This study explores the tunable Goos-Hanchen (GH) shift in a one-dimensional photonic crystal (PC) featuring a central defect layer composed of the optical phase-change material Ge2Sb2Se4Te1 (GSST). GSST exhibits a reversible phase transition between amorphous and crystalline states, typically occurring at a critical temperature of approximately 500 K. A polarization and phase-state-dependent defect mode is identified within the photonic bandgap, whose intensity and spectral position vary with key parameters such as the GSST phase ratio, defect layer thickness, incident angle, and PC periodicity. Near the defect mode, a pronounced negative GH shift is observed in reflection, while a positive shift emerges in transmission. Importantly, the negative GH shift disappears entirely as GSST transitions from the amorphous to the crystalline phase. These results demonstrate a viable strategy for dynamic control of light propagation, enabling GSST-based non-volatile photonic functionalities including optical switches, detectors, and sensors.

Phase change materials (PCMs) are attractive candidates for tunable devices due to their unique properties, such as high degree of scalability, thermal control, low power consumption, wide waveband operation, and the ability to switch between different optical phases. These properties can be enhanced by integrating PCMs with other materials, such as plasmonic nanoparticles. In this work, a core-shell nanostructure (Au@GSST) is proposed comprising a gold nanoparticle (AuNP) core coated with Ge2Sb2Se4Te1 (GSST), a PCM with high optical contrast, embedded in an aqueous medium. We demonstrate how the phase transition of GSST can be actively controlled by the light energy absorption of the Au@GSST. The integration of the Au core facilitates the phase change process of GSST due to its plasmonic effect, which leads to lower heat capacity and higher heat conductivity of the AuNP. These characteristics accelerate the GSST phase change process at a lower continuous wave (CW) laser intensity compared to a bare GSST nanoparticle. An induced photothermal process that includes heat transfer, the crystalline fraction, and the electric field enhancement of the Au@GSST, as functions of the laser wavelength and intensity is investigated. Our results show that through this process, the GSST shell can be tuned between fully amorphous, intermediate, and fully crystalline states. This phase transition leads to a substantial modification of the optical responses of the Au@GSST. The absorption, scattering and extinction cross-sections of the structure over a wide range of wavelengths before and after the GSST phase transition is studied. We focus on two specific wavelengths, 778 nm and 919 nm, which exhibit higher light absorption contrast in both the amorphous and crystalline phases of GSST. Such active tunning of Au@GSST without morphological variation can be utilized in reconfigurable nanophotonic devices, such as switches, modulators, and sensors.

This paper presents a reconfigurable plasmonic structure consisting of a phase-change material slab, Ge2Sb2Se4Te1 (GSST), overlaid with a fishnet pattern comprising two dielectric layers containing quantum dots, sandwiching a silver metal layer. We demonstrate the distinct optical tunability of this structure by exploiting the phase transition of GSST from its amorphous to crystalline state in the near-infrared spectrum. Our findings reveal significant tunable contrasts in the structure transmission coefficient that leads to achieve extraordinary optical transmission (EOT). These results highlight the structure's potential for adaptive optics and reconfigurable photonic applications. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

In this paper, we demonstrate a theoretical study of a multiphysics problem to solve for the photothermal response of a one-dimensional multilayer structure containing a layer doped with VO2@Au nanoshells. The VO2@Au nanoshell consists of a gold (Au) shell and a core of the phase change material vanadium dioxide (VO2) where the VO2 core transitions from a semiconductor state to a conductor state at the critical temperature of 68 degrees C. This behaviour results in thermal induced optical tunability through this reversible phase change of the VO2, due to the temperature dependent optical and thermal properties. The presence of the VO2 core, functioning as an ultra-fast and reversible optical phase-change material, leads to the emergence of photothermal induced bistability. The layer doped with the VO2@Au nanoshell is approximated as an effective medium using the Maxwell-Garnett Theory to enable an analytical solution. In this study, the optical response of the multilayer structure is obtained using the Transfer Matrix Method, while the thermal response for both stationary and transient states is solved using the Green's Function Method and Kirchhoff's Transformation. These equations are interconnected through the heat source term in the heat diffusion equations, representing the local heat generation induced by the continuous-wave laser applied to the structure. Our findings indicate that at the wavelengths of 658 nm and 747 nm, there are two distinct photothermal responses arising from the phase change of the VO2 core. At these wavelengths, the absorption of light increases and decreases, respectively, because of the VO2 phase change. This analytical method not only offers a thorough exploration of the fundamentals of induced photothermal responses in multilayer structures but also holds considerable potential for various applications, including solar cells, photothermal therapy, and nanothermal sensors.

