Efficiency estimates and practical aspects of an optical Kerr gate for time-resolved luminescence spectroscopy.

We report experience of assembling an optical Kerr gate setup at the Femtosecond Laser Center for collective use at the Institute of Physics of the National Academy of Sciences of Ukraine. This offers an inexpensive solution to the problem of time-resolved luminescence spectroscopy. Practical aspects of its design and alignment are discussed and its main characteristics are evaluated. Theoretical analysis and numerical estimates are performed to evaluate the efficiency and the response time of an optical Kerr gate setup for fluorescence spectroscopy with subpicosecond time resolution. The theoretically calculated efficiency is compared with the experimentally measured one of ~12% for Crown 5 glass and ~2% for fused silica. Other characteristics of the Kerr gate are analyzed and ways to improve them are discussed. A method of compensation for the refractive index dispersion in a Kerr gate medium is suggested. Examples of the application of the optical Kerr gate setup for measurements of the time-resolved luminescence of Astra Phloxine and Coumarin 30 dyes and both linear and nonlinear chirp parameters of a supercontinuum are presented.

Double heterojunction nanowire photocatalysts for hydrogen generation.

Charge separation and charge transfer across interfaces are key aspects in the design of efficient photocatalysts for solar energy conversion. In this study, we investigate the hydrogen generating capabilities and underlying photophysics of nanostructured photocatalysts based on CdSe nanowires (NWs). Systems studied include CdSe, CdSe/CdS core/shell nanowires and their Pt nanoparticle-decorated counterparts. Femtosecond transient differential absorption measurements reveal how semiconductor/semiconductor and metal/semiconductor heterojunctions affect the charge separation and hydrogen generation efficiencies of these hybrid photocatalysts. In turn, we unravel the role of surface passivation, charge separation at semiconductor interfaces and charge transfer to metal co-catalysts in determining photocatalytic H2 generation efficiencies. This allows us to rationalize why Pt nanoparticle decorated CdSe/CdS NWs, a double heterojunction system, performs best with H2 generation rates of ∼434.29 ± 27.40 µmol h(-1) g(-1) under UV/Visible irradiation. In particular, we conclude that the CdS shell of this double heterojunction system serves two purposes. The first is to passivate CdSe NW surface defects, leading to long-lived charges at the CdSe/CdS interface capable of carrying out reduction chemistries. Upon photoexcitation, we also find that CdS selectively injects charges into Pt NPs, enabling simultaneous reduction chemistries at the Pt NP/solvent interface. Pt nanoparticle decorated CdSe/CdS NWs thus enable reduction chemistries at not one, but rather two interfaces, taking advantage of each junction's optimal catalytic activities.

Supercontinuum spatial modulation spectroscopy: detection and noise limitations.

Supercontinuum spatial modulation spectroscopy is a facile tool for conducting single molecule/particle extinction spectroscopy throughout the visible and near infrared (420-1100 nm). The technique's capabilities are benchmarked using individual Au nanoparticles (NPs) as a standard since they are well studied and display a prominent plasmon resonance in the visible. Extinction spectra of individual Au NPs with diameters (d) ranging from d ~ 8 to 40 nm are resolved with extinction cross sections (σ(ext)) of σ(ext) ~ 1 × 10(-13)-1 × 10(-11) cm(2). Corresponding signal-to-noise ratios range from ~30 to ~1400. The technique's limit of detection is determined to be 4.3 × 10(-14) cm(2) (4.3 nm(2)). To showcase supercontinuum spatial modulation spectroscopy's broader applicability, extinction spectra are acquired for other model systems, such as individual single-walled carbon nanotubes (SWCNTs) and CdSe nanowires. We show for the first time extinction spectra of individual (8,3) and (6,5) SWCNTs. For both chiralities, their E11 [(8,3) 1.30 eV (952 nm); (6,5) 1.26 eV (986 nm)] and E22 [(8,3) 1.86 eV (667 nm); (6,5) 2.19 eV (567 nm)] excitonic resonances are seen with corresponding cross sections of σ(ext) ~ 10(-13) cm(2) µm(-1).