Recent Advances in Photonic Crystal and Optical Devices

In recent years, photonic crystals (PhCs) have garnered significant attention due to their extraordinary ability to control and manipulate light at the nanoscale [1]. These periodic structures, typically composed of dielectric materials with alternating high and low refractive indices, exhibit unique optical properties, including photonic bandgaps and strong light confinement [2]. Advancements in the fabrication techniques and theoretical understanding of PhCs have led to groundbreaking developments in various devices and applications [3]. One of the notable advances lies in the realm of optical communications [4]. Photonic crystal fibers have emerged as promising candidates for next-generation telecommunication systems. These fibers feature a periodic arrangement of air holes running along their length, allowing for unprecedented control over light propagation. By tailoring the geometry and composition of the PCF, researchers have achieved remarkable improvements in dispersion management, nonlinear effects mitigation, and bandwidth enhancement, paving the way for high-speed data transmission over long distances.

Moreover, PhCs have revolutionized the field of sensing and detection [5,6]. PhC sensors offer exceptional sensitivity to changes in the surrounding environment, making them ideal for various applications, including biosensing, environmental monitoring, and chemical analysis. By integrating functional materials or biomolecules into the PhC structure, researchers have developed ultrasensitive sensors capable of detecting trace amounts of analytes with high accuracy and specificity [7]. These advancements hold great promise for advancements in medical diagnostics, food safety, and environmental surveillance. In addition to communications and sensing, PhCs have found extensive use in the development of novel optical devices. From compact wavelength filters and splitters to efficient light-emitting diodes (LEDs) and lasers, PhC-based devices offer unprecedented performance metrics, including high efficiency, low power consumption, and wide tuning range [8,9,10]. By harnessing the unique properties of PhCs, researchers continue to push the boundaries of optical device miniaturization, integration, and functionality, enabling advancements in fields such as integrated photonics, quantum computing, and on-chip optical processing [11].

This Special Issue encapsulates a compendium of studies emblematic of the dedication, creativity, and collaborative spirit of researchers across the globe. Through these articles, readers are invited to delve into a realm brimming with valuable insights, inspiration, and a renewed sense of possibility, heralding the future of PhCs and optical devices. We extend a warm welcome to readers, offering a tantalizing glimpse into the frontier of sensing and filtering, made possible by the remarkable advancements in PhC technology. For a comprehensive overview of the published articles, we direct readers to the List of Contributions.

The first paper of this Special Issue contains a study, “A dual-core surface plasmon resonance-based photonic crystal fiber sensor for simultaneously measuring the refractive index and temperature”, conducted by Li et al., which introduced a novel photonic crystal fiber biosensor architecture leveraging surface plasmon resonance (SPR) to simultaneously measure the refractive index and temperature-sensitive material temperature. The innovative design involved coating both the central and external surfaces of the biosensor structure with a thin gold film. Additionally, a hole adjacent was incorporated to the inner gold film, filled with temperature-sensitive material. By integrating internal and external gold coatings alongside temperature-sensitive material, the proposed biosensor achieved dual functionality, enabling the measurement of refractive index and temperature with two distinct wavelength coverages. Through rigorous numerical simulations and calculation analyses, the author demonstrated the remarkable performance of the biosensor structure. Specifically, the results reveal an impressive average wavelength sensitivity of 7080 nm/RIU and 3.36 nm/°C, respectively, with refractive index coverage ranging from 1.36 to 1.41 and temperature coverage spanning from 0 to 60 °C. Notably, the strategic utilization of different wavelength regions for refractive index and temperature sensing enhances the versatility and efficacy of the proposed biosensor. It is anticipated that the pioneering biosensor structure presented by Li et al. holds significant promise for a wide range of applications, particularly in the realms of medical diagnostics and environmental assessments. By offering simultaneous and precise measurements of refractive index and temperature, the proposed device addressed critical needs in these fields, facilitating more accurate analyses and assessments. This advancement underscores the potential of photonic crystal fiber biosensors as indispensable tools for advancing healthcare and environmental monitoring technologies.