Phase change materials (PCMs) have received significant attention in various fields due to their remarkable ability to undergo phase transitions and induce substantial changes in their physical properties. One such material, vanadium dioxide (VO2), has emerged as a prominent PCM that exhibits a reversible metal–insulator transition near room temperature. These transitions are accompanied by rapid modifications in electrical conductivity and surface properties. Efforts have been made recently to enhance the performance and expand the utility of VO2 by combining it with other materials and structures. One effective approach is the use of plasmonic hybridization with vanadium dioxide (VO2), which enhances the optical and functional properties of VO2-based materials. This study offers a comprehensive review of previous research, with a specific focus on investigating the plasmonic hybridization in VO2@Au nanoshells. To analyze the plasmonic modes in this innovative core–shell structure, a combined theoretical and simulation-based approach is employed. The investigation encompasses both the semiconductor and metallic phases of the VO2 core, revealing the presence of sphere and cavity plasmonic modes. Remarkably, the results highlight that the cavity frequency becomes the dominant mode beyond wavelengths of 778 nm, particularly in the metallic phase. Furthermore, this study presents valuable insights into the charge distribution resulting from symmetric and asymmetric plasmon oscillations at specific wavelengths, particularly in the optimized scenario of the VO2@Au nanoshell.

This work introduces a fast semi-analytical algorithm for the inverse design and optimization of a one-dimensional beam deflector metagrating, utilizing Smart Pattern Search (SPS), an enhanced pattern search algorithm from MATLAB’s Global Optimization Toolbox. This algorithm demonstrates a significantly shorter processing time compared to machine learning based approaches for the same metagrating structure setup, parameters, and electromagnetic solver while achieving highly competitive efficiencies. At a wavelength of 1100 nm with angles of 60° and 70°, SPS even outperforms these methods. The SPS algorithm needs no state-of-the-art computers and completes the process in less than 27 min, while for counterpart methods at least several hours are needed on an Intel Core i7-3632QM CPU at 2.2 GHz, with 8 GB of DDR3 RAM.

In this paper, we present an in-depth examination of the absorption dynamics of a silver plasmonic grating incorporating Kerr-type nonlinear graphene-oxide (GO) nanoslits and a Ge2Sb2Se4Te1 (GSST) phase-change material, both in linear and nonlinear regimes. Linear absorption approaches unity at the resonant wavelength when GSST is amorphous. Upon exposure to a nanosecond Gaussian pulse laser irradiation with appropriate fluences, the amorphous GSST partially crystallizes through a thermoplasmonic-induced process, resulting in a significant reduction of the linear on-resonance absorption in the form of a step-like curve. Additionally, the weak off-resonance linear absorption of the system can be enhanced owing to the non-uniform phase-transition within the GSST. Transitioning to a nonlinear regime, considering the third-order nonlinearity of GO nanoslits, reveals distinct absorption behaviors. Temporal analysis reveals that at the on-resonance wavelength, the unit absorption decreases with increasing laser intensity, reaching a minimum at the pulse peak due to the Kerr nonlinearity in the GO. Intriguingly, the absorption exhibits a U-shaped curve at lower fluences, while at higher fluences, it stabilizes at a lower value post-pulse peak where the amorphous GSST undergoes a thermally driven partially crystallization phase transformation. In the off-resonance mode, the weak absorption enhances dramatically by increasing the fluence, tracing an imperfect parabolic trajectory that reaches nearly unit at the trailing edge of the pulse, attributed to the Kerr nonlinearity within the GO nanoslits. By further increasing the laser fluence, the photothermal response of the GSST emerges as the dominant factor, diminishing the unity absorption observed at the trailing edge of the pulse. These findings underscore the dynamic responsiveness of the proposed grating to pulsed laser illumination, driven by the nonlinear Kerr effect and GSST reconfigurability, offering promising opportunities for developing active optical switching devices.

We study infrared routing and switching with tunable spectral bandwidth using in-plane scattering of light by flat Au nanoantenna arrays. The base dimensions of these nanoantennas are approximately 250 by 850 nm, while their heights vary from 20 to 150 nm. Our results show that, with the increase in height, the arrays become more efficient scatterers while their spectra broaden within the 1-1.6 mu m range. Our findings demonstrate that such processes strongly depend on the incident light polarization. For a given polarization, the incident light is efficiently scattered in only two opposite directions along the plane of the arrays, with insignificant transmission. Switching such a polarization by 90 degrees, however, suppresses this process, allowing the light to mostly pass through the arrays with minimal scattering. These unique characteristics suggest a tunable beam splitter application in the 1-1.6 mu m range and even longer wavelengths.

