Low-index Layer Research Articles

The role of photonic crystal based architectures on tailoring the laser induced fluorescence from green emitting cesium lead bromide nanocrystals

Researchers have long been fascinated by fluorescence and its potential to be boosted as it unlocks a treasure trove of promising applications. In this context, this experimental study investigates the amplification of laser induced fluorescence (LIF) emission from cesium lead bromide (CsPbBr3) perovskite nanocrystals (PNC) using two distinct all-polymer photonic crystal (PC) architectures made of poly 9-vinyl carbazole (PVK) and cellulose acetate (CA) as high and low refractive index (RI) layers, respectively. First structure is a photonic crystal micro cavity (PCMC) where PNC incorporated PVK (PNC:PVK) polymer matrix forms the defect layer sandwiched between two similar PCs. In the second structure, PNC:PVK composite itself functions as high RI layers within the PC. These two designs exploit different mechanisms to achieve fluorescence enhancement. The findings demonstrate the promise of simple, solution processed polymer PCs on applications demanding intensified fluorescence output and pave way to the development of flexible photonic devices capable of manipulating light emission.

Multilayer dielectric reflector using low-index nanolattices.

Dielectric mirrors based on Bragg reflection and photonic crystals have broad application in controlling light reflection with low optical losses. One key parameter in the design of these optical multilayers is the refractive index contrast, which controls the reflector performance. This work reports the demonstration of a high-reflectivity multilayer photonic reflector that consists of alternating layers of TiO2 films and nanolattices with low refractive index. The use of nanolattices enables high-index contrast between the high- and low-index layers, allowing high reflectivity with fewer layers. The broadband reflectance of the nanolattice reflectors with one to three layers has been characterized with peak reflectance of 91.9% at 527 nm and agrees well with theoretical optical models. The high-index contrast induced by the nanolattice layer enables a normalize reflectance band of Δλ/λo of 43.6%, the broadest demonstrated to date. The proposed nanolattice reflectors can find applications in nanophotonics, radiative cooling, and thermal insulation.

Low-loss plasmonic resonance using surface Bloch waves in photoplasmonic metamaterials

We present a method for the suppression of the radiative loss of plasmonic resonant modes by efficiently coupling them with the nonradiative Bloch surface waves in photoplasmonic metamaterials, comprised of a 2D array of plasmonic resonators deposited on the low-index termination layer of a 1D photonic crystal. The number of photonic crystal bilayers plays a crucial role in the plasmonic radiative loss, with Q-values increasing with incrementing photonic crystal bilayers reaching values of ∼580 in the UV-Vis regime. We also show that the fundamental plasmonic mode of the metamaterial split into different energy levels corresponding to surface Bloch waves associated with each bilayer of the photonic crystal.

Tunable Emission Enhancement in Surface Plasmon‐Enhanced GaN Light‐Emitting Diode Based on the Ag–SiO2 (Ag‐Rich) Cermet Material

The metal–dielectric cermet (metal‐rich) material is utilized for turning the dispersion properties of surface plasmon in GaN‐light‐emitting diode. The designed structure contains a cermet layer coated on p‐GaN layer. The optimized ingredients in the cermet material and thickness of the cermet layer are obtained by the simulations. For enhancing the extraction of surface plasmon, the designed structure is further optimized by inserting a layer of low refractive index. The calculated results indicate that the Purcell factor in optimized structure is greater than 100 over the emission spectrum and the extraction efficiency of surface plasmon is effectively improved. The photoluminescence (PL) experiment by bottom excitation demonstrates that peaks of PL spectra of the designed and optimized samples are enhanced by 1.6 and 3.4 times, respectively, compared with the sample covered by Ag film. Compared with the naked sample, peaks of PL spectra of bottom pumping of optimized and designed samples are enhanced by 36.3 and 17.2 times, respectively. Peaks of normalized PL spectra by top excitation of optimized and designed samples are enhanced by 29.0 and 9.1 times, respectively. The results of numerical calculations are substantially consistent with the results of PL tests.

Ultrahigh-Q guided mode resonances in an All-dielectric metasurface

High quality(Q) factor optical resonators are indispensable for many photonic devices. While very large Q-factors can be obtained theoretically in guided-mode settings, free-space implementations suffer from various limitations on the narrowest linewidth in real experiments. Here, we propose a simple strategy to enable ultrahigh-Q guided-mode resonances by introducing a patterned perturbation layer on top of a multilayer-waveguide system. We demonstrate that the associated Q-factors are inversely proportional to the perturbation squared while the resonant wavelength can be tuned through material or structural parameters. We experimentally demonstrate such high-Q resonances at telecom wavelengths by patterning a low-index layer on top of a 220 nm silicon on insulator substrate. The measurements show Q-factors up to 2.39 × 105, comparable to the largest Q-factor obtained by topological engineering, while the resonant wavelength is tuned by varying the lattice constant of the top perturbation layer. Our results hold great promise for exciting applications like sensors and filters.

