arXiv subjects

Alina Karabchevsky

Publications and source records attributed to Alina Karabchevsky.

At least 19 recordsLinked to original sources

arXiv2026

Mo2TiC2Tx MXene Saturable Absorption for Neural Networks on a Chip

MXenes have attracted considerable interest for integrated nonlinear photonics owing to their broadband optical response and chemical tunability, yet investigations have focused predominantly on Ti-based compositions, leaving the nonlinear potential of double-transition-metal MXenes largely unexplored. Here we quantify the nonlinear optical response of the double-transition-metal MXene \ce{Mo2TiC2T_x} and demonstrate its functionality as a waveguide-integrated nonlinear activation element for neuromorphic photonics. Z-scan measurements at \SI{800}{\nano\metre} reveal strong saturable absorption in films of two different thicknesses, yielding effective nonlinear absorption coefficients ($β_\mathrm{eff}$) between approximately $-2.69\times10^{3}$ and $-0.88\times10^{3}$ \si{\centi\metre\per\giga\watt}, with the nonlinear response decreasing at higher excitation intensities. The extracted nonlinear absorption is approximately one order of magnitude larger than that reported for \ce{Ti3C2T_x} MXene under comparable conditions. We integrate an ultrathin \ce{Mo2TiC2T_x} MXene layer onto a silicon rib waveguide to realize a compact nonlinear optical activation function, which, when implemented in a neural-network emulator, achieves $98.39\%$ classification accuracy on the MNIST benchmark. This work expands the MXene material platform beyond Ti-based compositions and establishes double-transition-metal MXenes as promising candidates for high-performance integrated nonlinear photonic and neuromorphic computing technologies.

physics.optics↗

arXiv2026

Optoelectronic Reservoir Computing with an On-Chip True-Time-Delay Element

Compact delay elements remain a central challenge in photonic neuromorphic processors. Here, we demonstrate an optoelectronic delayed-feedback reservoir computer that incorporates a foundry-fabricated silicon nitride (Si$_3$N$_4$) true-time-delay circuit within its feedback loop. Eight cascaded Archimedean spirals provide a \SI{1.76}{\metre} on-chip optical path and a calculated passive group delay of \SI{11.63}{\nano\second}. At a feedback gain of $G=0.0223$, the system classifies sinusoidal and square waveforms without error (word error rate, WER\,$=$\,0 across all cross-validation folds), predicts the Mackey--Glass chaotic series with a best-fold normalized mean-square error (NMSE) of $0.0082$, and performs nine-class Japanese Vowels speaker classification with WER\,$=$\,0.0898. We also introduce a subcarrier phase-encoding method that maps the calculated $0.232π$ spiral-to-reference phase contrast onto the measured reservoir states through deliberate aliasing, achieving NMSE\,$=$\,0.0767 and WER\,$=$\,0 without increasing the insertion-loss-limited feedback gain. These results show that on-chip propagation delay in Si$_3$N$_4$ can operate as a functional component of an optoelectronic reservoir computer and motivate lower-loss, more-integrated implementations.

physics.optics↗

arXiv2025

Bound-state-in-the-continuum (BIC) induced narrow resonances in MXene-coated absorptive dielectric metasurfaces for Methane sensing

Strong light confinement is highly necessary for various applications, including sensing. MXene, a novel and emerging material with a broadband plasmonic response, has been highly utilized in electronic sensing systems, as well has garnered significant attention for its applicability in photonics. The loss imparted by MXene can be overcome through the Bound States in Continuum (BIC) physics. In this work, we report two important designs of a hybrid metasurface, comprising a silicon nanodisk metasurface and MXene. In both designs, narrow and high absorption resonance of quality factor of $\sim$150 is attained, where MD is the dominant multipole, governed by symmetry-protected BIC. Both hybrid metasurface designs are optimized to exhibit narrow resonance in the vicinity of 1650 nm with an absorption greater than 90 $\%$. The origin of high absorbance in such a hybrid metasurface is attributed to the momentum matching by the spacer layer of $SiO_2$. The spectral characteristics of the designed metasurface can be utilized for first overtone spectroscopy of Methane gas. Numerical simulations yield a bulk refractive index sensitivity of 171 nm/RIU with FOM = 17.56 $RIU^{-1}$ and sensitivity for Methane gas, S = 0.8 nm per unit percentage concentration, when a Cryptophane-E layer is used.

