Optical Tunneling Research Articles

The nontrivial effects of annealing on superconducting properties of Nb single crystals

The effect of annealing on the superconducting properties of niobium single crystals was studied using optical, magnetic, and scanning tunneling microscopy (STM) methods. Pieces of the same crystal boule were studied before and after the annealing at 800 ∘C , 1400 ∘C , and near the melting point of niobium (2477 ∘C ). The initial samples had a high hydrogen content and low-temperature imaging revealed large hydrides (hundreds of micrometers) appearing below 190 K. The formation of these large precipitates is already completely suppressed by annealing at 800 ∘C . However, the overall superconducting properties of the annealed samples did not improve and, in fact, worsened. In particular, the superconducting transition temperature decreased, the upper critical field increased, and the pinning strength increased. In the STM study, the sample was annealed initially at 400 ∘C , measured, annealed at 1700 ∘C , and measured again. The STM revealed a ‘dirty’ superconducting gap with a significant spatial variation in tunneling conductance after annealing at 400 ∘C . The clean gap was recovered after annealing at 1700 ∘C . This is likely due to oxygen redistribution near the surface, which is always covered by oxide layers in as-grown crystals. Our results indicate that vacuum annealing at least up to 1400 ∘C , while removing a large percentage of hydrogen, introduces additional nanosized defects, likely hydride precipitates, that act as efficient pair-breaking and pinning centers. The dimensionless scattering rate is estimated to have increased from Γ=0.2 to about Γ=0.4 after annealing at 1400 ∘C . These results on single crystals differ drastically from those obtained in polycrystalline bulk niobium (i.e. cut from superconducting radio-frequency cavities), where annealing is known to have a significant positive effect that is attributed to the improvement of the crystalline structure masking the more subtle influence of the hydrides.

A Silicon-Based ROTE Sensor for High-Q and Label-Free Carcinoembryonic Antigen Detection.

This paper presents a biosensor based on the resonant optical tunneling effect (ROTE) for detecting a carcinoembryonic antigen (CEA). In this design, sensing is accomplished through the interaction of the evanescent wave with the CEA immobilized on the sensor's surface. When CEA binds to the anti-CEA, it alters the effective refractive index (RI) on the sensor's surface, leading to shifts in wavelength. This shift can be identified through the cascade coupling of the FP cavity and ROTE cavity in the same mode. Experimental results further show that the shift in resonance wavelength increases with the concentration of CEA. The biosensor responded linearly to CEA concentrations ranging from 1 to 5 ng/mL with a limit of detection (LOD) of 0.5 ng/mL and a total Q factor of 9500. This research introduces a new avenue for identifying biomolecules and cancer biomarkers, which are crucial for early cancer detection.

A Compensator Microelectromechanical Acceleration Transducer with a Piezoelectric Sensing Element and Optical Reading

Introduction.Modern mobile control objects require the use of highly sensitive transducers of motion parameters, e.g., acceleration, with a wide measurement range. Increased sensitivity to measured parameters can be achieved by using precision optics, e.g., based on the tunneling effect. However, operating ranges of induced movements are less than a micrometer, which creates difficulties in positioning the sensing element. In order to improve manufacturability, to extend the measurement range and to reduce errors of acceleration transducers with optical tunneling, compensation circuits with a piezoelectric actuator as an active sensor can be used.Aim.To extend the measurement range of microelectromechanical acceleration transducers through the use of an integrated approach, including the introduction of a compensation circuit for sensor movements based on the inverse piezoelectric effect and detection of these movements by optical means.Materials and methods.An approach to compensating sensor movements is proposed. This approach consists in using a bimorph piezoelectric plate as an inertial element. The use of optical reading of sensor sub-micrometer displacements is considered.Results.A block scheme and a functional scheme of a compensator micro-opto-electromechanical acceleration transducer with a bimorph piezoelectric sensing element are developed. Deformations in the sensing element under the influence of accelerations (up to 100 m/s2) and compensation voltages, whose amplitude does not exceed several volts, are investigated to ensure the possibility of using the optical tunneling effect in the proposed transducer.Conclusion.A mathematical model of the transducer was developed and studied. A 2.5-fold increase in the measurement range was achieved. It was shown that the introduction of compensation feedback does not decrease the permissible frequency range of measured accelerations.

