Transition metal oxides such as TiO2 and CeO2 are wide bandgap semiconductors with photocatalytic properties used to drive electrical currents and enhance electrochemical reactions. The utility of wide bandgap semiconductors can be extended from the UV and into the visible by embedding metal nanoparticles into the semiconductor. Excitation of localized surface plasmon resonances in the metal nanoparticle generates free electrons that can be injected into the semiconductor and extend the photoactivity range using lower energy photons. In this work, ultrafast transient absorption spectroscopy is used to investigate the hot electron injection from Cu nanoparticles photodeposited onto a CeO2 aerogel.
Molecular vibrations can couple to optical cavities to create new hybrid states called polaritons. The magnitude of this coupling, measured as the vacuum Rabi splitting (Ω), correlates with modified materials processes such as photon emission, molecular energy transfer, and chemical reaction rates. In this talk, I will first discuss active control of cavity coupling strength. We demonstrate active tuning of excitonic strong coupling in a system where organic dyes strongly couple to propagating surface plasmon polaritons (SPPs) and modulation of vibrational strong coupling in a Fabry-Perot cavity coupled to an organic charge shuttling molecule. Next, I will discuss results indicating modified chemical reaction rates for an alcoholysis addition reaction forming urethane monomers. Cavity tuning was used to selectively couple to reactant, solvent, and product vibrational modes resulting in a chemical response that is cavity tuning dependent. Lastly, and in light of the search for an understanding of the mechanisms leading to modified chemical and physical properties, I will present a theoretical description of the density of polariton states relative to molecular dark states. This work will also discuss the differences between polariton states generated in microcavities, slabs, and in the bulk.
2D perovskites have broad technological appeal because of their tunable mechanical, optical, and electrical properties. For flexible optoelectronic applications, it is necessary to determine how mechanical stresses affect their optoelectronic properties. We compare the impact of strain on the photoluminescence (PL) spectra and charge carrier recombination rates of two different 2D perovskite materials, synthesized using either phenethylammonium or butylammonium cations. Both perovskite materials exhibit strong PL enhancement, redshifts of the PL emission wavelength, and longer recombination lifetimes for compressive strains of ≲1%. These results are discussed in relation to the materials’ band structures and trap states.
2D perovskites, consisting of alternating layers of metal halide sheets and cations, tend to be more environmentally robust compared to their bulk 3D counterpart and have broad technological appeal because of their tunable mechanical, optical, and electrical properties. While these materials have promise for flexible optoelectronic applications, it is necessary to determine the impact of strain on the perovskite optical and electronic properties. Here, we discuss our work in understanding how strain modifies the carrier dynamics of 2D perovskites using time resolved spectroscopy. We compare the photoluminescence lifetime of two different 2D perovskite materials, synthesized using either phenethylammonium or butylammonium cations. Both perovskite materials exhibit about a 50% decrease in the lifetime for tensile strains <1%. The decrease in the photoluminescence lifetime, indicating a decrease in the charge carrier lifetime, is discussed in relation the materials defect states and bands
Vibration-cavity polaritons, which are produced by strong coupling between an optical cavity and a molecular vibration, can modify chemical reaction rates and branching ratios. However the observed effects are poorly understood. To gain insight into how these polaritons might alter molecular processes, we used ultrafast pump-probe and two-dimensional infrared spectroscopies to characterize polariton excited state dynamics. Our earlier studies on vibration-cavity polaritons with tungsten hexacarbonyl demonstrated that much of the response is due to so-called reservoir or uncoupled excited state absorption as well as polariton contraction. In recent studies, we have used 2D IR and spectrally filtered pump-probe studies on the nitroprusside anion in methanol to determine the transition frequencies and dynamics of polariton excited states allowing us to extract polariton dephasing timescales as well as incoherent polariton population which at a significantly longer timescale.
Materials with adaptable properties could impact optoelectronics (tunable sensors or filters) and chemical reactivity (triggered reactivity). It is widely known that strong material absorptions resonant with an optical cavity can lead to the formation of new hybrid light-matter states called polaritons. Strikingly, cavity-modified material properties (e.g., electrical conductivity, optical emission/absorption, chemical reaction rates and branching ratios) have been demonstrated and, the degree to which they are modified, shown to depend on the energy positions of these new hybrid states. Our work shows real-time tuning of these states through electrochemical cycling and optical excitation of the coupled species.
The mid-infrared spectra of many polar materials are dominated by highly reflective reststrahlen bands that occur between the transverse and longitudinal optical phonons. Within the reststrahlen bands, light can couple with optical phonons to support phonon-polariton modes. These modes enhance light-matter interactions through the concentration of light to nanoscale dimensions, and therefore, are particularly promising for mid-infrared nanophotonic applications. Here, we discuss our work on expanding the spectral range over which phonon-polaritons are supported by using new material systems, as well as active tuning of the modes via carrier photoinjection. In particular, we report on the confinement of hyperbolic phonon-polaritons in calcite, a ubiquitous polar material. We also report the use of the LO-phonon-plasmon-coupling (LOPC) effect to actively tune the Berreman mode of a GaN thin film.