The second paper of this Special Issue provides a study, “Performance analysis of DAST material-assisted photonic crystal-based electrical tunable optical filter”, conducted by Goyal et al., a novel 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) material-assisted 1D-PhC-based tunable optical filter. The filter design features a bilayer 1D-PhC structure incorporating DAST as an electro-optic material. Specifically tailored device parameters are optimized to effectively filter out the 632.8 nm wavelength from the reflection spectrum. The study demonstrated that upon illuminating the device with polychromatic light at an incident angle of 45.07°, the filtered wavelength exhibits transmission maxima with a remarkable full width at half maximum of less than 1 nm. Additionally, the analytical outcomes showcased the exceptional post-fabrication electrical tuning capability, with a tuning range of 60 nm achievable with a mere ±5 V applied potential. Furthermore, the filter structure displayed exceptional stability, maintaining a consistent filter response even under variations of up to 40% in optical thickness. This remarkable stability, coupled with the advantages of low-voltage wavelength tuning, underscores the significance of the proposed design. With its ease of fabrication and integration capability into integrated circuits, the suggested tunable optical filter presented a promising solution for a wide array of applications, ranging from telecommunications to optical sensing and beyond.

Matar et al., in their article titled “Theoretical study on polycarbonate-based one-dimensional ternary photonic structures from far-UV to NIR regions of the electromagnetic spectrum”, presented a theoretical exploration of the photonic bandgap (PBG) properties within 1D photonic structures crafted from polycarbonate and a selection of non-glass materials. Specifically, their investigation delved into four distinct photonic structures denoted as PhC₁, PhC₂, PhC₃, and PhC₄, each comprising alternating layers of polycarbonate combined with Al₂O₃, MgF₂, BaF₂, and TiO₂ materials, respectively. Within the period of each photonic structure, a thin non-glass material layer was nestled between two identical polycarbonate layers, forming a precisely engineered configuration.

To dissect the transmission properties of PhC₁ through PhC₄, Matar et al. employed the transfer matrix method, enabling a comprehensive analysis of their behavior. Comparative examination of the transmission spectra unveiled intriguing findings, notably the emergence of three PBGs with zero transmission within the far-UV, visible, and NIR regions of the electromagnetic spectrum for the polycarbonate and TiO₂-based photonic structure (PhC₄). Notably, these PBGs persist under both normal and oblique incidence angles (θ₀ = 55°), exclusively corresponding to TE wave characteristics.

The selection of polycarbonate alongside Al₂O₃, TiO₂, MgF₂, or BaF₂ stemmed from the desirable heat resistance properties of polycarbonate coupled with the distinctive optical characteristics of oxide and fluoride materials, offering wide transparency across the UV to NIR regions of the electromagnetic spectrum. The proposed framework bears significant implications, particularly in the realm of solar cell technology enhancement. By integrating wavelength-selective reflectors composed of 1D-PhCs behind the active regions of solar cells, it anticipates tangible improvements in photovoltaic performance. Furthermore, envisioning advanced solar cell designs incorporating 1D photonic mirror-based luminescence and reflection concentrators holds promise for future applications. Moreover, the potential utilization of smart windows based on the proposed multilayer structures presents a viable solution for mitigating the low-temperature challenges encountered in satellite environments, exemplifying the multifaceted utility and innovation inherent in the research findings.

The focal point of the next research paper, “Employing the defective photonic crystal composed of nanocomposite superconducting material in the detection of cancerous brain tumors biosensor: Computational study”, written by Malek et al., revolved around the exploration of externally tunable defect mode properties within a 1D defective photonic crystal (DPhC), with a primary aim of enabling rapid detection of cancerous brain tumors. Their innovative design harnessed a conventional 1D-DPhC, where the cavity was precisely coated with SiO₂ nanoparticles embedded within a superconducting material layer, aptly termed a nanocomposite layer. This integration of a nanocomposite superconducting layer served a dual purpose: facilitating temperature-dependent external tuning of the defect mode within the PBG and enabling adjustments in the angle of incidence. Additionally, the incorporation of the nanocomposite layer enhanced the interaction between light and various brain tissue samples under scrutiny, bolstering the efficacy of the detection system.

