Photonic reservoir computer using speckle in multimode waveguide ring resonators.

Photonic reservoir computers (RC) come in single mode ring and multimode array geometries. We propose and simulate a photonic RC architecture using speckle in a multimode waveguide ring resonator that requires neither the ultra-high-speed analog-digital conversion nor the spatial light modulator used in other designs. We show that the equations for propagation around a multimode (MM) ring resonator along with an optical nonlinearity, and optical feedback can be cast exactly in the standard RC form with speckle mixing performing the pseudo-random matrix multiplications. The hyperparameters are the outcoupling efficiency, the nonlinearity saturation intensity, the input bias, and the waveguide properties. In particular, the number of waveguide modes is a measure of the number of effective neurons in the RC. Simulations show a ring using a strongly guiding 50-µm planar waveguide gives 206 effective neurons and excellent predictions of Mackey-Glass waveforms for a broad range of the hyperparameters, while a weakly guiding MM 200-µm diameter fiber gives 4,238 effective neurons and excellent predictions of chaotic solutions of the Kuramoto-Sivashinsky equation. We discuss physical realizations for implementing the system with a chip-scale device or with discrete components and a MM optical fiber.

Setting an Upper Bound to the Biexciton Binding Energy in CsPbBr3 Perovskite Nanocrystals.

Cesium lead halide perovskite nanocrystals are promising emissive materials for a variety of optoelectronic applications. To fully realize the potential of these materials, we must understand the energetics and dynamics of multiexciton states which are populated under device relevant excitation conditions. We utilized time-resolved and spectrally-resolved photoluminescence studies to investigate the biexciton binding energy as well as a red-shifted emission feature previously reported under high-flux excitation conditions. We determine that this red-shifted emission feature can be ascribed to sample sintering induced by air-exposure and high-flux irradiation. Furthermore, we determine that the biexciton binding energy at room temperature is at most ±20 meV, providing a key insight toward understanding many-body interactions in the lead halide perovskite lattice.

Markov Chain Monte Carlo Sampling for Target Analysis of Transient Absorption Spectra.

Global and target analysis techniques are ubiquitous tools for interpreting transient absorption (TA) spectra. However, characterizing uncertainty in the kinetic parameters and component spectra derived from these fitting procedures can be challenging. Furthermore, fitting TA spectra of inorganic nanomaterials where the component spectra of different excited states are nearly or completely overlapped is particularly problematic. Here, we present a target analysis model for extracting excited-state spectra and dynamics from TA data using a Markov chain Monte Carlo (MCMC) sampler to visualize and understand uncertainty in the model fits. We demonstrate the utility of this approach by extracting the overlapping component spectra and dynamics of single- and biexciton states in CsPbBr3 nanocrystals. Significantly, refinement of the component spectra is accomplished by fitting the entire fluence-dependent series of ensemble TA data using the Poisson statistics of photon absorption, providing multiple checks for internal consistency. The MCMC method itself is highly general and can be applied to any data set or model framework to accurately characterize uncertainty in the fit and aid model selection when choosing between different models.

High repetition-rate femtosecond stimulated Raman spectroscopy with fast acquisition.

Time-resolved femtosecond stimulated Raman spectroscopy (FSRS) is a powerful tool for investigating ultrafast structural and vibrational dynamics in light absorbing systems. However, the technique generally requires exposing a sample to high laser pulse fluences and long acquisition times to achieve adequate signal-to-noise ratios. Here, we describe a time-resolved FSRS instrument built around a Yb ultrafast amplifier operating at 200 kHz, and address some of the unique challenges that arise at high repetition-rates. The setup includes detection with a 9 kHz CMOS camera and an improved dual-chopping scheme to reject scattering artifacts that occur in the 3-pulse configuration. The instrument demonstrates good signal-to-noise performance while simultaneously achieving a 3-6 fold reduction in pulse energy and a 5-10 fold reduction in acquisition time relative to comparable 1 kHz instruments.

Relative reactivity of biogenic and chemogenic uraninite and biogenic noncrystalline U(IV).

Aqueous chemical extractions and X-ray absorption spectroscopy (XAS) analyses were conducted to investigate the reactivity of chemogenic uraninite, nanoparticulate biogenic uraninite, and biogenic monomeric U(IV) species. The analyses were conducted in systems containing a total U concentration that ranged from 1.48 to 2.10 mM. Less than 0.02% of the total U was released to solution in extractions that targeted water-soluble and ion exchangeable fractions. Less than 5% of the total U was solubilized via complexation with a 0.1 M solution of NaF. Greater than 90% of the total U was extracted from biogenic uraninite and monomeric U(IV) after 6 h of reaction in an oxidizing solution of 50 mM K2S2O8. Additional oxidation experiments with lower concentrations (2 mM and 10 mM) of K2S2O8 and 8.2 mg L(-1) dissolved oxygen suggested that monomeric U(IV) species are more labile than biogenic uraninite; chemogenic uraninite was much less susceptible to oxidation than either form of biogenic U(IV). These results suggest that noncrystalline forms of U(IV) may be more labile than uraninite in subsurface environments. This work helps fill critical gaps in our understanding of the behavior of solid-associated U(IV) species in bioremediated sites and natural uranium ore deposits.