Sensing at exceptional point in terahertz metasurfaces.

This study investigates the dynamics of two-qubit systems in open quantum environments, with a focus on the influence of exceptional points (EPs) and phase angle on entanglement dynamics. Two-qubit systems are pivotal in quantum information science, as they are the simplest system that reveals quantum entanglement, a crucial resource for quantum computing and communication. The research explores the effects of coupling, spontaneous emission, and environment on two-qubit states, specifically the $X$-states, within the framework of the Gorini–Kossakowski–Sudarshan–Lindblad (GKSL) or Lindblad master equation. EPs are unique to non-Hermitian systems. They affect the system’s dynamics significantly when the system approaches EP. Analytical and numerical analyses reveal that third-order EPs occur at specific parameters for two-qubit system, leading to critical changes in eigenvalues and eigenvectors of the Liouvillian superoperator. The study demonstrates that EPs can enhance or suppress entanglement. The concurrence, a quantitative measure of entanglement, peaks around the EPs under certain initial conditions. Furthermore, the impact of phase angle, $\theta$, that parameterizes the relative phase of maximally entangled states is analyzed. For initial states labeled as $\rho_\Psi(\theta,p)$ in this thesis, the phase angle significantly influences concurrence evolution, enabling tunability of entanglement dynamics. In contrast, for other class of initial states labeled as $\rho_\Phi(\theta,p)$, concurrence exhibits phase invariance, ensuring stability across the variations in $\theta$. Besides this, the analysis shows that the maximum concurrence occurs around EP for some states, the phase angle can also be tuned to increase the concurrence for some initial states. These findings highlight the importance of both EPs and phase angle in optimizing entanglement generation and control. This research offers valuable insights into the interplay between non-Hermitian physics and quantum entanglement, paving the way for advancements in quantum technologies. The outcomes provide possible practical implications in the field of quantum computation.

The intricate connection between the distribution of exceptional points and particular fluctuation properties of level spacings is discussed. The distribution of the exceptional points of the problem H0+λH1 is given for the situation of hard chaos. Predictions of local properties of exceptional points have recently been confirmed experimentally. Chiral properties associated with exceptional points are pointed out.

A rotation sensing mechanism based on higher-order exceptional points in parity-time-symmetric ternary-coupled microresonators is proposed. Due to the properties of third-order exceptional points, rotation-induced frequency splitting is proportional to the cube root of the rotation rate. Moreover, the sensitivity of the proposed rotation sensing mechanism is improved by at least 1 order of magnitude compared with the parity-time-symmetric binary ring rotation sensing mechanism. This kind of sensing mechanism makes on-chip ultrasensitive rotation sensing possible.

In recent years, non-Hermitian degeneracies, also known as exceptional points (EPs), have emerged as a new paradigm for engineering the response of optical systems. Among many different non-conservative photonic configurations, parity-time (PT) symmetric arrangements are of particular interest since they provide an excellent platform to explore the physics of exceptional points. In this talk, I will present some of the work in our group, where the intriguing properties of exceptional points that are arising in judiciously designed parity-time-symmetric systems are utilized to address two of the long standing challenges in the field of integrated photonics: enforcing single mode lasing in intrinsically multimode cavities, and enhancing sensitivity of micro-resonators.

Inspired by the optical and acoustic non-Hermitian 𝒫𝒯 symmetric system, a 𝒫𝒯 symmetric metamaterial beam for flexural waves is proposed here, based on shunted piezoelectric patches. Positive and negative shunting resistances are the key to constructing the balanced loss and gain components. The associated asymmetric flexural wave scattering, 𝒫𝒯 phase transition and exceptional points are then investigated in both analytical and numerical ways. In addition, properties of exceptional points are tunable in our proposed system, simply owing to the variable shunting parameters. Our design may contribute to asymmetric wave control, enhanced sensing, amplification, and localization of flexural waves.

One-dimensional (1D) photonic crystals (PCs) with two defect layers (DLs) have unique properties of localization of light and multimode low-threshold laser generation, they can also be used as highly sensitive optical sensors. This study investigates the distribution of light in 1D PCs with two DLs at the exceptional points of degeneracy, where defect modes (DMs) merge in the reflection spectrum. The four most significant types of symmetry in such structures are considered, and an unusual distribution of the field at DMs in the PC structure with different symmetry properties around the first and second DLs is shown. Unlike previous works, we demonstrate for the first time four important methods for achieving merging of DMs in 1D PCs with two DLs: merging due to changes in the symmetry of the PC, merging with an increase in the number of unit cells in the middle, merging by altering the thickness of the defects in the PC, and merging by varying the angle and polarization type of the incident light. Additionally, the transmission of light through the 1D PC with two DLs is investigated for the first time under varying angles and types of polarization of incident light. The novelty of this work also lies in the fact that we show the possibility of changing the number of DMs within the photonic bandgaps depending on the s - or p -waves (TE and TM polarizations) incident on the structure at large angles. Such versatility can be further used for the development of tunable optical sensors, filters, and other devices, based on the PCs.

