Advanced Ceramics for Photonic Applications: A Rich Landscape

2.1 Transparent ceramics: The transparency of strength

Transparent ceramics (TCs) present a fascinating convergence of material science and optics [6]. While the term “ceramic” often evokes images of robust, opaque materials, TCs defy this perception by exhibiting remarkable optical clarity alongside exceptional strength and durability. These attributes make TCs ideal candidates for a wide range of applications in photonics, pushing the boundaries of what can be achieved with light [6, 7]. TCs are crystalline materials capable of transmitting a significant portion of visible light. Unlike traditional ceramics, they lack scattering centers such as pores, grain boundaries, and inclusions, which typically deflect and absorb light, leading to opacity (Figure 1). The transparency of a ceramic is intricately linked to several key factors, including its crystal structure, grain size and distribution, porosity, and the presence of inclusions and impurities [22, 23]. A highly ordered, defect-free crystal structure is paramount for transparency. It allows light to pass through with minimal scattering. Similarly, smaller, uniformly sized grains contribute to reduced scattering. Conversely, large grain boundaries or uneven grain size increase light scattering, diminishing transparency. Pores act as scattering centers, further reducing transparency. Consequently, densification techniques are crucial for producing transparent ceramics with minimal porosity. Furthermore, foreign particles and impurities within the ceramic matrix can also scatter light and compromise transparency. Therefore, strict purity and meticulous control during processing are essential [23, 24]. The interaction of light with a material is governed by its refractive index, which determines the speed of light in the medium. In transparent materials, the refractive index is relatively uniform throughout, enabling light to pass through with minimal deviation [22, 25]. When the refractive index of the material closely matches that of the surrounding medium, such as air, minimal reflection occurs, maximizing transparency. However, variations in refractive index encountered at grain boundaries or pores cause light to scatter, thereby reducing transparency. Despite their potential, achieving high transparency in ceramics poses significant challenges due to the inherent nature of these materials. High-temperature sintering, a crucial step for densification and achieving good transparency, can introduce defects and impurities [26]. Additionally, precise control over grain growth is essential to minimize scattering. Achieving a uniform, fine-grained microstructure requires careful processing. Moreover, trace impurities can significantly impact transparency, necessitating rigorous purification methods to ensure the presence of only the desired elements [27, 28]. Additionally, the random arrangement of grains in ceramics is detrimental as it diminishes transparency. Crystallographic orientation plays a crucial role in improving transparency, particularly in birefringent ceramics, by minimizing birefringence at grain boundaries [29].

TCs can be broadly categorized based on their primary material composition. Following the successful demonstration of laser oscillation in Nd³⁺-doped Y₃Al₅O₁₂ (Nd:YAG) transparent ceramics by Ikesue et al. in 1995 [30], YAG transparent ceramics containing a range of rare-earth ions such as Nd³⁺, Yb³⁺, and Er³⁺ have established themselves as the optimal laser host material, attributed to their straightforward manufacturing process and exceptional physical and chemical attributes [28, 31]. Yttria (Y₂O₃), a highly refractive material, finds applications in optical windows, lenses, and high-power lasers. Its exceptional mechanical strength, high melting point, and chemical stability make it a highly desirable material [32, 33, 34]. Alumina (Al₂O₃), known for its high hardness and strength, also exhibits transparency [12, 35]. It is frequently used for windows, lenses, and protective coatings due to its high optical performance as well as resistance to physical damages [23]. Other noteworthy materials include magnesium aluminate spinel (MgAl₂O₄), known for its high refractive index and thermal stability, used in optical windows and high-power laser applications [36]. Spinel (MgAl₂O₄) offers excellent optical properties and thermal shock resistance and a wide transparency window from UV to MIR, making it suitable for optical windows and protective coatings [37]. Zinc sulfide (ZnS) holds promise for infrared optics due to its high transmittance in the infrared region [38]. Finally, calcium fluoride (CaF₂), known for its wide transmission range and its performance as a host for lanthanides, finds applications in ultraviolet and infrared optics especially in laser technology [39, 40].

