Air Cladding Research Articles

Ge2Sb2Se4Te on n = 1 ITO: a monolithic platform for integrated photonics with mode symmetrization and post-fabrication tuning

We demonstrate, to our knowledge, a novel monolithic platform for photonic integrated circuits (PICs) based on amorphous-Ge2Sb2Se4Te (am-GSST). Additionally, we explore the concept of mode symmetrization using the epsilon-near-zero behavior displayed by indium-tin-oxide (ITO) to achieve a substrate with n=1 at 1550 nm, the same as the air cladding. We designed, fabricated, and characterized various on-chip components using this platform, including waveguides with preliminary 5.57±0.365dB/mm propagation loss. Furthermore, we propose a post-fabrication tuning of the refractive index by using the phase change nature of GSST to crystallize local sections of the waveguides using electron beams. Our substrate-blind approach is a versatile platform for post-fabrication tunable PICs that could benefit intricate on-chip nanophotonic structures requiring enhanced and symmetric mode confinement.

Direct 3D-printed ring-resonator photonic circuit on a dual core fiber tip for remote sensing applications.

On-chip optical sensors using ring- and disk-resonators have many potential sensing applications, yet robust and efficient fiber-to-chip coupling and the differing form factor between the two pose deployment challenges. To resolve this, we 3D-printed a ring-resonator onto the tip of a dual-core fiber and demonstrate its use as a remote temperature sensor. The fiber-tip optical circuit is fabricated using direct laser writing (DLW) with two-photon absorption photopolymer material IP-Dip, forming micrometer-scale waveguide cores having a refractive index of 1.53 with a surrounding air cladding. We connect the two-fiber cores by a printed bus-waveguide, utilizing total internal reflection mirrors, allowing light launched into one core to be guided back to the other core. Furthermore, a DLW printed racetrack resonator evanescently coupled to the bus waveguide (Q ∼ 3000) imposes spectral dips on resonance wavelengths. Light sent down into one core is interrogated upon return from the second core, all from the distal end of the sensor. When the sensing end's temperature is varied, we find a sensitivity of 78 pm/K, due to the polymer's thermo-optic index variation. The ring-resonator could be functionalized for other sensing applications.

Manipulating terahertz guided wave excitation with Fabry-Perot cavity–assisted metasurfaces

Metasurfaces are emerging as powerful tools for manipulating complex light fields, offering enhanced control in free space and on-chip waveguide applications. Their ability to customize refractive indices and dispersion properties opens up new possibilities in light guiding, yet their efficiency in exciting guided waves, particularly through metallic structures, is not fully explored. Here, we present a new method for exciting terahertz (THz) guided waves using Fabry-Perot (FP) cavity-assisted metasurfaces that enable spin-selective directional coupling and mode selection. Our design uses a substrate-free ridge silicon THz waveguide with air cladding and a supporting slab, incorporating placed metallic metasurfaces to exploit their unique interaction with the guided waves. With the silicon thin layer and air serving as an FP cavity, THz waves enter from the bottom of the device, thereby intensifying the impact of the metasurfaces. The inverse-structured complementary metasurface could enhance excitation performance. We demonstrate selective excitation of TE00 and TE10 modes with directional control, confirmed through simulations and experimental validations using a THz vector network analyzer (VNA) system. This work broadens the potential of metasurfaces for advanced THz waveguide technologies.

Detection of different drinkable milk using photonic crystal fibre biosensor in IR regime

A simplified PCF sensor has been designed to detect the different drinkable milk that includes camel, cow and buffalo milk, and can also assess its quality. The sensor features a singular circular core design and two layers octagonal cladding air holes that was analysed using the Finite Element Method technique in COMSOL Multiphysics software and determine the sensing and optical performance parameters: power fraction, relative sensitivity, confinement loss, effective area, numerical aperture, V-Parameter, spot size, and beam divergence. At the optimum wavelength of 6.0 μm, the relative sensitivities are 96.58%, 96.78%, and 96.84%, and confinement losses of 3.51 × 10−8 dB/m, 1.47 × 10−8 dB m−1, and 8.59 × 10−9 dB/m, for camel, cow, and buffalo milk, respectively. The efficacy of the proposed PCF structure for sensing applications in the dairy industry in distinguishing between different types of milk is evidenced by these findings. Moreover, the results of confinement loss and chromatic dispersion suggest potential applications of this design in optical communication.

