The computation of Q-switched laser pulses

Two methods for computing the single pulse of three- and four-level lasers with instantaneous Q-switching are described. The first method makes it possible to calculate the energy, power, and duration of the pulse. The second method, based on the approximation of the population inversion by means of suitable functions, yields its shape. The analytical solutions are compared with those obtained on an electronic digital computer. The optimal losses and limits of applicability of the formulas derived in the case of noninstantaneous Q-switching are determined.The authors are deeply grateful to B. I. Stepanov for his valuable suggestions.     

Coherent excitation of molecular vibrational modes by high-power ultrashort infrared laser pulses

Coherent dynamics of multiphoton excitation of molecular vibrational modes by subpicosecond IR laser pulses differs greatly from that of picosecond pulse excitation. The resonance response of a molecule is primarily determined by the power broadening rather than the laser carrier frequency. Selective excitation of high vibrational levels is possible with the use of subpicosecond pulses.     

Role of the Anion on the Transport and Structure of Organic Mixed Conductors

Organic mixed conductors are increasingly employed in electrochemical devices operating in aqueous solutions that leverage simultaneous transport of ions and electrons. Indeed, their mode of operation relies on changing their doping (oxidation) state by the migration of ions to compensate for electronic charges. Nevertheless, the structural and morphological changes that organic mixed conductors experience when ions and water penetrate the material are not fully understood. Through a combination of electrochemical, gravimetric, and structural characterization, the effects of water and anions with a hydrophilic conjugated polymer are elucidated. Using a series of sodium‐ion aqueous salts of varying anion size, hydration shells, and acidity, the links between the nature of the anion and the transport and structural properties of the polymer are systematically studied. Upon doping, ions intercalate in the crystallites, permanently modifying the lattice spacings, and residual water swells the film. The polymer, however, maintains electrochemical reversibility. The performance of electrochemical transistors reveals that doping with larger, less hydrated, anions increases their transconductance but decreases switching speed. This study highlights the complexity of electrolyte‐mixed conductor interactions and advances materials design, emphasizing the coupled role of polymer and electrolyte (solvent and ion) in device performance.     

The Effect of Alkyl Spacers on the Mixed Ionic-Electronic Conduction Properties of N-Type Polymers

Conjugated polymers with mixed ionic and electronic transport are essential for developing the complexity and function of electrochemical devices. Current n-type materials have a narrow scope and low performance compared with their p-type counterparts, requiring new molecular design strategies. This work presents two naphthalene diimide-bithiophene (NDI-T2) copolymers functionalized with hybrid alkyl-glycol side chains, where the naphthalene diimide unit is segregated from the ethylene glycol (EG) units within the side chain by an alkyl spacer. Introduction of hydrophobic propyl and hexyl spacers is investigated as a strategy to minimize detrimental swelling close to the conjugated backbone and balance the mixed conduction properties of n-type materials in aqueous electrolytes. It is found that both polymers functionalized with alkyl spacers outperform their analogue bearing EG-only side chains in organic electrochemical transistors (OECTs). The presence of the alkyl spacers also leads to remarkable stability in OECTs, with no decrease in the ON current after 2 h of operation. Through this versatile side chain modification, this work provides a greater understanding of the structure-property relationships required for n-type OECT materials operating in aqueous media.     

Flexible Organic Photovoltaic Cells with In Situ Nonthermal Photoreduction of Spin‐Coated Graphene Oxide Electrodes

The first reduction methodology, compatible with flexible, temperature‐sensitive substrates, for the production of reduced spin‐coated graphene oxide (GO) electrodes is reported. It is based on the use of a laser beam for the in situ, non‐thermal, reduction of spin‐coated GO films on flexible substrates over a large area. The photoreduction process is one‐step, facile, and is rapidly carried out at room temperature in air without affecting the integrity of the graphene lattice or the flexibility of the underlying substrate. Conductive graphene films with a sheet resistance of as low as 700 Ω sq^?1 and transmittance of 44% can be obtained, much higher than can be achieved for flexible layers reduced by chemical means. As a proof of concept of our technique, laser‐reduced GO (LrGO) films are utilized as transparent electrodes in flexible, bulk heterojunction, organic photovoltaic (OPV) devices, replacing the traditional ITO. The devices displayed a power‐conversion efficiency of 1.1%, which is the highest reported so far for OPV device incorporating reduced GO as the transparent electrode. The in situ non‐thermal photoreduction of spin‐coated GO films creates a new way to produce flexible functional graphene electrodes for a variety of electronic applications in a process that carries substantial promise for industrial implementation.