Various methods are employed for synthesizing TCs, each offering distinct advantages and drawbacks. Traditional powder processing involves synthesizing fine powders of the desired material, followed by pressing and sintering at high temperatures to densify the material [41]. While cost-effective, this method presents challenges in achieving precise control over grain size and porosity [31]. Sol-gel processing, on the other hand, involves the controlled hydrolysis and condensation of precursor materials to form a gel [42]. The gel is then dried and sintered at lower temperatures, resulting in high purity and homogeneity. However, this process can be time-consuming and requires careful control of the precursor chemistry. Hot isostatic pressing (HIP) involves applying high pressure at elevated temperatures, allowing the ceramic to densify and achieve high transparency [15]. It is effective for materials with high melting points but necessitates specialized equipment. Recent advances in fabrication techniques have further propelled the development of TCs. Spark plasma sintering (SPS) utilizes pulsed electrical current to rapidly heat and sinter ceramic powders, enabling faster processing times and better control over microstructure [15, 43]. Laser-assisted processing allows for the fabrication of complex, three-dimensional structures in TCs, opening up new possibilities for photonic devices [44]. Additive manufacturing, or 3D printing, is being explored to create TCs with intricate geometries and customized optical properties [45, 46, 47].

TCs offer several unique advantages over other optical materials, making them essential for photonics [6, 22]. Their high refractive indices enable the creation of compact optical devices. Their high hardness, thermal stability, and chemical resistance make them suitable for demanding environments. Some TCs can transmit light across a wide range of wavelengths, from the ultraviolet to the infrared [39]. Additionally, their properties can be fine-tuned by varying the material composition and processing techniques [15]. The applications of TCs in photonics are diverse and rapidly expanding. Their high transparency, strength, and resistance to harsh environments make them ideal for optical windows in spacecraft, lasers, and high-intensity lighting applications [22]. TCs can be fabricated into high-performance lenses for various optical instruments, including microscopes, telescopes, and laser systems. They are also used as host materials for solid-state lasers, providing high-power output and excellent thermal stability [5]. TC fibers offer unique advantages for high-power laser transmission and sensing applications due to their exceptional strength and durability [32]. Transparent ceramics represent a rapidly evolving field with immense potential to revolutionize photonics. Their unique combination of optical clarity, strength, and durability unlocks new possibilities for developing advanced optical devices for various applications. As research and development continue, we can expect even more exciting breakthroughs in TC fabrication and applications, pushing the boundaries of what is possible with light.

2.2 Transparent glass/ceramics: Combining the best of both worlds

Transparent glass/ceramics (TGC) are a unique class of materials that combine the amorphous nature of glass with the crystalline properties of ceramics [16]. This hybrid nature allows TGCs to exhibit superior mechanical, thermal, and optical properties, making them highly suitable for various photonic applications. The formation of TGC involves controlled crystallization within a glass matrix, resulting in a material that maintains transparency while gaining the advantageous properties of ceramics [16, 48].

The formation of TGCs begins with the preparation of a glass precursor, which is then subjected to a controlled heat treatment process known as ceramization [49]. During this process, nucleation and growth of crystalline phases occur within the glass matrix [50]. The key to maintaining transparency lies in the size and distribution of these crystalline phases. If the crystallites are smaller than the wavelength of visible light (typically less than 100 nm), they do not scatter light significantly, thus preserving the material’s transparency [48, 49]. The transparency of TGCs is also influenced by the refractive index matching between the glass matrix and the crystalline phases [50]. A close match minimizes light scattering at the interfaces, further enhancing transparency. The absence of impurities and defects is also crucial, as these can introduce scattering centers and reduce optical clarity. The controlled crystallization process ensures that the crystalline phases are uniformly distributed with a control of their size below the critical threshold for light scattering. The mechanism of controlled crystallization in transparent glass/ceramics involves a series of carefully controlled steps that influence the formation of crystalline phases within a glassy matrix [50]. Several parameters, including heat treatment conditions and material composition, play crucial roles in this process [50]. The controlled heating and cooling cycle allow for the nucleation and growth of crystalline phases within the glassy matrix. The temperature and duration of the heat treatment are critical in determining the size, distribution, and composition of the crystalline phases in the TGC. The composition of the glassy matrix in TGC plays a significant role in controlling crystallization [48]. The composition of the glass material can be tailored to promote the formation of specific crystalline phases during heat treatment. Nucleation is the initial formation of small crystalline nuclei within the glassy matrix, which serve as the starting points for crystal growth [48]. Therefore, nucleating agents, such as titanium dioxide or zirconium dioxide, can be added to the glass composition to facilitate the nucleation of crystals. Adding nucleating agents such as zirconia or titania promotes the nucleation process and helps achieve a higher density of crystalline phases. The growth of crystals occurs as the material is further heated and cooled, leading to the development of larger crystalline structures within the glassy matrix [51]. So, crystal growth inhibitors, such as aluminum oxide or magnesium oxide, or bore oxide, can be used to control the growth of crystals and prevent their excessive growth, ensuring a fine and uniform distribution of crystalline phases [49, 52, 53]. By carefully adjusting control parameters, researchers can optimize the formation of crystalline phases and tailor the properties of the material to meet specific requirements, such as transparency, strength, and thermal stability.

