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.

Humidity- and Temperature-Tunable Metal-Hydrogel-Metal Reflective Filters.

A tunable reflectance filter based on a metal-hydrogel-metal structure responsive to humidity and temperature is reported. The filter employs a poly(N-isopropylacrylamide)-acrylamidobenzophenone (PNIPAm-BP) hydrogel as an insulator layer in the metal-insulator-metal (MIM) assembly. The optical resonance of the structure is tunable by water immersion across the visible and near-infrared range. Swelling/deswelling and the volume phase transition of the hydrogel allow continuous reversible humidity- and/or temperature-induced tuning of the optical resonance. This work paves the way toward low-cost large-area fabrication of actively tunable reversible photonic devices.

Nonresonant Local Fields Enhance Second-Harmonic Generation from Metal Nanoislands with Dielectric Cover.

We study second-harmonic generation from gold nanoislands covered with amorphous titanium oxide (TiO_{2}) films. As the TiO_{2} thickness increases, the plasmon resonance of the nanoislands shifts away from the second-harmonic wavelength of 532 nm, diminishing the resonant enhancement of the process at this wavelength. Nevertheless, the second-harmonic signal is enhanced by up to a factor of 45 with increasing TiO_{2} thickness. This unexpected effect arises from the scaling of local fields at the fundamental wavelength of 1064 nm-which is at the far tail of the resonance-due to a change in the dielectric environment of the nanoislands.