Photonic Structures Research Articles

Evolution of an Overlapped Bandgap and Topological Flat Bands in a Higher‐Order Valley Photonic Insulator Based on Dendritic Structure

AbstractRobust edge states and corner states in photonic topological insulators provide effective ways to manipulate the propagation of electromagnetic waves. Bandgaps of the previously reported photonic topological insulators are independent of each other. In this paper, a higher‐order valley photonic insulator composed of arrays of dendritic structure is designed. The band structure shows an overlapped bandgap is observed, and the overlapped bandgap divides the band structure into three bandgaps. The evolution of the overlapped bandgap is investigated by changing the geometric parameters and increasing the fractal to break the C3v symmetry drastically. Besides, it is notable that the band structure of the proposed valley photonic insulator is flat bands. It is demonstrated that undistorted transmission can be observed as a plane electromagnetic wave transmits through the proposed valley photonic insulator. The interface consisting of two valley photonic structures with distinct topological nontrivial phases is constructed. Edge states and corner states with strong energy localization are obtained in multi‐band frequencies. Remarkably, two triangular structures with different Wannier center configurations both have corner states in the same bandgap, which does not obey the valley selectivity. The phenomenon is caused by the weak valley locking property due to the overlapped bandgap. The proposed valley photonic insulators are expected to benefit applications in optical devices such as topological lasers.

Impact of Tamm plasmon structures on fluorescence and optical nonlinearity of graphene quantum dots

Graphene Quantum Dots (GQDs) are crucial in biomedicine for sensitive biosensing and high-resolution bioimaging and in photonics for their nonlinear optical properties. Integrating GQDs with photonic structures enhances optical properties by optimizing light-matter interactions and enabling precise control over their emission wavelengths. In this work, we explore a facile synthesis method for GQDs by pulsed laser irradiation in chlorobenzene and highlight the transformative potential of Tamm Plasmon Cavity (TPC) structures for tuning and amplifying the photoluminescence and nonlinear optical properties of GQDs. The characterization of GQDs revealed their exceptional properties, including efficient optical limiting and stable photoluminescence. The study demonstrated that the TPC structure significantly amplifies nonlinear optical effects due to the high light-matter interaction, indicating the potential for advanced optical systems, including optical limiters and nonlinear optical devices. Furthermore, introducing GQDs into the TPC structure leads to a significant enhancement and tuning of fluorescence emission. The Purcell effect, in combination with the confined electromagnetic fields within the TPC, increases the spontaneous emission rate of GQDs and subsequently enhances the fluorescence intensity. This enhanced and tunable fluorescence has exciting implications for high-sensitivity applications such as biosensing and single-molecule detection.

Large-scale photonic inverse design: computational challenges and breakthroughs

Abstract Recent advancements in inverse design approaches, exemplified by their large-scale optimization of all geometrical degrees of freedom, have provided a significant paradigm shift in photonic design. However, these innovative strategies still require full-wave Maxwell solutions to compute the gradients concerning the desired figure of merit, imposing, prohibitive computational demands on conventional computing platforms. This review analyzes the computational challenges associated with the design of large-scale photonic structures. It delves into the adequacy of various electromagnetic solvers for large-scale designs, from conventional to neural network-based solvers, and discusses their suitability and limitations. Furthermore, this review evaluates the research on optimization techniques, analyzes their advantages and disadvantages in large-scale applications, and sheds light on cutting-edge studies that combine neural networks with inverse design for large-scale applications. Through this comprehensive examination, this review aims to provide insights into navigating the landscape of large-scale design and advocate for strategic advancements in optimization methods, solver selection, and the integration of neural networks to overcome computational barriers, thereby guiding future advancements in large-scale photonic design.