Combining the characteristics of glass and ceramics, TGCs offer several unique properties. One of the most significant advantages is their mechanical strength [44, 54]. TGCs exhibit higher mechanical strength and toughness than conventional glasses, making them more resistant to mechanical damage. This is particularly important in applications where durability and longevity are critical. Another advantage is the enhanced thermal stability provided by the crystalline phases within TGCs. This allows TGCs to withstand higher temperatures without deformation, making them suitable for high-temperature applications [54]. Additionally, TGCs maintain high optical clarity, which is essential for photonic applications [51]. The controlled crystallization process ensures that the material remains transparent while gaining the beneficial properties of ceramics. Chemical durability is another important property of TGCs [48, 55]. They are often more resistant to chemical attack than pure glasses, extending their lifespan in harsh environments. This makes TGCs suitable for applications in chemically aggressive environments where conventional glasses would degrade.

TGCs can be classified based on their material composition. Some crucial types include tellurites [49], phosphates [54], and silicates [51, 56]. Tellurite-based TGCs are known for their high refractive indices and low phonon energies, making them excellent for nonlinear optical applications. Phosphate-based TGCs offer good chemical durability and are used in laser and optical amplifier applications. Silicate-based TGCs are widely used due to their excellent mechanical properties and thermal stability. Several methods are used to synthesize TGCs, including melt quenching [56], sol-gel process [11], and spark plasma sintering (SPS) [51]. Melt quenching involves melting the raw materials and rapidly cooling them to form a glass, followed by controlled heat treatment to induce crystallization. This method is widely used due to its simplicity and scalability. The sol-gel process is a chemical method that involves the transition of a solution into a gel, which is then dried and heat-treated to form TGCs [57]. The sol-gel method offers better control over composition and microstructure, allowing for the synthesis of TGCs with tailored properties. However, it is more complex and time-consuming compared to melt quenching. Spark plasma sintering (SPS) is a technique that uses pulsed electric current to sinter the material at lower temperatures and shorter times, preserving transparency [51]. The SPS technique also provides rapid processing and allows for synthesizing TGCs with high density and uniform microstructure. However, it requires specialized equipment and is less scalable compared to melt quenching. Recent advances in fabrication techniques, such as additive manufacturing and advanced sintering methods, have expanded the possibilities for TGCs [58, 59]. Additive manufacturing allows for the precise control of material composition and microstructure, enabling the synthesis of TGCs with tailored properties. Advanced sintering methods, such as SPS, provide rapid processing and high-density materials with uniform microstructure.

TGCs are crucial for photonic applications due to their unique combination of properties. Significant applications include high-performance optics, displays, and optical fibers [16, 48]. In high-performance optics, TGCs are used in lenses, windows, and other optical components that require high transparency and durability. Their optical clarity and mechanical strength make TGCs ideal for protective covers and substrates in display technologies. In optical fibers, TGCs with tailored refractive indices are used in the core and cladding, enhancing signal transmission and durability [14, 18, 60]. The ability to precisely control the refractive index and maintain high optical clarity makes TGCs suitable for advanced optical fiber applications. State-of-the-art applications of TGCs include laser systems, photonic integrated circuits, and biomedical optics [61, 62]. In laser systems, TGCs are used in high-power laser components due to their excellent thermal and optical properties. The integration of TGCs in photonic circuits enhances performance and miniaturization, enabling the development of compact and efficient photonic devices. In biomedical optics, TGCs are being explored for use in medical imaging and diagnostic devices due to their biocompatibility and optical clarity [62].