Chalcogenide phase-change materials (PCMs) are particularly suited for dynamically controlling the response of photonic devices because they offer high-speed phase switching, non-volatility, reversibility, high thermal stability, and multi-level structure capability. Ge2Sb2Se4Te1 (GSST) has recently received intense attention due to the remarkable differences between the optical and electrical features of its two states (amorphous and crystalline). The crystallization is rapid with significantly low optical loss across the visible and near-infrared spectral ranges. In this paper, we propose a reconfigurable one-dimension (1D) gold Fabry-Perot grating filled with GSST irradiated by a nanosecond Gaussian pulse laser. By using the finite element method (FEM), we calculate the dynamics of partially crystallized GSST through a thermoplasmonic induced process. Our results show that with well-chosen pulse laser fluences, we can achieve a different layout of alternating amorphous and crystalline GSST states in the grating grooves where all the structural variations remain constant. These induced multi-layered formations in the grating grooves results in a tunable light absorber nanostructure where the absorption peaks experience a red shift gradually decreasing in value as the number of layers is increased. The findings of this study not only provide the fundamental concepts for the suggested tunable nanostructure but also suggest potential applications in various nanophotonic devices including thermal emission controllers, multi-level memories, color displays, and cognitive computing devices.

In this paper, we investigate the nonlinear optical response characteristics of a metallic nanograting with nonlinear Kerr media within its slits using the finite element method. The proposed nanograting system is illuminated by a nanosecond Gaussian pulse laser under normal incidence and the electric filed pointing across the slits. The results show a perfect linear absorption at resonance wavelength thanks to the coupling of the surface plasmon resonance (SPR) mode and photon cavity mode. We simulate the transient nonlinear absorption variation of the system when the pulse laser is set up at either resonance or off-resonance wavelengths. The results indicate that the unit linear absorption drastically decreases by increasing the laser fluence around the center of the pulse. Interestingly, one can also enhance the weak linear off-resonance absorption to the value of unit by increasing the pulse laser fluence. The higher the laser fluence, the higher the maximum absorption contrast between linear and nonlinear regimes occurs owing to the nonlinear Kerr effect. Indeed, when the laser fluence reaches a critical value, it can excite the Kerr nonlinearity, which changes the coupling strength of SPR mode and the photon cavity mode leading to the absorption adjustment in the nanograting. These properties indicate the possibility of utilizing the proposed nanograting in dual functional absorber and nonabsorber systems, which make it an appropriate candidate for agile optical switching devices.

The thermally triggered semiconductor-metal phase transition of vanadium dioxide (VO2) is a frequent subject in the study of nanostructure responses due to the high speed of the transition. Here we report on the molecular energy transfer near a hybrid VO2@Au nanoshell during the VO2 phase transition when induced by a continuous-wave (CW) laser. The presence of VO2 causes a bistable and reversible change in the optical response of the nanoshell through the thermo-optical process at the resonance wavelength of the VO2@Au nanoshell. This behavior is achieved by controlling the laser intensity during the heating and cooling processes. In this paper we couple the thermodynamics with the Forster-Dexter theory of energy transfer between molecules which is generalized to use a nearby VO2@Au nanoshell. The bistable and reversible change in the response of the nanoshell causes the molecular energy transfer in the same manner over an intensity range of 1.52 (GW/m(2)). This work also provides general guidelines for designing switchable surface plasmon based biosensors, switchable molecular junction devices and switching the energy exchange between proteins.

Gold nanoparticles (AuNPs) are capable of localizing and enhancing light on a nano-sized scale, making these particles a unique structure to overcome the diffraction limit to allow for higher optical resolution in photonic devices. The optical response of AuNPs is controlled by their size, shape, and the refractive index contrast with the surrounding medium. Using phase change materials (PCMs) in the AuNP design introduces a dynamic tunability to the optical response without any modification to the geometrical properties. In this work, we have studied the induced photothermal response of the vesicle nanoparticle (VNP) consisting of a vanadium dioxide (VO2) core (80 nm), as a PCM, and an Au shell (10 nm) covered by Au-seeds (10 nm). When the VNP is irradiated by a CW laser, the light energy absorbed by the particle provides enough heat for the VO2 core to undergo a phase change from the semiconductor state to the metallic state. To do this, we simultaneously solve a selfconsistent multiphysics problem consisting of electromagnetism and thermodynamics. Our calculations show that the maximum tunability of the extinction cross section corresponds to the scattering part achieved at the near-infrared wavelength of lambda = 790 nm for the incident threshold intensity of 101.5 kW/cm2 at which the VO2 core experiences the semiconductor-metal phase change. We also show how the temperature is localized inside the VNP at both the semiconductor and metal phases of the VO2 core. It is expected that our results will offer a promising potential application in active photonic devices, near infrared imaging, detectors, and tunable scatterers.