Temperature dependence analysis of intersubband absorption in asymmetric ZnO/MgZnO quantum wells: Potential for terahertz emission applications

In this paper, in ZnO/MgxZn1−xO quantum well structures, with 20% of Mg, we have calculated the electronic states of confined levels and the intersubband absorption coefficient with or without the effect of non-parabolicity. We use the finite difference approach to do our computations inside the envelope function formalism approximation. The results show that the wavelength range covered by the E12 transition (9–16 μm) is mid-infrared to far-infrared. According to our findings, ZnO/MgZnO quantum well structures, which have extremely similar lattice characteristics and the benefit of an unconstrained structure, have E23 transition energies that provide terahertz frequencies (1.1—20THz) that are utilized in airport security and explosives detection. This promotes intersubband photonic structure design and epitaxial development. Furthermore, we have examined the impact of temperature on the Fermi level and the load carrier population inside each level for these remarkable structures. In addition, each transition's oscillation force and intersubband absorption are considered.

Hierarchical Gradient Structural Porous Metamaterial with Selective Spectral Response for Daytime Passive Radiative Cooling

AbstractPorous photonic structures have greatly advanced large‐scale radiative cooling application owing to its satisfying optical properties, easy‐designation, and low cost. However, current reported porous radiative cooling materials mainly focuses on uniform or random pore distribution structure, which failed to achieve precise spectrum control for sunlight and mid‐infrared light, thereby weakening the radiative cooling performance. Herein, gradient structural porous metamaterials (GSPMs) are proposed and successfully constructed for maximizing cooling effectiveness based on step‐by‐step freeze‐casting technology. The designed GSPMs with gradient micro‐nano porous structure can wide‐range scatter entire solar spectrum while possess gradual refractive index transition to increase air‐medium interface absorption at mid‐infrared regions. Notably, solar reflectivity and mid‐infrared emissivity of GSPMs reach 97.3% and 97.6%, achieving maxinum cooling temperature of 8.7 °C and net cooling power of 94.1 W m−2, presenting a higher cooling effect than random porous materials. Moreover, GSPMs enable achieve compatibility with color aesthetic and cooling superiorities due to selective spectra response behavior, as well as can be applied in building materials, realizing efficient energy saving and CO2 emission reduction. This work proposed gradient porous structure can achieve highly efficient daytime radiative cooling, offering new possibilities in the structure design of next‐generation porous radiative cooling materials.

Resonator embedded photonic crystal surface emitting lasers

The finite size of 2D photonic crystals results in them being a lossy resonator, with the normally emitting modes of conventional photonic crystal surface emitting lasers (PCSELs) differing in photon lifetime via their different radiative rates, and the different in-plane losses of higher order spatial modes. As a consequence, the fundamental spatial mode (lowest in-plane loss) with lowest out-of-plane scattering is the primary lasing mode. For electrically driven PCSELs, as current is increased, incomplete gain clamping results in additional spatial (and spectral) modes leading to a reduction in beam quality. A number of approaches have been discussed to enhance the area (power) scalability of epitaxy regrown PCSELs through careful design of the photonic crystal atom1–3. None of these approaches tackle the inflexibility in being unable to independently modify the photon lifetime of the different modes at the Γ2 point. As a method to introduce design flexibility, resonator embedded photonic crystal surface emitting lasers (REPCSELs) are introduced. This device, combining comparatively low coupling strength photonic crystal structures along with perimeter mirrors, allow a Fabry–Pérot resonance effect to be realised that provides wavelength selective modification of the photon lifetime. We show that surface emission of different surface emitting modes may be selectively enhanced, effectively changing the character of the modes at the Γ2 point. This is a consequence of the selective modification of in-plane loss for particular modes, and is dependent upon the alignment of the photonic crystal (PhC) band-structure and distributed Bragg reflectors’ (DBRs) reflectance spectrum. These findings offer new avenues in surface emitting laser diode engineering. The use of DBRs to reduce the lateral size of a PCSEL opens the route to small, low threshold current (Ith), high output efficiency epitaxy regrown PCSELs for high-speed communication and power sensitive sensing applications.