In conclusion, transparent glass/ceramics represent a vital class of materials in photonics, offering a unique blend of properties that make them indispensable for advanced optical applications. Continued research and development in synthesis methods.

2.3 Rare-earth doped ceramics: Illuminating photonics applications

Rare-earth (RE) elements, renowned for their unique electronic configurations and sharp, narrow emission lines, have profoundly impacted photonics. Their integration into ceramic matrices, resulting in rare-earth doped ceramics (RE-Cs), has unlocked a plethora of opportunities for enhancing and customizing optical properties, leading to innovative photonic devices [8, 15].

The core principle behind RE doping lies in introducing specific RE ions into the host ceramic lattice. These ions, typically trivalent (RE³⁺), possess partially filled 4f orbitals shielded by outer 5s and 5p orbitals. This unique electronic structure is responsible for the characteristic sharp emission bands and long fluorescence lifetimes exhibited by RE ions, rendering them ideal for a variety of photonic applications [8, 63]. RE doping significantly influences the optical properties of ceramics, manifesting in several ways. Notably, RE³⁺ ions act as luminescent centers, absorbing energy and emitting light at specific wavelengths. The emitted color is determined by the specific RE³⁺ ion and the host material. Certain RE³⁺ such as Er³⁺, Tm³⁺, Ho³⁺ and Nd³⁺ ions exhibit upconversion photoluminescence, a process where they absorb multiple low-energy photons and emit a higher-energy photons, effectively converting longer wavelengths to shorter ones [8, 15, 62, 63]. This capability has immense potential for applications like bioimaging and solar energy conversion. Conversely, some RE ions such as Eu³⁺, Tb³⁺, Ce³⁺ and Dy³⁺ display downconversion luminescence, where they absorb high-energy photons and emit lower-energy photons, which is advantageous for improving the efficiency of light-emitting diodes (LEDs) by shifting blue light to longer wavelengths within the visible spectrum [15, 25, 57, 61, 62]. Additionally, RE doping can influence the refractive index of the host ceramic, enabling the development of optical components with specific refractive index profiles [62]. A few common mechanisms of down and upconversion emissions are demonstrated schematically in Figure 2.

RE-Cs play a pivotal role in various photonic applications. They are widely used as gain media in solid-state lasers due to their high optical gain, narrow emission linewidth, and tunability [62]. These characteristics make them suitable for a wide array of applications, including telecommunications, medical diagnostics, and scientific research [4, 8, 18, 62, 64, 65]. RE-Cs contribute to energy-efficient solid-state lighting (SSL) by providing white light emission. They act as phosphors in white LEDs, converting blue light emitted from the semiconductor chip into longer wavelengths to produce white light [61, 62]. RE-Cs are also essential components in optical amplifiers, which boost signal strength in optical communication systems [18, 62]. They amplify optical signals without significant signal distortion, enabling long-distance communication.

The unique properties of RE-Cs have led to exciting state-of-the-art applications. They are being explored for their potential in quantum computing and communication due to their long coherence times and narrow emission lines, making them suitable for storing and processing quantum information [18, 65]. In biomedical applications, RE-Cs, especially in nanoparticle form, serve as biocompatible luminescent probes for bioimaging and diagnostics, their narrow emission bands and high quantum yields enable high sensitivity and specificity in biological applications [8, 21, 66, 67, 68]. RE-Cs are also being incorporated into advanced displays, offering enhanced color purity and efficiency, leading to broader color gamut and improved display quality [25, 50, 69].

In conclusion, rare-earth doped ceramics have revolutionized photonics, offering a wide array of optical functionalities and applications. Their unique luminescent properties and advancements in fabrication techniques continue to drive innovation in lasers, solid-state lighting, optical amplifiers, and emerging fields such as quantum computing and biomedicine. As research progresses, RE-Cs are poised to play an increasingly crucial role in shaping the future of photonics.