Bragg reflector one-dimensional multi-layer structure sensor for the detection of thyroid cancer cells

In the proposed work, a defect cavity multi-layer Bragg reflector structure is proposed theoretically to find the presence of thyroid cancer cells in the given sample. The modelling, design and analysis of the sensor is performed using characteristic matrix method (CMM). Proposed structure has central defect cavity with 6 pairs of low and high refractive index layers on each side of the defect. To enhances the sensor sensitivity, the incident light in mid-infrared frequency range is used as input light source. The refractive index of normal and thyroid cancer cells is analysed for the performance of the sensor. The obtained Q factor and sensitivity of the sensor design is 3729 and 2828 nm/RIU respectively. The proposed sensor is a best choice of optical sensor for the detection of thyroid cancer cells in the given test sample for accurate analysis in medical applications.

Silicon Oxy-Nitride for the Low Refractive Index Layers in the Mirror Coatings of the Cryogenic Laser Interferometer Gravitational Waves Detector.

The low refractive index layers in the mirror coatings of the room-temperature laser interferometer gravitational waves detectors are silica deposited by the ion beam sputter method. However, the silica film suffers from the cryogenic mechanical loss peak, hindering its application for the next generation detector operated at cryogenics. New low refractive index materials need to be explored. We study amorphous silicon oxy-nitride (SiON) films deposited using the method of plasma-enhanced chemical vapor deposition. By changing the N_{2}O/SiH_{4} flow rate ratio, we can tune the refractive index of the SiON smoothly from nitridelike to silicalike of ∼1.48 at 1064nm, 1550nm, and 1950nm. Thermal anneal reduced the refractive index down to ∼1.46 and effectively reduced the absorption and cryogenic mechanical loss; the reductions correlated with the N─H bond concentration decrease. Extinction coefficients of the SiONs at the three wavelengths are reduced down to 5×10^{-6}∼3×10^{-7} by annealing. Cryogenic mechanical losses at 10K and 20K (for ET and KAGRA) of the annealed SiONs are significantly lower than the annealed ion beam sputter silica. They are comparable at 120K (for LIGO-Voyager). Absorption from the vibrational modes of the N─H terminal-hydride structures dominates over the absorption from other terminal hydrides, the Urbach tail, and the silicon dangling bond states in SiON at the three wavelengths.

Suppressing the mechanochromism of flexible photonic crystals.

Photonic crystal slabs (PCS) are a promising platform for optical biosensing. Yet, flexible applications based on PCS for biosensing have been limited, as the mechanical properties influence the optical ones. Here, we show the suppression of the mechanochromism effect for flexible PCS. We obtained flexible photonic crystal slabs by sputtering of a dielectric 100 nm Nb2O5 high refractive index layer onto a flexible nanostructured polydimethylsiloxane (PDMS) substrate with 370 nm grating period. The PCS exhibit a guided mode resonance at around 650 nm. We demonstrate that these flexible photonic crystal slabs show less than 0.5 nm resonance shift for 4% strain and call them stabilized PCS (sPCS). We compare this to a resonance shift of ∼21 nm for ∼4% strain of a flexible photonic crystal with a flexible nanoparticle high index layer (mechanochromatic PCS, mPCS). This high resonance shift is expected from the Bragg equations, where 4% grating period change correspond to approximately 4% change of the resonance wavelength (i.e., ∼26 nm at a resonance wavelength of 650 nm), if changes in the mode effective refractive index are neglected. In a stretch series we obtain color-to-strain dependencies of 4.79 nm/% strain for mPCS and 0.11 nm/% strain for our stabilized sPCS. We analyze the suppression of the mechanochromism with detailed microscopy results. We observe that fissures and fractures form in the rigid waveguiding layer of the sPCS upon mechanical stress. An algorithm based on Holistically-Nested Edge Detection (HED) is used for automated counting of cracks. Rigid photonic crystal cells with sizes on the order of 10 µm to 100 µm are formed that explain the stable optical properties. Even more stable optical properties with less than 0.03 nm wavelength shift per 1% strain are demonstrated for sPCS with an additional dielectric 100 nm SiO2 low index layer beneath the Nb2O5 waveguide layer decoupling the waveguide further from the flexible PDMS substrate.

