[ { "id": "authors:07h66-ee088", "collection": "authors", "collection_id": "07h66-ee088", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20211215-155246537", "type": "conference_item", "title": "Entangled photon spectroscopy of molecules", "author": [ { "family_name": "Cushing", "given_name": "Scott Kevin", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" } ], "abstract": "In this talk, I will discuss the growing field of entangled-photon spectroscopy as related to mol. systems. An entangled\nstate is represented by a wavefunction, describing a group of particles, which cannot be factored into sep. states of\nindividual particles. For photons, these correlations lead to non-classical light-matter interactions, potentially creating new opportunities for spectroscopic techniques. For example, second-order processes with two entangled photons, such as two-photon absorption or sum frequency generation, occur at a linear rate with a cross section that should be close to that of one-photon classical processes. However, early results in the field have produced conflicting measurements as to its magnitude. I will discuss the role of excited state dephasing and time-ordering for entangled photon interactions. I will also briefly remark on other interesting prospects of entangled photons, such as their ability to break the temporal-spectral Fourier transform limit. For instance, few-femtosecond spectroscopy is predicted to be possible with entangled light sources of MHz linewidths. Given the entangled photons quantum correlations, this property may lead to interesting opportunities in directing quantum coupled quantum states, although the techniques are still at an early stage.", "publisher": "Caltech Library", "publication_date": "2021-08" }, { "id": "authors:fgkrm-v2r80", "collection": "authors", "collection_id": "fgkrm-v2r80", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20201221-090410882", "type": "conference_item", "title": "Role of polarons, interfaces, and phonons in photocatalytic junctions", "author": [ { "family_name": "Cushing", "given_name": "Scott Kevin", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" } ], "abstract": "Femtosecond to picosecond processes are often considered to be a thermalizing bath that has little effect on longer time\nscale photocatalytic reactions. In this talk, we will discuss when strong electron-phonon interactions can be the rate\nlimiting step in photocatalytic materials and junctions. Specifically, high harmonic generation is used to create X-ray\npulses with attosecond pulse widths and energies covering 10-150 eV. The energy range allows for multiple elemental\nX-ray edges to be measured simultaneously. The XUV pulse is combined with a 3-5 fs excitation pulse and a delay line\nto allow for transient absorption measurements. The response of the elemental X-ray edges to visible light\nphotoexcitation is sepd. into electronic and thermal structural components using the Bethe-Salpeter equation with d.\nfunctional theory. This allows for the heat and charge transfer to be measured as a function of an element. For example,\nthe conversion of photoexcited electrons to polarons is measured in Fe\u2082O\u2083. The time-resolved energy of electrons and\nholes, as well as their correlation with the acoustic and optical phonon bath, is measured in Si and Ge. Finally, the\ncharge and heat transport is measured between each component of a metal-insulator-semiconductor junction (Ni-TiO\u2082-Si) using the Ni, Ti, and Si X-ray edges.", "publisher": "Caltech Library", "publication_date": "2020-08" }, { "id": "authors:twffm-kdr21", "collection": "authors", "collection_id": "twffm-kdr21", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190812-085857670", "type": "conference_item", "title": "Element-specific measurement of hole transport and kinetics in a Ni-TiO_2-Si photolectrode using transient extreme ultraviolet spectroscopy", "author": [ { "family_name": "Cushing", "given_name": "Scott Kevin", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" } ], "abstract": "A passivating oxide layer is crit. for the stability and the performance of solar-fuel photoelectrodes. While the semiconductor surface can be passivated by a few nanometer oxide film, the best performance often correlates with a thicker and defect-rich amorphous TiO_2 layer. The defect states are suggested to facilitate hole transport between the semiconductor and metal catalyst. In this presentation, transient extreme UV (XUV) absorption spectroscopy quantifies the electron and hole transport between each element of a photoexcited Ni-TiO_2 -Si photoelectrode. A ballistic hole tunneling is measured from the p-type Si to