Free-standing microscale photonic lantern spatial mode (De-)multiplexer fabricated using 3D nanoprinting

Photonic lantern (PL) spatial multiplexers show great promise for a range of applications, such as future high-capacity mode division multiplexing (MDM) optical communication networks and free-space optical communication. They enable efficient conversion between multiple single-mode (SM) sources and a multimode (MM) waveguide of the same dimension. PL multiplexers operate by facilitating adiabatic transitions between the SM arrayed space and the single MM space. However, current fabrication methods are forcing the size of these devices to multi-millimeters, making integration with micro-scale photonic systems quite challenging. The advent of 3D micro and nano printing techniques enables the fabrication of freestanding photonic structures with a high refractive index contrast (photopolymer-air). In this work we present the design, fabrication, and characterization of a 6-mode mixing, 375 µm long PL that enables the conversion between six single-mode inputs and a single six-mode waveguide. The PL was designed using a genetic algorithm based inverse design approach and fabricated directly on a 7-core fiber using a commercial two-photon polymerization-based 3D printer and a photopolymer. Although the waveguides exhibit high index contrast, low insertion loss (−2.6 dB), polarization dependent (−0.2 dB) and mode dependent loss (−4.4 dB) were measured.

Optimization of waveguide fabrication processes in lithium-niobate-on-insulator platform.

Lithium niobate (LN) is used in diverse applications such as spectroscopy, remote sensing, and quantum communications. The emergence of lithium-niobate-on-insulator (LNOI) technology and its commercial accessibility represent significant milestones. This technology aids in harnessing the full potential of LN's properties, such as achieving tight mode confinement and strong overlap with applied electric fields, which has enabled LNOI-based electro-optic modulators to have ultra-broad bandwidths with low-voltage operation and low power consumption. Consequently, LNOI devices are emerging as competitive contenders in the integrated photonics landscape. However, the nanofabrication, particularly LN etching, presents a notable challenge. LN is hard, dense, and chemically inert. It has anisotropic etch behavior and a propensity to produce material redeposition during the reactive-ion plasma etch process. These factors make fabricating low-loss LNOI waveguides (WGs) challenging. Recognizing the pivotal role of addressing these fabrication challenges for obtaining low-loss WGs, our research focuses on a systematic study of various process steps in fabricating LNOI WGs and other photonic structures. In particular, our study involves (i) careful selection of hard mask materials, (ii) optimization of inductively coupled plasma etch parameters, and finally, (iii) determining the optimal post-etch cleaning approach to remove redeposited material on the sidewalls of the etched photonic structures. Using the recipe established, we realized optical WGs with total (propagation and coupling) loss value of -10.5 dB, comparable to established values found in the literature. Our findings broaden our understanding of optimizing fabrication processes for low-loss lithium-niobate waveguides and can serve as an accessible resource in advancing LNOI technology.

Cephalopod-inspired Mxene-SEBS/TiO2 composite film for synergistic solar and infrared radiative heat management

Flexible biomimetic photonic structures, capable of achieving dynamic radiation modulation in both solar and infrared (IR) bands as required, are increasingly catering to low-energy thermal management and sustainable development. Herein, the cephalopod-inspired composite film of styrene-ethylene-butene-styrene block copolymer (SEBS)/titanium dioxide (TiO2)-Mxene has been designed and prepared. Analogous to squid leucophores, the SEBS/TiO2 layer of is able to regulate solar reflection and transmit infrared radiation. Mimicking squid iridocytes, the Mxene layer can regulate infrared reflection and absorb sunlight. The optimal structural parameters of the film and the regulation contrast were estimated via theoretical analysis. The theoretical design also illustrates that the two layers collaborate without interference. According to the optimal parameters, the Mxene-SEBS/TiO2 film was prepared. A dynamic regulation between radiative heating and cooling, namely, the solar reflection between 38 % and 66 % and infrared emissivity between 68 % and 89 % can be realized via mechanical actuation under 200 % strain. As the solar radiation is ∼650 W/m2, the composite film achieves ∼6.2 °C heating/∼1.4 °C cooling of the skin under mechanical actuation. When reducing the natural convection loss, the film further achieves ∼11.6 °C heating/∼0.8 °C cooling of the skin. This work provides a novel system and strategy for radiation modulation in solar-infrared bands for possible applications in thermal management.