Optimizing Bulk and Surface Sensitivity With Fiber-Tip Leaky Mode Resonances Excited by Low Refractive Index Overlay

In this work, an all-dielectric fiber-tip sensing platform is proposed by which multiple nonpolarized leaky mode resonances (LeMRs) are generated for sensitively decoupling bulk and surface characteristics. The key point is to excite fiber-tip leaky modes with a layer of low refractive index (RI) material (here magnesium fluoride thin film) deposited on the fiber-tip end facet, but not high RI films. In this configuration, the fiber-tip thin film works as a planar film waveguide, which is confirmed by the mode equation obtained here. Both the excitation and sensing principles are obtained by analyzing the influence of bulk and surface characteristics on the mode dispersion. Similar with the case of high RI films, the results also verify the capability of decoupling bulk and surface properties with a single fiber-tip LeMR excited with low RI film. Another important advance is that both bulk and surface sensitivities can be greatly improved, respectively, up to 107.6 dB/RIU and 16.9 nm/nm (being several tenfolds higher than previous reports). The proposed fiber-tip LeMR configuration could open a new paradigm in the domain of “lab-on-optrode” technology, which is highly appealing for multifunctional plug and play biosensing.

Broadband antireflection coatings with low polarization splitting.

A major problem for optical systems is the polarization splitting occurring at any interface in the event of oblique light. Low-index nanostructured silica layers were produced by overcoating an initial organic structure with silica and subsequent removal of the organic constituents. The nanostructured layers can be tailored to achieve defined low effective refractive indices down to 1.05. They can also be stacked together with homogeneous layers to produce broadband antireflective coatings with very low polarization splitting. Especially thin interlayers that separate the low-index structured layers turned out to be useful to optimize the polarization properties.

Fabrication of three-dimensional opal nanolattices using template-directed colloidal assembly

Three-dimensional (3D) nanostructures play a crucial role in nanophotonics, lasers, and optical systems. This article reports on the fabrication of 3D nanostructures consisting of opal structures that are spatially aligned to an array of holes defined in the photoresist. The proposed method uses colloidal lithography to pattern a hexagonal array of holes, which are then used to direct the subsequent 3D assembly of colloidal particles. This approach allows the 3D opal structures to be aligned with the 2D array of holes, which can enhance spatial-phase coherence and reduce defects. The polymer patterns can be used as a sacrificial template for atomic layer deposition and create free-standing nanolattices. The final structure consists of a combination of nanolattice, upon which controlled deposition of opal structures is achieved. These structures result in nanostructured materials with high porosity, which is essential to create low-index materials for nanophotonics. A thick layer of titanium oxide with high refractive index is deposited over nanolattices to demonstrate the mechanical stability of underlying structures. These nanolattice structures with precisely controlled height can serve as a low-index layer and can find applications in Bragg reflectors, nanophotonics, and optical multilayers.

Anisotropic slot waveguides with bulk transition metal dichalcogenides for crosstalk reduction and high-efficiency mode conversion.

Conventional slot waveguides (CSWs) consisting of an isotropic low-index material sandwiched by two high-index silicon wires have been extensively used in functional photonic devices, including chemical sensing, optical modulating, and all-optical signal processing, due to its significantly enhanced electric field perpendicular to the interfaces in the slot layer. However, there are two drawbacks to be improved if the CSWs are used for signal transmission in photonic integrated circuits, including the crosstalk between waveguides and direct butting mode conversion efficiency (MCE) to a silicon (Si)-strip waveguide. In this study, we propose an anisotropic SW with bulk transition metal dichalcogenide (ASWTMD) to relieve the two shortcomings by replacing the isotropic low-index slot layer with a bulk molybdenum disulfide layer having a high refractive index and giant optical anisotropy. We demonstrated the crosstalk reduction (CR) of the proposed ASWTMD by analyzing the mode profile, power confinement, and coupling strength. We also investigated the MCE by examining the mode overlap ratio and power evolution. The proposed ASWTMD shows significant CR and superior MCE for the transverse electric and transverse magnetic modes compared to those of a CSW with a SiO2-slot layer. The present design paves the possible extensibility to other transition metal dichalcogenides (TMDs) for designing state-of-the-art TMD-based photonic devices exploiting their extraordinary optical properties.

