Time-resolved optical studies of colossal magnetoresistance and charge -density wave materials

This thesis presents measurements of collective modes and ultrafast carrier relaxation dynamics in charge-density-wave (CDW) conductors and colossal magnetoresistance (CMR) manganites. A femtosecond laser pump pulse excites a broad frequency spectrum of low-energy collective modes and electron-hole pairs thereby changing its optical properties. The low-energy collective excitations and quasiparticle relaxation and recombination processes are monitored by measuring the resulting photoinduced absorption as a function of probe pulse wavelength and time delay.;A general model was developed for the photogeneration and detection mechanism of collective modes based on light absorption in two-color pump-probe experiments. A broad spectrum of collective modes (phasons and amplitudons) with frequencies down to a few GHz is excited and propagates normal to the surface into the material. The dispersion of the long-wavelength phason and amplitudon can be measured by changing the probe wavelength.;The first pump-probe spectroscopy was performed from the ultraviolet to mid-infrared wavelength range to study low-frequency collective excitations, including temperature evolution, dispersion, damping, and anisotropy of amplitude mode and transverse phason in quasi-one dimensional CDW conductors, K 0.3MoO3 and K0.33MoO3 on ultrafast time scale. The transverse phason exhibits an acoustic-like dispersion relation in the frequency range from 5--40 GHz. The phason velocity is strongly anisotropic with a very weak temperature dependence. In contrast, the amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.66 THz at 30 K.;The studies were extended to doped perovskite manganite thin films and single crystals. A low-energy collective mode is observed and discussed in terms of the opening of a pseudogap resulting from charge/orbital ordering phases. The softening of the collective mode is necessary to explain by combining a cooperative Jahn-Teller type distortion of the MnO6 octahedra with the collective mode. The quasiparticle dynamics in the vicinity of the metal-insulator transition is strongly affected by the presence of a pseudogap, phase separation and percolation, which are strongly dependent on temperature. A very long-lived relaxation process is observed due to a slow spin relaxation process. The dynamics of the spin system is further investigated in strained and unstrained thin films, which show a strong strain effect

Ultrafast Collective Dynamics in the Charge-Density-Wave Conductor K0.3_{0.3}MoO3_{3}

Low-energy coherent charge-density wave excitations are investigated in blue bronze (K$_{0.3}$MoO$_{3}$) and red bronze (K$_{0.33}$MoO$_{3}$) by femtosecond pump-probe spectroscopy. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K$_{0.33}$MoO$_{3}$. The amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.Low-energy coherent charge-density wave excitations are investigated in blue bronze (K$_{0.3}$MoO$_{3}$) and red bronze (K$_{0.33}$MoO$_{3}$) by femtosecond pump-probe spectroscopy. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K$_{0.33}$MoO$_{3}$. The amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.Comment: 10 pages, 4 figure

Energy barrier at the N719-dye/CsSnI3 interface for photogenerated holes in dye-sensitized solar cells

This report is to address the question if black γ-polymorph of cesium tin tri-iodide (B-γ-CsSnI3) can be used as a solid-state hole-transport material in the conventional DSSCs with the N719 dye to replace the liquid electrolyte as reported by I. Chung et al. on Nature 485, 486, (2012). Here we demonstrate rigorously that B-γ-CsSnI3 is not energetically possible to collect photogenerated holes because of the large energy barrier at the interface of N719/B-γ-CsSnI3. Therefore, it cannot serve as a hole-transporter for the conventional DSSCs although it is a good hole-conducting material. A solution-based method was employed to synthesize the B-γ-CsSnI3 polycrystalline thin-films used for this work. These thin-films were then characterized by X-ray diffraction, Hall measurements, optical reflection, and photoluminescence (PL). Particularly, spatially resolved PL intensity images were taken after B-γ-CsSnI3 was incorporated in the DSSC structure to insure the material integrity. The means of ultraviolet photoemission spectroscopy (UPS) was used to reveal why B-γ-CsSnI3 could not act as the substitute of liquid electrolyte in the conventional DSSCs. For the completeness, other two related compounds, one is the yellow polymorph of CsSnI3 and other is Cs2SnI6 with tetravalent tin instead of double-valent tin in CsSnI3 were also investigated by UPS

Microstructural Origin of the High-Energy Storage Performance in Epitaxial Lead-Free Ba(Zr 0.2 Ti 0.8 )O 3 Thick Films

In our previous work, epitaxial Ba(Zr 0.2 Ti 0.8 )O 3 thick films (~1–2 μ m) showed an excellent energy storage performance with a large recyclable energy density (~58 J/cc) and a high energy efficiency (~92%), which was attributed to a nanoscale entangled heterophase polydomain structure. Here, we propose a detailed analysis of the structure–property relationship in these film materials, using an annealing process to illustrate the effect of nanodomain entanglement on the energy storage performance. It is revealed that an annealing-induced stress relaxation led to the segregation of the nanodomains (via detailed XRD analyses), and a degraded energy storage performance (via polarization-electric field analysis). These results confirm that a nanophase entanglement is an origin of the high-energy storage performance in the Ba(Zr 0.2 Ti 0.8 )O 3 thickfilms

Mechanics Design for Stretchable, High Areal Coverage GaAs Solar Module on an Ultrathin Substrate,”

The trench design of substrate together with curvy interconnect formed from buckling provides a solution to stretchable electronics with high areal coverage on an ultrathin substrate, which are critically important for stretchable photovoltaics. In this paper, an improved trench design is proposed and verified by finite element analysis (FEA), through use of a heterogeneous design, to facilitate strain isolation and avoid possible fracture/delamination issue. A serpentine design of interconnect is also devised to offer $440$90%

Broadband nonlinear modulation of incoherent light using a transparent optoelectronic neuron array

Nonlinear optical processing of ambient natural light is highly desired in computational imaging and sensing applications. A strong optical nonlinear response that can work under weak broadband incoherent light is essential for this purpose. Here we introduce an optoelectronic nonlinear filter array that can address this emerging need. By merging 2D transparent phototransistors (TPTs) with liquid crystal (LC) modulators, we create an optoelectronic neuron array that allows self-amplitude modulation of spatially incoherent light, achieving a large nonlinear contrast over a broad spectrum at orders-of-magnitude lower intensity than what is achievable in most optical nonlinear materials. For a proof-of-concept demonstration, we fabricated a 10,000-pixel array of optoelectronic neurons, each serving as a nonlinear filter, and experimentally demonstrated an intelligent imaging system that uses the nonlinear response to instantly reduce input glares while retaining the weaker-intensity objects within the field of view of a cellphone camera. This intelligent glare-reduction capability is important for various imaging applications, including autonomous driving, machine vision, and security cameras. Beyond imaging and sensing, this optoelectronic neuron array, with its rapid nonlinear modulation for processing incoherent broadband light, might also find applications in optical computing, where nonlinear activation functions that can work under ambient light conditions are highly sought.Comment: 20 Pages, 5 Figure