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Physica Scripta

Time-resolved measurements of optical gain and photoluminescence in silicon nanocrystals

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

Published Nov 1, 2010• 11 citations
Publisher

Abstract

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 .

DOI
10.1088/0031-8949/2010/t141/014011

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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 .
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 work, we focus on the interaction of SiV color centers in diamond with femtose- cond light pulses. In the theoretical part, a script is developed to simulate the experimen- tal varying stripe length method and the effects of saturated optical gain are investigated. In the experimental part, photoluminescence and transient differential absorption under non-resonant excitation are studied. The result of the luminescence measurements are estimates of the effective cross-sections of single-photon absorption for the wavelengths 620 nm, 515 nm and 340 nm, as well as the effective cross-section of three-photon absorp- tion for 2000 nm. The result of the differential absorption measurements is the relaxation constant of the excited level of the SiV center and the upper estimate of the time duration of the initial dynamics. 1
In this work, we focus on the interaction of SiV color centers in diamond with femtose- cond light pulses. In the theoretical part, a script is developed to simulate the experimen- tal varying stripe length method and the effects of saturated optical gain are investigated. In the experimental part, photoluminescence and transient differential absorption under non-resonant excitation are studied. The result of the luminescence measurements are estimates of the effective cross-sections of single-photon absorption for the wavelengths 620 nm, 515 nm and 340 nm, as well as the effective cross-section of three-photon absorp- tion for 2000 nm. The result of the differential absorption measurements is the relaxation constant of the excited level of the SiV center and the upper estimate of the time duration of the initial dynamics. 1
Net optical gain in silicon nanocrystals (Si-nc) has been measured by pump and probe transmission experiments. Si-nc active layers have been produced by plasma enhanced chemical vapor deposition on transparent quartz substrates. Continuous and pulsed pump and probe transmission measurements have shown clear evidences of net probe amplification with fast (nanosecond) response time. Transfer matrix and rate equations modeling of the nonlinear signal transmission allowed to extract the gain spectra for different pump intensities yielding good qualitative agreement with the experimental data. Gain cross sections per nanocrystals of the order of 3×10−16cm2 have been deduced. The physical origin of the optical amplification is interpreted within a four level recombination model describing the dynamics of strongly localized excitons at the Si-nc∕SiO2 interface.
We use a rate equation approach to model the conditions for optical gain in nanocluster sensitized erbium in a slot waveguide geometry. We determine the viability of achieving net gain for the range of reported values of the carrier absorption cross section for silicon nanoclusters. After accounting for the local density of optical states modification of the emission rates, we find that gain is impossible in continuous wave pumping due to carrier absorption, regardless of the carrier absorption cross section. We, therefore, propose a pulsed electrical operation scheme which mitigates carrier absorption by taking advantage of the short lifetime of silicon nanoclusters compared to erbium. We show that pulsed excitation of a 10 nm layer achieves a modal gain of 0.9 dB/cm during each pulse. Furthermore this gain can be increased to 2 dB/cm by pumping a 50 nm layer.
The response of matter to an optical excitation consists essentially of absorption and emission. Traditional spectroscopy accesses the frequency-resolved and time-integrated response, while the temporal evolution stays concealed. However, we will demonstrate here that the temporal evolution of a virtual single-photon transition can be mapped out by a second pulsed electromagnetic field. The resulting optical signal shows optical gain and loss, which can be gated and controlled via the relative delay of the electromagnetic fields.