2.4 Advanced ceramics for nonlinear optics: Gateway beyond linearity

The realm of photonics is being reshaped by the burgeoning field of nonlinear optics (NLO), which explores the fascinating interactions of light with materials that exhibit nonlinear optical responses. Unlike linear optical phenomena, where the light’s behavior is directly proportional to the strength of the applied field, nonlinear optics involves complex interactions where the response of the material is dependent on the intensity of the light. This opens up a vast array of possibilities for manipulating and controlling light in ways previously inconceivable, leading to the development of advanced photonic devices with unique functionalities.

At the heart of nonlinear optics lies the concept of optical nonlinearity, a phenomenon where the polarization of a material changes non-linearly with the electric field strength of the incident light. This nonlinear response gives rise to various intriguing effects, including second or third harmonic generation (SHG, THD), sum and difference of frequency, two or three-photon absorption (2PA, 3PA) and saturable absorption (SA) [2, 70]. In SHG, light interacting with a non-centrosymmetric material is converted to a new frequency, double the original frequency. This process is crucial for generating new light sources and finds applications in frequency conversion, laser design, and microscopy. In 2PA (3PA), two (three) photons at a similar frequency are simultaneously absorbed by the NLO material. Saturable absorbers, on the other hand, have the unique property of absorbing light at low intensities but becoming transparent at high intensities. This property arises from the energy level structure of the dopant ions. This phenomenon is critical for laser Q-switching and mode-locking, enabling the generation of ultrashort laser pulses.

The search for materials possessing specific nonlinear optical properties has ignited intense research efforts, particularly focusing on ceramics. These materials offer several advantages over conventional materials for nonlinear optics. Ceramics exhibit a high resistance to laser-induced damage, making them ideal for high-power laser applications. Their robust chemical stability ensures their performance over time, even under harsh environmental conditions. Moreover, their composition and microstructure can be precisely controlled, allowing for fine-tuning of their optical properties, including nonlinear refractive index and SHG efficiency.

Significant progress has been made in developing new ceramic materials with enhanced nonlinear optical properties. Minerals, particularly perovskite oxides, like strontium barium niobate (SBN), lithium niobate (LiNbO₃), barium titanate (BaTiO₃), and KTiOPO₄ (KTP), have demonstrated remarkable SHG efficiencies and are under investigation for frequency conversion applications in telecommunications and laser technologies [1, 71, 72]. Rare-earth compounds with noncentrosymmetric crystal structures, like La₃Ga₅SnO₁₄, LiCs₂Y₂(PO₄)₃, Cd₄LuO(BO₃)₃, Na₈Lu₂(CO₃)₆F₂, Na₄La₂(CO₃)₅, Y(OH)₂NO₃ and much more, continue to garner widespread attention as an important class of nonlinear optical (NLO) materials [1]. This is because the highly distorted structural arrangements centered around the rare-earth ions can significantly enhance the materials’ second harmonic generation (SHG) properties. Chromium (Cr³⁺) and Cobalt (Co²⁺) are popular dopants for SA due to their strong absorption in the visible and near-infrared (NIR) region for a variety of laser applications [13, 73]. Cr:YAG, Cr:Sapphire, Cr:ZnS as well as Co:MgAl₂O₄, Co:ZnSe, Co:ZnAl₂O₄ are well-established materials for passive Q-switching and high-power applications with good thermal properties, enabling the generation of nanosecond, picosecond and femtosecond pulses and also used in integrated optics for pulse shaping and switching applications [13, 73].

The potential applications of nonlinear ceramic materials span various fields within photonics. Frequency conversion using SHG enables efficient signal generation in telecommunications, enhancing bandwidth and data transmission rates. SHG microscopy allows for label-free imaging of biological samples, providing insights into cellular structure and dynamics. Nonlinear ceramics are explored for advanced imaging modalities, like nonlinear optical microscopy, for non-invasive diagnostics and drug delivery [1, 70]. SA ceramics play a vital role in laser Q-switching, enabling the production of high-energy, short laser pulses used in diverse applications like laser cutting, marking, and micromachining [2, 13, 70, 73]. Nonlinear ceramics with sensitivity to specific parameters like temperature, pressure, or chemical species are being explored for developing robust optical sensors.