Preparation of CO2‐based waterborne polyurethane/cellulose nanocrystal composite films and research of their radiative cooling properties

AbstractConventional colored radiative cooling (ColPRC) has a single color and limited reflection of visible light bands, which affects its radiative cooling effect, and the introduction of multiple photonic crystal structures can improve its radiative cooling performance. Therefore, in this study, a composite film was established by cross‐linking cellulose nanocrystals (CNC) with CO2‐based waterborne polyurethane (WPU) to obtain multistage structural colors. X‐ray diffraction pattern, thermogravimetric analysis, scanning electron microscopy, spectral analysis, and so on were used to examine the structure and properties of the films. The results showed that with the increase of CNC content, the color shifted blue, the crystallinity increased, the flame retardancy increased, but the thermal stability decreased; the addition of WPU assists in resisting moisture; the reflectivity of the multistage structured films is improved in the VIS–NIR range compared with the monochromatic films (an average increase of 5%), which helps to reduce solar irradiation in the visible wavelengths; high emissivity (≈90%) in the atmospheric window, which facilitates deep‐space heat exchange. The maximum temperature difference of the multistage structural color film can reach 6°C under strong solar irradiation. This research can be applied in photovoltaic devices or outdoor thermal management to provide new ways to address human energy efficiency.

Directional thermal emission and display using pixelated non-imaging micro-optics

Thermal radiation is intrinsically broadband, incoherent and non-directional. The ability to beam thermal energy preferentially in one direction is not only of fundamental importance, but it will enable high radiative efficiency critical for many thermal sensing, imaging, and energy devices. Over the years, different photonic materials and structures have been designed utilizing resonant and propagating modes to generate directional thermal emission. However, such thermal emission is narrowband and polarised, leading to limited thermal transfer efficiency. Here we experimentally demonstrate ultrabroadband polarisation-independent directional control of thermal radiation with a pixelated directional micro-emitter. Our compact pixelated directional micro-emitter facilitates tunable angular control of thermal radiation through non-imaging optical principles, producing a large emissivity contrast at different view angles. Using this platform, we further create a pixelated infrared display, where information is only observable at certain directions. Our pixelated non-imaging micro-optics approach can enable efficient radiative cooling, infrared spectroscopy, thermophotovoltaics, and thermal camouflaging.

Integrated photonic encoder for low power and high-speed image processing.

Modern lens designs are capable of resolving greater than 10 gigapixels, while advances in camera frame-rate and hyperspectral imaging have made data acquisition rates of Terapixel/second a real possibility. The main bottlenecks preventing such high data-rate systems are power consumption and data storage. In this work, we show that analog photonic encoders could address this challenge, enabling high-speed image compression using orders-of-magnitude lower power than digital electronics. Our approach relies on a silicon-photonics front-end to compress raw image data, foregoing energy-intensive image conditioning and reducing data storage requirements. The compression scheme uses a passive disordered photonic structure to perform kernel-type random projections of the raw image data with minimal power consumption and low latency. A back-end neural network can then reconstruct the original images with structural similarity exceeding 90%. This scheme has the potential to process data streams exceeding Terapixel/second using less than 100 fJ/pixel, providing a path to ultra-high-resolution data and image acquisition systems.

Waveguide-Coupled Light Photodetector Based on Two-Dimensional Molybdenum Disulfide.