Performance evaluation of photonic crystal fibre sensor for controlled drugs detection: a simulation approach

This study proposes a simple and efficient Photonic Crystal Fibre sensor design for the detection of controlled drugs such as cocaine, amphetamine, and ketamine. The design uses a pentagonal core hole and circular cladding air holes in 2 layers, made of fused silica substrate. The sensing performance of the proposed PCF design is evaluated using COMSOL Multiphysics and Finite Element Method, operating in the visible and infrared range from 0.4 to 3.2 μm. Results show that the proposed PCF design achieves high relative sensitivities of 99.55%, 99.75%, and 99.99% for cocaine, amphetamine, and ketamine, respectively, at the optimum wavelength of 0.4 μm. Additionally, the design is robust, showcasing minimal variations in relative sensitivities with changes in pitch distance, air hole diameter, and core hole length. These findings make the proposed PCF design a promising candidate for practical controlled drug sensing.

Reinforcement learning for photonic component design

We present a new fab-in-the-loop reinforcement learning algorithm for the design of nano-photonic components that accounts for the imperfections present in nanofabrication processes. As a demonstration of the potential of this technique, we apply it to the design of photonic crystal grating couplers fabricated on an air clad 220 nm silicon on insulator single etch platform. This fab-in-the-loop algorithm improves the insertion loss from 8.8 to 3.24 dB. The widest bandwidth designs produced using our fab-in-the-loop algorithm can cover a 150 nm bandwidth with less than 10.2 dB of loss at their lowest point.

Alternative Approach to Design and Optimization of High-Q Ring Resonators for Membrane-Free Acoustic Sensors.

Membrane-free acoustic sensors based on new principle and structure are becoming a research hotspot, because of many advantages, e.g., their wide bandwidth and high sensitivity. It is proposed that a membrane-free acoustic sensor employs a semi-buried optical waveguide ring resonator (SOWRR) as a sensing element. Using air as the upper cladding medium, the excited evanescent field in the air cladding medium would be modulated by acoustic wave. On this basis, the acoustic sensing model is established. Taking high Q factor and resonance depth as design criteria, the optimal design parameters are given. The optimal values of the air/SiO2: Ge/SiO2 waveguide resonator length and coupling spacing are obtained as 50 mm and 5.6 μm, respectively. The Q factor of the waveguide resonator of this size is as high as 8.33 × 106. The theoretical simulation indicates that the frequency response ranges from 1 Hz to 1.58 MHz and that the minimum detectable sound pressure is 7.48 µPa using a laser with linewidth of 1 kHz. Because of its advantages of wide bandwidth and high sensitivity, the membrane-free sensor is expected to become one of the most promising candidates for the next-generation acoustic sensor.

Residual dispersion compensation photonic crystal fiber based on simulated annealing algorithm and Livelink data interaction

An octagonal photonic crystal fiber (O-PCF) structure with equal air hole diameter of the cladding is proposed for residual dispersion compensation, and a high refractive index material SF57 is introduced in the core region to make it have the advantages of high negative dispersion, low attenuation loss, low fabrication difficulty. Due to the symmetric structure, no additional polarization control is required in the dispersion compensation process, which further reduces the production cost. Using Livelink data interaction not only accelerates the modeling of O-PCF, but also enables the effective refractive index of HE11 to be unattended continuously searched. Furthermore, simulated annealing algorithm is used to reverse design the structural parameters of cladding air hole diameter and central solid hole diameter. The results show that, based on the simulated annealing algorithm interacting with Livelink data, the optimize efficiency, design accuracy and dispersion characteristics are improved. The ultra-flat high negative dispersion O-PCF can be reverse designed in the S + C + L and S + C + L + U bands which shows the average dispersion of −847.34 ps/(nm·km) and −752.90 ps/(nm·km) with the absolute dispersion of only ±7.25 ps/(nm·km) and ±10.28 ps/(nm·km), respectively. The designed RDCF is tolerant to ±1% and ±2% variation of the central solid hole diameter and the cladding air hole diameter during fabrication, respectively. The combination of simulated annealing algorithm and Livelink data interaction provides an efficient method for reverse design of PCF.

