Metal-clad Waveguide Research Articles

Clearing a path for light through non-Hermitian media

Abstract The performance of all active photonic devices today is greatly limited by loss. Here, we show that one can engineer a low loss path in a metal-clad lossy multi-mode waveguide while simultaneously achieving high-performance active photonic devices. We leverage non-Hermitian systems operating beyond the exceptional point to enable the redistribution of losses in a multi-mode photonic waveguide. Consequently, our multi-mode waveguide offers low propagation losses for fundamental mode while other higher order modes experience prohibitively high losses. Furthermore, we show an application of this non-Hermitian waveguide platform in designing power-efficient thermo-optic phase shifters with significantly faster response times than conventional silicon-based thermo-optic phase shifters. Our device achieves a propagation loss of less than 0.02 dB μm−1 for our non-Hermitian waveguide-based phase shifters with high performance efficiency of P π ⋅ τ = 19.1 mW μs. In addition, our phase shifters have significantly faster response time (rise/fall time), τ ≈ 1.4 μs, compared to traditional silicon based thermo-optic phase shifters.

Experimental study of DC Kerr effect of chalcogenide glass film by free space coupling method

Chalcogenide glass is an important nonlinear optical material that has attracted much attention in the areas of integrated photonics, supercontinuum sources, and all-optical switches in recent years. However, the current research mainly focuses on the nonlinear effect excited by light, and the research on its properties under the action of a DC field is still deficient. Here, a metal-cladding optical waveguide, which is composed of a chalcogenide glass film coated on a glass substrate, is presented to analyze the quadratic electro-optic (QEO) effect of the chalcogenide glass film. The DC Kerr coefficient and the whole components of the QEO tensor of the sample were experimentally determined by the free space coupling method.

Ultra-high order mode-assisted optical differentiator for edge detection with high tunability

An optical spatial differentiator based on the photonic spin Hall effect (PSHE) with high tunability is presented. By utilizing the characteristics of ultra-high order modes in the symmetrical metal cladding waveguide, the Fresnel reflection coefficient spectrum exhibits a narrow peak width and low trough at the resonant incident angles, resulting in high sensitivity to changes in the incident angle-induced spatial shift caused by the PSHE (the highest ∂(|r s /r p |)/∂θ value can reach 107). After polarization transformation and extinction, the output field demonstrates differential operation with respect to the input field. When applied to edge detection, our differentiator can achieve tunable resolution edge images by adjusting the incident angle. Our proposed edge detection scheme has potential applications for cellular and molecular imaging through two-dimensional extension via the target rotation.

Waveguide‐based Raman enhancement strategies

AbstractWaveguide‐enhanced Raman scattering (WERS) is a powerful branch of enhanced Raman technologies that has gained significant progress in recent years because of its advantages, such as reproducibility and robustness. As a complementary tool to surface‐enhanced Raman spectroscopy (SERS), WERS provides a powerful solution for reproducible quantification of analytes. According to different Raman enhancement mechanisms, five major WERS implementation strategies, namely, (1) single‐mode dielectric waveguide, (2) liquid core waveguide, (3) metal cladding waveguide, (4) resonance mirror waveguide, and (5) double metal cladding waveguide, are classified and described in detail in this review. The flexibility of WERS structures makes them easy to be integrated with 2D devices to obtain a complete on‐chip detection scheme, allowing the WERS chip to combine excitation, detection, and data analysis in integrated chips, providing a powerful prospect for real‐time and on‐site analysis of target samples. This article highlights the principles, implementations, and application scenarios of WERS techniques and evaluates their advantages and limitations, respectively. Finally, the strengths and weaknesses of WERS techniques are summarized, and promising future applications are proposed. This review provides a panoramic view for researchers interested in waveguide‐enhanced Raman technology.

