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Ultrafast acoustic phonon scattering in CH <sub/>3</sub> NH <sub/>3</sub> PbI <sub/>3</sub> revealed by femtosecond four-wave mixing
摘要： Carrier scattering processes are studied in CH3NH3PbI3 using temperature-dependent four-wave mixing experiments. Our results indicate that scattering by ionized impurities limits the interband dephasing time (T2) below 30 K, with strong electron-phonon scattering dominating at higher temperatures (with a time scale of 125 fs at 100 K). Our theoretical simulations provide quantitative agreement with the measured carrier scattering rate and show that the rate of acoustic phonon scattering is enhanced by strong spin-orbit coupling, which modifies the band-edge density of states. The Rashba coefficient extracted from fitting the experimental results (γc = 2 eV ?) is in agreement with calculations of the surface Rashba effect and recent experiments using the photogalvanic effect on thin films.
关键词： electron-phonon scattering,carrier scattering,CH3NH3PbI3,four-wave mixing,Rashba effect
更新于2025-09-19 17:13:59
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Visualization of subnanometric phonon modes in a plasmonic nano-cavity via ambient tip-enhanced Raman spectroscopy
摘要： Phonons provide information on the physicochemical properties of a crystalline lattice from the material’s vibrational spectrum. Optical phonons, in particular, can be probed at both micrometre and nanometre scales using light-based techniques, such as, micro-Raman and tip-enhanced Raman spectroscopy (TERS), respectively. Selection rules, however, govern the accessibility of the phonons and, hence, the information that can be extracted about the sample. Herein, we simultaneously observe both allowed and forbidden optical phonon modes of defect-free areas in monolayer graphene to study nanometre scale strain variations and plasmonic activation of the Raman peaks, respectively, using our home-built TERS system in ambient. Through TERS imaging, strain variations and nanometre-sized domains down to 5 nm were visualised with a spatial resolution of 0.7 nm. Moreover, such subnanometric con?nement was found to activate not only the D and D’ forbidden phonon modes but also their D + D’ combination mode. With our TERS in ambient system, the full phonon characterisation of defect-free graphene and other 2D nanomaterials is now possible, which will be useful for subnanometre strain analysis and exploring the inherent properties of defect-free materials.
关键词： graphene,plasmonic nano-cavity,tip-enhanced Raman spectroscopy,phonon modes,strain analysis
更新于2025-09-19 17:13:59
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Highly indistinguishable single photons from incoherently excited quantum dots
摘要： Semiconductor quantum dots with dimensions exceeding the free-exciton Bohr radius are appealing because of their high oscillator strengths. While this property has received much attention in the context of cavity quantum electrodynamics, little is known about the degree of indistinguishability of single photons consecutively emitted by such dots and on the proper excitation schemes to achieve high indistinguishability. A prominent example is represented by GaAs quantum dots obtained by local droplet etching, which recently outperformed other systems as triggered sources of entangled photon pairs. On these dots, we compare different single-photon excitation mechanisms, and we find (i) poor indistinguishability for conventional excitation via excited states and (ii) photon indistinguishablities above 90% for both strictly resonant and for incoherent phonon-assisted excitation. Among the explored excitation schemes, optical phonon-assisted excitation allows straightforward laser rejection and is thus worth of further investigation and optimization for quantum dots embedded in high-brightness photonic structures.
关键词： quantum dots,indistinguishability,single-photon emission,GaAs,phonon-assisted excitation
更新于2025-09-19 17:13:59
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Control of Charge Carrier Dynamics in Plasmonic Au Films by TiO <sub/><i>x</i> </sub> Substrate Stoichiometry
摘要： Plasmonic excitations in noble metals have many fascinating properties and give rise to a broad range of applications. We demonstrate, using non-adiabatic molecular dynamics combined with time-domain density functional theory, that chemical composition and stoichiometry of substrates can have a strong influence on charge dynamics. By changing oxygen content in TiO2, including stoichiometric, oxygen rich and oxygen poor phases, and Ti metal, one can alter lifetimes of charge carriers in Au by a factor of 5, and control the ratio of electron-to-hole relaxation rates by a factor of 10. Remarkably, a thin TiOx substrate alters so much charge carrier properties in much thicker Au films. Such large variations stem from the fact that the Ti and O atoms are much lighter than Au, and their vibrations are much faster at dissipating the energy. The control over a particular charge carrier and an energy range depends on the Au and TiOx level alignment, and the interfacial interaction strength. These factors are easily influenced by the TiOx stoichiometry. In particular, oxygen rich and poor TiO2 can be used to control holes and electrons, respectively, while metallic Ti affects both charge carriers. The detailed atomistic analysis of the interfacial and electron-vibrational interactions generates the fundamental understanding of the properties of plasmonic materials needed to design photovoltaic, photocatalytic, optoelectronic, sensing, nanomedical and other devices.