The Design, Modeling and Experimental Investigation of a Micro-G Microoptoelectromechanical Accelerometer with an Optical Tunneling Measuring Transducer

This treatise studies a microoptoelectromechanical accelerometer (MOEMA) with an optical measuring transducer built according to the optical tunneling principle (evanescent coupling). The work discusses the design of the accelerometer’s microelectromechanical sensing element (MSE) and states the requirements for the design to achieve a sensitivity threshold of 1 µg m/s2 at a calculated eigenvalue of the MSE. The studies cover the selection of the dimensions, mass, eigenfrequency and corresponding stiffness of the spring suspension, gravity-induced cross-displacements. The authors propose and experimentally test an optical transducer positioning system represented by a capacitive actuator. This approach allows avoiding the restrictions in the fabrication of the transducer conditioned by the extremely high aspect ratio of deep silicon etching (more than 100). The designed MOEMA is tested on three manufactured prototypes. The experiments show that the sensitivity threshold of the accelerometers is 2 µg. For the dynamic range from minus 0.01 g to plus 0.01 g, the average nonlinearity of the accelerometers’ characteristics ranges from 0.7% to 1.62%. For the maximum dynamic range from minus 0.015 g to plus 0.05 g, the nonlinearity ranges from 2.34% to 2.9%, having the maximum deviation at the edges of the regions. The power gain of the three prototypes of accelerometers varies from 12.321 mW/g to 26.472 mW/g. The results provide broad prospects for the application of the proposed solutions in integrated inertial devices.

Photonic Spin Hall Effect as a Highly Sensitive Refractive Index Sensing Platform for Glucose

AbstractThis study presents an innovative refractive index (RI) sensor that measures glucose concentration by utilizing the photonic spin Hall effect (SHE) in a resonant optical tunneling effect (ROTE) structure. The ROTE structure consists of three InP layers with the high RI and two analyte layers (with a high‐low‐high‐low‐high RI distribution), in which glucose solution samples with the low RI are injected. By subjecting the InP layers to external bias‐assisted light, the photonic SHE can be flexibly manipulated, enabling the modulation of the sensing performance accordingly. It is found that the transverse shift of photonic SHE presents a large variation in response to the tiny change in glucose concentrations. By optimizing the parameters (i.e., intensity or wavelength) of bias light, the sensitivity of this sensor can reach as high as 735.7 µm RIU−1. Compared to traditional glucose sensors, this original work implements the novel photonic SHE with the superior sensing performance. Therefore, the proposed design shows promising potential for biomedical applications, such as medical diagnoses and drug discovery.

Entanglement generation and steering implementation in a double-cavity-magnon hybrid system.

We demonstrate a scheme for the generation of bipartite and tripartite entanglement, as well as he implementation of stable and controllable long-distance one-way and asymmetric two-way steering in a cavity-magnon hybrid system. This system consists of a magnon mode and two coupled microwave cavities. The first cavity is driven by a flux-driven Josephson parametric amplifier, which generates squeezed vacuum fields, and is coupled to the other cavity through optical tunneling interaction. The second cavity and magnon mode are coupled through magnetic dipole interaction. We find that under weak coupling between the two cavities, and strong coupling between the second cavity and magnon mode, remote controllable one-way steering and tripartite entanglement can be achieved. Our scheme may have potential applications in quantum information.

Three-dimensional polarization effects in optical tunneling.

We consider the three-dimensional (3D) polarimetric properties of an evanescent optical field excited in the gap of a double-prism system by a random plane wave. The analysis covers the case of frustrated total internal reflection (FTIR), i.e.,optical tunneling, and relies on the characteristic decomposition of the 3×3 polarization matrix. We find in particular that, for any incident partially polarized plane wave, the evanescent field inside the gap is necessarily in a nonregular, genuine 3D polarization state. We also show that the 3D polarimetric properties of the field at the second boundary are sensitive to the changes of the gap width and that the relevant effects occur for the smaller widths when the angle of incidence of the plane wave becomes larger. The results of this work uncover new aspects of the polarimetric structure of genuine 3D evanescent fields and may find applications in near-field optics and surface nanophotonics.

Inelastic Light Scattering in the Vicinity of a Single-Atom Quantum Point Contact in a Plasmonic Picocavity.