the Ni metal in <100 fs after photoexcitation. The measured hole injection efficiency is 26\u00b14%. The transient hole population is then measured to back-diffuse through the TiO_2 on a picoseconds timescale, followed by an increased electron-hole recombination at the Si-TiO_2 interface. By temporally resolving the\npopulation of electrons and holes in each layer of the junction, the hole transport velocity in the TiO_2 , the hole mobility in the Si, the diffusion const. of holes in the TiO_2 , and the surface recombination velocity at the Si/TiO_2 interface are quantized.", "publisher": "Caltech Library", "publication_date": "2019-08" }, { "id": "authors:afn7z-pf075", "collection": "authors", "collection_id": "afn7z-pf075", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180702-131437834", "type": "conference_item", "title": "Carrier Lifetime Dependence on Size Confinement in Silicon Nanoparticles Using Extreme Ultraviolet Spectroscopy", "author": [ { "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": "Carneiro", "given_name": "Lucas M.", "clpid": "Carneiro-L-M" }, { "family_name": "Porter", "given_name": "Ilana J.", "orcid": "0000-0001-8692-9950", "clpid": "Porter-I-J" }, { "family_name": "Chang", "given_name": "Hung-Tzu", "orcid": "0000-0001-7378-8212", "clpid": "Chang-Hung-Tzu" } ], "abstract": "[no abstract]", "publisher": "Caltech Library", "publication_date": "2018-01-24" }, { "id": "authors:x4537-g5851", "collection": "authors", "collection_id": "x4537-g5851", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180627-101154152", "type": "conference_item", "title": "Evidence for small polaron formation leading to intrinsic photoexcited charge trapping in \u03b1-Fe_2O_3", "author": [ { "family_name": "Carneiro", "given_name": "Lucas M.", "clpid": "Carneiro-L-M" }, { "family_name": "Cushing", "given_name": "Scott", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" }, { "family_name": "Liu", "given_name": "Chong", "clpid": "Liu-Chong" }, { "family_name": "Leone", "given_name": "Stephen R.", "orcid": "0000-0003-1819-1338", "clpid": "Leone-S-R" } ], "abstract": "Hematite has long been a promising photoanode material for photoelectrochem. water splitting due to its suitable\nbandgap (1.9-2.2 eV), low cost, and photostability. However, photoexcited carriers with short diffusion lengths and short\nrecombination times, thought to be caused by ultrafast trapping, have limited hematite's conversion efficiency to a third of\nits theor. value. Herein we discuss evidence that the ultrafast trapping of photoexcited carriers in hematite (\u03b1-Fe_2O_3)\noccurs by small polaron formation. Using visible pump-extreme UV (XUV) probe spectroscopy we observe a pulse-width\nlimited shift in charge d. from the oxygen center to the iron center through a change in the multiplet splitting of the Fe\nM_(2,3)-edge, also understood as a change in oxidn. state of the iron center from Fe^(3+) to Fe^(2+). Small polaron formation\nthen starts on a sub-100 fs time scale, as evidenced by a breaking of the Fe 3p core level's degeneracy in the XUV\ntransition. This spectral signature continues increasing in amplitude up to 2-3 ps. The obsd. small polaron localization\ntimescales match that commonly attributed to mid-gap or surface trap states in hematite. However, the pre-edge\nabsorption or bleach expected for mid-gap or surface trap states in the XUV spectrum is not obsd. Small polaron\nformation would lead to a decrease in mobility of photoexcited carriers since conduction must occur by thermally\nactivated hopping. Accordingly, we observe that the probability of photoexcited charge trapping with increasing\nexcitation energy matches trends in the photoconversion efficiency with increasing excitation energy, particularly in the\nt2g conduction band. This presentation provides insight into the role of small polaron formation in photoexcited charge\ntrapping in hematite, and more generally into the charge trapping process in transition metal oxide catalysts thought to be\nlimited by small-polaron conduction.", "publisher": "Caltech Library", "publication_date": "2017-04" }, { "id": "authors:chfvz-yrg55", "collection": "authors", "collection_id": "chfvz-yrg55", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180627-095920460", "type": "conference_item", "title": "Attosecond quantum kinetics of photoexcited Germanium", "author": [ { "family_name": "Kraus", "given_name": "Peter", "clpid": "Kraus-P-M" }, { "family_name": "Kaplan", "given_name": "Christopher", "orcid": "0000-0002-5873-9487", "clpid": "Kaplan-C-J" }, { "family_name": "Zuerch", "given_name": "Michael W.", "clpid": "Z\u00fcrch-M-W" }, { "family_name": "Chang", "given_name": "Hung-Tzu", "orcid": "0000-0001-7378-8212", "clpid": "Chang-Hung-Tzu" }, { "family_name": "Borja", "given_name": "Lauren J.", "clpid": "Borja-L-J" }, { "family_name": "Jager", "given_name": "Marieke F.", "clpid": "Jager-M-F" }, { "family_name": "Cushing", "given_name": "Scott", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" }, { "family_name": "Neumark", "given_name": "Daniel M.", "orcid": "0000-0002-3762-9473", "clpid": "Neumark-D-M" }, { "family_name": "Leone", "given_name": "Stephen R.", "orcid": "0000-0003-1819-1338", "clpid": "Leone-S-R" } ], "abstract": "The electronic motion in a semiconductor after light absorption is the central aspect of modern opto-electronics.