关键词： non-adiabatic molecular dynamics,substrate layers,time-domain density functional theory,metallic films,electron-phonon energy relaxation,surface plasmons
更新于2025-09-19 17:13:59
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Heterogeneous Cationa??Lattice Interaction and Dynamics in Triple-Cation Perovskites Revealed by Infrared Vibrational Nanoscopy
摘要： Hybrid organic-inorganic perovskites exhibit extraordinary photovoltaic performance. This is believed to arise from almost liquid-like low-energy interactions among lattice ions and charge carriers. While spatial variations have recently been identified over multiple length scales in the optoelectronic responses, the relationship of the heterogeneity and the soft cation-lattice interactions has remained elusive. Here, we apply multivariate infrared vibrational nano-imaging to a formamidinium (FA)-methylammonium (MA)-cesium triple cation perovskite. By using the FA vibrational resonance as a probe of its local chemical environment, through spatial variation and correlation in composition, strength of cation-lattice coupling and its associated few-ps vibrational dynamics, we reveal how a varying local reaction field is correlated with the cation-lattice coupling. We attribute the associated heterogeneous lattice contraction to a non-uniform distribution of cesium cations. The observed varying elasticity of the lattice leads to disordered charge-phonon coupling and polaron formation, the control of which is central to improving perovskite photovoltaics.
关键词： charge-phonon coupling,infrared vibrational nano-imaging,Hybrid organic-inorganic perovskites,lattice contraction,photovoltaic performance,cation-lattice coupling,polaron formation,vibrational dynamics
更新于2025-09-19 17:13:59
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Graphene based plasma-wave devices for terahertz applications
摘要： Unique properties of graphene are combined to enable graphene plasmonic devices that could revolutionize the terahertz (THz) electronic technology. A high value of the carrier mobility allows us to excite resonant plasma waves. The graphene bipolar nature allows for different mechanisms of plasma wave excitation. Graphene bilayer and multilayer structures make possible improved THz device configurations. The ability of graphene to form a high quality heterostructure with h-BN, black phosphorus, and other materials systems supports advanced heterostructure devices comprised of the best properties of graphene and other emerging materials. In particular, using black phosphorus compounds for cooling electron–hole plasma in graphene could dramatically improve the conditions for THz lasing. High optical phonon energy allows for reaching higher plasma frequencies that are supported by high sheet carrier densities in graphene. Recent improvements in graphene technology combined with a better understanding of the device physics of graphene THz plasmonics and graphene plasmonic device designs hold promise to make graphene THz plasmonic technology one of the key graphene applications. Commercialization of plasmonic graphene technology is facing the same challenges as other graphene applications, which have difficulties in producing uniform large graphene layers, bilayers, and heterostructures of high quality and making good low resistance stable Ohmic contacts. The time projection for large scale graphene electronic device applications now extends into the 2030s. However, emerging graphene mass production technologies might bring commercial applications of the graphene plasmonic terahertz technology closer.
关键词： heterostructures,graphene,black phosphorus,Ohmic contacts,plasma waves,THz technology,terahertz,plasmonic devices,optical phonon energy,carrier mobility
更新于2025-09-19 17:13:59
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Soft Lattice and Defect Covalency Rationalize Tolerance of ?2a??CsPbI3 Perovskite Solar Cells to Native Defects
摘要： Although all-inorganic lead halide perovskite solar cells have shown tremendous improvement over the past few years, they are still inferior to the hybrid organic-inorganic perovskites in the solar power conversion efficiency. Recently, a conceptually new β-CsPbI3 perovskite has demonstrated an impressive 18.4% efficiency combined with good thermodynamic stability at ambient conditions. We use ab initio non-adiabatic molecular dynamics to show that native point defects in β-CsPbI3 are generally benign for nonradiative charge recombination, regardless of whether they introduce shallow or deep trap states. Moreover, formation of new covalently bound species in the presence of defects slows down the recombination. These results indicate that halide perovskites do not follow the simple models used to explain defect-mediated charge recombination in the conventional semiconductors. The strong tolerance of electron-hole recombination against defects arises due to the softness of the perovskite lattice, which permits separation of electrons and holes upon defect formation, and allows only low-frequency vibrations to couple to the electronic subsystem. Both factors decrease significantly the non-adiabatic coupling and slow down the dissipation of electronic energy to heat. We suggest that a halide-rich synthesis environment may further improve the efficiency, and propose that strong defect tolerance is general to metal halide perovskites because they exhibit much lower bulk moduli compared to the conventional semiconductors used in photovoltaic, photocatalytic, electrocatalytic, lasing, light-emitting, detecting and other opto-electronic devices.