Finite difference time domain simulations of absorption enhancement in thin GaAs solar cells with textured back surface reflectors

Three-dimensional finite difference time domain simulations were performed to evaluate the absorption enhancement of near-band edge light in 500 nm-thick GaAs solar cells with textured rear reflectors. Three types of the textures were examined: Triangular-prisms, pyramids, and a pseudo-random etch. The tested design parameters were the period and aspect ratios of the texture and then the thickness of the transparent rear scattering layer. It was found that a low-index layer between semiconductor and metal was required to achieve a high degree of absorption enhancement, as this layer greatly reduced parasitic absorption in the metal reflector. In terms of absorption enhancement, pyramids of nearly any period and aspect ratio were superior to triangular prisms by a factor between 2 and 3. The pseudo-random etch achieved performance generally between the two other texture types. The most promising texture in terms of feasibility and performance was pyramids with a period of 2 µm and aspect ratio of 0.3, as this texture is accessible to wet-etching and granted especially high absorption enhancement. A cell thusly textured achieved 1-sun terrestrial short-circuit current of 27.4 mA/cm 2 compared to 28.3 mA/cm 2 for a conventional, 3 µm-thick on-substrate cell. Current loss was due to parasitic absorption and light escaping through the front surface. • Textured rear reflectors allow GaAs solar cell active region to be thinned. • Thin GaAs cells thus far struggle to reach predicted performance. • Thins cells with back surface reflector textures were simulated and optimized. • Optimized pyramid textures demonstrated absorption enhancement factors of about 20.

P‐81: Enhanced Color‐Conversion Efficiency of Quantum‐Dot Layer Using Low‐Refractive‐Index Layer

In displays that use QD as color conversion materials, the QD layer must re‐absorb the blue light that passes through it to achieve high‐luminance red and green pixels. In this study, a low refractive index layer was introduced between the color filter and the QDs layer to reuse the light that passes through the QDs layer. This result shows that there is an 8% increase in luminescence compared to that in Ref.

Femtosecond laser micromachined one-dimensional photonic crystal channel waveguides

We present the sol-gel fabrication and characterization of SiO2–TiO2 one-dimensional photonic crystal (1DPhC) waveguides embedded with a low-index defect layer as the core. We demonstrate femtosecond laser micromachining as an attractive technique for microfabrication on multilayer structures over other conventional techniques like, electron beam lithography and focused ion beam milling. 1DPhC channel waveguides of widths 15.3 ± 0.5 and 17.8 ± 0.5 μm were fabricated using optimised femtosecond laser micromachining technique, and their waveguiding characteristics were studied in the end-fire coupling geometry. The modes in the fabricated structures exhibit confinement in the core, which creates an efficient way of light guiding via leaky modes for applications in optical integrated circuits. The waveguiding in the fabricated 1DPhC channel waveguides are attributed to light confinement due to the photonic stop-band in the out-of-plane direction and total internal reflection in the in-plane direction. Polarization dependent studies show that the modes supported by the 1DPhC planar and channel waveguides preserve the state of input polarization during propagation. Finally, a comparative study made between planar and channel waveguides of two different widths shows that the propagation losses decrease with the reduction in the width of the channel waveguides.

Design of Distributed Bragg Reflectors for Green Light-Emitting Devices Based on Quantum Dots as Emission Layer

Light-emitting diodes based on quantum dots as an active emission can be considered as a promising next generation for application in displays and lighting. We report a theoretical investigation of green emission at 550 nm of microcavity inorganic–organic light-emitting devices based on Zn (Te, Se) alloy quantum dots as an active layer. Distributed Bragg Reflector (DBR) has been applied as a bottom mirror. The realization of high-quality DBR consisting of both high and low refractive index structures is investigated. The structures applied for high refractive index layers are (ZrO2, SiNx, ZnS), while those applied for low index layers are (Zr, SiO2, CaF2). DBR of ZnS/CaF2 consisting of three pairs with a high refractive index step of (Δn = 0.95) revealed a broad stop bandwidth (178 nm) and achieved a high reflectivity of 0.914.

