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We present our latest work and investigation of exceptional point in plasmonic nanostructures. We show that the non-Hermitian singularity can be effectively deployed in small scale sensors and pave the path for engineering metamaterials with novel properties.

We have demonstrated the first plasmonic exceptional points (EPs) at subwavelength scale. The plasmonic EPs are based on the hybridization of detuned resonances in multilayered plasmonic crystals to reach a critical complex coupling rate between nanoantennas arrays, and, exhibit the dispersion of exceptional points around the non-Hermitian singularity and enhanced nanosensing was observed. The ability to drive plasmons to EPs lays the foundation to explore topological physics at small scales and to novel sensors and optoelectronic devices based on topological polaritonic effects

We present progress on the experimental observation of exceptional points (EPs) in passive plasmonic nanostructures. The system has EPs which are degeneracies in open wave systems where at least two energy levels and their corresponding eigenstates coalesce. They manifest themselves by the simultaneous degeneracy of both resonant frequencies and its linewidths. We consider a plasmonic system based on a multilayer plasmonic structure with structural offset [1, 2]. The realization of an EP via hybridized modes requires the control of at least two physical parameters. The two parameters used for the above system to reach an EP are the shift between bars and the periodicity.

Steering the beam of a wave source has been demonstrated using mechanical and non-mechanical techniques. While mechanical techniques are bulky and slow, non-mechanical techniques rely on breaking the symmetry of the refractive index profile either using asymmetric structure or injecting a non-uniform current. In this contribution, we theoretically and experimentally demonstrated a new type of topological steering of light sources in which the phase offset is provided by Floquet-Bloch phase in periodic structure. It was shown that in periodic structures, there exist singular states in the radiation region of the band diagram that exhibit diverging quality factor. Thus light sources can operate at these states with lower power threshold. The existence of these singular states are topologically protected, and their momentum are very sensitive to any small perturbations, which is used to control the steering angle. By uniformly controlling some parameters in the system, such as a physical dimension or injecting current uniformly, the beam of the light source steers. Our experimental demonstrations open new paradigm in the implementation of light steering with applications in data communications, bio imaging and sensing.

We propose a rectangular resonator sensor structure with butterfly MMI coupler using SOI. It consists of the rectangular resonator, total internal reflection (TIR) mirror, and the butterfly MMI coupler. The rectangular resonator is expected to be used as bio and chemical sensors because of the advantages of using MMI coupler and the absence of bending loss unlike ring resonators. The butterfly MMI coupler can miniaturize the device compared to conventional MMI by using a linear butterfly shape instead of a square in the MMI part. The width, height, and slab height of the rib type waveguide are designed to be 1.5 μm, 1.5 μm, and 0.9 μm, respectively. This structure is designed as a single mode. When designing a TIR mirror, we considered the Goos-Hanchen shift and critical angle. We designed 3:1 MMI coupler because rectangular resonator has no bending loss. The width of MMI is designed to be 4.5 μm and we optimize the length of the butterfly MMI coupler using finite-difference time-domain (FDTD) method for higher Q-factor. It has the equal performance with conventional MMI even though the length is reduced by 1/3. As a result of the simulation, Qfactor of rectangular resonator can be obtained as 7381.

In this paper, we propose temperature sensing method by using optical beating. When temperature changes, a peak wavelength of the sensing laser varies slightly. However, with limitation of the optical spectrum analyzer’s (OSA) spectral resolution (sub-nm), it is hard to measure the exact quantity of the wavelength variation. Therefore, we used electrical spectrum analyzer (ESA) and two lasers to obtain the wavelength shift. We used DFB-LD (distributed feedback laser diode) and TLS (tunable laser source) to get beating signal. Each of laser has 1550 nm of wavelength, -20 dBm of intensity and 108 of Q factor. We varied temperature by 0.1 °C from 17.4 °C to 18.4 °C using TEC (temperature controller). We observed 0.01 nm/°C of wavelength change through OSA and 9.5 GHz/°C of beating frequency change through ESA. With this result, we verified that we can measure relative temperature change with having ultra-fine resolution of 9.5×10-7 °C theoretically for the ESA resolution bandwidth of 1 kHz. This detecting ability can be applied to highly sensitive temperature sensor.

We consider gold plasmonic nanorods in the infrared domain. Such elements are very anisotropic and only polarizable along their longer dimension. Varying the nanorod length from 150 to 500 nm changes the resonant frequency of the element, which allows us to tune the phase-shift provided to an incident plane wave which electric field is parallel to the long axis. On the contrary, the nanorod is transparent to an incoming plane wave with a polarization perpendicular to its main axis. In order to provide a 0 to 2pi phase shift, we chose to work in reflection with metasurfaces made of elements with random positions and orientation. We emphasize that the length of each nanorod is not random, but strongly depends on the position of the element. It is chosen accordingly so that the reflected phase shift follows a parabolic law. The focusing efficiency strongly depends on the density of nanorods but also of the dimensionality and of the symmetry of the metasurface. Using full wave simulations, we design ordered and random metalens and compare their characteristics. Unfortunately, simulating 2D large area metasurface is numerically challenging. Hence, we extract the transmission matrix parameters for single elements from our FDTD simulation, and model the metasurface as an array of two level atom scatterers Finally, we present an experimental realization of such random metalens. The latter is made with conventional top-down fabrication techniques and e-beam lithography. We will show that the resulting lens focus light on diffraction limited focal spots for the two polarizations.

In this paper, total internal reflection (TIR) mirror is carefully simulated for silicon nitride polygonal ring resonator sensor structure. Polygonal resonator has recently attracted much attention for applications in bio and chemical sensors because it does not have a bending loss, and it has an advantage of using MMI coupler. In polygonal resonator sensor design, high Q-factor and low TIR mirror loss are extremely significant factors. Therefore, critical angle and Goos-Hanchen shift should be considered in the design of TIR mirror. When cladding material is SiO2, the critical angle of SiNx waveguide is about 44.99 degrees and the Goos-Hanchen shift is about 400 nm at 1.55 μm wavelength. For the rib type waveguide, we designed it to have 3 μm width, 1 μm height, and 0.5 μm etching depth for decreasing TIR mirror loss. As simulation results of FDTD, reflectivitities of polygonal TIR mirrors are 79% for pentagon, 95% for hexagon and 98% for octagon, respectively. According to the simulations, Q-factors for hexagonal and octagonal resonators can be obtained as high as 1.55 x 104 and 1.72 x 104, respectively.

Recently, photo-alignment technology has been the focus of research efforts because lowering the pre-tilt angle is essential for complete elimination of the off-axis light leakage. However, even though photo-alignment can provide zero pre-tilt angle, it has not yet been widely applied in mass production because of its weak surface anchoring, high curing energy, and strong image sticking. In this paper, we demonstrate that the zero pre-tilt angle can be obtained by employing the field-induced UV-alignment method. We have shown electro-optical characteristics and parameters related to the image quality of a fringe-field switching cell fabricated using the proposed method as functions of the monomer concentration and the UV irradiation time.