The high optical losses of metal-based plasmonic materials have driven an extensive search for alternative lower-loss materials that can support plasmonic-like effects, such as sub-diffraction confinement of optical fields. One such alternative employs phonon-mediated collective-charge oscillations (surface phonon polaritons, SPhPs) that can be optically excited in nanostructured polar dielectric materials. Similar to plasmonics, tailoring the geometry of polar-dielectric resonators results in resonances that can be spectrally tuned throughout the spectral range between the LO and TO phonons. However, generally, the spectral position and amplitude of these resonances remain fixed after sample fabrication. In this presentation, we discuss recent advancements made by our group in achieving actively tunable localized SPhP resonances in the long-wave- and far-infrared spectral regimes. In particular, we focus on three experiments that demonstrate active modulation of resonances. The first and second experiments focus on tuning the spectral position of localized SPhP resonances in cylindrical nanopillars that are etched into indium phosphide and silicon carbide substrates. In both of these cases we are able to induce resonance shifts as large as 15 cm-1 by optically injecting free-carriers into the pillars. The optical injection introduces a reversible, free-carrier perturbation to the dielectric permittivity that results in a spectral shift of the resonances. While the effects investigated for both the InP and SiC systems are similar, each material allows us to explore a different aspect of the phenomena. For InP we investigate the effects in the far-infrared (303-344 cm-1) with steady-state carrier photoinjection, while for SiC we investigate the dynamics of frequency modulated resonances in the long-wave infrared (797-972 cm-1) via transient reflection spectroscopy. Lastly, in the third experiment we demonstrate the ability to modulate the amplitude of resonances by coating SiC nanopillars with vanadium dioxide, a well-known phase change material that undergoes a metal-to-insulator transition near a temperature of 70 C. As such, we show that by exploiting this phase change we are able to modulate the reflectance and thermal emission of nanopillar arrays. The results described in this work may open the door to tunable, narrow-band thermal sources that operate in the long-wave to far-infrared spectral regimes.
We will focus on approaches which make use of light-matter interactions to alter the chemical behavior of a target molecular species. This is done through cavity coupling to a molecular vibration. Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations. This approach is a viable method to explore controlling chemical reactivity and energy relaxation. Here, we demonstrate frequency domain results for vibrational bands strongly coupled to optical cavities. We experimentally and numerically describe strong coupling between a Fabry-Pérot cavity and several molecular species (e.g., poly-methylmethacrylate, thiocyanate, hexamethyl diisocyanate). We investigate strong and weak coupling regimes through examination of cavities loaded with varying concentrations of a urethane monomer. Rabi splittings are in excellent agreement with an analytical description using no fitting parameters. We show that coupling strength is a function of molecule/cavity mode overlap by systematically altering the position of a molecular slab throughout a first order cavity with results agreeing well with analytical and transfer matrix predictions. Further, remote molecule-molecule interaction will be explored by placing discrete and separated molecular layers throughout a cavity. In addition to establishing that coupling to an optical cavity modifies the energy levels accessible to the coupled molecules, this work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied in steady state and dynamically.
Transient absorption spectra were measured to demonstrate carrier injection in multi-layered stacks of Au nanoparticles sandwiched in between TiO2 atomic layer deposited (ALD) thin films. Similar structures were fabricated with ALD Al2O3 for control samples. Sub-percolation thin films of Au resulted in <20 nm particles with plasmon resonances at ~650 nm (~590 nm) in the TiO2 (Al2O3) samples. Two separate pump-probe experiments were preformed to monitor transient heating of the metal and carrier injection in the TiO2. In the first experiment, the metal nanoparticles were excited at 400 nm, and the metal electron dynamics were probed at wavelengths around the plasmon resonance. We measured a decay time of ~1.7 ps in the TiO2-Au layered samples compared to ~2.2 ps in the Al2O3-Au layered samples. The decay times are attributed to electron-phonon coupling. The faster decay in TiO2 may be the result of charge injection into the TiO2. In the second experiment, carriers were excited in the Au nanoparticles by pumping on the plasmon resonance, and the system was probed in the mid-IR to measure free carrier absorption in the TiO2. The TiO2-Au layered sample exhibited transient signals similar to the free carrier absorption signals following excitation of TiO2 films, however, no signal was observed on the Al2O3-Au layered sample. This provides clear evidence that the signal measured in the TiO2-Au layered sample was not from the Au nanoparticles alone but instead originated from charge injection from the Au into the TiO2.
Coherent coupling between an optical transition and confined optical mode have been investigated for electronic-state transitions, however, only very recently have vibrational transitions been considered. Here, we demonstrate both static and dynamic results for vibrational bands strongly coupled to optical cavities. We experimentally and numerically describe strong coupling between a Fabry-Pérot cavity and carbonyl stretch (~1730 cm 1) in poly-methylmethacrylate and provide evidence that the mixed-states are immune to inhomogeneous broadening. We investigate strong and weak coupling regimes through examination of cavities loaded with varying concentrations of a urethane monomer. Rabi splittings are in excellent agreement with an analytical description using no fitting parameters.
