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1. Expanding the bandwidth of fluorescence-detected two-dimensional electronic spectroscopy using a broadband continuum probe pulse pair

   https://doi.org/10.1364/OE.516963  

   Sanders, Stephanie_E; Zhang, Muyi; Javed, Ariba; Ogilvie, Jennifer_P (February 2024, Optics Express)

   We demonstrate fluorescence-detected two-dimensional electronic spectroscopy (F-2DES) with a broadband, continuum probe pulse pair in the pump-probe geometry. The approach combines a pump pulse pair generated by an acousto-optic pulse-shaper with precise control of the relative pump pulse phase and time delay with a broadband, continuum probe pulse pair created using the Translating Wedge-based Identical pulses eNcoding System (TWINS). The continuum probe expands the spectral range of the detection axis and lengthens the waiting times that can be accessed in comparison to implementations of F-2DES using a single pulse-shaper. We employ phase-cycling of the pump pulse pair and take advantage of the separation of signals in the frequency domain to isolate rephasing and non-rephasing signals and optimize the signal-to-noise ratio. As proof of principle, we demonstrate broadband F-2DES on a laser dye and bacteriochlorophylla. 

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2. On optimality and bounds for internal solutions generated from boundary data-driven Gramians.

   https://doi.org/10.1088/1361-6420/ae0fbe  

   Druskin, Vladimir; Moskow, Shari; Zaslavskiy, Mikhail (October 2025, Inverse Problems)

   Abstract We consider the computation of internal solutions for a time domain plasma wave equation with unknown coefficients from the data obtained by sampling its transfer function at the boundary. The computation is performed by transforming known background snapshots using the Cholesky decomposition of the data-driven Gramian. We show that this approximation is asymptotically close to the projection of the true internal solution onto the subspace of background snapshots. This allows us to derive a generally applicable bound for the error in the approximation of internal fields from boundary data for a time domain plasma wave equation with an unknown potential $$q$$. For general $$q\in L^\infty$$, we prove convergence of these data generated internal fields in one dimension for two examples of initial waves. The first is for piecewise constant initial data and sampling $$\tau$$ equal to the pulse width. The second is piecewise linear initial data and sampling at half the pulse width. We show that in both cases the data generated solutions converge in $L^2$ at order $$\sqrt{\tau}$$. We present numerical experiments validating the result and the sharpness of this convergence rate. 

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3. Experimental and Computational Analysis of Broadband THz Photoconductive Antennas

   https://doi.org/10.1155/2023/6682627  

   Uttley, Zachary; Santos Batista, Jose; Pirzada, Bilal; El-Shenawee, Magda (February 2023, International Journal of Antennas and Propagation)

   Gennarelli, Claudio (Ed.)

   The work presents the fabrication and measurements of four LT-GaAs photoconductive terahertz (THz) antennas with different geometries of metallic electrodes. The goal is to analyze the overall bandwidth of the antennas through a comparison between the spectra of the generated photocurrent in the antenna gap, the radiated electric field THz pulse, and the S11 parameter of the metallic electrodes. The photocurrent density and the S11 parameters are computed using COMSOL multiphysics, while the generated THz pulse was experimentally measured using a time-domain spectroscopy system. The polarizations of the photoconductive antennas are experimentally measured, using x-cut quartz crystal halfwave plates, showing polarization in the direction of the electrode’s long axis. Pinholes are used to verify system alignment and quality of the radiated signal spectra. The results show that the spectra of the radiated THz pulses in all four antennas considered in this work are dominated by the behavior of the S11 parameter at the lower part of the frequency band, but with the decreasing photocurrent dominating the spectra at higher frequencies. 

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4. Measuring carrier diffusion in MAPbI3 solar cells with photocurrent-detected transient grating spectroscopy

   https://doi.org/10.1063/5.0159301  

   Ouyang, Zhenyu; Gan, Zijian; Yan, Liang; You, Wei; Moran, Andrew M (September 2023, The Journal of Chemical Physics)

   Conventional time-of-flight methods can be used to determine carrier mobilities for photovoltaic cells in which the transit time between electrodes is greater than the RC time constant of the device. To measure carrier drift on sub-ns timescales, we have recently developed a two-pulse time-of-flight technique capable of detecting drift velocities with 100-ps time resolution in perovskite materials. In this method, the rates of carrier transit across the active layer of a device are determined by varying the delay time between laser pulses and measuring the magnitude of the recombination-induced nonlinearity in the photocurrent. Here, we present a related experimental approach in which diffractive optic-based transient grating spectroscopy is combined with our two-pulse time-of-flight technique to simultaneously probe drift and diffusion in orthogonal directions within the active layer of a photovoltaic cell. Carrier density gratings are generated using two time-coincident pulse-pairs with passively stabilized phases. Relaxation of the grating amplitude associated with the first pulse-pair is detected by varying the delay and phase of the density grating corresponding to the second pulse-pair. The ability of the technique to reveal carrier diffusion is demonstrated with model calculations and experiments conducted using MAPbI3 photovoltaic cells. 

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5. Investigation of Photoconductive Antenna Electrodes on Terahertz Signal Generation

   Santos, J. (January 2021, IEEE Antennas and Propagation Symposium)

   null (Ed.)

   This paper presents an investigation of terahertz (THz) signals generated using photoconductive antennas (PCAs) based on different electrode geometries. The geometry of the electrodes influences the shape of the THz pulse and its frequency spectrum. The geometry can be used to provide improvements to the bandwidth of the PCA. We used the transient solution of the RF module of COMSOL Multiphysics to simulate the radiated electric field based on using the photoconductive current as a time domain source in the gap of the antenna. The dimensions of a previously published bowtie H-dipole antenna were used as reference that was modified to three new configurations. The results show that the bowtie-H-dipole antenna provided broader bandwidth compared with the fractal, slot, and/or circular shape electrodes PCAs. 

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