physics.optics↗

arXiv2025

Quantum photonics on a chip

Optical chips for quantum photonics are cutting-edge technology, merging photonics and quantum mechanics to manipulate light at the quantum level. These chips are crucial for advancing quantum computing, secure communication, and precision sensing by integrating photonic components like waveguides, beam splitters, and detectors to manipulate single photons, the fundamental carriers of quantum information. Key advancements in optical chips include low-loss waveguides, efficient single-photon sources, and high-fidelity quantum gates, all essential for scalable quantum circuits. Integrating these circuits on a chip offers significant advantages in miniaturization, stability, and reproducibility over traditional bulk optics setups. Recent breakthroughs in materials science and nanofabrication have propelled the field forward, enabling the production of chips with higher precision and lower defect rates. Silicon photonics, in particular, has become a prominent platform due to its compatibility with existing semiconductor manufacturing processes, facilitating the integration of quantum photonic circuits with classical electronic systems. Here, we share our vision of the future of optical chips for quantum photonics, which hold promise for various applications. In quantum computing, they enable the development of compact and scalable quantum processors. In communication, they provide the foundation for ultra-secure quantum networks through quantum key distribution. In sensing, they allow for high-precision measurements that surpass classical limits. As research progresses, optical chips are expected to play a critical role in realizing the full potential of quantum technologies.

quant-ph↗

arXiv2025

Phase Topology Stability of an Optical Vortex via an Electrically Controlled Twist-Planar Oriented Liquid Crystal Fresnel Lens

Optical vortices (OVs) have emerged as a revolutionary concept in modern photonics, offering a unique method of manipulating light beyond conventional Gaussian beams. Despite their vast potential, phase topology stability remains unaddressed, limiting their widespread adoption and performance in real-world environments. Here, we reveal the missing link to assessing the stability of optical vortices using an electrically tunable twist-planar liquid crystal (LC) Fresnel lens. The proposed LC-based lens leverages the birefringence and voltage-controlled reconfigurability of liquid crystals to dynamically probe the phase topology of singular beams. By modulating the LC orientation with an applied voltage, we restructure the optical phase in real-time without requiring modifications to the optical setup. The 3V and 35V voltage supply allows for the switch between the "topological charge detection" and "optical singular beam propagation" modes. This eliminates the need for additional optical elements, significantly simplifying the detection and characterization of vortex beams. Experimental and theoretical investigations demonstrate that the vortex topology can be unambiguously identified from the intensity profile observed in the Fourier plane of a lens. Furthermore, the designed device features low power consumption, compact form factor, and seamless integration potential, making it a promising candidate for scalable vortex-based photonic systems.

physics.optics↗

arXiv2025

In Situ Optimization of an Optoelectronic Reservoir Computer with Digital Delayed Feedback

Reservoir computing (RC) is an innovative paradigm in neuromorphic computing that leverages fixed, randomized, internal connections to address the challenge of overfitting. RC has shown remarkable effectiveness in signal processing and pattern recognition tasks, making it well-suited for hardware implementations across various physical substrates, which promise enhanced computation speeds and reduced energy consumption. However, achieving optimal performance in RC systems requires effective parameter optimization. Traditionally, this optimization has relied on software modeling, limiting the practicality of physical computing approaches. Here, we report an \emph{in situ} optimization method for an optoelectronic delay-based RC system with digital delayed feedback. By simultaneously optimizing five parameters, normalized mean squared error (NMSE) of 0.028, 0.561, and 0.271 is achieved in three benchmark tasks: waveform classification, time series prediction, and speech recognition outperforming simulation-based optimization (NMSE 0.054, 0.543, and 0.329, respectively) in the two of the three tasks. This method marks a significant advancement in physical computing, facilitating the optimization of RC and neuromorphic systems without the need for simulation, thus enhancing their practical applicability.

