Publications by authors named "Conrad A Kocoj"

Article Synopsis
  • Detection of mid-infrared (MIR) light is vital for various technologies like night vision and thermal imaging, yet traditional methods often require complex setups or cooling.
  • This study introduces a novel approach using two-dimensional metal halide perovskites (2D-MHPs) that enables high-sensitivity detection of MIR light at room temperature, with capabilities down to 1 nanowatt per square micrometer.
  • The technology achieves further sensitivity improvements using unique membrane structures and photonic strategies, covering a range of infrared wavelengths from 2 to 10.6 micrometers, paving the way for advancements in areas like environmental monitoring and molecular sensing.
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Intermolecular distance largely determines the optoelectronic properties of organic matter. Conventional organic luminescent molecules are commonly used either as aggregates or as single molecules that are diluted in a foreigner matrix. They have garnered great research interest in recent decades for a variety of applications, including light-emitting diodes, lasers and quantum technologies, among others.

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In two-dimensional chiral metal-halide perovskites, chiral organic spacers endow structural and optical chirality to the metal-halide sublattice, enabling exquisite control of light, charge, and electron spin. The chiroptical properties of metal-halide perovskites have been measured by transmissive circular dichroism spectroscopy, which necessitates thin-film samples. Here, by developing a reflection-based approach, we characterize the intrinsic, circular polarization-dependent complex refractive index for a prototypical two-dimensional chiral lead-bromide perovskite and report large circular dichroism for single crystals.

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Advancement toward dye-sensitized photoelectrochemical cells to produce solar fuels by solar-driven water splitting requires a photosensitizer that is firmly attached to the semiconducting photoelectrodes. Covalent binding enhances the efficiency of electron injection from the photoexcited dye into the metal oxide. Optimization of charge transfer, efficient electron injection, and minimal electron-hole recombination are mandatory for achieving high efficiencies.

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In recent years, optical pump-probe microscopy (PPM) has become a vital technique for spatiotemporally imaging electronic excitations and charge-carrier transport in metals and semiconductors. However, existing methods are limited by mechanical delay lines with a probe time window up to several nanoseconds (ns) or monochromatic pump and probe sources with restricted spectral coverage and temporal resolution, hindering their amenability in studying relatively slow processes. To bridge these gaps, we introduce a dual-hyperspectral PPM setup with a time window spanning from nanoseconds to milliseconds and single-nanosecond resolution.

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Alkali metals have low optical losses in the visible to near-infrared (NIR) compared with noble metals. However, their high reactivity prohibits the exploration of their optical properties. Recently sodium (Na) has been experimentally demonstrated as a low-loss plasmonic material.

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Two-dimensional organic-inorganic hybrid perovskites (2DHPs) are natural quantum-well-like materials, in which strong quantum and dielectric confinement effects due to the organic spacers give rise to tightly bound excitons with large binding energy. To examine the mutual interactions between the organic spacer cations and the inorganic charge-residing octahedral framework in 2DHPs, here we perform femtosecond pump-probe spectroscopy by direct vibrational pumping of the organic spacers, followed by a visible-to-ultraviolet probe covering their excitonic resonances. Measurements on prototypical lead-bromide based 2DHP compounds, (BA)_{2}PbBr_{4} and (BA)_{2}(FA)Pb_{2}Br_{7} (BA^{+}=butylammonium; FA^{+}=formamidinium), reveal two distinct regimes of the temporal response.

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