Search Results (8)

Bacterial cellulose, a biopolymer synthesised by microorganisms, exhibits remarkable structural, optical, and electronic properties. This study utilised a range of photon- and electron-based techniques, including X-ray diffraction, proton nuclear magnetic resonance (¹H-NMR), photoacoustic spectroscopy, and scanning electron microscopy, to thoroughly characterise BC. While XRD and NMR directly employ photons to probe the structure and composition, PAS indirectly converts absorbed photons into phonons to evaluate optoelectronic features. SEM revealed a dense nanofibrillar network with fibrils measuring 10–75 nm in diameter. XRD confirmed the crystalline nature of BC, identifying characteristic peaks associated with cellulose Iα. ¹H-NMR relaxation analysis differentiated between the ordered and disordered cellulose regions. PAS determined an optical bandgap of 2.97 eV and identified defect states between 3.6 and 2.9 eV, including a prominent peak at 3.35 eV, likely resulting from oxygen vacancies, hydroxyl modifications, or UV-induced rearrangements. These defects modify BC’s electronic structure, suggesting potential for bandgap engineering. The integration of these complementary techniques provides a multidimensional understanding of BC’s morphology, crystallinity, and electronic behaviour, underscoring its potential in bioelectronics, advanced composites, and biomedical applications. Full article

Two-dimensional transition metal carbides/nitrides (MXenes) have shown potential in biosensors, cancer theranostics, microbiology, electromagnetic interference shielding, photothermal conversion, and thermal energy storage due to their unique electronic structure, ability to absorb a wide range of light, and tunable surface chemistry. In spite of the growing interest in MXenes, there are relatively few studies on their applications in phase-change materials for enhancing thermal conductivity and weak photo-responsiveness between 0 °C and 150 °C. Thus, this study aims to provide a current overview of recent developments, to examine how MXenes are made, and to outline the combined effects of different processes that can convert light into heat. This study illustrates the mechanisms that include enhanced broadband photon harvesting through localized surface plasmon resonance, electron–phonon coupling-mediated nonradiative relaxation, and interlayer phonon transport that optimizes thermal diffusion pathways. This study emphasizes that MXene-engineered 3D thermal networks can greatly improve energy storage and heat conversion, solving important problems with phase-change materials (PCMs), like poor heat conductivity and low responsiveness to light. This study also highlights the real-world issues of making MXene-based materials on a large scale, and suggests future research directions for using them in smart thermal management systems and solar thermal grid technologies. Full article

Anti-Stokes photoluminescence (ASPL) is an up-conversion phonon-assisted process of radiative recombination of photoexcited charge carriers when the ASPL photon energy is above the excitation one. This process can be very efficient in nanocrystals (NCs) of metalorganic and inorganic semiconductors with perovskite (Pe) crystal structure. In this review, we present an analysis of the basic mechanisms of ASPL and discuss its efficiency depending on the size distribution and surface passivation of Pe-NCs as well as the optical excitation energy and temperature. When the ASPL process is sufficiently efficient, it can result in an escape of most of the optical excitation together with the phonon energy from the Pe-NCs. It can be used in optical fully solid-state cooling or optical refrigeration. Full article

We propose the use of an n-doped periodic AlN/GaN quantum cascade structure for the optical up-conversion of multiple near-infrared (near-IR) photons into deep-ultraviolet (deep-UV) radiation. Without applying an external bias voltage, the active region of such a device will (similar to an un-biased quantum cascade laser) resemble a sawtooth-shaped inter-subband structure. A carefully adjusted bias voltage then converts this sawtooth pattern into a ‘quantum-stair’. Illumination with λ = 1.55 µm radiation results in photon absorption thereby lifting electrons from the ground state of each main well into the first excited state. Three additional GaN quantum wells per period then provide by LO-phonon-assisted tunneling a diagonal transfer of these electrons towards the ground level of the neighboring period. From there, the next near-infrared (near-IR) photon absorption, electron excitation, and partial relaxation takes place. After 12 such absorption, transfer, and relaxation processes, the excited electrons have gained a sufficiently high amount of energy to undergo in the final AlN-based p-type contact layer an electron-hole band-to-band recombination. By employing this procedure, multiple near-IR photons will be up-converted to produce deep-UV radiation. Since for a wavelength of 1.55 µm very powerful near-IR pump lasers are readily available, such an up-conversion device will (even at a moderate overall conversion efficiency) potentially result in an equal or even higher output power than the one of an AlN-based p-n-junction light-emitting diode. The proposed structures are therefore very interesting for applications such as ultra-high-resolution photolithography or printing, water purification, medical equipment disinfection, white light generation, or the automotive industry. Full article