\nHowever, a real-time observation of the initial electronic response following single-photon excitation has so far defied\nexptl. approaches due to its extreme time and energy scales. Here, attosecond transient reflectivity is developed to\nmeasure the attosecond time-resolved dielec. function in the extreme UV following carrier promotion by visible-to-IR sub-\n5 fs (fs) pulses from the valence into the conduction band in germanium. The buildup of holes and electrons in the\nvalance and conduction band is monitored on attosecond timescales by the change in the reflection at the M_(4,5) edge (30\neV). The electron and hole features are found to exhibit a 1.4 fs oscillation, which is indicative of a field-induced\npolarization of the bands. The measurement of the attosecond dielec. function further enables the buildup of screening\nof the core-hole potential due to the collective electronic motion in the valence and conduction band to be tracked. It is\nexptl. obsd. through the real part of the dielec. function that a bare, unscreened Coulomb potential is formed\ninstantaneously after photoexcitation. A subsequent broadening of the real part of the dielec. function over a few fs is\nattributed to the screening of the Coulomb potential due to the collective electronic motion in the valence and conduction\nbands. Simultaneously, the imaginary part of the dielec. function tracks the buildup of new absorption channels. The\nexpt. shows that two sharp features appear instantaneously due to the change in state-filling in the valence and\nconduction band, with an addnl. broadening occurring on a few fs time scale. This broadening is attributed to a response\ntime of the collective electronic motion, which screens the Coulomb potential and changes the absorption due to a carrier\nredistribution. The time scale of the screening clocks in well with the inverse plasma frequency of the electron-hole\nplasma created by photoexcitation. Similar electronic responses after photoexcitation are anticipated to occur in all\nmaterials. The results presented here provide an exptl. measurement of the fundamental timescales of charged particle\ninteractions in solids.", "publisher": "Caltech Library", "publication_date": "2017-04" }, { "id": "authors:f1832-dvz73", "collection": "authors", "collection_id": "f1832-dvz73", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180627-100741086", "type": "conference_item", "title": "Decomposition of contributions from core-levels exhibiting spin-orbit splitting in XUV core-level spectroscopy", "author": [ { "family_name": "Chang", "given_name": "Hung-Tzu", "orcid": "0000-0001-7378-8212", "clpid": "Chang-Hung-Tzu" }, { "family_name": "Zuerch", "given_name": "Michael W.", "clpid": "Z\u00fcrch-M-W" }, { "family_name": "Kraus", "given_name": "Peter", "clpid": "Kraus-P-M" }, { "family_name": "Kaplan", "given_name": "Christopher", "orcid": "0000-0002-5873-9487", "clpid": "Kaplan-C-J" }, { "family_name": "Borja", "given_name": "Lauren J.", "clpid": "Borja-L-J" }, { "family_name": "Cushing", "given_name": "Scott", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" }, { "family_name": "Neumark", "given_name": "Daniel M.", "orcid": "0000-0002-3762-9473", "clpid": "Neumark-D-M" }, { "family_name": "Leone", "given_name": "Stephen R.", "orcid": "0000-0003-1819-1338", "clpid": "Leone-S-R" } ], "abstract": "Attosecond transient absorption spectroscopy (ATAS) is a versatile technique that allows observing ultrafast charge\ndynamics in solid-state samples. In ATAS, the transient absorption of a core-level excitation in the extreme UV (XUV) is\nmeasured following optical excitation of the sample. The lack of spin-selectivity in the XUV pulse results in overlapping\nXUV absorption spectra from spin-orbit split core-levels and leads to difficulty in disentangling the spectral signatures.