[ { "id": "authors:0f950-mxs44", "collection": "authors", "collection_id": "0f950-mxs44", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220517-214305241", "type": "monograph", "title": "Ab Initio Prediction of Excited State and Polaron Effects in Transient XUV Measurements of \u03b1-Fe\u2082O\u2083", "author": [ { "family_name": "Klein", "given_name": "Isabel M.", "clpid": "Klein-Isabel-M" }, { "family_name": "Liu", "given_name": "Hanzhe", "orcid": "0000-0001-9001-725X", "clpid": "Liu-Hanzhe" }, { "family_name": "Nimlos", "given_name": "Danika", "orcid": "0000-0002-5414-0039", "clpid": "Nimlos-Danika" }, { "family_name": "Krotz", "given_name": "Alex", "orcid": "0000-0001-8189-7902", "clpid": "Krotz-Alex" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" } ], "abstract": "Transient X-ray and extreme ultraviolet (XUV) spectroscopies have become invaluable tools for studying photoexcited dynamics due to their sensitivity to carrier occupations and local chemical or structural changes. One of the most studied mate-rials using transient XUV spectroscopy is \u03b1-Fe\u2082O\u2083 because of its rich photoexcited dynamics, including small polaron formation. The interpretation of carrier and polaron effects in \u03b1-Fe\u2082O\u2083 is currently done using a semi-empirical method that is not transferrable to most materials. Here, an ab initio, Bethe-Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for arbitrary materials systems. The accuracy of this approach is proven by calculating the XUV absorption spectra for the ground, photoexcited, and polaron states of \u03b1-Fe\u2082O\u2083. Furthermore, the theoretical approach allows for the projection of the core-valence excitons and different components of the X-ray transition Hamiltonian onto the band structure, providing new insights into old measurements. From this information, a physical intuition about the origins and nature of the transient XUV spectra can be built. A route to extracting electron and hole energies is even shown possible for highly angular momentum split XUV peaks. This method is easily generalized to K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.", "doi": "10.48550/arXiv.2204.06129", "publisher": "arXiv", "publication_date": "2022-04-13" }, { "id": "authors:mf1mb-9sn54", "collection": "authors", "collection_id": "mf1mb-9sn54", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220707-170621365", "type": "monograph", "title": "Single Photon Scattering Can Account for the Discrepancies Between Entangled Two-Photon Measurement Techniques", "author": [ { "family_name": "Hickam", "given_name": "Bryce P.", "orcid": "0000-0003-2120-4769", "clpid": "Hickam-Bryce-P" }, { "family_name": "He", "given_name": "Manni", "clpid": "He-Manni" }, { "family_name": "Harper", "given_name": "Nathan", "clpid": "Harper-Nathan" }, { "family_name": "Szoke", "given_name": "Szilard", "clpid": "Szoke-Szilard" }, { "family_name": "Cushing", "given_name": "Scott", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" } ], "abstract": "Entangled photon pairs are predicted to linearize and increase the efficiency of two-photon absorption, allowing continuous wave laser diodes to drive ultrafast time-resolved spectroscopy and nonlinear processes. Despite a range of theoretical studies and experimental measurements, inconsistencies persist about the value of the entanglement enhanced interaction cross section. A spectrometer is constructed that can temporally and spectrally characterize the entangled photon state before, during, and after any potential two-photon excitation event. For the molecule Rhodamine 6G, which has a virtual state pathway, any entangled two-photon interaction is found to be equal to or lower than classical, single photon scattering events. This result can account for the discrepancies between the wide variety of entangled two-photon absorption cross sections reported from different measurement techniques. The reported instrumentation can unambiguously separate classical and entangled effects and therefore is of importance for the growing field of nonlinear and multiphoton