Ge/BaF2 thin-films for surface micromachined mid-wave and long-wave infrared reflectors

High performance distributed Bragg reflectors (DBRs) are key elements to achieving high finesse MEMS-based Fabry–Pérot interferometers (FPIs). Suitable mechanical parameters combined with high contrast between the refractive indices of the constituent optical materials are the main requirements. In this paper, Germanium (Ge) and barium fluoride (BaF2) optical thin-films have been investigated for mid-wave infrared (MWIR) and long-wave infrared (LWIR) filter applications. Thin-film deposition and fabrication processes were optimised to achieve mechanical and optical properties that provide flat suspended structures with uniform thickness and maximum reflectivity. Ge-BaF2-Ge 3-layer solid-material DBRs have been fabricated that matched the predicted simulation performance, although a degradation in performance was observed for wavelengths beyond 10 μm that is associated with optical absorption in the BaF2 material. Ge-Air-Ge 3-layer air-gap DBRs, in which air rather than BaF2 served as the low refractive index layer, were realized to exhibit layer flatness at the level of 10 to 20 nm across lateral DBR dimensions of several hundred micrometers. Measured DBR reflectance was found to be ≳90 % over the entire wavelength range of the MWIR band and for the LWIR band up to a wavelength of 11 μm. Simulations based on the measured DBR reflectance indicates that MEMS-based FPIs are able to achieve a peak transmission of ≳90 % over the entire MWIR band and up to 10 μm in the LWIR band, with a corresponding spectral passband of ≲50 nm in the MWIR and <80 nm in the LWIR.

Mid-infrared quasi-BIC resonances with sub-wavelength slot mode profiles in germanium-based coupled guided-mode resonance structures

We experimentally demonstrate a novel quasi-bound state in the continuum (BIC) resonance in the mid-infrared wavelength region with the resonant electric field confined as a slot mode within a low-refractive-index medium sandwiched between high-index layers. The structures studied here comprise coupled amorphous germanium guided-mode resonance (GMR) structures with a top one-dimensional grating layer and bottom uniform layer separated by a low-index silicon nitride layer. The slot-mode profile within the silicon nitride layer with mode field confinement &gt; 30 % is achieved as a solution to the electromagnetic wave propagation through the coupled GMR structure with the dominant field component being perpendicular to the layers. The quasi-BIC resonance in symmetric 1D grating structures can be observed even at normal incidence when considering a realistic excitation beam with finite angular spread. The measured transmission peak is found to redshift (remain almost unchanged) under classical (full-conical) mounting conditions. The highest quality factor of ∼ 400 is experimentally extracted at normal incidence under a classical mounting condition with a resonance peak at 3.41 μm wavelength. Such slot-mode GMR structures with appropriately chosen low-index intermediate layers can find applications in resonantly enhanced sensing and active photonic devices.

Low-Threshold Nanolaser Based on Hybrid Plasmonic Waveguide Mode Supported by Metallic Grating Waveguide Structure.

A high Q-factor of the nanocavity can effectively reduce the threshold of nanolasers. In this paper, a modified nanostructure composed of a silver grating on a low-index dielectric layer (LID) and a high-index dielectric layer (HID) was proposed to realize a nanolaser with a lower lasing threshold. The nanostructure supports a hybrid plasmonic waveguide mode with a very-narrow line-width that can be reduced to about 1.79 nm by adjusting the thickness of the LID/HID layer or the duty ratio of grating, and the Q-factor can reach up to about 348. We theoretically demonstrated the lasing behavior of the modified nanostructures using the model of the combination of the classical electrodynamics and the four-level two-electron model of the gain material. The results demonstrated that the nanolaser based on the hybrid plasmonic waveguide mode can really reduce the lasing threshold to 0.042 mJ/cm2, which is about three times lower than the nanolaser based on the surface plasmon. The lasing action can be modulated by the thickness of the LID layer, the thickness of the HID layer and the duty cycle of grating. Our findings could provide a useful guideline to design low-threshold and highly-efficient miniaturized lasers.

Exploring superlattice DBR effect on a micro-LED as an electron blocking layer.

The role of a superlattice distributed Bragg reflector (SL DBR) as the p-type electron blocking layer (EBL) in a GaN micro-light-emitting diode (micro-LED) is numerically investigated to improve wall-plug efficiency (WPE). The DBR consists of AlGaN/GaN superlattice (high refractive index layer) and GaN (low refractive index layer). It is observed that the reflectivity of the p-region and light extraction efficiency (LEE) increase with the number of DBR pairs. The AlGaN/GaN superlattice EBL is well known to reduce the polarization effect and to promote hole injection. Thus, the superlattice DBR structure shows a balanced carrier injection and results in a higher internal quantum efficiency (IQE). In addition, due to the high refractive-index layer replaced by the superlattice, the conductive DBR results in a lower operation voltage. As a result, WPE is improved by 22.9% compared to the identical device with the incorporation of a conventional p-type EBL.