Recently, interest in the use of hot electron photodetectors (HEPDs) for energy harvesting has increased. In this paper, we introduce a switchable structure consisting of an absorber and metal/semiconductor Schottky-junction HEPD that incorporates a phase change material in the design. The structure is a purely planer configuration composed of a vanadium dioxide (VO 2 )/molybdenum disulfide (MoS 2 )/photonic crystal-distributed Bragg reflector (PC-DBR). VO 2 is a phase change material that exhibits a reversible semiconductor to metal phase transition at the critical temperature of 68 °C. The nanostructure response relies on the VO 2 phase transition under thermal heating. The transition of the VO 2 phase from the semiconductor to the metal phase causes the formation of the Schottky-junction between the VO 2 /MoS 2 interface, causing the structure to switch from the absorber to the hot electron photodetector in the near infrared region. In addition to, the switchable absorber–HEPD response; the absorption–photoresponsivity shows spectral tunability via the chosen thickness of the VO 2 and MoS 2 layers. This hybrid nanostructure is also sensitive to the light polarization state of both TE and TM modes. The proposed hybrid nanostructure is a promising candidate for switchable absorber–HEPD structures, with applications as switchable hot electron-based photovoltaic and sensing systems.

Plasmonic nanoparticles (PNPs) are considered as a proper mediator in photothermal therapy due to their capability to efficiently convert the absorbed light energy into localized heat. As the localized heat diffuses during therapy processes, it is crucial to control and detect the temperature in a non-invasive way. Synthesizing a new generation of PNPs that utilize optical phase-change materials leads to a tunable photothermal response without geometrical variation. This tunability originates from the prompt variation of optical and thermal properties during the phase-change transition. In this paper, we numerically study the photothermal response of a VO2@Au nanoshell in an aqueous medium when irradiated by a nanosecond pulsed laser (5 ns). For the temperature profiles, we simultaneously solve a self-consistent multiphysics problem consisting of electromagnetism and thermodynamics. We do our calculations for two wavelengths of 658 and 737 nm where the absorption spectrum of the nanoshell has two extrema during the VO2 core's phase-change transition. Finally, for the two selected wavelengths, by coupling the structural dynamic physics to the problem, we calculate the acoustic pressure signals generated by the photothermal expansion of both the nanoshell and its surrounding medium. This photoacoustic signal could be considered as a non-invasive method to measure the local temperature in deep tissues accurately.

In this study, the finite‐element method is used to numerically calculate a photo‐thermally induced reconfigurability in a 1D gold grating structure filled with the phase change material Ge2Sb2Se4Te1 (GSST). GSST features a reversible and stable phase change between amorphous and crystalline phases around a critical temperature of 410 K with broadband low optical loss. In this study, the required heating for the transition between phases is provided by a nanosecond Gaussian pulse laser through a photo‐thermal absorption process. A comprehensive heat transfer analysis is performed to investigate the thermal characterizations of the proposed structure. The results show that in the amorphous state, the structure has a near unity absorption band in the infrared region. As the temperature increases during the pulse, the GSST undergoes the amorphous to crystalline phase change. In the intermediate states (partial crystallization) of the GSST two resonance peaks are excited and the absorption finally reaches its minimum value of 0.2 in the GSST crystalline phase. The findings of this study not only provide the fundamental concepts for the suggested tunable structure, but also have potential applications in a variety of nanophotonic devices including thermal emission controllers, sensors, and optical detection devices.

In this paper, the optical properties of two small silver metallic nanoparticles (sAgNPs) coupled to a quantum dot (QD) are studied. The interaction between sAgNP and QD is investigated theoretically using the compact density matrix method. In this paper, due to the small size of the AgNP, the dielectric function does not follow the classical models and quantum-size effects must be considered. The excitation of surface plasmons in sAgNP is observed using the finite element method. The main result of the current study shows that when AgNPs are small, the absorption spectrum profile of the QD is strongly affected due to the plasmon–exciton–plasmon interaction. The absorption spectrum profile of the QD shows an electromagnetically induced transparency with two peaks and a minimum in the transition frequency. Then, the near-field enhancement of the sAgNP, the field experienced by the QD, the exciton transition energy shift, and the Förster-enhanced broadening of the excitonic transition are also examined.