2.5 Ceramic in periodic structures: Controlling light with periodicity

Ceramic materials have emerged as pivotal components in the design and fabrication of periodic structures such as photonic crystals and metamaterials. These structures are engineered to manipulate electromagnetic waves in ways impossible with conventional materials. Photonic crystals, characterized by their periodic dielectric structures, influence the propagation of photons similarly to how semiconductor crystals affect electron movement [3]. These artificial crystals have a photonic bandgap, which is a range of frequencies where light cannot propagate through the material. This property allows for the control and manipulation of light at the nanoscale level, enabling the creation of devices with unique optical properties. Metamaterials, conversely, are artificially engineered to exhibit properties not found in nature, enabling unprecedented control over electromagnetic waves. Metamaterials are typically composed of artificially engineered structures made from a combination of metals, insulators, or both [74, 75]. The structure of metamaterials consists of subwavelength unit cells that are arranged in a periodic pattern. These unit cells can be made from various materials, such as metals like gold, silver, or copper, or insulators like dielectric materials. The choice of materials and the design of the unit cells determine the electromagnetic properties of the metamaterial, allowing it to exhibit unique optical characteristics. By designing the structure of these materials at the subwavelength scale, it is possible to manipulate light in ways that are not achievable with natural materials. Metamaterials can exhibit negative refractive index, cloaking abilities, and other extraordinary optical properties that go beyond what is possible with traditional materials [74]. Integrating ceramics into these periodic structures has opened new avenues in photonics, offering enhanced performance and novel functionalities [75].

The use of ceramics in fabricating periodic structures is important due to their exceptional properties, such as high-temperature resistance, chemical stability, mechanical strength, and diverse dielectric properties [3, 74, 75]. These properties make ceramics ideal for applications where traditional materials may not be suitable, especially for environments that demand high durability and performance under extreme conditions, such as in high-temperature environments or corrosive conditions. Furthermore, ceramics can be engineered to possess specific refractive indices, such as high refractive index contrast, which is crucial for the precise manipulation of light. The ability to tailor the optical properties of ceramics enhances their utility in creating photonic structures that require exact control over light propagation and interaction [76].

Recent advancements in fabrication techniques have significantly improved the ability to create complex ceramic photonic structures [77]. Techniques such as lithography, etching, and deposition processes have been optimized to create intricate structures with subwavelength features [76]. Additive manufacturing methods, such as 3D printing, allow for the layer-by-layer construction of ceramic materials with nanoscale precision and have also been explored for rapid prototyping of complex photonic structures [78]. This technique enables the production of intricate geometries that are essential for the functionality of photonic crystals and metamaterials. Sol-gel processing is another versatile approach, offering the ability to create ceramics with tailored porosity and refractive indices [17]. Additionally, laser-assisted fabrication techniques provide a means to directly write photonic structures with sub-micron resolution [79]. These methods collectively enhance the precision and functionality of ceramic photonic devices.

Ceramic photonic crystals and metamaterials have found extensive applications in various photonic devices, including optical filters, lasers, and sensors [3, 75, 80]. In optical filters, the periodic structure of photonic crystals creates photonic band gaps that can be engineered to selectively block or transmit specific wavelengths of light. This capability is crucial in applications such as wavelength division multiplexing in optical communications, where precise wavelength control is essential for efficient data transmission. In the realm of lasers, ceramic photonic crystals and metamaterials are employed to manipulate light within the laser cavity [77]. By incorporating these materials, it is possible to achieve enhanced control over the emission properties of the laser, such as wavelength, beam shape, and coherence [3]. The high sensitivity of photonic crystals to environmental changes makes them ideal for sensor applications [17]. This sensitivity is particularly beneficial in fields such as environmental monitoring, medical diagnostics, and industrial process control, where precise and reliable sensing is paramount.

2.6 Integrated photonics: Combining ceramics with miniaturization

Integrated photonics, the integration of optical functions onto a single chip, is revolutionizing various fields, including telecommunications, sensing, and computing [9]. The inherent properties of ceramics make them particularly well-suited for integrated photonic applications. Additionally, ceramics excel in heat dissipation, effectively mitigating thermal effects that can degrade device performance. This is crucial for high-power applications like lasers and optical amplifiers, where heat management is critical [9, 19]. Furthermore, ceramics are robust, resistant to harsh chemicals and environmental conditions, ensuring the long-term reliability and durability of photonic devices. Finally, many ceramics exhibit transparency across a wide range of wavelengths, enabling the development of devices operating in various spectral regions, from visible to infrared [6].

