 nm. Compared with the other two structures, the DBR structure has a high reflectance peak of 75% with a reflectance window from 300 to 650 nm.

Table 3

Comparative study of different sensors proposed in recent years.

Minimizing of surface reflectance is an effective way to increase the efficiency of solar cells. A silicon solar cell without an antireflecting coating reflects almost 40% of the incident light in the surrounding. The inclusion of a single layer of silica can reduce the reflectance by occurring destructive interference between the reflected waves in the surface layers. The use of PhC in photovoltaic devices can increase the absorption rate as PhC has the unique property of trapping light. In an article published in 2021, Mercier et al.¹⁵⁵ proposed a PhC structure superimposed on the top surface of the solar cell to minimize the reflectance losses. The introduction of photonic quasicrystal nanostructuring can reduce effective average reflectance significantly. Though bifacial solar cells have the capability of higher power generation, the problem arises in enhancing their performance due to insufficient infrared absorption. The use of DBR in the backside of solar cells can enhance light absorption in longer wavelengths. For the different bottom angles of the pyramid surface and different thicknesses of the silicon wafer, light absorption at the rear side of the solar cell is analyzed.¹⁵⁶ A reduction in the pyramid bottom angle enhances the light trapping and reaches its maximum at an angle of 20 deg. A comparative study of PhC based solar cells is given in Table 4.

3.4.

PhC-Based Beam Power Splitters and Polarizers

In optical communication systems, and photonic integrated circuits, optical beam splitter, and polarization beam splitter are the fundamental component. The optical beam splitter divides the input optical signal into several parts at the output^158–160 whereas the polarization beam splitter (PBS) is used to split the optical signal into two polarization states (TE and TM).^161–163 As the photonic devices can operate in only one polarization state, therefore, the unpolarized light or light having partial polarization cannot propagate through the device. So, PBS is the most efficient device in photonics to separate the light into two orthogonal states of polarization. To analyze the efficiency of a PBS, polarization extinction ratio (PER) is a vital figure of merit^164,165 and is the indicator of the degree of polarization for a polarization-maintaining device.

Small footprints of PhC-based PBS have made it more attractive among researchers.^166–170 The combined effect of self-collimation and PBG is utilized in Ref. 170 to achieve two orthogonal states of polarization of light. Self-collimation in a PhC structure allows light with a particular state of polarization to pass through it in a straight line without any diffraction. To separate different polarized light, a heterostructure of PhC is used. Two structures of a PhC having rectangular air holes (PhC2) and circular air holes (PhC1) are inclined at an angle of the 45-deg interface (Fig. 18). Two outputs have been obtained at the two ports placed at the end of two different types of PhC structures (PhC1 and PhC2). Due to PBG, PhC2 does not allow TE-polarized light to pass through it and allows only TM-polarized light. On the other hand, Both TE and TM polarized light can pass through the PhC1. The source is placed at one end of the PhC1. When self-collimated TE polarized light is incident from the source, then after passing through the PhC1 it strikes at the interface of the two PhC structures and then reflected with a 90° bend, and output is received at output 2. When self-collimated TM polarized light has emerged from the source, then after marginal refraction at the interface, the light passes through the PhC2, and output is received at output 1. In the self-collimation wavelength interval of 1500 to 1615 nm, transmissivity is about 60% and 78% for TE and TM polarized input light. Additionally, at the wavelength of 1550 nm, the transmissivity is increased and reached up to 94.9% and 88% with high PER of 17 and 41 dB for TE and TM polarized light.

Fig. 18

PhC heterostructure to design a PBS.¹⁷⁰

Table 4

Comparative study of different solar cells proposed in recent years.

Unlike conventional optical fiber, photonic crystal fiber (PCF) cladding comprises periodically arranged air holes and attracts researchers for its endlessly single-mode guiding operation, high nonlinear effect, low loss, and the controllability of dispersion. The surface plasmon resonance (SPR) effect is also utilized in PCF by different researchers to design PBS.^171,172 In a PCF, air holes of the cladding are filled with metal nanowires to enhance the SPR effect.¹⁷¹ The light having two polarization states can be filtered out at two communication bands. At the resonance wavelength of $1.24\mu \mathrm{m}$, confinement loss is 10738.726 and 272.2325 dB/m for $x$ and $y$ polarization, respectively. Also, the obtained crosstalks are $-90.9$ and 85 dB for $x$ and $y$ polarizations at resonance wavelengths of 1.24 and $1.32\mu \mathrm{m}$, respectively.

In Ref. 172, a novel PCF is designed based on SPR to be used as a polarization filter. Some air holes are selectively coated with a gold layer and filled with water. The peak loss obtained for the polarization filter in the direction of $y$-polarization is 1209.57 dB/cm at 1310 nm wavelength whereas for the $x$-
polarization peak loss is almost zero. The position and the intensity of the loss peak can also be varied by changing the air hole diameter and the metal layer thickness.

A PBS is designed in the THz region is designed based on 2D PhC cavities.¹⁷³ PhC line defects and two PhC cavities are combined to design the PBS, which can separate the TE and TM polarized light into two propagation directions in the THz wave frequency ranging from 1.16 to 1.19 THz (Fig. 19). In the THz wave region, Tellurium has a very negligible absorption and for this reason, a square lattice structure of Tellurium rods is used to design the PBS. The internal PhC section is highly transmissive for TM mode but highly reflective for TE mode. A Gaussian shape pulse source (TE/TM) of center frequency $0.236(a/\lambda )$ when inserted at the input port, then the light having TE polarization reflected back from the internal section of the PhC structure with high reflection efficiency, and the output is collected at output A. But the light having TM polarization transmits through the PhC internal section with high transmission efficiency and is finally collected at output B (Fig. 19). The obtained extinction ratio is larger than 20 dB for both TE and TM polarization. A comparative study of different PhC based PBS is given in Table 5.

Fig. 19

PhC structure for PBS in THz region.¹⁷³

Table 5

Comparative study of different PBS proposed in recent years.