Biosensors based on terahertz (THz) metasurfaces have recently attracted widespread attention. However, few have been reported so far because it is a challenge to achieve ultrasensitive multidimensional detection in the THz spectrum. Here, we propose a novel THz biosensor that consists of a metasurfaces and a metal oxide semiconductor-like structure (MOSLS), which is based on patterned graphene-polyimide-perovskite. We varied the photoconductivity of the MOSLS via the electrostatic doping effect. The biosensor could detect whey protein down to a concentration limit of 6.25ng/mL. Significant responses in frequency, phase, and transmission amplitude were all detected for different protein concentrations. The transmission value difference, frequency shift, and phase difference increased with the concentration of whey protein, clearly demonstrating multidimensional biosensing. Moreover, by applying lasers with different wavelengths, we have realized reversible biosensing in THz region for the first time. These results are very promising for applications of THz metasurfaces in the field of biosensing.

A nonreciprocal‐amplified scheme for signal transmission is proposed in a coupled rotation‐cavity system, where the two cavities rotate with opposite directions, and an optical parametric amplifier (OPA) is attached to one cavity. The coupling between cavities can effectively compensate and enhance the frequency shift of the Sagnac effect, and a perfect nonreciprocity can be achieved. Owing to the introduction of an OPA medium, eigenvalues of the dynamic matrix exhibit exceptional points (EPs), and the amplified‐nonreciprocal transmission can be obtained around EPs.

The concept of non‐Hermitian systems, also known as exceptional points (EPs), has recently emerged as a novel approach for designing the response of systems that exchange energy with the external environment. The system behaves as a non‐Hermitian Hamiltonian (NH Hamiltonian), the real part of which is related to the internal dynamics of the system, and the imaginary part reflects the energy exchange between the system and the outside environment. By utilizing double EIT windows and adjusting the intensity of the dressing field, the number and positions of eigenvalues can be modulated to construct quantum entanglement sources for non‐Hermitian systems, thereby laying the foundation for quantum communication and quantum computing. To prepare photon entanglement sources, it is necessary to study the correlation of high‐dimensional quantized energy levels under PT symmetry and symmetry breaking induced by the dressing field; Hamiltonian correlation matrix of the non‐Hermitian system characterizes the relationship between gain and loss, detuning and dephasing rate in the reaction energy‐level system, and the coupling relationship of the dressing field. The energy‐level system itself is non‐Hermitian, and the study of symmetry breaking is conducted by controlling EPs during the level control process through the dressing field, thus eliminating the need for introducing artificial photonic structures. The third‐order non‐Hermitian characteristics of the PT‐NH system reflect the positional relationship between the cascade field and the nested field. When the Rabi frequency (dephase rate) of the dressing field is the dominant factor, the real (imaginary) part of the eigenvalues is split, resulting in in‐phase (out‐of‐phase) constructive (destructive) transformations of the dressing order.

This paper presents a terahertz metasurface based sensor design incorporating graphene and other plasmonic materials for highly sensitive detection of different chemicals. The proposed sensor employs the combination of multiple resonator designs - including circular and square ring resonators - to attain enhanced sensitivity among other performance parameters. Machine learning techniques like Random Forest regression, are employed to enhance the sensor design and predict its performance. The optimized sensor demonstrates excellent sensitivity of 417 GHzRIU and a low detection limit of 0.264 RIU for ethanol and benzene detection. Furthermore, the integration of machine learning cuts down the simulation time and computational requirements by approximately 90% without compromising accuracy. The sensor's unique design and performance characteristics, including its high-quality factor of 14.476, position it as a promising candidate for environmental monitoring and chemical sensing applications. Moreover, it also demonstrates potential for 2-bit encoding applications through strategic modulation of graphene chemical potential values. On the other hand, it also shows prospects of 2-bit encoding applications via the modulation of graphene chemical. This work provides a major advancement to the terahertz sensing application by proposing new materials, structures, and methods in computation in order to develop a high-performance chemical sensor.