cs.ET↗

arXiv2023

On-chip polarization-encoded single-qubit gates with twisted waveguides

Integrated photonics is a remarkable platform for scalable classical and quantum light-based information processing. However, polarization manipulation on a chip despite of its fundamental significance in information processing remains elusive. Polarization manipulation capabilities have been recently demonstrated in femtosecond laser-inscribed twisted waveguides, although the systematic theoretical description of polarization manipulation has not been established for this architecture. In this work we develop a rigorous theory of a twisted waveguide unveiling its eigenmodes and transmission matrix in the closed form. Utilizing the developed theory, we demonstrate that twisted waveguides can realize virtually arbitrary polarization transformations while satisfying reasonable design constraints. This fact combined with low cost and ease of prototyping of laser inscribed photonic integrated circuits allows us to suggest twisted waveguide as a robust building block for on-chip polarization-encoded information processing.

quant-ph↗

arXiv2022

On-chip beam rotators, adiabatic mode converters, and waveplates through low-loss waveguides with variable cross-sections

Photonics integrated circuitry would benefit considerably from the ability to arbitrarily control waveguide cross-sections with high precision and low loss, in order to provide more degrees of freedom in manipulating propagating light. Here, we report a new method for femtosecond laser writing of optical-fibre-compatible glass waveguides, namely spherical phase induced multi-core waveguide (SPIM-WG), which addresses this challenging task with three dimensional on-chip light control. Fabricating in the heating regime with high scanning speed, precise deformation of cross-sections is still achievable along the waveguide, with shapes and sizes finely controllable of high resolution in both horizontal and vertical transversal directions. We observed that these waveguides have high refractive index contrast of 0.017, low propagation loss of 0.14 dB/cm, and very low coupling loss of 0.19 dB coupled from a single mode fibre. SPIM-WG devices were easily fabricated that were able to perform on-chip beam rotation through varying angles, or manipulate polarization state of propagating light for target wavelengths. We also demonstrated SPIM-WG mode converters that provide arbitrary adiabatic mode conversion with high efficiency between symmetric and asymmetric non-uniform modes; examples include circular, elliptical modes and asymmetric modes from ppKTP (periodically-poled potassium titanyl phosphate) waveguides which are generally applied in frequency conversion and quantum light sources. Created inside optical glass, these waveguides and devices have the capability to operate across ultra-broad bands from visible to infrared wavelengths. The compatibility with optical fibre also paves the way toward packaged photonic integrated circuitry, which usually needs input and output fibre connections.

physics.optics↗

arXiv2021

Super-resolution imaging and optomechanical manipulation using optical nanojet for non-destructive single-cell research

Advanced photonic tools may enable researchers and clinicians to visualize, track, control and manipulate biological processes at the single-cell level in space and time. Biological systems are complex and highly organized on both spatial and temporal levels. If we are to study, perturb, engineer or heal biological entities, we must be able to visualize key players in such systems and to track, control and manipulate them precisely and selectively. To achieve this goal, the engineering of non-destructive tools will allow us to interrogate and manipulate the function of proteins, pathways and cells for physicians, enabling the design of 'smart materials' that can direct and respond to biological processes on-demand. Among the potentially exploitable non-destructive tools, light-based actuation is particularly desirable, as it enables high spatial and temporal resolution, dosage control, minimal disturbance to biological systems and deep tissue penetration. Here, we overview existing approaches toward the engineering of light-activated tools for the interrogation and manipulation of single-cell processes, and list the types of studies and types of functions that can be controlled by light. Timely applications, such as studies of inflammation and of crossing brain barrier systems - via super-resolution imaging and optomechanical manipulation - are two representative examples of emerging applications so far never addressed.

physics.optics↗

arXiv2021

Green-extraction of carbon thin films from natural mineral Shungite

Conventional fabrication methods to produce graphene are cumbersome, expensive and are not ecology friendly. This is due to the fact that a large volume of raw materials requires a large number of acids and alkalis, which in turn requires special disposal. Therefore, it is necessary to develop new technologies or refine existing technologies for the production of graphene and create new ecology-safe and effective methods to produce graphene. Here, we utilized the physical dispergation to extract graphene films from natural mineral shungite rock. We studied the structure of shungite via Raman spectrometry and X-ray phases analysis and found that the shungite refers to graphite-like mineral structures. From spectral data, we learned that the main constituent of shungite is amorphous carbon in sp2 and sp3 forms. Transmission electron microscopy images of the processed material revealed that the obtained graphene films with well-developed surfaces are as small as 200 nanometers. Our green fabrication method of graphene can be widely used in a variety of fields from electronics (electrodes), optics, biotechnology (biosensors), ecology (sorbents for wastewater, air purification) to list a few.