These advantages are already being harnessed in the development of various photonic devices. Ceramic-based waveguides with minimal propagation losses have been demonstrated for applications like high-speed data transmission, optical sensing, and optical interconnects [9]. Integrating multiple optical components like waveguides, splitters, and couplers on a single ceramic chip enables compact, efficient photonic integrated circuits (PICs) for various applications [19]. The high thermal conductivity of ceramics has also enabled the fabrication of high-power lasers with enhanced stability and efficiency.

However, challenges remain in the fabrication and integration of ceramic-based photonic devices. Achieving high-quality, low-loss waveguides with precise dimensions requires advanced microfabrication techniques, which can be complex and costly [10]. Furthermore, finding ceramics with the ideal combination of optical, thermal, and mechanical properties for specific applications can be challenging. Additionally, combining ceramic components with other materials like polymers or silicon requires careful consideration of compatibility and fabrication processes.

Despite these challenges, the potential of ceramics in integrated photonics is significant [9, 10]. Ongoing research focuses on developing new fabrication techniques, exploring novel ceramic materials, and enhancing integration with other photonic components. As this field progresses, we can expect to see miniaturized, high-performance photonic devices based on advanced ceramics, revolutionizing various sectors and opening up exciting possibilities for the future.

2.7 Ceramic-based biophotonics: A new frontier in harnessing light for biomedicine

The burgeoning field of biophotonics seeks to harness the power of light for biomedical applications, enabling advancements in diagnostics, therapeutics, and personalized medicine [8, 64]. While traditional silicon-based photonics has seen considerable success, ceramic materials are increasingly recognized for their unique advantages in biophotonic applications. Their biocompatibility, chemical inertness, and tailored optical properties make them promising candidates for developing advanced biophotonic devices.

Biocompatible ceramics, like hydroxyapatite (HA), bioactive glass, and alumina (Al₂O₃), are gaining traction in biophotonics due to their excellent biocompatibility and favorable optical properties [36, 81]. These materials can be engineered to possess specific optical properties like bioluminescence, fluorescence, or even upconversion, making them suitable for various biomedical applications. For example, HA, with its biocompatibility and excellent bioactivity, has shown potential for optical sensing platforms in bone regeneration and drug delivery applications [81].

Ceramic-based biophotonic devices are designed to leverage the unique optical properties of these materials for biomedical applications. These devices can be designed for optical diagnostics, drug delivery, and therapeutic applications [66, 68]. Upconversion nanoparticles (UCNPs), especially, composed of lanthanide-doped ceramics, exhibit remarkable properties. They can convert low-energy photons (e.g., near-infrared light) into higher-energy photons (e.g., visible light). This upconversion phenomenon has opened new possibilities for biophotonics, particularly in bioimaging and sensing (Figure 3) [8, 68]. The use of UCNPs allows for deep tissue imaging with minimal autofluorescence and improved signal-to-noise ratio. Moreover, UCNPs can be used to deliver light to specific targets for photodynamic therapy, where they generate reactive oxygen species for cancer cell destruction [66].

Biophotonic ceramics are poised to play a crucial role in the future of personalized medicine and advanced healthcare [68]. The ability to tailor their optical properties and biocompatibility opens up exciting avenues for developing novel biophotonic devices. Integrating ceramics with other materials like polymers and metals can create hybrid devices with enhanced functionality, enabling advancements in personalized medicine, targeted drug delivery, and minimally invasive therapies [8, 82]. The development of biocompatible ceramic micro- and nanostructures, coupled with advances in upconversion technology, could revolutionize optical diagnostics and therapeutics.

In conclusion, biophotonic ceramics represent a promising frontier in biomedical applications, offering enhanced biocompatibility, tailored optical properties, and a wide range of functionalities. Their integration into biophotonic devices holds immense potential for revolutionizing optical diagnostics, therapies, and personalized medicine. Further research and development in this field can pave the way for transformative innovations in healthcare.