In this work, we put forward a rigorous study on ultraviolet (355-nm) laser irradiation of polyimide for the realization of high-quality laser-induced graphene (LIG) with micron-scale features. High-quality material and micron-scale features are desirable—but often at odds—given that small features demand tightly focused beam spots, with a predisposition to ablation. As such, we investigate the synthesis of LIG by correlating the material characteristics, as gleaned from scanning electron microscopy and Raman spectroscopy, to the incident optical fluence, as a measure of applied optical energy per unit area. The study reveals that high-quality LIG, with ratios of Raman 2D-to-G peak heights approaching 0.7, can be synthesized with micron-scale features, down to 18 ± 2 μm, given suitable attention to the optical fluence. Optimal characteristics are seen at optical fluences between 40 and 50 J/cm2, which promote graphenization and minimize ablation. It is hoped that these findings will lay a foundation for the application of LIG in future integrated technologies.
In this work, we introduce an electro-absorption (EA)-based retro-modulator for effective realization of free-space optical communications via passive downlinks. Demands for deep modulation and broad directionality in such links are met by its corner-cube assembly of EA-modulators. The EA-modulators use semi-insulating InP as its band edge absorption exhibits an Urbach tail near the 980-nm wavelength of the laser light. This enables Urbach-edge-assisted EA, which allows the field-induced absorption to be optimized via temperature. The theory, from a uniting of the Einstein model and Franz–Keldysh effect, and experiments, from a prototype, show good agreement with deep (greater than 15%) modulation depths. Such functionality can meet the key demands of emerging free-space optical communication links.
The foundational Franz-Keldysh effect and Einstein model are applied in this work to characterize semiconductor band-edge absorption-and its departures from ideality. We unify the Franz-Keldysh and Einstein models to fully characterize the field-induced tunneling of photoexcited electrons from degenerate valence bands into the conduction band, with encroachment into the band gap arising as an Urbach tail. Our unified model is implemented for semi-insulating indium phosphide (SI-InP) with strong agreement seen between the theoretical and experimental results for varied photon energies and electric fields.
The proposed work introduces time-captured Raman and terahertz spectroscopic analyses as orthogonal probes of intramolecular and intermolecular modes in biomolecular structures. The work focuses on glucose given the complexity and dynamics of its anomeric conversion and crystallization. The Raman analyses capture the dynamics of its intramolecular modes – revealing conversion between α and β anomers. At the same time, the terahertz analyses capture the dynamics of its intermolecular modes – showing an evolution from amorphous to crystalline morphology. It is shown that time-captured Raman and terahertz spectroscopy together render a more complete depiction, and deeper understanding, of the biomolecular structure of glucose.
In this work, we explore the band edge absorption characteristics of semiconductors as applied to optoelectronic modulation—with careful consideration to the departures from ideality in the semiconductors. To this end, we develop a rigorous model of electroabsorption in semiconductors that characterizes the electric-field-induced constriction/narrowing of the bandgap and the resulting increase in absorption of photons, whose energies are slightly below the bandgap energy. The model unifies the Franz-Keldysh effect, characterizing the electric-field-induced tunneling of photoexcited electrons from valence band states to conduction band states, and the Einstein model, quantifying the encroachment of valence and conduction band states into the bandgap. Careful consideration is given here to the nonidealities in the semiconductor, which arise within the valence band as degenerate states, due to light and heavy holes, and within the bandgap, as encroaching Urbach tail states. We apply the model in characterizing optoelectronic modulation of 980-nm photons with semi-insulating indium phosphide (SI-InP), and we see strong agreement between our theoretical and experimental results over a wide range of electric fields and photon energies. Ultimately, the findings show that optoelectronic modulation can be had with large modulation depths over short propagation lengths through the semiconductor. This opens the door to highly effective implementations of optoelectronic modulators in emerging free-space optical communication systems—given that such modulators do not allow for prolonged (guided-wave) propagation and have thus exhibited small modulation depths.
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