Electromagnetic fields can be confined in the presence of metal nanostructures. Recently, subnanometer scale confinement has been demonstrated to occur at atomic protrusions on plasmonic nanostructures. Such an extreme field may dominate atomic-scale light-matter interactions in "picocavities". However, it remains to be elucidated how atomic-level structures and electron transport affect plasmonic properties of a picocavity. Here, using low-temperature optical scanning tunneling microscopy (STM), we investigate inelastic light scattering (ILS) in the vicinity of a single-atom quantum point contact (QPC). A vibration mode localized at the single Ag adatom on the Ag(111) surface is resolved in the ILS spectrum, resulting from tip-enhanced Raman scattering (TERS) by the atomically confined plasmonic field in the STM junction. Furthermore, we trace how TERS from the single adatom evolves as a function of the gap distance. The exceptional stability of the low-temperature STM allows to examine distinctly different electron transport regimes of the picocavity, namely, in the tunneling and QPC regimes. This measurement shows that the vibration mode localized at the adatom and its TERS intensity exhibits a sharp change upon the QPC formation, indicating that the atomic-level structure has a crucial impact on the plasmonic properties. To gain microscopic insights into picocavity optomechanics, we scrutinize the structure and plasmonic field in the STM junction using time-dependent density functional theory. The simulations reveal that atomic-scale structural relaxation at the single-atom QPC results in a discrete change of the plasmonic field strength, volume, and distribution as well as the vibration mode localized at the single atom. These findings give a qualitative explanation for the experimental observations. Furthermore, we demonstrate that strong ILS is a characteristic feature of QPC by continuously forming, breaking, and reforming the atomic contact and how the plasmonic resonance evolves throughout the nontunneling, tunneling, and QPC regimes.

On-line identification method of railway tunnels and slopes based on Brillouin scattering signal

Wide coverage and complex environment along the railway make a huge demand for calibration in critical areas such as tunnels or slopes. In this paper, an on-line identification method of railway tunnels and slopes based on Brillouin scattering signal is proposed. By monitoring Brillouin scattering signal of a redundant communication optical fiber, tunnels and slopes can be recognized by feature analysis. Experimental results indicate that the on-line identification method of railway tunnels and slopes based on Brillouin scattering signal is effective, invulnerable and accurate.

An Optical Measuring Transducer for a Micro-Opto-Electro-Mechanical Micro-g Accelerometer Based on the Optical Tunneling Effect

Micro-opto-electro-mechanical (MOEM) accelerometers that can measure small accelerations are attracting growing attention thanks to their considerable advantages—such as high sensitivity and immunity to electromagnetic noise—over their rivals. In this treatise, we analyze 12 schemes of MOEM-accelerometers, which include a spring mass and a tunneling-effect-based optical sensing system containing an optical directional coupler consisting of a fixed and a movable waveguide separated by an air gap. The movable waveguide can perform linear and angular movement. In addition, the waveguides can lie in single or different planes. Under acceleration, the schemes feature the following changes to the optical system: gap, coupling length, overlapping area between the movable and fixed waveguides. The schemes with altering coupling lengths feature the lowest sensitivity, yet possess a virtually unlimited dynamic range, which makes them comparable to capacitive transducers. The sensitivity of the scheme depends on the coupling length and amounts to 11.25 × 103 m−1 for a coupling length of 44 μm and 30 × 103 m−1 for a coupling length of 15 μm. The schemes with changing overlapping areas possess moderate sensitivity (1.25 × 106 m−1). The highest sensitivity (above 6.25 × 106 m−1) belongs to the schemes with an altering gap between the waveguides.

Overview and Research on Signal Readout Technology of Microcantilever Sensor

Since 1986, the first microcantilever applied in atomic force microscope, the device manufacturing technology, driving method and readout technology have improved significantly. In this paper, five readout technologies will be introduced in detail, and they are optical readout, piezoelectric readout, piezoresistive readout, capacitance readout and electron tunneling readout, respectively. Among them, the piezoresistive readout technology attracts a large number of researchers due to simple detection equipment, easy integration, and lower cost. In order to improve the detection accuracy of the piezoresistive microcantilever, a detection system based on FPGA is proposed in this paper. Output signal of the piezoresistive microcantilever array sensor for alpha-fetoprote (AFP) detection was researched. The detection accuracy of 0.05 Hz is achieved, the system stability is within 0.6 Hz, and it has a precise detection level for AFP with concentration of 50 pg/ml, which makes microcantilever has potential in early diagnosis and detection of liver cancer.

Nonlinear Landau-Zener tunneling under higher-order dispersion

We studied nonlinear Landau-Zener tunneling under the effect of higher-order dispersion, transformed the Gross-Pitaevskii (GP) equation with higher-order dispersion terms into a nonlinear two-energy level form using a two-mode approximation, and comprehensively analyzed the loop structure of the lowest-energy band and the nonlinear Landau-Zener tunneling. The results show that, when third-order dispersion coefficient is not zero, there is a spatial inversion symmetry breaking of the energy band structure and unbalanced Landau-Zener tunneling. By analyzing the fixed point's nature of the classical Hamiltonian, we obtain a law for the variation of the loop structure and adiabatic tunneling probability with the dispersion coefficient, consistent with our numerical analysis results. In addition, we analyzed the real-time evolution of solitons with transverse bias and observed the tunneling phenomenon. Consistent with our analysis, the adjustment of the dispersion term can effectively control optical tunneling, which provides an alternative idea for optical switching.

