Transmission Spectrum Of Waveguide Research Articles

Planar terahertz multi-resonator meta-waveguide based upon strongly coupled nested split-ring resonator

In this paper, we propose terahertz (THz) meta-waveguide geometry based on one-dimensional array of nested split-ring resonators (SRRs) comprising three split gaps. The meta-waveguide geometry consists of three layers of materials namely copper, dielectric and meta-molecules. The meta-molecules of aluminum with nested split gaps are placed periodically in one dimension along the wave propagation direction. The numerically obtained waveguide transmission spectrum shows the excitation of multiple resonances, resulting from three split gaps and a strong coupling between them. We found that the resonant frequencies of the guided modes can be tuned by varying the inter-split gaps separation which controls the coupling. The meta-waveguide geometry offers greater interaction of meta-molecules with the propagating terahertz radiation which can be sensitive to various applications. Further, the proposed design offers a flexibility in tuning resonant behavior owing to nested split gaps and coupling between them. We have examined the electric field distribution to confirm the multiple resonance excitations supported by the meta-waveguide geometry. We have also suggested a coupled harmonic oscillator model to interpret and analyze the numerically simulated waveguide transmission results. The study opens up new avenues for terahertz meta-waveguide based highly sensitive devices such as sensors, storage applications etc. where properties can be controlled via coupling within a resonator in a nested split gaps approach.

Fano resonance from a one-dimensional topological photonic crystal

An ultra-compact one-dimensional topological photonic crystal (1D-TPC) is designed in a single mode silicon bus-waveguide to generate the Fano resonance lineshape. The Fano resonance comes from the interference between the discrete topological boundary state of the 1D-TPC and the continuum high-order leaky mode of the bus-waveguide. Standalone asymmetric Fano resonance lineshapes are obtained experimentally in the waveguide transmission spectrum with a maximum extinction ratio of 33 dB and a slope ratio of 10 dB/nm over a broadband flat background.

Integrating two-photon nonlinear spectroscopy of rubidium atoms with silicon photonics.

We study an integrated silicon photonic chip, composed of several sub-wavelength ridge waveguides, and immersed in a micro-cell with rubidium vapor. Employing two-photon excitation, including a telecom wavelength, we observe that the waveguide transmission spectrum gets modified when the photonic mode is coupled to rubidium atoms through its evanescent tail. Due to the enhanced electric field in the waveguide cladding, the atomic transition can be saturated at a photon number ≈80 times less than a free-propagating beam case. The non-linearity of the atom-clad Si-waveguide is about 4 orders of magnitude larger than the maximum achievable value in doped Si photonics. The measured spectra corroborate well with a generalized effective susceptibility model that includes the Casimir-Polder potentials, due to the dielectric surface, and the transient interaction between flying atoms and the evanescent waveguide mode. This work paves the way towards a miniaturized, low-power, and integrated hybrid atomic-photonic system compatible with CMOS technologies.

3D printed hollow core terahertz Bragg waveguides with defect layers for surface sensing applications.

We study a 3D-printed hollow core terahertz (THz) Bragg waveguide for resonant surface sensing applications. We demonstrate theoretically and confirm experimentally that by introducing a defect in the first layer of the Bragg reflector, thereby causing anticrossing between the dispersion relations of the core-guided mode and the defect mode, we can create a sharp transmission dip in the waveguide transmission spectrum. By tracking changes in the spectral position of the narrow transmission dip, one can build a sensor, which is highly sensitive to the optical properties of the defect layer. To calibrate our sensor, we use PMMA layers of various thicknesses deposited onto the waveguide core surface. The measured sensitivity to changes in the defect layer thickness is found to be 0.1 GHz/μm. Then, we explore THz resonant surface sensing using α-lactose monohydrate powder as an analyte. We employ a rotating THz Bragg fiber and a semi-automatic powder feeder to explore the limit of the analyte thickness detection using a surface modality. We demonstrate experimentally that powder layer thickness variations as small as 3μm can be reliably detected with our sensor. Finally, we present a comparative study of the time-domain spectroscopy versus continuous wave THz systems supplemented with THz imaging for resonant surface sensing applications.

