A theoretical design of evanescent wave biosensors based on gate-controlled graphene surface plasmon resonance

Circular quantum wire symmetrically loaded with a graphene strip as the plasmonic micro/nano laser: threshold conditions analysis

We study, apparently for the first time, the threshold conditions for the time-harmonic natural modes of the micro-to-nanosize plasmonic laser shaped as a circular quantum wire with a flat graphene strip, placed symmetrically inside it, in the H-polarization case. We suppose that the quantum wire is made of a nonmagnetic gain material, characterized with the aid of the "active" imaginary part of the complex refractive index. The emergence of lasers integrating plasmonic effects marks a significant trend in contemporary photonics. Here, the graphene offers a promising alternative to the noble metals as it exhibits the capacity to sustain plasmon-polariton natural surface waves across the infrared and terahertz (THz) spectra. The used innovative approach is the lasing eigenvalue problem (LEP), which is classical electromagnetic field boundary-value problem, adapted to the presence of active region. It is tailored to deliver both the mode-specific emission frequency, which is purely real at the threshold, and the value of the gain index of the active region, necessary to make the frequency real-valued. The conductivity of graphene is characterized using the quantum Kubo formalism. We reduce the LEP for the considered nanolaser to a hyper-singular integral equation for the current on the strip and discretize it by the Nystrom-type method. This method is meshless and computationally economic. After discretization, a matrix equation is obtained. The sought for mode-specific pairs {the frequency and the threshold gain index} correspond to the zeros of the matrix determinant. It should be noted that the convergence to exact LEP eigenvalues is guaranteed mathematically if the discretization order is taken progressively larger. Two families of modes are identified and studied: the modes of the quantum wire, perturbed by the presence of the graphene strip and the plasmon modes of the strip. The frequencies of all plasmon modes and the lowest mode of the quantum wire are found to be well-tuned by changing the chemical potential of graphene. Engineering analytic formulas for the plasmon-mode frequencies and thresholds are derived. We believe that the presented results can be used in the creation of single-mode tunable micro and nanolasers.

Design of a Reconfigurable Ultra-Wideband Terahertz Polarization Rotator Based on Graphene Metamaterial

In this work, a reconfigurable ultra-wideband transmissive terahertz polarization rotator based on graphene metamaterial is proposed that can switch between two states of polarization rotation within a broad terahertz band by changing the Fermi level of graphene. The proposed reconfigurable polarization rotator is based on a two-dimensional periodic array of multilayer graphene metamaterial structure, which is composed of metal grating, graphene grating, silicon dioxide thin film, and a dielectric substrate. The graphene metamaterial can achieve high co-polarized transmission of a linearly polarized incident wave at the off-state of the graphene grating without applying the bias voltage. Once the specially designed bias voltage is applied to change the Fermi level of graphene, the polarization rotation angle of linearly polarized waves is switched to 45° by the graphene metamaterial at the on-state. The working frequency band with 45-degree linear polarized transmission remaining above 0.7 and the polarization conversion ratio (PCR) above 90% is from 0.35 to 1.75 THz, and the relative bandwidth reaches 133.3% of the central working frequency. Furthermore, even with oblique incidence at large angles, the proposed device retains high-efficiency conversion in a broad band. The proposed graphene metamaterial offers a novel approach for the design of a terahertz tunable polarization rotator and is expected to be applied in the applications of terahertz wireless communication, imaging, and sensing.

Environmental Monitoring: A Comprehensive Review on Optical Waveguide and Fiber-Based Sensors

Globally, there is active development of photonic sensors incorporating multidisciplinary research. The ultimate objective is to develop small, low-cost, sensitive, selective, quick, durable, remote-controllable sensors that are resistant to electromagnetic interference. Different photonic sensor designs and advances in photonic frameworks have shown the possibility to realize these capabilities. In this review paper, the latest developments in the field of optical waveguide and fiber-based sensors which can serve for environmental monitoring are discussed. Several important topics such as toxic gas, water quality, indoor environment, and natural disaster monitoring are reviewed.

Spoiling of tunability of on-substrate graphene strip grating due to lattice-mode-induced transparency

We report a prediction of the optical effect apparently not discussed earlier. As known both from theory and experiment, the gratings of flat graphene strips lying on dielectric substrates display moderate-Q resonances on the strip plasmon modes in the H-polarization case. In the plasmon resonances, high reflectance and absorbance are observed. These characteristics are tunable with the aid of the graphene chemical potential, which controls the plasmon-mode frequency. However, if this frequency coincides with the high-Q lattice-mode frequency, a narrow-band regime of electromagnetically induced transparency (EIT) appears. A new point in our finding is that, in the EIT regime, the tunability of the reflectance and absorbance of a grating of narrow graphene strips get spoiled profoundly. This is established using a full-wave meshless code based on the method of analytical regularization, which leads to a Fredholm second-kind matrix equation that guarantees the code convergence. Numerical results are presented for the strip width and period, having the microsize dimensions so that all resonances lie in the THz range. However, the same effect takes place in the infrared range for narrower strips and smaller periods. The lattice modes are caused by the periodicity and can have ultra-high Q-factors; however, they do not exist if the substrate is absent. The loss of tunability at EIT is explained by the lattice-mode field pattern, which has deep E-field minima at the strips.

We report a loss of tunability of the scattering from the dielectric substrate-backed graphene-strip grating in a narrow-band all-optical electromagnetically induced transparency regime, appearing due to the excitation of the high-Q lattice mode.

Near-infrared tunable surface plasmon resonance sensors based on graphene plasmons via electrostatic gating control

A tunable near-infrared surface plasmon resonance sensor based on graphene plasmons via electrostatic gating control is investigated theoretically. Instead of the traditional refractive index sensing, the sensor can respond sensitively to the change of the chemical potential in graphene caused by the attachment of the analyte molecules. This feature can be potentially used for biological sensing with high sensitivity and high specificity. Theoretical calculations show that the chemical potential sensing sensitivities under wavelength interrogation patterns are 1.5, 2.21, 3, 3.79, 4.64 nm meV-1 at different wavebands with centre wavelengths of 1100, 1310, 1550, 1700, 1900 nm respectively, and the full width half maximum (FWHM) is also evaluated to be 10, 25.5, 43, 55.5, 77 nm at these different wavebands respectively. It can be estimated that the theoretical limit of detection (LOD) in DNA sensing of the proposed sensor can reach the femtomolar level, several orders of magnitude superior to that of noble metal-based SPR sensors (nanomolar or subnanomolar scale), and is comparable to that of noble metal-based SPR sensors with graphene/Au-NPs as a sensitivity enhancement strategy. The FWHM is much smaller than that of the noble metal-based SPR sensors, making the proposed sensor have a potentially higher figure of merit (FOM). This work provides a new way of thinking to detect in an SPR manner the analyte that can cause chemical potential change in graphene and provides a beneficial complement to refractive index sensing SPR sensors.