Ultrafast pump-probe measurements reveal transient absorption signals over a frequency range well-separated from the vibrational band, as well as drastically modified relaxation rates. We speculate these modified kinetics are a consequence of the energy proximity between the vibration-cavity polariton modes and excited state transitions and that polaritons offer an alternative relaxation path for vibrational excitations. Varying the polariton energies by angle-tuning yields transient results consistent with this hypothesis. Furthermore, Rabi oscillations, or quantum beats, are observed at early times and we see evidence that these coherent vibration-cavity polariton excitations impact excited state population through cavity losses.
Together, these results indicate that cavity coupling may be used to influence both excitation and relaxation rates of vibrations. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied in steady state and dynamically.
Coherent coupling between an optical-transition and confined optical mode, when sufficiently strong, gives rise to new modes separated by the vacuum Rabi splitting. Such systems have been investigated for electronic-state transitions, however, only very recently have vibrational transitions been considered. Here, we bring strong polaritonic-coupling in cavities from the visible into the infrared where a new range of static and dynamic vibrational processes await investigation.
First, we experimentally and numerically describe coupling between a Fabry-Perot cavity and carbonyl stretch (~1730 cm 1) in poly-methylmethacrylate. As is requisite for “strong coupling”, the measured vacuum Rabi splitting of 132 cm 1 is much larger than the full width of the cavity (34 cm-1) and the inhomogeneously broadened carbonyl-stretch (24 cm-1). Agreement with classical theories providea evidence that the mixed-states are relatively immune to inhomogeneous broadening. Next, we investigate strong and weak coupling regimes through examination of cavities loaded with varying concentrations of urethane. Rabi splittings increases from 0 to ~104 cm-1 with concentrations from 0-20 vol% and are in excellent agreement to an analytical description using no fitting parameters. Ultra-fast pump-probe measurements reveal transient absorption signals over a frequency range well-separated from the vibrational band as well as modifications of energy relaxation times. Finally, we demonstrate coupling to liquids using the C-O stretching band (~1985 cm-1) of Mo(CO)6 in an aqueous solution.
Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied statically and dynamically.
Sub-diffractional confinement of light has led to advancements in imaging, metamaterials, nano-manufacturing, plasmonics, and other fields. One potential route to sub-diffractional confinement is via stimulated surface phonon polaritons (SPhPs). SPhPs couple infrared photons with optical phonons and consequently their lifetimes can be longer than surface plasmon polaritons (SPPs), whose lifetimes are dominated by electron scattering events. Thus, materials capable of generating SPhPs are of general interest to study. SPhPs are activated by photons with energies near the Reststrahlen band of semiconductors such as SiC. In this work we examine aspects of carrier dynamics by photo-injecting electrons into the SiC conduction band using a pulsed 355 nm laser and probe the resulting dynamics near the Reststrahlen band using a tunable CO2 laser. The fluence of the pump laser was varied to provide photo-injection levels ranging from ~1x10^17 to 1x10^19 free carriers. Probing the excited-state dynamics near the blue-edge of the Reststrahlen band resulted in complex transient behavior, showing both positive and negative changes in transient reflectance depending on the level of photo-injected carriers and probe energy. Numerical calculations of the SiC reflectance spectra with different doping levels were done to simulate the initial photo-injection level provided by the transient experiment. The computed spectra and the experimentally measured excited spectra for different photo-injection levels were compared and resulted in qualitative agreement.
A data fusion-based, multisensory detection system, called "Volume Sensor", was developed under the Advanced
Damage Countermeasures (ADC) portion of the US Navy's Future Naval Capabilities program (FNC) to meet reduced
manning goals. A diverse group of sensing modalities was chosen to provide an automated damage control monitoring
capability that could be constructed at a relatively low cost and also easily integrated into existing ship infrastructure.
Volume Sensor employs an efficient, scalable, and adaptable design framework that can serve as a template for
heterogeneous sensor network integration for situational awareness. In the development of Volume Sensor, a number of
challenges were addressed and met with solutions that are applicable to heterogeneous sensor networks of any type.
These solutions include: 1) a uniform, but general format for encapsulating sensor data, 2) a communications protocol
for the transfer of sensor data and command and control of networked sensor systems, 3) the development of event
specific data fusion algorithms, and 4) the design and implementation of modular and scalable system architecture. In
full-scale testing on a shipboard environment, two prototype Volume Sensor systems demonstrated the capability to
provide highly accurate and timely situational awareness regarding damage control events while simultaneously
imparting a negligible footprint on the ship's 100 Mbps Ethernet network and maintaining smooth and reliable
operation in a real-time fashion.
Nonlinear gating of cw infrared lasers using ultrafast dye lasers permits a versatile infrared probe for
investigating photochemical and vibrational dynamics of various molecules in condensed phases. Specific examples are
presented for studies of optically excited bacteriorhodopsin, hemoglobin and myoglobin, as well as infrared excited ions
and metal carbonyls.
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