Fabrication of waveguide directional couplers using 2-photon lithography.

Advances in 2-photon lithography have enabled in-lab production of sub-micron resolution and millimeter scale 3D optical components. The potential complex geometries are well suited to rapid prototyping and production of waveguide structures, interconnects, and waveguide directional couplers, furthering future development and miniaturization of waveguide-based imaging technologies. System alignment is inherent to the 2-photon process, obviating the need for manual assembly and allowing precise micron scale waveguide geometries not possible in traditional fused fiber coupler fabrication. Here we present the use of 2-photon lithography for direct printing of multi-mode waveguide couplers with air cladding and single mode waveguide couplers with uncured liquid photoresin cladding. Experimental results show reproducible coupling which can be modified by selected design parameters.

Ultracompact optical fiber high‐temperature sensor based on the angled fiber end face

AbstractAn ultracompact optical fiber Fabry–Perot interferometric sensor is proposed and demonstrated for high‐temperature measurement. The device is simply a section of single‐mode fiber with angled end face of 45°, the reflected light from the angled fiber end face is directed sequentially to the fiber core–cladding and cladding–air interfaces, respectively, and reflected back, and finally returned to the fiber core after being reflected by the angled fiber end face again. As a result, a Fabry–Perot interferometer is constructed. Because of the thermal‐expansion and thermal‐optical effect of the fiber material, the optical path difference between the core–cladding and cladding–air surfaces is changed with temperature, hence the device is well suitable for temperature measurement. In particular, the fiber device has good high‐temperature sustainability up to 1100°C, which makes it attractive for high‐temperature measurement in a narrow or hard‐to‐reach spatial environment.

Dispersion Engineering of Waveguide Microresonators by the Design of Atomic Layer Deposition

In this work, we demonstrate dispersion engineering of silicon nitride waveguide resonators with atomic layer deposition (ALD). We conducted theoretical and experimental analyses on the waveguide dispersion with air cladding, hafnium oxide (HfO2) cladding, and aluminum oxide (Al2O3) cladding. By employing ALD HfO2 as the cladding layer, the dispersion of waveguide can be tuned to a finer degree in the normal regime at a wavelength of 1550 nm. On the other hand, using ALD Al2O3 cladding provides the waveguide dispersion that spans regimes in normal, near-zero, and anomalous dispersion.

Inverse-Designed Ultra-Compact Polarization Splitter–Rotator in Standard Silicon Photonic Platforms With Large Fabrication Tolerance

Polarization splitter-rotators (PSRs) are crucial components for controlling the polarization states of the input light in photonic integrated circuits. In this paper, we designed, fabricated and characterized a high-performance CMOS-compatible O-band PSR based on silicon-on-insulator (SOI) platform by using inverse design technique. The PSR mainly consists of a polarization rotator designed by our new proposed bi-level shape adjoint method and a mode demultiplexer designed by topology adjoint method. The device robustness against fabrication errors is significantly strengthened by properly designing optimization strategies. The proposed PSR displays a low insertion loss (<1 dB) and low crosstalk (<−20 dB) within a bandwidth of 50 nm by measurements. The device footprint is 3 × 17 μm, making this the smallest PSR based on standard silicon photonic platforms with no extra layers or an air cladding.

