Humidity-Controlled Tunable Emission in a Dye-Incorporated Metal-Hydrogel-Metal Cavity.

Actively controllable photoluminescence is potent for a wide variety of applications from biosensing and imaging to optoelectronic components. Traditionally, methods to achieve active emission control are limited due to complex fabrication processes or irreversible tuning. Here, we demonstrate active emission tuning, achieved by changing the ambient humidity in a fluorescent dye-containing hydrogel integrated into a metal-insulator-metal (MIM) system. Altering the overlapping region of the MIM cavity resonance and the absorption and emission spectra of the dye used is the underlying principle to achieving tunability of the emission. We first verify this by passive tuning of cavity resonance and further experimentally demonstrate active tuning in both air and aqueous environments. The proposed approach is reversible, easy to integrate, and spectrally scalable, thus providing opportunities for developing tunable photonic devices.

All-optical switching based on plasmon-induced Enhancement of Index of Refraction.

In quantum optical Enhancement of Index of Refraction (EIR), coherence and quantum interference render the atomic systems to exhibit orders of magnitude higher susceptibilities with vanishing or even negative absorption at their resonances. Here we show the plasmonic analogue of the quantum optical EIR effect in an optical system and further implement this in a linear all-optical switching mechanism. We realize plasmon-induced EIR using a particular plasmonic metasurface consisting of a square array of L-shaped meta-molecules. In contrast to the conventional methods, this approach provides a scheme to modulate the amplitude of incident signals by coherent control of absorption without implementing gain materials or nonlinear processes. Therefore, light is controlled by applying ultra-low intensity at the extreme levels of spatiotemporal localization. In the pursuit of potential applications of linear all-optical switching devices, this scheme may introduce an effective tool for improving the modulation strength of optical modulators and switches through the amplification of input signals at ultra-low power.

Suppressing the spectral shift of a polarization-independent nanostructure with multiple resonances.

Resonances are the cornerstone of photonic applications in many areas of physics and engineering. The spectral position of a photonic resonance is dominated by the structure design. Here, we devise a polarization-independent plasmonic structure comprising nanoantennas with two resonances on an epsilon-near-zero (ENZ) substrate in order to loosen this correlation to obtain less sensitivity to geometrical perturbations of the structure. Compared with the bare glass substrate, the designed plasmonic nanoantennas on an ENZ substrate exhibit a nearly three-fold reduction only in the resonance wavelength shift near the ENZ wavelength as a function of antenna length.

Independently tunable electromagnetically induced transparency effect and dispersion in a multi-band terahertz metamaterial.

In this article, we experimentally and numerically investigate a planar terahertz metamaterial (MM) geometry capable of exhibiting independently tunable multi-band electromagnetically induced transparency effect (EIT). The MM structure exhibits multi-band EIT effect due to the strong near field coupling between the bright mode of the cut-wire (CW) and dark modes of pair of asymmetric double C resonators (DCRs). The configuration allows us to independently tune the transparency windows which is challenging task in multiband EIT effect. The independent modulation is achieved by displacing one DCR with respect to the CW, while keeping the other asymmetric DCR fixed. We further examine steep dispersive behavior of the transmission spectra within the transparency windows and analyze slow light properties. A coupled harmonic oscillator based theoretical model is employed to elucidate as well as understand the experimental and numerical observations. The study can be highly significant in the development of multi-band slow light devices, buffers and modulators.

Enhanced terahertz emission bandwidth from photoconductive antenna by manipulating carrier dynamics of semiconducting substrate with embedded plasmonic metasurface.

In this article, we demonstrate a technique to enhance the Terahertz (THz) emission bandwidth from photo-conductive antenna (PCA) based on semiconducting substrates by manipulating the surface carrier dynamics of the semiconductor. Bandwidths in PCAs are limited by the decay of the photogenerated charge carriers, which in case of SI-GaAs is in the orders of 50 picoseconds. We show, with an embedded design of plasmonic meta-surface in the photoconductive gap of a PCA, it is possible to enhance the emission bandwidths by more than 50 percent. This is due to the fact that these nano-structures act as local recombination sites for the photogenerated carriers, effectively reducing the carriers' lifetime. Additionally, the defect sites reduce the terminal current, thereby reducing the Joule heating in the device. Furthermore, the meta-surface also facilitates higher in-coupling of the exciting infrared light on to the PCA, thereby increasing the optical-to-THz conversion efficiency of the device.