\nHere, we demonstrate the successful retrieval of the contribution of a single spin-orbit level on the XUV transient\nabsorption signal. Under the approxn. that the spin-orbit split levels yield identical absorption spectra which are\nenergetically shifted and weighted by the degeneracy of the spin-orbit states, the contribution of one single spin-orbit\nstate can be retrieved by the Fourier transform of the measured signal. In this case the energy shift of the spectra from\nthe spin-orbit split states becomes a phase shift. By dividing out this phase factor and taking the inverse Fourier\ntransform, the underlying signal referenced to a single spin-orbit level can be retrieved. We apply this method to an\nATAS measurement at the germanium M_(4,5)-edge (\uf07e30 eV), where the spin-orbit energy splitting (0.57 eV) is comparable\nto the germanium band gap (0.66 eV). The successful decompn. of the transient absorption signals yields clear,\nspectrally-resolved signatures of electrons and holes such that carrier dynamics can be simultaneously measured and\ncharacterized. The presented method allows decompg. contributions of spin-orbit split core-levels in a transient\nabsorption spectrum. This allows clearer assignment of spectroscopic features and reveals weak signal contributions not\nvisible in the exptl. raw data. In the presented case of germanium M-edge spectroscopy, a clear assignment of features\nassocd. with electrons, holes and the bandgap becomes possible only after applying this method.", "publisher": "Caltech Library", "publication_date": "2017-04" }, { "id": "authors:9g7cw-sbx97", "collection": "authors", "collection_id": "9g7cw-sbx97", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180627-101640903", "type": "conference_item", "title": "Ultrafast extreme-ultraviolet reflection spectroscopy of electro-phonon dynamics in Germanium", "author": [ { "family_name": "Kaplan", "given_name": "Christopher", "orcid": "0000-0002-5873-9487", "clpid": "Kaplan-C-J" }, { "family_name": "Kraus", "given_name": "Peter", "clpid": "Kraus-P-M" }, { "family_name": "Borja", "given_name": "Lauren J.", "clpid": "Borja-L-J" }, { "family_name": "Zuerch", "given_name": "Michael W.", "clpid": "Z\u00fcrch-M-W" }, { "family_name": "Chang", "given_name": "Hung-Tzu", "orcid": "0000-0001-7378-8212", "clpid": "Chang-Hung-Tzu" }, { "family_name": "Jager", "given_name": "Marieke F.", "clpid": "Jager-M-F" }, { "family_name": "Cushing", "given_name": "Scott", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-S-K" }, { "family_name": "Neumark", "given_name": "Daniel M.", "orcid": "0000-0002-3762-9473", "clpid": "Neumark-D-M" }, { "family_name": "Leone", "given_name": "Stephen R.", "orcid": "0000-0003-1819-1338", "clpid": "Leone-S-R" } ], "abstract": "Ultrafast transient reflection spectroscopy in the extreme UV (XUV) is developed as a new technique for following bandgap\ndynamics after photoexcitation. Signatures of carrier and lattice dynamics are obsd. through the measurement of\nreflectivity changes in the XUV of single-cryst. germanium after exciting the sample with a sub-5 fs visible-to-IR pulse. In\nthe expt., a 5 fs near IR (NIR) pulse excites carriers across the band gap. The subsequent dynamics are probed by\nmeasuring the transient reflectivity changes at the Ge M_(4,5) edge around 30 eV, corresponding to transitions from the 3d\ncore levels to the unoccupied conduction and valence band states. The energetically sepd. spectral signatures of\nphotoexcited electrons and holes in the XUV enable to measure the thermalization dynamics of the carriers. Importantly,\nshifts in photon energy of the electron and hole features allow real time tracking of the energy sepn. of valence and\nconduction bands change over time. The dynamics obsd. in the expts. suggest dynamics on different time scales.\nWithin 500fs, decays and energetic shifts of the obsd. reflectivity changes occur, which are assigned to electron-electron\nand electron-optical phonon scattering thermalizing the photoexcited carrier distribution. Within 2 ps, a blue shift of the\nvalence band features is obsd., whereas a red shift of the conduction band features is present. This striking observation\nis assigned to a renormalization of the band gap due to the lattice expansion of the crystal. This band-gap\nrenormalization sets in once momentum of the excited carriers has been transferred to phonons by electron-phonon and\nphonon-phonon scattering. Transient reflectivity is shown to overcome the thin-film and x-ray membrane related issues\nof transmission geometries. It will be shown how transient reflectivity can further be employed to ext. the time-dependent\ncomplex dielec. function over the timescales governing carrier-carrier, carrier-phonon, and phonon-phonon interactions.", "publisher": "Caltech Library", "publication_date": "2017-04" } ]