Anti-Stokes photoluminescence (ASPL), which is an up-conversion phonon-assisted process of the radiative recombination of photoexcited charge carriers, was investigated in methylammonium lead bromide (MALB) perovskite nanocrystals (NCs) with mean sizes that varied from about 6 to 120 nm. The structure properties of the MALB NCs were investigated by means of the scanning and transmission electron microscopy, X-ray diffraction and Raman spectroscopy. ASPL spectra of MALB NCs were measured under near-resonant laser excitation with a photon energy of 2.33 eV and they were compared with the results of the photoluminescence (PL) measurements under non-resonant excitation at 3.06 eV to reveal a contribution of phonon-assisted processes in ASPL. MALB NCs with a mean size of about 6 nm were found to demonstrate the most efficient ASPL, which is explained by an enhanced contribution of the phonon absorption process during the photoexcitation of small NCs. The obtained results can be useful for the application of nanocrystalline organometal perovskites in optoelectronic and all-optical solid-state cooling devices. Full article

Stimulated Brillouin scattering (SBS) lasers based on silicon waveguides with large SBS gain have been widely used in frequency tunable laser emissions, mode-locked pulse lasers, low-noise oscillators, optical gyroscopes and other fields. However, among SBS lasers, the realization of Brillouin laser output often requires a longer waveguide length, which not only increases waveguide loss but also increase the size of the device. As a new medium, lithium niobate has been fabricated into a new type of hybrid structure. Meanwhile, the width of a suspended waveguide is adjusted to tune the phonon frequency of an SBS laser based on lithium niobate substrate. Simulation results show that the tunable forward SBS effect is realized in a lithium niobate-suspended optical waveguide, showing a larger forward stimulated Brillouin scattering gain of 0.31 W⁻¹m⁻¹. The tunable phonon frequency ranges from 1 to 15 GHz. Therefore, utilizing the photon–phonon conversion effect, the waveguide system with LiNbO₃ will pave a new way forward with better integration. Full article

We theoretically investigate the optical output fields of a photonic-molecule optomechanical system in an optomechanically induced transparency (OMIT) regime, in which the optomechanical cavity is optically driven by a strong pump laser field and a weak probe laser field and the mechanical mode is driven by weak coherent phonon driving. The numerical simulations indicate that when the driven frequency of the phonon pump equals the frequency difference of the two laser fields, we show an enhancement OMIT where the probe transmission can exceed unity via controlling the driving amplitude and pump phase of the phonon driving. In addition, the phase dispersion of the transmitted probe field can be modified for different parametric regimes, which leads to a tunable delayed probe light transmission. We further study the group delay of the output probe field with numerical simulations, which can reach a tunable conversion from slow to fast light with the manipulation of the pump laser power, the ratio parameter of the two cavities, and the driving amplitude and phase of the weak phonon pump. Full article

The power conversion efficiency (PCE) of single-junction solar cells was theoretically predicted to be limited by the Shockley–Queisser limit due to the intrinsic potential loss of the photo-excited electrons in the light absorbing materials. Up to now, the optimized GaAs solar cell has the highest PCE of 29.1%, which is close to the theoretical limit of ~33%. To pursue the perfect photovoltaic performance, it is necessary to extend the lifetimes of the photo-excited carriers (hot electrons and hot holes) and to collect the hot carriers without potential loss. Thanks to the long-lived hot carriers in perovskite crystal materials, it is possible to completely convert the photon energy to electrical power when the hot electrons and hot holes can freely transport in the quantized energy levels of the electron transport layer and hole transport layer, respectively. In order to achieve the ideal PCE, the interactions between photo-excited carriers and phonons in perovskite solar cells has to be completely understood. Full article