Viewing Optical Processes at the Nanoscale: Combining Scanning Tunneling Microscopy and Optical Spectroscopy

The combination of optical spectroscopy and scanning tunneling microscopy techniques has the potential to yield the unambiguous single-molecule level insight required to fully understand the complex structure–property relationship of emerging material systems. In this Perspective, we highlight the recent progress of single-molecule absorption detected by scanning tunneling microscopy (SMA-STM) to investigate light–matter interactions of supported (nano)particles, quantum dots, and molecular and thin films at the nanoscale. We further show how the SMA-STM technique has been recently extended to expand the classes of materials that can be investigated as well as recent design modifications that enable both time-resolved imaging and tomography. Lastly, we hypothesize how these modifications can be further leveraged to advance our current knowledge of light–matter interactions and how the resulting nanoscale insight can be implemented for future material applications.

Combining grating-coupled illumination and image recognition for stable and localized optical scanning tunneling microscopy.

Combining scanning tunneling microscopy (STM) and optical excitation has been a major objective in STM for the last 30 years to study light-matter interactions on the atomic scale. The combination with modern pulsed laser systems even made it possible to achieve a temporal resolution down to the femtosecond regime. A promising approach toward a truly localized optical excitation is featured by nanofocusing via an optical antenna spatially separated from the tunnel junction. Until now, these experiments have been limited by thermal instabilities introduced by the laser. This paper presents a versatile solution to this problem by actively coupling the laser and STM, bypassing the vibration-isolation without compromising it. We utilize optical image recognition to monitor the position of the tunneling junction and compensate for any movement of the microscope relative to the laser setup with up to 10Hz by adjusting the beamline. Our setup stabilizes the focus position with high precision (<1μm) on long timescales (>1 h) and allows for high resolution STM under intense optical excitation with femtosecond pulses.

Externally-triggerable optical pump-probe scanning tunneling microscopy with a time resolution of tens-picosecond

Photoinduced carrier dynamics of nanostructures play a crucial role in developing novel functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of real-space imaging of such carrier dynamics with nanoscale spatial resolution. However, combining the advanced technology of ultrafast pulsed lasers with STM for stable time-resolved measurements has remained challenging. The recent OPP-STM system, whose laser-pulse timing is electrically controlled by external triggers, has significantly simplified this combination but limited its application due to nanosecond temporal resolution. Here we report an externally-triggerable OPP-STM system with a temporal resolution in the tens-picosecond range. We also realize the stable laser illumination of the tip-sample junction by placing a position-movable aspheric lens driven by piezo actuators directly on the STM stage and by employing an optical beam stabilization system. We demonstrate the OPP-STM measurements on GaAs(110) surfaces, observing carrier dynamics with a decay time of sim 170 ps and revealing local carrier dynamics at features including a step edge and a nanoscale defect. The stable OPP-STM measurements with the tens-picosecond resolution by the electrical control of laser pulses highlight the potential capabilities of this system for investigating nanoscale carrier dynamics of a wide range of functional materials.

Strong enhancement of Goos-Hänchen shift through the resonant optical tunneling effect.

The resonant optical tunneling effect (ROTE) originates from the frustrated total reflection effect because unique transmission characteristics are used to study high-sensitivity sensors. In this study, we theoretically demonstrated that choosing a suitable transmission gap made it possible for the ROTE structure based on hexagonal boron nitride and graphene to obtain a large Goos-Hänchen shift as high as tens of thousands of times the incident wavelength at a specific incident angle. The amplitude of the Goos-Hänchen shift was found to be sensitive to the central layer thickness but was also modulated by the tunneling gap on both sides. In addition, adjusting the chemical potential and relaxation time of the graphene sheets could alter the Goos-Hänchen shift. Our work provides a new way to explore the Goos-Hänchen effect and opens the possibility for the application of high-precision measurement technology based on the ROTE.

Low-loss, geometry-invariant optical waveguides with near-zero-index materials.