Spatial differentiation based on resonant absorption on symmetrical metal-cladding waveguide

We propose a new spatial differentiation scheme based on resonant absorption on a symmetrical metal-cladding waveguide (SMCW) with the guiding layer as birefringent crystal. The differentiator consists of a SWCW and two polarizers. When the resonant condition and critical thickness are satisfied for SMCW, the destructive interference between the directedly reflected field and the leaked field makes differentiation in the x-direction possible, and the destructive interference between the left-handed and right-handed circularly polarized field components makes differentiation in the y-direction possible. So, the x-direction, y-direction and two-dimensional spatial differentiation can be obtained just by adjusting the orientation of the first polarizer. The differentiator has the advantages of miniaturization and simple operation.

A novel tunable metal-clad planar waveguide with 0.62PMN-0.38PT material for detection of cancer cells.

A dynamically tunable metal clad planar waveguide having 0.62PMN-0.38PT material is simulated and optimized for detection of cancer cells. Angular interrogation of the TE0 mode of waveguide shows that critical angle increases greater than the resonance angle with increasing of cover refractive index, which limits the detection range of waveguide. To overcome this limitation, proposed waveguide applies a potential on the PMN-PT adlayer. Although a sensitivity of 105.42 degree/RIU was achieved at 70 Volts in testing the proposed waveguide, it was found that the optimal performance parameters were obtained at 60 Volts. At this voltage, the waveguide demonstrated detection range 1.3330-1.5030, a detection accuracy 2393.33, and a figure of merit 2243.59 RIU-1 , which enabled the detection of the entire range of the targeted cancer cells. Therefore, it is recommended to apply a potential of 60 Volts to achieve the best performance from the proposed waveguide.

Plain quantification of subwavelength-gap metal-clad planar waveguides by the approximate analytical approach

A theoretical study that enables us to make a plain quantitative analysis of subwavelength-gap metal-clad planar waveguides (SMWs) via the analytical approach with a few-percentage-point discrepancy error compared to the complicated computational approach is presented. A variety of the characteristic features of SMWs, such as effective refractive index, propagating loss, mode width, and Fabry–Perot multiple-pass transmittance, have been analytically quantified by simple functions of SMW parameters, for the first time, to the best of the author’s knowledge. By applying the theoretical results to various real metal SMWs, their maximum achievable nanophotonic performances have been demonstrated.

Bistable Reflection Assisted by Fano Resonance in DMDMW With Low-Threshold and Large Modulation Depth

Owing to an additional thinner planar waveguide with micron scale is attracted to the symmetrical metal cladding waveguide (SMCW) with sub-millimeter scale, the interference interaction between different Q-value guided modes would generate a Fano reflectivity curve. In our double metal-dielectric-metal waveguides (DMDMW) structure, a Kerr nonlinear medium is located in the guiding layer of the SMCW where the excited oscillating wave is enormously enhanced. Numerical calculations show that a minute variation of the incident light intensity will give rise to a change in the dielectric constant of the Kerr nonlinear medium and lead to positive feedback. A bistable reflection can be achieved with a low threshold and a large modulation depth because of the sharpness and the asymmetricity of the Fano reflectivity curve.

Ultra-narrowband filter based on the metal-cladding resonant waveguide.

The simple and effective optical filter is the significantly scientific and technical interest in optical signal processing and communication. Especially, the development of microsystem integration is limited in traditional optical filters, due to the complicated structure, small choice, large cost, etc. In this paper, we report an ultra-narrowband filter based on a metal-cladding resonant waveguide. Therein, the ultra-narrowband resonant mode is achieved based on the resonance screening of incident light and cavity modes. According to the experimental data, the full width at half maximum (FWHM) can reach less than 0.1 nm. Furthermore, the resonant peak of FWMH is determined by the thickness of the waveguide, and the resonant wavelength can be selected by changing the incident angle.

Theoretical study on fabrication of sub-wavelength structures via combining low-order guided mode interference lithography with sample rotation

Abstract Guided mode interference lithography based on a symmetric metal-cladding dielectric waveguide structure (a resist between two metal layers), is studied. The low-order guided modes in the waveguide structure excited by 325 nm laser, and then sub-wavelength gratings with half-periods of 47 nm ( λ /6.91) and 60 nm ( λ /5.42) can be obtained by excited TM0 and TE0 guided modes, respectively. Among them, the period of the sub-wavelength grating fabricated by TM0 guided mode interference is smaller, but it is limited by the thickness of resist. TE0 guided mode interference is more suitable for thick resist. On this basis, taking TM0 guided mode as an example, periodic sub-wavelength structures, such as square lattice and quasi-hexagonally close-packed structure, can be obtained by rotating and exposing the sample in different ways. The proposed method of combining guided mode interference lithography with sample rotation has simple operation and low cost, and provides a theoretical reference for fabricating two-dimensional sub-wavelength structures.