Van Der Waals Materials for Subdiffractional Light Guidance

Photonics is a natural next technological step after an era of electronics. However, the diffraction limit of light poses severe limitations on photonic elements and dictates their size. Herein, we demonstrate that layered semiconductors solve this challenge thanks to their giant optical anisotropy. In particular, waveguides with molybdenum disulfide (MoS2) and tungsten disulfide (WS2) claddings can operate in a transparency region slightly above (20%) the diffraction limit and even overcome it by 10% around 700 nm, providing an even better confinement than air cladding, but with excitonic losses. Further analysis reveals that van der Waals materials with an in-plane refractive index of about five or an out-of-plane index around two provide subdiffractional and lossless guidance. Therefore, our results establish the route for ultra-dense photonic integration based on layered materials.

A Novel Structured Magnetic Field Sensor Based on Photonic Crystal Fiber Filled with Magnetic Fluid

A novel structured magnetic field sensor based on dual-core photonic crystal fiber is proposed which has two elliptical central holes filled with magnetic fluid and some elliptical cladding air holes. The transmission characteristics of the dual-core photonic crystal fiber such as coupling length, birefringence, and coupling loss are analyzed with changing structural parameters. Enhanced birefringence is obtained by the novel structure of two elliptical central holes and some elliptical cladding air holes. The sensitivity of the magnetic sensor is calculated, and the numerical simulation result shows that the highest sensitivity of 1200 pm/Oe can be achieved.

A gold film coated dual-core photonic crystal fiber for refractive index and temperature sensing with high isolation

• When measuring RI of an analyte, the measurement is often affected by temperature. the simultaneous measurement of RI and temperature has important significance. • In this paper, a gold film coated dual-core photonic crystal fiber (DC-PCF) was proposed and numerically investigated for both RI and temperature sensing. • The maximum sensitivity of the RI reached 8100 nm/RIU, while the maximum sensitivity of temperature reached 1.3 nm/°C. • The design of dual independent cores in DC-PCF not only avoids the problem of cross-sensitivity; but also improves the isolation of the sensor. When measuring the refractive index (RI) of an analyte, the measurement is often affected by temperature. Therefore, the simultaneous measurement of RI and temperature has important research significance. In this paper, a gold film coated dual-core photonic crystal fiber (DC-PCF) was proposed and numerically investigated for both RI and temperature sensing. In order to stimulate the surface plasmon resonance (SPR) with high sensitivity to the surrounding, two gold films were coated on the polished surface and inner wall of a cladding air hole in the DC-PCF, respectively. Numerical results showed that the measurement sensitivities of RI and temperature reached 8100 nm/RIU based on Core I and 1.3 nm/°C based on Core II, respectively. It should also be noted that the designed sensor showed a high isolation. This designed sensor can be used where the RI of analyte should be detected at a high temperature accuracy.

Inverse design of an air-cladding and fully-etched silicon polarization rotator based on a taper-based mode hybridization

In the regular photonic integrated circuits, many optical devices are designed on a silicon-on-insulator platform with a top silicon layer and an air cladding by the fully-etched procedure because of the simple fabrication. Based on a taper-based mode hybridization, a polarization rotator consisting of the polarization-conversion and mode-conversion parts is realized by the forward-design-assisted inverse design. The polarization rotator transforms the fundamental transverse magnetic mode to the first order transverse electric mode and then converted to the fundamental transverse electric mode. The polarization rotator with a total length of about 10μm exhibits the insertion loss of less than 1.1 dB and the extinction ratio of larger than 20.5 dB from 1540 nm to 1560 nm. In addition, we design another same-functionality polarization rotator with a total length of about 13μm. The insertion loss and the extinction ratio are less than 0.8 dB and larger than 24.3 dB, which investigate that appropriately improving the initial structure quality is conducive to the direct-binary-search algorithm convergence.

Suspended-core fiber with embedded GaSe nanosheets for second harmonic generation.

