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Siberian Journal of Physics

Optical Gain Coefficient Measurement Of Colloidal Solution Of Spasers

Fedor A. Benimetskiy, Aleksandr I. Plekhanov

Published Jun 1, 2015• 0 citations
Publisher

Abstract

Optical gain coefficient of colloidal solution of spasers was measured by the variable stripe length method, where the intensity of the amplified spontaneous emission from the sample edge is measured as a function of excited volume. Spasers consist of gold core-silica (porous) shell nanoparticles penetrated with fluorescein. Under these conditions, we observed the exponential growth and narrowing of line and directivity of stimulated emission. The obtained optical gain coefficient of spaser colloids in fluorescein solution in ethanol is g = 16,2 cm–1. Separately, fluorescein solution in ethanol had g = 3,7 cm–1.

DOI
10.54362/1818-7919-2015-10-2-5-9

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Optical gain coefficient of colloidal solution of spasers was measured by the variable stripe length method, where the intensity of the amplified spontaneous emission from the sample edge is measured as a function of excited volume. Spasers consist of gold core-silica (porous) shell nanoparticles penetrated with fluorescein. Under these conditions, we observed the exponential growth and narrowing of line and directivity of stimulated emission. The obtained optical gain coefficient of spaser colloids in fluorescein solution in ethanol is g = 16,2 cm–1. Separately, fluorescein solution in ethanol had g = 3,7 cm–1.
In this paper we present time-resolved optical gain spectroscopy using the variable stripe length technique in combination with the shifting excitation spot technique under pumping with nanosecond laser pulses. Measurements reveal positive optical gain on a nanosecond time scale at 430 nm (F-band), accompanied by spectral narrowing and a threshold behaviour of the amplified spontaneous emission as a function of the excitation intensity. We show that the presence of the slow-red (S-band) emission component critically influences the observation of stimulated emission from the F-band.
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Improving and controlling the efficiency of a gain medium is one of the most challenging problems of laser research. By measuring the gain length in an opal-based photonic crystal (PhC) doped with laser dye, we demonstrate that optical amplification is more than twenty -fold enhanced along the Γ-K symmetry directions of the face centered-cubic photonic crystal. The samples under study are dye doped thin-film artificial opals, from which gain is extracted by using the stripe length technique: the spontaneous emitted photons, traveling in the direction of the pumped stripe, undergo an exponential amplification of the intensity as they pass through an area of population inversion. This can be quantified by the gain coefficient G which we demonstrate do depend from the propagation direction.
The widely used technique for determining optical gain and absorption from single-pass, amplified spontaneous emission (ASE) is analyzed with particular attention given to active-volume saturation and three-dimensional effects. The results for homogeneously broadened media show that the population inversion and hence the gain in the active region is diminished by high levels of spontaneous and ASE. The externally measured gain is further reduced for large output collection apertures that admit incompletely amplified off-axis rays. Criteria are given for the excitation and detection geometries required to ensure accurate (∼10-percent) gain measurement. The simultaneous measurement of the equilibrium absorption coefficient can be made under less restrictive conditions.
A silica colloidal photonic crystal in ethanol solution, whose lattice constant varies with the height of the crystal, was prepared. The Raman intensities of th e ethanol solution induced by a picosecond intense laser were measured at differ ent heights, and two phenomena were found: (1) although the frequency of Raman s ignals generated within the photonic crystal is equal to that of the photonic ps eudogap of the photonic crystal, the Raman intensity attenuation measured is not reduced to the extent of that in the conventional transmission experiment; (2) although the transmission of an incident beam with the wavelength of the Raman s ignal is strongly dependent on the height of crystal, the intensity of th e Raman scattering generated within the silicaethanol colloidal photonic crystal does n ot change with the height, i.e. it is insensitive to the pseudogap. These anti i ntuitive phenomena may be directly interpreted using the fact that the propagati on mechanism of the monodirectional incidental laser is intrinsically different from that of the Raman signals from inner light sources inside a photonic crysta l, and the Raman signals scattered by defects deep inside the crystal and genera ted in the surface layer contribute largely to the measured intensities.
Emission spectra of concentrated (~10 18 –10 19 cm –3 ) ethanol solutions of Rhodamine C excited by high-power (~10 25 –10 26 cm –2 s –1 ) laser radiation were measured. The emission spectrum consists of two narrow bands (with half-widths of ~200–300 cm –1 ). The long-wavelength band was interpreted as cooperative spontaneous emission, and the short-wavelength band was assigned to the amplified spontaneous emission. The ratio of their intensities depends on the dye concentration and the pump power density. The ratio of band intensities is assumed to be determined by the dephasing rate of excited molecules responsible for cooperative spontaneous emission.