Coherent control of spatial and angular Goos-Hänchen shifts with spontaneously generated coherence and incoherent pumping.

We propose an efficient scheme to manipulate the Goos-Hänchen (GH) shift of a reflected beam from a metal-clad waveguide, where a coherent atomic medium with a Λ-type configuration is employed as the substrate. Using experimentally achievable parameters, we identify the conditions under which spontaneously generated coherence (SGC) allows us to enhance the spatial and angular GH shifts of the reflected beam. With the help of SGC, the relative phases of the probe and control fields can alter the absorption gain and refractive index of the atomic medium, thereby manipulating the magnitudes, signs, and positions of the spatial and angular shifts. Furthermore, the spatial and angular GH shifts can be coherently controlled via adjusting the incoherent pumping rate and the intensity of the control field. Our proposal provides an avenue for the manipulation of spatial and angular GH shifts and potential applications in optical switching and optical steering.

Adsorption independent silver colloids index measurement with metal-cladding optical waveguide

The refractive index measurements of silver nanoparticles colloids can provide chemical composition and nanostructure details. Although the standard surface plasmon resonance (SPR) approach improves index measurement precision, it is restricted by silver nanoparticles adsorption and incapable of detecting the colloid index due to the evanescent nature of the surface wave. Herein, a method utilizing metal-cladding optical waveguide (MCOW) was proposed to measure the volume refractive index of silver nanoparticles colloids, importantly, which is independent of particle adsorption. The experimental results show that the sensitivity of the refractive index measurement using the MCOW approach is at least 10−6 RIU (Refractive Index Unit) owing to the oscillating ultrahigh-order modes, this work not only presents a high-sensitivity, high-yield, simple-to-process, and effective refractive index measuring technique, but also recommends a more practical and adaptable colloidal particle characterization approach.

Fiber-optic surface waveguide resonance in gaseous medium: Tunable generation with all fiber modes

The integration of metal clad with dielectric waveguide offers numerous advantages for generating optical surface waves. Here we perform a comprehensive study on the principle and tunable characteristics of fiber-optic surface waveguide resonances (SWRs) excited with a metal-clad film waveguide deposited on optical fiber in gaseous medium. A versatile approach is proposed for the analysis of fiber-optic SWRs and it is also available for analyzing other kinds of optical surface waves. The essential mechanism for generating surface waveguide modes is obtained. The results show that the film waveguide supports both P- and S-polarized discrete surface waveguide modes including guided modes, cladding modes and leaky modes. It is found that the introduction of a metal clad strengthens the guidance of these modes and enhances the mode coupling with fiber modes. This makes it possible to generate strongly polarization-dependent fiber-optic SWRs with all fiber modes by simply tailoring the thickness of film waveguide.

Theoretical study of sub-wavelength gratings fabrication by TM0 mode interference in symmetric metal-cladding dielectric waveguide

Theoretical study of sub-wavelength gratings fabrication by TM0 mode interference in symmetric metal-cladding dielectric waveguide

Continuous Goos-Hänchen Shift of Vortex Beam via Symmetric Metal-Cladding Waveguide.

Goos-Hänchen shift provides a way to manipulate the transverse shift of an optical beam with sub-wavelength accuracy. Among various enhancement schemes, millimeter-scale shift at near-infrared range has been realized by a simple symmetrical metal-cladding waveguide structure owing to its unique ultrahigh-order modes. However, the interpretation of the shift depends crucially on its definition. This paper shows that the shift of a Gaussian beam is discrete if we follow the light peak based on the stationary phase approach, where the M-lines are fixed to specific directions and the beam profile is separated near resonance. On the contrary, continuous shift can be obtained if the waveguide is illuminated by a vortex beam, and the physical cause can be attributed to the position-dependent phase-match condition of the ultrahigh-order modes due to the spatial phase distribution.