Infrared neural stimulation techniques have potential applications in the diagnosis and treatment of numerous neurological and psychiatric disorders. There has been little progress in the computational modeling of these techniques and further improvement is needed in this area. In this paper, a comprehensive computational model is presented for simulating the complete mechanism of direct and plasmonic nanoparticle-mediated infrared neural stimulation techniques in schematic samples of experimental setups. The simulation process involves three phases: 1) Simulating the light transmission and absorption in setups containing pure water or a gold nanorod solution using developed 3D, time-independent, and time-dependent Monte Carlo models, 2) calculating the spatiotemporal evolutions of temperature within the setup using the finite difference method and a presented novel method, and 3) simulating the thermally induced responses of lipid membranes using an improved method compared to existing theoretical models. The model is validated by comparing the computational results with existing experimental data. The effect of the laser pulse characteristics, nanofluid properties, and some other related parameters on the thermally induced membrane responses is investigated. The computational results help to optimize the parameters selection and maximize the overall efficiency of the infrared neural stimulation techniques.

Chalcogenide phase-change materials (PCMs) offer a unique feature that can be used to dynamically control the response of the photonic devices and achieve fast, nonvolatile, reversible, multilevel, and specific optical modulation. The phase-change material Ge2Sb2Se4Te1 (GSST) has recently received a lot of attention due to the large index contrast between its amorphous and crystalline states with significantly low optical loss in the optical to near-IR spectrum. In this paper, we propose a tunable and reconfigurable hybrid PCM plasmonic nanostructure composed of a spacer layer of GSST sandwiched between a Ag back reflector and a 1D Ag Fabry-Perot grating structure. We use the finite element method (FEM) to numerically calculate the light absorption, absorption contrast, and figure of merit of the plasmonic nanostructure for both the amorphous and crystalline state of the GSST. Our calculations show that with constant structural variation the observed multimode absorption is drastically modified when the GSST undergoes a phase change from the amorphous to the crystalline state. The absorption contrast spectrum, which is defined as the absorption difference between the amorphous and crystalline state of GSST, shows four extrema modes between 70% and 89%. The figure of merit spectrum shows two large values of 44.39 and 37.78 at the 1502 nm and 2063 nm wavelengths, respectively. We also address the observed modes in the absorption contrast spectrum through spatial representation of the enhanced electric field distribution at their corresponding wavelengths. We show how the phase change in the GSST spacer can control the coupling between the optical cavity modes and the Ag surface plasmon resonance modes in the cavities and GSST spacer strip boundaries. The findings in this paper may open new avenues toward the design of next-generation photonic systems such as thermal emission controllers, sensors, ranging holograms, modulators and optical detection devices.

We theoretically investigate the bistable spontaneous emission behavior of a single emitter in the vicinity of a hybrid V O 2 –Au nanoshell. The hybrid nanoshell is illuminated by a continuous wave pump laser. The incident beam generates heat through light absorption and causes the V O 2 to undergo a phase change from semiconductor to metallic mode. Our calculation shows that for certain values, over a range of incident laser intensity [ 0.25 ( G W / m 2 ) < I < 0.84 ( G W / m 2 )], at the resonance wavelength of the V O 2 (semiconductor)–Au nanoshell ( λ = 630 n m ), there is a reasonable bistable contrast for the two spontaneous emission rate values: the radiative and nonradiative decay rates. Our results provide important general guidelines for enabling platforms for optical switching sources and tunable sensors.

Tunable absorption features are theoretically investigated in a symmetrical Fibonacci (FB) structure hybridized with a 40 nm-Ge2Sb2Te5 (GST) film in the near-infrared region. GST is one of the most prominent phase change materials whose optical properties can drastically change during its phase transition from an amorphous to a crystalline structure. In particular, its non-volatile behavior which makes it a stable material in either phase for years, is of great importance in optoelectronics. Indeed, we utilize the phase switching of GST via the thermooptical effect to engineer a tunable absorbing device. Our results show a high contrast in the absorption levels between the amorphous and crystalline states of GST for the proposed structure. Considering the sixth FB generation, for example, the amorphous-to-crystalline phase transition of GST changes the absorption value from nearly zero to similar to 0.9 under a TE-polarized wave at an angle of incidence of 45 degrees. We can also achieve enhanced absorption in amorphous GST with the appropriate choice of parameters. Consequently, the structure can be reconfigured from nearly transparent to absorbent and vice versa. This work suggests a promising approach for designing tunable temperature-assisted GST-based absorbers and optical switching devices.