physics.app-ph↗

arXiv2021

Ti$_3$C$_2$T$_x$ MXene Enabled All-Optical Nonlinear Activation Function for On-Chip Photonic Deep Neural Networks

Neural networks are one of the first major milestones in developing artificial intelligence systems. The utilisation of integrated photonics in neural networks offers a promising alternative approach to microelectronic and hybrid optical-electronic implementations due to improvements in computational speed and low energy consumption in machine-learning tasks. However, at present, most of the neural network hardware systems are still electronic-based due to a lack of optical realisation of the nonlinear activation function. Here, we experimentally demonstrate two novel approaches for implementing an all-optical neural nonlinear activation function based on utilising unique light-matter interactions in 2D Ti$_3$C$_2$T$_x$ (MXene) in the infrared (IR) range in two configurations: 1) a saturable absorber made of MXene thin film, and 2) a silicon waveguide with MXene flakes overlayer. These configurations may serve as nonlinear units in photonic neural networks, while their nonlinear transfer function can be flexibly designed to optimise the performance of different neuromorphic tasks, depending on the operating wavelength. The proposed configurations are reconfigurable and can therefore be adjusted for various applications without the need to modify the physical structure. We confirm the capability and feasibility of the obtained results in machine-learning applications via an Modified National Institute of Standards and Technology (MNIST) handwritten digit classifications task, with near 99% accuracy. Our developed concept for an all-optical neuron is expected to constitute a major step towards the realization of all-optically implemented deep neural networks.

physics.optics↗

arXiv2021

Purcell effect in PT-symmetric waveguides

This chapter overviews the principles of the spontaneous emission rate increase, that is the Purcell effect, in relation to the photonic parity-time (PT) symmetry. Being focused on the system of coupled PT-symmetric optical waveguides, we consider behaviors of the Purcell factor in PT-symmetric and broken-PT-symmetric regimes. Surprisingly, exceptional points in a coupled waveguide do not influence on the Purcell factor.

physics.optics↗

arXiv2020

Modal Purcell factor in $\mathcal{PT}$-symmetric waveguides

We study the spontaneous emission rate of a dipole emitter in $\mathcal{PT}$-symmetric environment of two coupled waveguides using the reciprocity approach generalized to non-orthogonal eigenmodes of non-Hermitian systems. Considering emission to the guided modes, we define and calculate the modal Purcell factor composed of contributions of independent and interfering non-orthogonal modes leading to the emergence of cross-mode terms in the Purcell factor. We reveal that the closed-form expression for the modal Purcell factor within the coupled mode theory slightly alters for the non-Hermitian coupled waveguide compared to the Hermitian case. It is true even near the exceptional point, where the eigenmodes coalesce and the Petermann factor goes to infinity. This result is fully confirmed by the numerical simulations of active and passive $\mathcal{PT}$-symmetric systems being the consequence of the mode non-orthogonality.

physics.optics↗

arXiv2020

Tailored optical potentials for Cs atoms above waveguides with focusing dielectric nano-antenna

Tuning the near-field using all-dielectric nano-antennae offers a promising approach for trapping atoms, which could enable strong single-atom/photon coupling. Here we report the simulation results of an optical trapping concept, in which a silicon nano-antenna produces a trapping potential for atoms in a chip-scale configuration. Using counter-propagating incident fields, bichromatically detuned from the atomic cesium D-lines, we numerically investigate the dependence of the optical potential on the nano-antenna geometry. We tailor the near-field potential landscape by tuning the evanescent field of the waveguide using a toroidal nano-antenna, a configuration that enables trapping of ultracold Cs atoms.

physics.optics↗

arXiv2020

Simulation and experimental observation of tunable photonic nanojet and photonic hook upon asymmetric illumination of a mesoscale cylinder with mask