Perovskite solar cell with light trappers fabricated by double metal cladding waveguide technology

Local surface plasmon resonance (LSPR) based on metal nanostructures was widely used in enhanced light capture and light absorption. In addition, LSPR can also facilitate processes such as exciton dissociation and charge transfer (CT) in solar cells. We developed perovskite solar cells (PSCs) by coating chips with hexagon-like nanostructures on the backside of Indium tin oxide (ITO) conductive glass using the double metal cladding waveguide (DMCW) technology. The optical absorption and photoelectric conversion efficiency (PCE) of the PSCs with different morphologies of chips were analyzed. The LSPR of the chips corresponded with the absorption capacity of perovskite. LSPR enhanced the scattering, gathering, and capture of light energy. Because the strong absorption rates at the long wavelengths after 600 nm in the ultraviolet visible (UV–Vis) spectrum, the properties of PSCs with the chips are improved. The PCE and short circuit current density (Jsc) increased by 27.21%, and 6.78%, respectively, attributed to the increase in the optical path of incident light and the light absorption capacity of the active layer.

Sensitive detection of orthogonal polarization intensity ratio via metal-cladding waveguide

A rapid and sensitive detection method of incident light with different orthogonal polarization intensity ratios based on the double metal-cladding waveguide has been demonstrated. The middle layer of the waveguide is a uniaxial negative crystal lithium niobate, resulting in distinct coupling angles for TE and TM polarization. The isolation of the device can reach 42 dB due to well separation of TE and TM polarization. The quantitative experiments to measure ratio of TE to TM incident light have been carried out. Meanwhile, this method can support polarization detection in ultraviolet, visible, and infrared region. This method will play a significant role in optical information processing system and the integrated system.

All-photonic synapse based on iron-doped lithium niobate double metal-cladding waveguides

In an artificial neural network composed of neuromorphic computing units, the resistance of the memristor can be modulated dynamically and repeatedly under external stimuli, such as electric fields, magnetic fields, and light illumination, leading to variations in local conductivity and memory effects. Here we show, using an all-photonic memristor, that the memristance arises naturally in an optical system in which solid-state ionic transport is realized under low-power external incident light. These results show an extremely stable multilevel storage weight and a high signal-to-noise ratio. In all-photonic memristors, the light signal is employed as the extra stimuli of the memristor devices to ensure a large memory window and a variation margin of multiple storage levels.

Maskless nanostructure photolithography by ultrahigh-order modes of a symmetrical metal-cladding waveguide.

To fabricate fine patterns beyond the diffraction limit, a nanostructure photolithography technique is required. In this Letter, we present a method that allows sub-100-nm lines to be patterned photolithographically using ultrahigh-order modes from a symmetrical metal-cladding waveguide (SMCW) in the near field, which are excited by continuous-wave visible light without focusing. The etching depth of the nanopattern reaches more than 200 nm. The localized light intensity distribution can be used to map the photoresist exposure pattern, which agrees well with our theoretical model. This technique opens up the possibility of localizing light fields below the diffraction limit using maskless and lower power visible light.

A High-Sensitivity Method Based on Advanced Optical Waveguide Technology to Detect HBsAg.

A novel method for the detection of the hepatitis B surface antigen (HBsAg) at low concentrations, using the ultrahigh-order guided mode acting as the probe excited by a symmetrical metal-cladding waveguide, is proposed. The method using the fact of the minimum value of the absorption peaks is proportional to the concentration of the sample to be detected to realize the detection of the hepatitis B virus at extremely low concentrations. It is realized that the low concentration of the HBsAg measurement relied on the principle of the minimum value of the absorption peak and the concentration having a good linear relationship. The measurement results indicate that this new method can precisely detect HBsAg at the concentrations in the lower region of the clinical gray area (i.e., below 20 ng/mL), the lower region of the current clinical gray area of HBsAg (below 20 ng/ml) can be measured, and the resolution can be reached (2 ng/mL).