The integration of transition metal dichalcogenides with photonic structures such as sol-gel SiOx:TiOy optical waveguides (WGs) makes possible the fabrication of photonic devices with the desired characteristics in the visible spectral range. In this study, we propose and experimentally demonstrate a MoS2-based photodetector integrated with a sol-gel SiOx:TiOy WG. Based on the spectroscopic measurements performed for our device, we concluded that the light entering the WG is almost completely channeled out from the WG and absorbed by the MoS2 flake, which is deposited on the WG. Therefore, this device works as a photodetector. The light coupling into the MoS2 region in this device construction is due to the high contrast of refractive index between the van der Waals crystal and the sol-gel WG, which is ∼4 and ∼1.8, respectively. The obtained MoS2-based photodetectors exhibit a photoresponsivity of 0.3 A W-1 (n-type MoS2) and 7.53 mA W-1 (p-type MoS2) at a bias voltage of 2 V. These results reveal great potential in the integration of sol-gel WGs with van der Waals crystals in optoelectronic applications.

Chirality tuning and reversing with resonant phase-change metasurfaces.

Dynamic control of circular dichroism in photonic structures is critically important for compact spectrometers, stereoscopic displays, and information processing exploiting multiple degrees of freedom. Metasurfaces can help miniaturize chiral devices but only produce static and limited chiral responses. While external stimuli can tune resonances, their modulations are often weak, and reversing continuously the sign of circular dichroism is extremely challenging. Here, we demonstrate the dynamically tunable chiral response of resonant metasurfaces supporting chiral bound states in the continuum combining them with phase-change materials. Phase transition between amorphous and crystalline phases allows for control of chiral response and varies chirality rapidly from -0.947 to +0.958 backward and forward via the chirality continuum. Our demonstrations underpin the rapid development of chiral photonics and its applications.

Strong Raman enhancement in structured colloids: localization of light

Raman spectroscopy is a powerful technique for studying the interaction between light and matter. Here we show a significant enhancement of Raman emission over a broad range of pumping wavelengths from strongly scattering media comprising spatially correlated photonic structures of core–shell TiO2@Silica scatterers mixed with silica nanoparticles and suspended in ethanol. Long-range Coulomb interactions between nanoparticles inside these photonic colloidal structures induce a correlation in the scatterers’ positions (TiO2@Silica), affecting local and global photonic properties. The anomalous enhancement in Raman signal increases as the scattering strength is increased (through either scatterer concentration or pumping wavelength); however, the signal strength continues to behave linearly with excitation power, ruling out classical nonlinear and interferential phenomena. These observations may indicate strong photon correlation in strongly localized electromagnetic modes, inducing successive photon interactions with the atoms or molecules. Aside from the fundamental relevance to understanding measurable properties in this regime of strongly localized electromagnetic modes, our demonstration of strongly enhanced Raman emission over a broad range of pumping wavelengths provides new opportunities for the development of advanced photonic materials and devices.

Simulation Computation of Two-dimensional Hexagonal Honeycomb Photonic Crystals Energy Band Structure Based on COMSOL

This study employs COMSOL software, integrating the plane wave expansion method and finite element method, to simulate and analyze the energy band structure of two-dimensional hexagonal honeycomb photonic crystals. By setting parameters and conducting scans, this paper not only maps out the electric field intensity and energy band distribution but also discovers a significant enhancement of electric field intensity at the lattice's high symmetry points, indicating a higher likelihood of electromagnetic waves' presence at these points. Furthermore, the notable energy gaps between the second and third energy bands provide directions for subsequent research. The findings of this study are of significant value for research and development in optoelectronic integration and communication, demonstrating an effective comprehensive method for analyzing the energy band structure of two-dimensional hexagonal honeycomb photonic crystals.

Visible-infrared compatible and independent camouflage with multicolor patterns and tunable emissivity

Abstract With the rapid development and wide application of visible (VIS) and infrared (IR) detections, it is necessary to explore visible-infrared (VIS-IR) compatible camouflage. Here, we report a VIS-IR compatible and independent camouflage device which is composed of the upper IR-transparent VIS-color-patterned layer and the lower electrochromic IR layer. The upper layer has amorphous photonic structure of polystyrene nanospheres (PSNSs). By customizing the PSNS size, various colors can be realized for VIS camouflage. The lower electrochromic IR layer takes advantage of multiwall carbon nanotubes (MWCNTs) as the electrode as well as the IR active material. Experimental results reveal that different colors (including blue, green, and purple) have been obtained, and the IR emissivity can be electrically regulated from 0.43 to 0.9. Moreover, the prototype also exhibits good electrical stability as well as hydrophobic characteristic (the water contact angle of the outmost surface exceeds 120°). These output performances demonstrate the success of our design strategy for promoting the finding applied in camouflage fields as well as energy conservation fields.

