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Journal of Physics D Applied Physics

Optical gain at the F-band of oxidized silicon nanocrystals

Kateřina Dohnalová, Karel Žídek, Lukáš Ondič, Kateřina Kůsová, Ondřej Cibulka, I. Pelant

Published Jun 10, 2009• 35 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3727/42/13/135102

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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.
In this paper, we present time-resolved optical gain spectroscopy using a combination of the variable stripe length and the shifting excitation spot techniques under pulsed nanosecond excitation at 355 nm. Optical gain measurements in the temporal detection window of 10 ns width, coincident with the excitation pulse, revealed induced absorption losses, whereas measurements with a different detection gate width and delay in two main photoluminescence components (a fast band at ∼430 nm decaying in nanoseconds and a slow band at ∼620 nm decaying in microseconds) show a positive optical gain of the order of tens of cm -1 .
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.
Argon gas excited by resonant femtosecond ultraviolet pulses gives rise to cavity-free lasing emission in the near-infrared (NIR) range. Here we reported on a pump-probe study of the optical gain of this lasing phenomenon. With the injection of an external seeding pulse, the forward signal was significantly enhanced, confirming the existence of optical gain. The temporal dynamics of the optical gain were characterized by a time-resolved measurement. It was found that the optical gain decays on a time scale of ∼ 10 ps and it does not present a significant dependence on the gas pressures. Moreover, the intensity of the forward NIR emission signal shows a linear dependence on the gas pressure. These features suggest that the nature of this forward NIR radiation is amplified spontaneous emission, not superradiance when multiple-photon resonant excitation is involved.
We pump low-pressure nitrogen gas with ionizing femtosecond laser pulses at 1.5 $\ensuremath{\mu}\mathrm{m}$ wavelength. The resulting rotationally excited N2+{\mathrm{N}}_{2}^{+}N2+​ molecular ions generate directional, forward-propagating stimulated and isotropic spontaneous emissions at 428 nm wavelength. Through high-resolution spectroscopy of these emissions, we quantify rotational population distributions in the upper and lower emission levels. We show that these distributions are shifted with respect to each other, which has a strong influence on the transient optical gain in this system. Although we find that electronic population inversion exists in our particular experiment, we show that sufficient dissimilarity of rotational distributions in the upper and lower emission levels could, in principle, lead to gain without net electronic population inversion.
The luminescence and laser characteristics of a synthetic opal matrix filled with organic dyes are studied upon excitation by nanosecond laser pulses. The appearance of stimulated emission in a partially ordered scattering medium is investigated. It is shown that if the luminescence spectrum of a dye (oxazine-17) is located far outside the photonic bandgap of the opal matrix, stimulated emission along a preferential direction in the (111) plane is observed when pumping exceeds a threshold even without an external optical cavity. The stimulated emission spectrum is considerably narrower than the luminescence spectrum and consists of several narrow lines located within the dye luminescence band. If the luminescence spectrum of a dye (rhodamine 6G) overlaps with the photonic bandgap of the opal matrix, a different picture is observed. The loss of radiation in the matrix leads to the red shift of the luminescence spectrum, while the stimulated emission as in the case of oxazine-17 lies is observed within the luminescence band.