Modification of Relative Reflectance Curves in Plasmonic Thin Films with Gain

Dissipation within a plasmonic system remains the Achilles heel towards development of plasmonic circuits, that are now acknowledged as a promising disruptive technology towards ultrasmall communication devices. Therefore, plasmonic loss compensation becomes a critical requirement towards realizability of plasmonic devices. In this paper, we report our investigations on probe amplification from a plasmonic thin film, covered by a gain medium.

The subtle phenomenon of loss compensation in a plasmonic system is studied via the shapes of the reflectance curve from the system, which depends on both film thickness and the gain coefficient. Using Fresnel coefficients in a prism-metal-gain medium three layer model, we first theoretically investigate relative reflectance curves (ratio of reflectance with gain to that without gain) for various metal film thicknesses and gain coefficients. Next, we experimentally investigate loss compensation in a silver film. The film (thickness ~60 nm) was coated on an equilateral prism fabricated out of SF4 glass (refractive index 1.75) onto which a solution of Rhodamine 6G in methanol was kept. The large refractive index of SF4 glass ensured a convenient SPR (Surface Plasmon Resonance) angle for robust experimental observations. The solution was pumped using a pulsed nanosecond Nd:YAG laser (532 nm), while the probe was also a pulsed beam of wavelength 565 nm. Nanosecond excitation helps in avoiding population transfer of the dye molecules into the triplet state, thus offering a high gain in contrast to continuous wave excitation. It was ensured that the probe and pump pulses coincide in space and time, at the region of interest in the plasmonic sample. The gain was varied by changing the pump power, for two dye concentrations. Using a focusing lens to illuminate the probe beam at various angles, single-shot observations were made in the Kretschmann configuration. Data revealed monotonic modifications in the relative reflectance curves with increasing gain, with the peak value increasing from about 4% to about 20% over unity.

mujumdar@tifr.res.in