关键词： Electron-phonon coupling,Nonradiative recombination,All-inorganic perovskites,Time-dependent density functional theory,Defects
更新于2025-09-19 17:13:59
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High-speed modulation of a terahertz quantum cascade laser by coherent acoustic phonon pulses
摘要： The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology. In the terahertz-frequency range, the quantum-cascade laser (QCL) is a high-power source with the potential for high-frequency modulation. However, conventional electronic modulation is limited fundamentally by parasitic device impedance, and so alternative physical processes must be exploited to modulate the QCL gain on ultrafast timescales. Here, we demonstrate an alternative mechanism to modulate the emission from a QCL device, whereby optically-generated acoustic phonon pulses are used to perturb the QCL bandstructure, enabling fast amplitude modulation that can be controlled using the QCL drive current or strain pulse amplitude, to a maximum modulation depth of 6% in our experiment. We show that this modulation can be explained using perturbation theory analysis. While the modulation rise-time was limited to ~800 ps by our measurement system, theoretical considerations suggest considerably faster modulation could be possible.
关键词： terahertz,quantum-cascade laser,modulation,acoustic phonon,ultrafast
更新于2025-09-19 17:13:59
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Surface Polariton‐Like s‐Polarized Waveguide Modes in Switchable Dielectric Thin Films on Polar Crystals
摘要： Surface phonon polaritons (SPhPs) and surface plasmon polaritons (SPPs), evanescent modes supported by media with negative permittivity, are a fundamental building block of nanophotonics. These modes are unmatched in terms of field enhancement and spatial confinement, and dynamical all-optical control can be achieved, e.g., by employing phase-change materials. However, the excitation of surface polaritons in planar structures is intrinsically limited to p-polarization. On the contrary, waveguide modes in high-permittivity films can couple to both p- and s-polarized light, and in thin films, their confinement can become comparable to surface polaritons. Here, it is demonstrated that the s-polarized waveguide mode in a thin Ge3Sb2Te6 (GST) film features a similar dispersion, confinement, and electric field enhancement as the SPhP mode of the silicon carbide (SiC) substrate, while even expanding the allowed frequency range. Moreover, it is experimentally shown that switching the GST film grants nonvolatile control over the SPhP and the waveguide mode dispersions. An analytical model is provided for the description of the GST/SiC waveguide mode and it is shown that the concept is applicable to the broad variety of polar crystals throughout the infrared spectral range. As such, complementarily to the polarization-limited surface polaritons, the s-polarized waveguide mode constitutes a promising additional building block for nanophotonic applications.
关键词： waveguide modes,infrared spectral range,phase-change materials,surface plasmon polaritons,silicon carbide,nanophotonics,Ge3Sb2Te6,surface phonon polaritons,s-polarized light
更新于2025-09-19 17:13:59
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Unusual Bimodal Photovoltaic Performance of Perovskite Solar Cells at Real-World Operating Temperatures
摘要： A deep understanding of environmental effects on perovskite solar cell (PSC) performance is highly desirable for further progress towards large-scale deployment of this technology. We investigate the operation of PSCs in the temperature range (15?50 C) and report an unusual bimodal behavior in photovoltaic (PV) performance, with positive and negative temperature coefficients (TCs) below and above room temperature (RT), respectively. Furthermore, the performance metrics exhibit hysteresis, as their values depend on whether the measurements are made during the heating or cooling stages of the experiment. Conventional semiconductor solar cells, in contrast, exhibit a monotonic and non-hysteretic performance decline in this temperature range. The variations in power conversion efficiency primarily follow changes in open-circuit voltage and fill factor. Photoluminescence data suggest that the performance variations below RT are accompanied by a reduction in defect-related traps in the perovskite absorber and a drop in interfacial built-in potential at the perovskite/transport layer interface. The behavior above RT follows the conventional trend and can hence be explained by charge-phonon interactions. Our findings offer significant insight into the salient PV properties and photophysics of perovskite materials that define their performance in the real-world operating temperature range.
关键词： charge-phonon coupling,temperature-dependent hysteresis,Perovskite solar cells,Temperature coefficient
更新于2025-09-19 17:13:59