entangled spectroscopy.", "doi": "10.48550/arXiv.acs.jpclett.2c00865", "publisher": "arXiv", "publication_date": "2022-02-23" }, { "id": "authors:8k5jc-1h339", "collection": "authors", "collection_id": "8k5jc-1h339", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20210831-203959863", "type": "monograph", "title": "Measuring photoexcited electron and hole dynamics in ZnTe and modeling excited state core-valence effects in transient XUV reflection spectroscopy", "author": [ { "family_name": "Liu", "given_name": "Hanzhe", "orcid": "0000-0001-9001-725X", "clpid": "Liu-Hanzhe" }, { "family_name": "Michelsen", "given_name": "Jonathan M.", "clpid": "Michelsen-Jonathan-M" }, { "family_name": "Klein", "given_name": "Isabel M.", "clpid": "Klein-Isabel-M" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" } ], "abstract": "Transient XUV spectroscopy is growing in popularity for the measurement of solar fuel and photovoltaic materials as it can separately measure electron and hole energies for multiple elements at once. However, interpretation of transient XUV measurements is complicated by changes in core-valence exciton and angular momentum effects after photoexcitation. Here, we report the photoexcited electron and hole dynamics for ZnTe, a promising material for CO\u2082 reduction, following 400 nm excitation. We apply a newly developed, ab-initio theoretical approach based on density functional theory and the Bethe-Salpeter equation to accurately predict the excited state change in the measured transient XUV spectra. Electrons excited to the conduction band are measured with a thermalization rate of 70 \u00b1 40 fs. Holes are excited with an average excess energy of ~1 eV and thermalize in 1130 \u00b1 150 fs. The theoretical approach also allows an estimated assignment of inter- and intra-valley relaxation pathways in k-space using the relative amplitudes of the core-valence excitons.", "doi": "10.48550/arXiv.2108.02262", "publisher": "arXiv", "publication_date": "2021-08-04" }, { "id": "authors:898wg-b8y52", "collection": "authors", "collection_id": "898wg-b8y52", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220816-192448471", "type": "monograph", "title": "Element-specific electronic and structural dynamics using transient X-ray spectroscopy", "author": [ { "family_name": "Liu", "given_name": "Hanzhe", "orcid": "0000-0001-9001-725X", "clpid": "Liu-Hanzhe" }, { "family_name": "Klein", "given_name": "Isabel M.", "orcid": "0000-0001-6134-6732", "clpid": "Klein-Isabel-M" }, { "family_name": "Michelsen", "given_name": "Jonathan M.", "clpid": "Michelsen-Jonathan-M" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" } ], "abstract": "Transient X-ray absorption techniques can measure ultrafast dynamics of the elemental edges in a material or multiple layer junction, giving them immense potential for deconvoluting concurrent processes. However, the interpretation of the photoexcited changes to an X-ray edge is not as simple as directly probing a transition with optical or infrared wavelengths. The core hole left by the core-level transition distorts the measured absorption and reflection spectra, both hiding and revealing different aspects of a photo-induced process. In this perspective, we describe the implementation and interpretation of transient X-ray experiments. This description includes a guide of how to choose the best wavelength and corresponding X-ray sources when designing an experiment. As an example, we focus on the rising use of extreme ultraviolet (XUV) spectroscopy for understanding performance limiting behaviors in solar energy materials, such as measurements of polaron formation, electron and hole kinetics, and charge transport in each layer of a metal-oxide-semiconductor junction. The ability of measuring photoexcited carriers in each layer of a multilayer junction could prove particularly impactful in the study of molecules, materials, and their combinations that lead to functional devices in photochemistry and