Silicon nanocrystals for nonlinear optics and secure communications

Silicon nanocrystals (Si‐nc) are reviewed here for their interesting optical applications. On the one hand, they do exhibit quantum confinement effects. This allows turning silicon into a light‐emitting material where luminescence can be excited by electrical injection. On the other hand, small sizes, large surfaces, and dielectric mismatch between the core and the surrounding matrix increase dramatically the nonlinear optical coefficients. This allows using Si‐nc as a nonlinear material in different waveguide configurations. In this paper, we discuss specifically two different applications of Si‐nc: (i) as a nonlinear material in various devices, e.g., in bistable optical cavities, in waveguide optical mode monitors that are based on two‐photon excited luminescence detection, and in wavelength shifters by using four‐wave mixing (FWM); (ii) as an entropy source for quantum random number generation, the key device for cryptography.(a) Top view optical image of a Si‐nc‐based whispering‐gallery mode (WGM) microdisk resonator, vertically coupled with a bus waveguide. (b) Detail of the coupling zone. (c) Normalized waveguide transmission spectrum over a wide spectral range. The inset shows a blow up of a resonance with the radial and azimuthal mode orders and the Q factor value indicated.

Intermode reactive coupling induced by waveguide-resonator interaction

We report on a joint theoretical and experimental study of an integrated photonic device consisting of a single mode waveguide vertically coupled to a disk-shaped microresonator. Starting from the general theory of open systems, we show how the presence of a neighboring waveguide induces reactive inter-mode coupling in the resonator, analogous to an off-diagonal Lamb shift from atomic physics. Observable consequences of this coupling manifest as peculiar Fano lineshapes in the waveguide transmission spectra. The theoretical predictions are validated by full vectorial 3D finite element numerical simulations and are confirmed by the experiments.

Efficient coupling between dielectric waveguide modes and exterior plasmon whispering gallery modes

Inefficient coupling between dielectric guided mode and plasmon mode has been overlooked in the past. The coupling mechanism is essentially different from the conventional coupling between dielectric modes. Based on qualitative theoretical analysis, we proposed two methods to strengthen the coupling between dielectric waveguide modes and exterior plasmon whispering gallery modes. One is using a U-shaped bent waveguide to break the adiabatic mode conversion process, and the other is to utilize higher-order dielectric mode to reach phase matching with plasmon mode. Both the transmission spectrum of waveguide and the energy spectrum of cavity demonstrate that the coupling efficiency can be greatly improved. These simple configurations are potential for wide applications, for example, tunable integrated optical devices and sensors.

Planar terahertz waveguides based on complementary split ring resonators

We experimentally demonstrate planar plasmonic THz waveguides using metal films that are periodically perforated with complementary split ring resonators (CSRRs). The waveguide transmission spectra exhibit numerous transmission resonances. While the geometry is commonly used in developing negative index materials, the excitation geometry used here does not allow for conventional metamaterial response. Instead, we show that all of the observed resonances can be determined from the geometrical properties of the CSRR apertures. Surprisingly, the Bragg condition does not appear to limit the frequency extent of the observed resonances. The results suggest that metamaterial-inspired geometries may be useful for developing THz guided-wave devices.

Lateral Waveguiding Properties of VCSELs for Integrated Optical Monitoring

We have investigated experimentally the optical behaviour of a Vertical-Cavity Surface-Emitting Lasers (VCSEL) when it is used as a lateral waveguide. This provides a better understanding of the mechanisms involved in VCSEL optical power monitoring by integrated lateral detection. The lateral optical spectra as well as the waveguide transmission spectrum have been measured. Their analysis and comparison demonstrate that the lateral signal is mostly coming from the spontaneous emission escaping laterally from the VCSEL and that light is filtered during the propagation and detection processes. This allows us to explain previous results obtained in two different configurations of lateral detection. These results are useful to optimize the lateral detection performances for concrete applications in monitoring or telemetry systems.