In solid mechanics, the resonance frequency of a material is typically linked to its mechanical properties. A frequency shift therefore typically denotes a change in stiffness. It has already been shown that temperature variation causes stiffness changes in tissues. The amplitude of the ultrasound-stimulated acoustic emission (USAE) signal has also been shown to vary with temperature as well as indicate temperature elevation. Its amplitude, however, is sensitive to both acoustical and mechanical parameters, which at most frequencies have opposite effects resulting from temperature changes. In this paper, the measurement of a frequency shift of the USAE resonant peaks is used for monitoring of the tissue stiffness variation with temperature. In a numerical simulation, the variation of the frequency shift (ranging between −150 and 500 Hz) at different temperatures is shown. Then, in a series of experiments involving a gel phantom and porcine muscle tissue, the frequency shift variation (ranging between −200 and 200 Hz) is shown to follow the known stiffness changes due to temperature. Both simulation and experimental results indicate that the USAE frequency shift method can dissociate the mechanical from the acoustical parameter dependence and detect tissue coagulation. [Work supported by NIH Grant CA82275.]

A two-parameter sensor that can detect the variation of temperature and refractive index is realized in a multilayer dielectric structure obeying parity-time (PT) symmetry. The sensor can operate near exceptional points (EPs), which have been shown to provide dramatic variations of their eigenvalues in response to small parameter changes. The optical sensing behavior is theoretically investigated based on the transfer matrix method. The results show that the sensor can work within the surrounding temperature (tp) ranging from 0 to 30℃, and the refractive index (ng) of incident medium ranging from 1.0 to 1.4. The detectable variation △ng of the sensor can reach 0.02. The sensitivity of ng and tp can reach 372496.53 RIU-1 and 249.18℃-1, respectively. Our structures show great promise in temperature monitoring in cold environment and identification of chemical gases or liquids.

Magnetic field signals contain a large amount of power grid and equipment operation information, which is of great significance for the realization of power system information management. Traditional magnetic sensor has low sensitivity, high cost and large volume, and its application range is limited. To solve this problem, a novel magnetic sensing enhancement method for power systems based on quantum non-Hermitian EP is proposed. First, the natural microfluidic channel of the microbubble whispering gallery mode micro-resonator is used to make the magnetic fluid interact fully with the evanescent field of the echo wall, and the magnetic field intensity is detected by measuring the change of the resonant frequency, the light field distribution of microbottle resonator is also studied. Then, a second order EP energy exchange dynamic equation based on PT symmetry is established, and the sensitivity enhancement of Eigen-frequency splitting to perturbations in quantum mechanics is studied in detail. Finally, theoretical analysis and simulation show that the system can change the resonance frequency shift and the magnetic field intensity into a nonlinear relationship, so that the weak magnetic signal presents 4 orders of magnitude amplification, and the detection limit is at least 0.0001mT. Our solution provides a novel magnetic field strength detection scheme, which has a broad application prospect in the field of new power systems.

This work proposes a sensitivity-enhanced film bulk acoustic resonator (FBAR) humidity sensor based on a nonlinear parity-time (PT) symmetric system, which comprises three key components: an FBAR sensor, a series LC network, and a negative resistance circuit implemented with an operational amplifier. The PT -symmetric system operates at the exceptional point (EP), characterized by coalesce eigenfrequencies, leading to enhanced responsiveness to asymmetric perturbations. The nonlinear gain of the negative resistance drives the perturbed PT-symmetric system into a stable oscillatory state. Theoretical analysis demonstrates that the EP-operated PT-symmetric configuration significantly amplifies frequency shifts compared with a traditional single FBAR sensor. Experiments using an electrospun cellulose acetate/ graphene oxide (CA/GO) nanofiber-modified FBAR humidity sensor validated the theoretical predictions, with a 4 %RH perturbation inducing frequency deviations 16-fold higher than those observed in conventional configurations.

In this study, the multi-peak terahertz metamaterials sensors are designed and fabricated, whose structures are the asymmetrical single split ring (SSR) and three split rings (TSR). The resonant formation and sensing mechanism of the two structures are investigated by using the finite-difference time-domain (FDTD) method. Vitamin B6 (VB6) and its reactants with bovine serum protein (BSA) are tested as the medium, and the sensing experiments of the SSR and TSR are carried out. The experimental and simulation results indicate the consistent law, which is the sensitivity of the resonance in the transverse magnetic (TM) mode is much greater than that in the transverse electric (TE) mode. According to the weighted average method and the law for unequal precision measuring, the quality factor of the resonance is used as the weighting coefficient to calculate the comprehensive evaluation parameter (CEP) of the multi-peak metamaterials sensors in the TE and TM modes based on the experimental data. When the CEP and frequency shifts are as the evaluation parameter in experiments, the law’s variation of the CEP is consistent with that of the frequency shift, indicating that it is feasible to characterize the sensing characteristics of metamaterials with the CEP, which presents simplified characteristics of multi-peak metamaterials at different polarization modes. The method implies that the different influencing factors may be integrated into the CEP with the idea of weight, which promotes the practical application of the metamaterials sensor. The revelation of the sensing law also provides a method for the design of the terahertz metamaterials sensor with the high sensitivity.