Plasmonic Hot-Electron Injection Driving Ultrafast Phase Transition in Self-Supported VO₂ Films for All-Optical Modulation

VO2 has been widely used in optical modulation devices due to its large change in permittivity across its first-order metal–semiconductor phase transition. People have proved that VO2’s phase transition can be driven by plasmonic hot-electron injection, providing a new approach to realizing ultrafast all-optical devices with a low pump fluence. Here, we report on ultrafast VO2 all-optical modulation with a dramatically low pump fluence utilizing the composite structure of a self-supported VO2 film on a gold nanoshell grating. Plasmonic hot electrons in the gold excited by femtosecond pulses are injected into the VO2 film and trigger its phase transition, reducing the pump fluence threshold for the structural transformation to only 147.8 μJ/cm2. With a pump fluence above this threshold, ultrafast phase transition can be triggered within 1 ps, and a high modulation depth of 50% can be achieved. Moreover, an ultrafast modulation with an on–off time of 650 fs can also be achieved using a pump fluence below the threshold. This work may explore new applications of plasmonic hot-electron-driven phase transition in optical modulation devices and configurable devices

Disparity in Optical Charge Generation and Recombination Processes in Upright and Inverted PbS Quantum-Dot Solar Cells

The role of optical charge generation and nongeminate recombination on the photocurrent of upright and inverted colloidal-PbS quantum-dot solar cells is investigated. With a controlled active layer thickness, upright (PbS/fullerene) devices are found to present overall better photovoltaic performance relative to inverted devices, notwithstanding the better NIR photoconversion efficiency in the latter. Through detailed analysis and numerical optoelectronic simulations, we show that beyond incidental differences, these two device architectures have fundamentally dissimilar properties that stem from their particular optical generation characteristics and the nature of the recombination processes at play, with the inverted devices affected only by trap-assisted losses and the upright ones suffering from enhanced bimolecular recombination. This study unveils the role of device geometry and inherent material properties on the carrier generation and collection efficiency of the light-generated photocurrent in colloidal quantum-dot solar cells

Gains and Losses in PbS Quantum Dot Solar Cells with Submicron Periodic Grating Structures

Corrugated structures are integral to many types of photoelectronic devices, used essentially for the manipulation of optical energy inputs. Here, we have investigated the gains and losses incurred by this microscale geometrical change. We have employed nanostructured electrode gratings of 600 nm pitch in PbS colloidal quantum dot (PbS-CQD) solar cells and investigated their effect on photovoltaic properties. Solar cells employing grating structure achieved a 32% and 20% increase in short-circuit current density (<i>J</i>_sc) and power conversion efficiency, respectively, compared to nonstructured reference cells. The observed photocurrent increase of the structured devices mainly stems from the enhancement of photon absorption due to the trapping of optical energy by the grating structures. This optical absorption enhancement was particularly high in the near-infrared portion of the sun spectrum where PbS-based solar cells commonly present poor absorption. We have interestingly observed that the open-circuit voltage of all the devices increase with the increase in the absorbed photon energy (at a fixed light intensity), indicating a significant shift in Fermi energy level due to localization of low photon energy generated carriers in the tail of the density of states. We elucidate the role of the grating structure on charge dynamics and discuss the feasibility of these structures for construction of cheap and efficient photovoltaic devices

Dynamic Optical Gratings Accessed by Reversible Shape Memory

Shape memory polymers (SMPs) have been shown to accurately replicate photonic structures that produce tunable optical responses, but in practice, these responses are limited by the irreversibility of conventional shape memory processes. Here, we report the intensity modulation of a diffraction grating utilizing two-way reversible shape changes. Reversible shifting of the grating height was accomplished through partial melting and recrystallization of semicrystalline poly­(octylene adipate). The concurrent variations of the grating shape and diffraction intensity were monitored via atomic force microscopy and first order diffraction measurements, respectively. A maximum reversibility of the diffraction intensity of 36% was repeatable over multiple cycles. To that end, the reversible shape memory process is shown to broaden the functionality of SMP-based optical devices