CdS@CS/PVA inverse opal films as photocatalyst

We prepared CdS@CS/PVA inverse opal (IO) films by the colloidal photonic crystal (CPC) template method. The photonic structure of the CdS@CS/PVA IO films consists of three-dimensional periodic macropores interconnected by CS/PVA polymers. The as-prepared CdS@CS/PVA IO films were utilized as photocatalysts for the degradation of methylene blue dyes. The degradation rate of methylene blue by the CdS@CS/PVA IO film catalyst was 92%, while that of the CdS@CS/PVA film catalyst was 55% for 6 h. The photocatalytic degradation effect of the CdS@CS/PVA IO film catalyst on methylene blue is [Formula: see text]1.7 times higher than that of the CdS@CS/PVA film catalyst, which proves that the introduction of the inverse opal structure into the photocatalyst is beneficial to improve the efficiency of the photocatalyst for degrading dyes.

Synthesis of Controllable Superparamagnetic Nano Fe3O4 Based on Reduction Method for Colloidal Clusters of Magnetically Responsive Photonic Crystals.

In this paper, we designed and investigated a reduction-based method to synthesize controllably monodisperse superparamagnetic nano Fe3O4 colloidal clusters for magnetically responsive photonic crystals. It was shown that the addition of ascorbic acid (VC) to the system could synthesize monodisperse superparamagnetic nano Fe3O4 and avoided the generation of γ-Fe2O3 impurities, while the particle size and saturation magnetization intensity of nano Fe3O4 gradually decreased with the increase of VC dosage. Nano Fe3O4 could be rapidly assembled into photonic crystal dot matrix structures under a magnetic field, demonstrating tunability to various diffraction wavelengths. The nano Fe3O4 modified by polyvinylpyrrolidone (PVP) and silicon coated could be stably dispersed in a variety of organic solvents and thus diffracted different wavelengths under a magnetic field. This is expected to be applied in various scenarios in the field of optical color development.

A novel photonic crystal hydrogel inverse opal fluorescence enhancement platform for highly sensitive immunoassay of tumor markers

Cancer is a significant global health issue, necessitating early detection for effective diagnosis and treatment. We presented a novel method for detecting tumor markers using fluorescence-enhanced photonic crystal hydrogel inverse opal (PC-GELIO) membranes. We efficiently fabricated large-area, low-crack photonic crystal membranes using a homemade microfluidic chip. Composed of PEG-DA and PEG, the hydrogel replicated the photonic crystal structure, yielding photonic crystal hydrogel inverse opal membranes with a unique photonic band gap and inverse opal porous structure. These properties afford the PC-GELIO platform several advantages: Firstly, the high stability of the internal nanostructure in the large-area, low-crack photonic crystals film, prepared using the homemade microfluidic chip, is crucial for preserving the platform's exceptional fluorescence-enhanced sensing capabilities. Secondly, the peak wavelength emitted by fluorescent molecules aligns with the photonic band gap (PBG), significantly enhancing the fluorescence signal through the low-photon effect. Lastly, the high specific surface area provides numerous binding targets for biomolecule modification, and the hydrogel offers a uniform liquid environment conducive to biomolecule binding. We evaluated the fluorescence-enhanced sensing performance of the platform using alpha-fetoprotein (AFP). The limit of quantification (LOQ) for AFP was determined to be as low as 23 pg/mL, representing an order of magnitude improvement compared to non-enhanced platforms (0.29 ng/mL and 0.25 ng/mL). Consequently, this novel fluorescence-enhanced photonic crystal hydrogel inverse opal platform holds significant potential as a straightforward and efficient method for biomolecular detection.