In this letter, we report on numerical study, fabrication and experimental observations of tunable photonic nanojet and photonic hook. Here, the curved photonic nanojets (photonic hooks) are generated by single mesoscale microcylinder that we fabricated from polydimethylsiloxane (PDMS), upon its boundary illumination and adjustable area at an incident wavelength of λ = 405 nm. Experimental observations were conducted for PDMS microcylinder of diameter d = 5 μm deposited on a silicon substrate with the aluminum mask. Measurements were performed with a scanning optical microscope. Our experimental results are in good agreement with numerical predictions performed with the finite-difference time-domain method. The observed the full widths at half-maximum of photonic hooks are 0.48λ, 0.56λ, and 0.76λ for tilt angles of θ = 0°, 5.7°, and 20.1° respectively, at the mask heights of h = 0, 0.25d and 0.5d, respectively, displaying the specificities of the field localization. These photonic hooks have great potential in complex manipulation such as super-resolution imaging, surface fabrication, and optomechanical manipulation in curved trajectories smaller than wavelength.

physics.optics↗

arXiv2019

Magnetic Octupole Response of Dielectric Oligomers

The development of new approaches to tuning the resonant magnetic response of simple all-dielectric nanostructures is very important in modern nanophotonics. Here we show that a resonant magnetic octupole (MOCT) response can be obtained by dividing a solid rectangular silicon block to an oligomer structure with the introduction of narrow gaps between four nanocubes. We control and tune the spectral position of the MOCT resonance by varying the distance between the nanocubes. We demonstrate that several magnetic hot-spots related to the MOCT resonance can be located in the gaps creating a strong magnetic field gradient in free space. We observe that the resonant excitation of the MOCT moment leads to a significant enhancement of light absorption in the system at the spectral region, where light absorption in bulk silicon is weak. The results of this work can be applied to design new composite antennas and metamaterials based on complex building blocks, energy harvesting devices and molecular trapping with magnetic hot-spots.

physics.optics↗

arXiv2019

Transverse scattering and generalized Kerker effects in all-dielectric Mie-resonant meta-optics

All-dielectric resonant nanophotonics lies at the heart of modern optics and nanotechnology due to the unique possibilities to control scattering of light from high-index dielectric nanoparticles and metasurfaces. One of the important concepts of dielectric Mie-resonant nanophotonics is associated with the Kerker effect that drives the unidirectional scattering of light from nanoantennas and Huygens' metasurfaces. Here we suggest and demonstrate experimentally a novel effect manifested in the nearly complete simultaneous suppression of both forward and backward scattered fields. This effect is governed by the Fano interference between an electric dipole and off-resonant quadrupoles, providing necessary phases and amplitudes of the scattered fields to achieve the transverse scattering. We extend this concept to dielectric metasurfaces that demonstrate zero reflection with transverse scattering and strong field enhancement for resonant light filtering, nonlinear effects, and sensing.

physics.optics↗

arXiv2019

Broadband plasmonic nanoparticles: fabrication, optical properties, and implications in liquid light chemiluminescence enhancement

Chemiphores are entities, which exhibit wide-band light emission without any external light source but just due to the chemical reaction resulting in the chemiluminescence effect. Since the chemiphores usually have low quantum efficiency, chemiluminescence is a weak optical effect. We found that plasmonic nanoparticles can efficiently enhance the peculiar effect of chemiluminescence due to the acceleration of the radiative decay of the chemiphore excited state which, in turn, enlarges the chemiluminescence yield. Correspondingly, plasmonic nanoparticles are nanoparticles with sub-wavelength sizes experiencing the absorption band in specific wavelength which are characterized by unique optical properties, as well as high localization of electromagnetic radiation. However, the broadband properties of plasmonic nanoparticles and their implications in liquid light, the chemiluminescence effect, is overlooked. Therefore, they can attract attention as novel materials for photonics, sensing, and forensic science. Here, fabrication techniques of broadband plasmonic nanoparticles are reported, and their interesting optical properties together with their applications in chemiluminescence effect are discussed, as well. We fabricated the nanoparticles with laser ablation in liquids (LAL) technique and propose the physical vapor deposition (PVD) synthesis with annealing-assisted treatment for further studies. Both techniques are accessible and allow production of ensembles of nanoparticles having shape and size distributions to exhibit broad plasmonic resonance which fit the wide-band emission of a chemiphore. Our results, in particular, a specific design for plasmonic nanoparticles placed on the dielectric material, lead the way toward a new generation of chemiluminescence-based devices starting from sensing, healthcare, biomedical research and quantum systems such as pump-free laser sources.

physics.optics↗