We report an all-fiber scheme for the second harmonic generation (SHG) by embedding gallium selenide (GaSe) nanosheets into a suspended-core fiber (SCF). Based on modes analysis and theoretical calculations, the phase-matching modes from multiple optional modes in the SHG process and the optimal SCF length are determined by calculating the effective refractive index and balancing the SHG growth and transmission loss. Due to the long-distance interaction between pumped fundamental mode and GaSe nanosheets around the suspended core, an SHG signal is observed under a milliwatt-level pump light, and exhibits a quadratic growth with the increased pump power. The SHG process is also realized in a broad wavelength range by varying the pump in the range of 1420∼1700 nm. The SCF with the large air cladding and suspended core as an excellent platform can therefore be employed to integrate low-dimensional nonlinear materials, which holds great promise for the applications of all-fiber structures in new light source generating, signal processing and fiber sensing.

High-Q asymmetrically cladded silicon nitride 1D photonic crystals cavities and hybrid external cavity lasers for sensing in air and liquids.

In this paper we show a novel design of high Q-factor silicon nitride (SiN) 1D photonic crystal (PhC) cavities side-coupled to curved waveguides, operating with both silica and air cladding. The engineering of the etched 1D PhC cavity sidewalls angle allows for high Q-factors over a wide range of upper cladding compositions, and the achievement of the highest calculated Q-factor for non-suspended asymmetric SiN PhC structures. We show the employment of these type of SiN PhC cavities in hybrid external cavity laser (HECL) configuration, with mode-hop free single mode laser operation over a broad range of injected currents (from 25 mA to 65 mA), milliwatts of power output (up to 9 mW) and side-mode suppression ratios in the range of 40 dB. We demonstrate the operation of these devices as compact and energy efficient optical sensors that respond to refractive index changes in the surrounding medium the measurement of sodium chloride (from 0% to 25%) and sucrose (from 0% to 25%) in aqueous solution. In HECL configuration, the RI sensor exhibits a 2 orders of magnitude improvement in detection limit compared to the passive microcavity. We also discuss the possibility for applying these devices as novel transducers for refractive index changes that are induced by analyte specific absorption of infrared radiation by the target analytes present in gas or liquid phase.

High-performance silicon TE-pass polarizer assisted by anisotropic metamaterials.

The polarizer is a key component for integrated photonics to deal with the strong waveguide birefringence, especially for silicon photonics. A high-performance silicon TE-pass polarizer covering all optical communication bands with low insertion loss (IL) and high polarization extinction ratio (PER) is proposed here. This polarizer is based on anisotropic subwavelength grating (SWG) metamaterials, which maintain the fundamental TE mode as a guided mode but make the fundamental TM mode leaky. Furthermore, based on this working mechanism, the proposed polarizer can work well for any upper cladding material, including air and silicon dioxide (SiO2). The numerical results show that our proposed TE-pass polarizer has a remarkable performance with IL < 0.34 dB over 420 nm (PER > 23.5 dB) or 380 nm (PER > 30 dB) for the air cladding, and IL < 0.3 dB over 420 nm (PER > 25 dB) or 320 nm (PER > 30 dB) for the SiO2 cladding. The fabricated polarizer shows IL < 0.8 dB and PER > 23 dB for the bandwidths of 1.26-1.36 µm and 1.52-1.58 µm (other bandwidths were not measured due to the limited instrument in our research center, but it still covers the most important O-band and C-band).

High optical coupling efficiency of polymer microlens and pillar on single mode fiber for silicon photonics

Silicon photonics technology has attracted considerable attention these days. However, the low coupling efficiency due to the difference in spot size between silicon photonic (SiPh) chips and single-mode fibers (SMFs) remains a challenging issue. We have already proposed a unique combination of a microlens and a pillar on the facet of SMF. However, the pillar may have difficulty in keeping a single mode for a signal beam of 1. 55 μm wavelength due to air cladding. In this study, we clarified the length of the pillar that can support the single mode through simulations and experiments. By the optimum designing, the spot size as the same level as the SiPh chip was obtained. We could show that our coupling device provides high coupling efficiency with a test sample of SiPh chip, and this device also can be applied to multi-core fibers.