Optical materials with nearly zero refractive indices have driven emerging applications ranging from geometry-invariant optical tunneling, nonlinear optics, optical cloaking to thermal emission manipulation. In conventional dielectric photonic circuits, light scattering and back reflection at the waveguide bends and crossings leads to significant optical loss. Here we propose to use near-zero-index materials as a cladding layer for low-loss optical waveguides, where optical modes are tightly confined within the dielectric core region. Compared to conventional waveguides, the near-zero-index waveguides are superior in maintaining a high mode-filling factor for small device sizes close to the diffraction limit and reducing the crosstalk in between at a sub-wavelength separation. In addition, we found that light propagation is robust to waveguide bends in a small radius (∼µm) and geometry variation in the cross section. Hollow waveguides with near-zero-index cladding layers further support low-loss light propagation because materials absorption is minimized from the air core. Our work offers critical insights into future designs of low-loss and miniaturized photonic devices.

Goos–Hänchen shift of a light beam tunable by graphene in the resonant optical tunneling structure

The structure for implementing resonant optical tunneling effect is a simple layered system of dielectrics that provides full light transmission for resonance condition, despite the presence of barrier layers partially locking light. The presence of a sharp resonant peak both for the intensity and for the spatial shift of the transmitted light beam makes such a structure promising for the creation of sensors and light control devices. This paper focuses on the spatial shift called the Goos–Hänchen shift of such a structure with interfaces of the waveguide layer coated by graphene. The effect of Goos–Hänchen shift near the resonance in this case may be controlled by small changes in the chemical potential or the Fermi energy of graphene, which can be controlled both chemically and by electrical bias. The characteristics of transmitted light beam strongly depend on the beam width for the selected optimal focusing condition.

Control of Contactless Profilometer for Scanning Surfaces of Complex Profile

The research problem of a surface’s profile with an extended precision detail of irregular shape with a given accuracy is considered by the contactless scanning profilometer. The contact of the measuring block with a controlled surface needs to be excluded because of a possibility of damage of a product and, at the same time, to provide the necessary accuracy of assessment of its profile. For the solution of this task in work the double-circuit system containing two sensors is used: the distance converter on the basis of optical tunneling for the “exact” channel and the converter on the basis of a chromatic aberration for “rough” channel of scanning. It is shown that based on these measurements of the “rough” channel providing the guaranteed exception of contact of “rough” channel’s contact with a surface of the studied product. It is possible to predict situation and traverse speed of the sensor of the “exact” channel for the purpose of an exception of contact with the studied surface and ensuring necessary accuracy of measurements. The mathematical model of a dynamic system of scanning based on a prior information about entering it elements and blocks is developed. The analysis of the main dynamic properties of a system of scanning is carried out and the law of its management providing the required quality of transition processes based on mathematical modeling is constructed. The algorithm of the forecast of the subsequent position of the sensor of the “exact” channel based on these measurements of “rough” channel, which provides the required speed and accuracy of scanning depending on the predicted object’s surface profile height is developed. The flowchart of the algorithm determining the value of movement of the sensor of the “exact” channel in the vertical direction depending on the received assessment of measurements of “rough” channel is developed. The conducted researches allowed developing the block diagram of a double-circuit measuring system. Modeling of this system in the environment of MATLAB/SIMULINK, which allowed evaluating efficiency of its functioning for the different studied profiles was carried out. Results of modeling showed efficiency of the offered scheme of a profilometer’s control system.

Quantum Dot Molecule Devices with Optical Control of Charge Status and Electronic Control of Coupling

AbstractTunnel‐coupled pairs of optically active quantum dots—quantum dot molecules (QDMs)—offer the possibility to combine excellent optical properties such as strong light‐matter coupling with two‐spin singlet–triplet () qubits having extended coherence times. The basis formed using two spins is inherently protected against electric and magnetic field noise. However, since a single gate voltage is typically used to stabilize the charge occupancy of the dots and control the inter‐dot orbital couplings, operation of the qubits under optimal conditions remains challenging. Here, an electric field tunable QDM that can be optically charged with one (1h) or two holes (2h) on demand is presented. A four‐phase optical and electric field control sequence facilitates the sequential preparation of the 2h charge state and subsequently allows flexible control of the inter‐dot coupling. Charges are loaded via optical pumping and electron tunnel ionization. One‐ and two‐hole charging efficiencies of (93.5 ± 0.8)% and (80.5 ± 1.3)% are achieved, respectively. Combining efficient charge state preparation and precise setting of inter‐dot coupling allows for the control of few‐spin qubits, as would be required for the on‐demand generation of 2D photonic cluster states or quantum transduction between microwaves and photons.