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Simultaneous Measurement of Mid-Infrared Refractive Indices in Thin-Film Heterostructures: Methodology and Results for GaAs/AlGaAs
Authors:
Lukas W. Perner,
Gar-Wing Truong,
David Follman,
Maximilian Prinz,
Georg Winkler,
Stephan Puchegger,
Garrett D. Cole,
Oliver H. Heckl
Abstract:
We present our results for simultaneous measurement of the refractive indices of gallium arsenide (GaAs) and aluminum gallium arsenide (Al$_\mathrm{x}$Ga$_\mathrm{1-x}$As) from $2.0$ to $7.1\,\mathrm{μm}$ ($5000$ to $1400\,\mathrm{cm^{-1}}$). We obtain these values from a monocrystalline superlattice Bragg mirror of excellent purity (background doping $\leq 1 \times 10^{-14}\,\mathrm{cm^{-3}}$), g…
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We present our results for simultaneous measurement of the refractive indices of gallium arsenide (GaAs) and aluminum gallium arsenide (Al$_\mathrm{x}$Ga$_\mathrm{1-x}$As) from $2.0$ to $7.1\,\mathrm{μm}$ ($5000$ to $1400\,\mathrm{cm^{-1}}$). We obtain these values from a monocrystalline superlattice Bragg mirror of excellent purity (background doping $\leq 1 \times 10^{-14}\,\mathrm{cm^{-3}}$), grown via molecular beam epitaxy. To recover the refractive indices over such a broad wavelength range, we fit a dispersion model for each material. In a novel combination of well-established methods, we measure both a photometrically accurate transmittance spectrum of the Bragg mirror via Fourier-transform infrared spectrometry and the individual physical layer thicknesses of the structure via scanning electron microscopy. To infer the uncertainty of the refractive index values, we estimate relevant measurement uncertainties and propagate them via a Monte-Carlo method. This highly-adaptable approach conclusively yields propagated relative uncertainties on the order of $10^{-4}$ over the measured spectral range for both GaAs and Al$_{0.929}$Ga$_{0.071}$As. The fitted model can also approximate the refractive index for MBE-grown Al$_\mathrm{x}$Ga$_\mathrm{1-x}$As for $0\leq x \leq 1$. Both these updated values and the measurement approach will be essential in the design, fabrication, and characterization of next-generation active and passive optical devices in a spectral region that is of high interest in many fields, e.g., laser design and cavity-enhanced spectroscopy in the mid-infrared spectral region.
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Submitted 16 May, 2023; v1 submitted 18 January, 2023;
originally announced January 2023.
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Precise Measurement of Refractive Indices in Thin Film Heterostructures
Authors:
Lukas W. Perner,
Gar-Wing Truong,
David Follman,
Maximilian Prinz,
Georg Winkler,
Stephan Puchegger,
Garrett D. Cole,
Oliver H. Heckl
Abstract:
We present a robust, precise, and accurate method to simultaneously measure the refractive indices of two transparent materials within an interference coating. This is achieved by measuring both a photometrically accurate transmittance spectrum and the as-grown individual layer thicknesses of a thin-film multilayer structure. These measurements are used for a TMM-based curve-fitting routine which…
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We present a robust, precise, and accurate method to simultaneously measure the refractive indices of two transparent materials within an interference coating. This is achieved by measuring both a photometrically accurate transmittance spectrum and the as-grown individual layer thicknesses of a thin-film multilayer structure. These measurements are used for a TMM-based curve-fitting routine which extracts the refractive indices and their measurement uncertainties via a Monte-Carlo-type error propagation. We demonstrate the performance of this approach by experimentally measuring the refractive indices of both, GaAs and Al$_{0.929}$Ga$_{0.071}$As, as present in an epitaxial distributed Bragg reflector. A variety of devices can be used to obtain the transmittance spectrum (e.g., FTIR, grating-based spectrophotometer) and layer thicknesses (e.g., SEM, TEM, AFM), the discussed approach is readily adaptable to virtually any wavelength region and many transparent material combinations of interest. The subsequent model-fitting approach yields refractive index values with $10^{-4}$-level uncertainty for both materials.
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Submitted 18 January, 2023;
originally announced January 2023.
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The role of the ligand layer for photoluminescence spectral diffusion of CdSe/ZnS nanoparticles
Authors:
Daniel Braam,
Andreas Mölleken,
Günther M. Prinz,
Christian Notthoff,
Martin Geller,
Axel Lorke
Abstract:
The time-resolved photoluminescence (PL) characteristics of single CdSe/ZnS nanoparticles, embedded in a PMMA layer is studied at room temperature. We observe a strong spectral jitter of up to 55 meV, which is correlated with a change in the observed linewidth. We evaluate this correlation effect using a simple model, based on the quantum confined Stark effect induced by a diffusing charge in the…
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The time-resolved photoluminescence (PL) characteristics of single CdSe/ZnS nanoparticles, embedded in a PMMA layer is studied at room temperature. We observe a strong spectral jitter of up to 55 meV, which is correlated with a change in the observed linewidth. We evaluate this correlation effect using a simple model, based on the quantum confined Stark effect induced by a diffusing charge in the vicinity of the nanoparticle. This allows us to derive a mean distance between the center of the particle and the diffusing charge of approximately 3.3 nm on average, as well as a mean charge carrier displacement within the integration time. The distances are larger than the combined radius of particle core and shell of about 3 nm, but smaller than the overall radius of 5 nm including ligands. These results are reproducible, even for particles which exhibit strong blueing, with shifts of up to 150 meV. Both the statistics and its independence of core-shell alterations lead us to conclude that the charge causing the spectral jitter is situated in the ligands.
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Submitted 22 March, 2013;
originally announced March 2013.