To comprehensively explore the transmission properties of the proposed structure, Malek et al. employed a combination of the transfer matrix formulation and MATLAB computational tools. Initially, internal parameters were optimized under normal incidence to achieve peak performance. Subsequently, they delved into the effects of altering the angle of incidence to further enhance performance metrics such as sensitivity, quality factor, figure of merit, and limit of detection, ensuring robust external tuning of the defect mode. Their investigations yielded promising results, with a maximum sensitivity value of 4139.24 nm/RIU observed at an angle of θ = 63°, corresponding to a sample containing brain tissue. Furthermore, they delved into the impact of temperature variations within the nanocomposite layers on both the position and intensity of the defect mode within the PBG. While temperature-induced changes resulted in subtle shifts in sensitivity, they led to significant amplification of defect mode intensity, a crucial aspect in any photonic biosensing design. The insights gleaned from this study hold immense potential for the development of diverse biosensing structures poised to play pivotal roles in biomedical applications. By facilitating rapid and precise detection of cancerous brain tumors, their research contributes to advancing the frontiers of medical diagnostics and underscores the transformative potential of photonics in healthcare.

The next paper, “Fabrication and investigation of spectral properties of a dielectric slab waveguide photonic crystal-based Fano-filter”, written by Khan et al., delved into the intricacies of fabricating a dielectric PhC-based Fano-filter device, accompanied by a comprehensive numerical investigation of its spectral characteristics. They thoroughly explored the process parameters that influenced both the structural and physical properties of the fabricated device, shedding light on their profound impact on the spectral properties of the filter. The experimental focus revolved around a three-layered PhC structure, precisely crafted utilizing focused ion-beam (FIB) technology, tailored for operation within the NIR range. The parameters under scrutiny encompassed a spectrum of factors, including the shape of PhC elements, the depth of the structures, cladding layer thicknesses, and the refractive index of the constituent material.

Leveraging an open-source Python-based finite-difference time-domain simulation tool, Khan et al. undertook rigorous numerical design and simulations to unravel the intricate interplay between these parameters and the device’s spectral behavior. Central to their investigation is the operational principle of guided-mode resonance, underpinning the functionality of the proposed optical filter device. Through careful experimentation and simulation, a maximum quality factor value within the impressive range of 800 was achieved, underscoring the efficacy and precision of the design. Their findings not only deepened the understanding of PhC-based optical filters but also paved the way for the development of high-performance devices poised to revolutionize spectral manipulation in the NIR regime.

The last paper, “MATLAB simulation-based theoretical study for detection of a wide range of pathogens using 1D defective photonic structure”, authored by Aly et al., presented a 1D photonic biosensor that comprises two sub-PhCs, featuring alternate layers of GaP and SiO₂. With a fixed period number of 3 for each PhC, these structures were seamlessly joined via a cavity region filled with different analytes, meticulously examined within the scope of the study. Leveraging the renowned transfer matrix method, they formulated the theoretical framework underpinning their findings, with all computations executed using MATLAB software. The theoretical exploration scrutinized the impact of varying cavity thickness and angle of incidence on the transmittance of the structure (AB)ND(AB)N, crucial determinants of the biosensor’s performance. They delved into a comprehensive array of parameters, including sensitivity (S), signal-to-noise ratio (SNR), figure of merit (FOM), resolution (RS), limit of detection (LOD), quality factor (Q), and dynamic range (DR), providing a holistic assessment of the proposed design’s capabilities.

Notably, the biosensor exhibited a sensitivity ranging from 337.3626 nm/RIU to 333.0882 nm/RIU, corresponding to water samples containing Pseudomonas aeruginosa and Bacillus anthracia cells, respectively, under normal incidence conditions with a cavity thickness of 2.0 µm. The resolution and LOD values attained underscore the precision and significance of the design, offering discerning capabilities crucial for distinguishing various microbiological samples and facilitating potential applications in discriminating bacterial cells from spores. This study not only enriched the understanding of photonic biosensing principles but also holds promise for practical applications, particularly in microbiological analysis and detection, where precise discrimination between different cell types is paramount.

This editorial synthesis encapsulates the essence of each contribution to this Special Issue, celebrating the collective endeavor to push the boundaries of PhCs for sensing and filtering applications. The research presented herein not only pioneers novel ideas in the design and implications of PhCs but also lays a robust foundation for future explorations in this dynamic and ever-evolving field.