photoelectrochemistry.", "doi": "10.48550/arXiv.2106.04793", "publisher": "arXiv", "publication_date": "2021-06-09" }, { "id": "authors:r8yq2-36z78", "collection": "authors", "collection_id": "r8yq2-36z78", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220816-192451845", "type": "monograph", "title": "Characterization of Carrier Cooling Bottleneck in Silicon Nanoparticles by Extreme Ultraviolet (XUV) Transient Absorption Spectroscopy", "author": [ { "family_name": "Porter", "given_name": "Ilana J.", "orcid": "0000-0001-8692-9950", "clpid": "Porter-Ilana-J" }, { "family_name": "Lee", "given_name": "Angela", "orcid": "0000-0001-5388-8400", "clpid": "Lee-Angela" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" }, { "family_name": "Chang", "given_name": "Hung-Tzu", "orcid": "0000-0001-7378-8212", "clpid": "Chang-Hung-Tzu" }, { "family_name": "Ondry", "given_name": "Justin C.", "orcid": "0000-0001-9113-3420", "clpid": "Ondry-Justin-C" }, { "family_name": "Alivisatos", "given_name": "A. Paul", "orcid": "0000-0001-6895-9048", "clpid": "Alivisatos-A-Paul" }, { "family_name": "Leone", "given_name": "Stephen R.", "orcid": "0000-0003-1819-1338", "clpid": "Leone-Stephen-R" } ], "abstract": "Silicon nanoparticles have the promise to surpass the theoretical efficiency limit of single-junction silicon photovoltaics by the creation of a \"phonon bottleneck\", a theorized slowing of the cooling rate of hot optical phonons that in turn reduces the cooling rate of hot carriers in the material. To verify the presence of a phonon bottleneck in silicon nanoparticles requires simultaneous resolution of electronic and structural changes at short timescales. Here, extreme ultraviolet transient absorption spectroscopy is used to observe the excited state electronic and lattice dynamics in polycrystalline silicon nanoparticles following 800 nm photoexcitation, which excites carriers with 0.35 \u00b1 0.03 eV excess energy above the \u0394\u2081 conduction band minimum. The nanoparticles have nominal 100 nm diameters with crystalline grain sized of about ~16 nm. The extracted carrier-phonon and phonon-phonon relaxation times of the nanoparticles are compared to those for a silicon (100) single crystal thin film at similar carrier densities (2 x 10\u00b9\u2079 cm\u207b\u00b3 for the nanoparticles and 6 x 10\u00b9\u2079 cm\u207b\u00b3 for the thin film). The measured carrier-phonon and phonon-phonon scattering lifetimes for the polycrystalline nanoparticles are 870 \u00b1 40 fs and 17.5 \u00b1 0.3 ps, respectively, versus 195\u00b120 fs and 8.1 \u00b1 0.2 ps, respectively, for the silicon thin film. The reduced scattering rates observed in the nanoparticles are consistent with the phonon bottleneck hypothesis.", "doi": "10.48550/arXiv.2104.05825", "publisher": "arXiv", "publication_date": "2021-04-12" }, { "id": "authors:vpj47-jpd75", "collection": "authors", "collection_id": "vpj47-jpd75", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180627-161213458", "type": "monograph", "title": "Terahertz Waveguiding in Silicon-Core Fibers", "author": [ { "family_name": "Bas", "given_name": "Derek A.", "clpid": "Bas-D-A" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" }, { "family_name": "Ballato", "given_name": "John", "clpid": "Ballato-J" }, { "family_name": "Bristow", "given_name": "Alan D.", "clpid": "Bristow-A-D" } ], "abstract": "We propose the use of a silicon-core optical fiber for terahertz (THz) waveguide applications. Finite-difference time-domain simulations have been performed based on a cylindrical waveguide with a silicon core and silica cladding. High-resistivity silicon has a flat dispersion over a 0.1 - 3 THz range, making it viable for propagation of tunable narrowband CW THz and possibly broadband picosecond pules of THz radiation. Simulations show the propagation dynamics and the integrated intensity, from which transverse mode profiles and absorption lengths are extraced. It is found that for 140 - 250 micron core diameters the mode is primarily confined to the core, such that the overall absorbance is only slightly less than in bulk polycrystalline silicon.", "doi": "10.48550/arXiv.1305.0520", "publisher": "arXiv", "publication_date": "2013-05-02" } ]