Element-Selective Ultrafast Magnetization Dynamics with a Tabletop Light Source

Next-generation hard-disk drives will require smaller magnetic bits and faster magnetization switching; hence, better understanding of nanoscale magnetic material is one of the key factors in developing of these devices. Here, I present the first ultrafast magnetization dynamics studies by use of extreme ultraviolet radiation from a tabletop high-harmonic generation source. This new probing technique offers three advantages over conventional ones: ultrafast time resolution, element selectivity, and the tabletop size. I report three experiments showing that high harmonics are a powerful tool for probing magnetization in magnetic materials. First, our group measures simultaneously the magnetizations of Ni and Fe in Permalloy using the transverse magneto-optical Kerr effect. Second, we study laser-induced demagnetization dynamics in two ferromagnetic alloys: Permalloy and Permalloy-Cu. Contrary to a common expectation that the dynamics in strong exchange-coupled alloys would be identical, we discover that the magnetization of Fe decays earlier than that of Ni during the first 60 fs. To explain this delay, we propose a simple model incorporating a finite exchange-time factor into the magnetization rate equations. Finally, to confirm the observed sequence of dynamics in alloys, we conduct the magnetization study of elemental Fe and Ni with identical experimental conditions. The results indicate that the order of demagnetizations in the elemental forms is the same as that in Permalloy: Fe demagnetizes faster than Ni does

Optical Properties of Organometallic Perovskite: An ab initio Study using Relativistic GW Correction and Bethe-Salpeter Equation

In the development of highly efficient photovoltaic cells, solid perovskite systems have demonstrated unprecedented promise, with the figure of merit exceeding nineteen percent of efficiency. In this paper, we investigate the optical and vibrational properties of organometallic cubic perovskite CH3NH3PbI3 using first-principles calculations. For accurate theoretical description, we go beyond conventional density functional theory (DFT), and calculated optical conductivity using relativist quasi-particle (GW) correction. Incorporating these many-body effects, we further solve Bethe-Salpeter equations (BSE) for excitons, and found enhanced optical conductivity near the gap edge. Due to the presence of organic methylammonium cations near the center of the perovskite cell, the system is sensitive to low energy vibrational modes. We estimate the phonon modes of CH3NH3PbI3 using small displacement approach, and further calculate the infrared absorption (IR) spectra. Qualitatively, our calculations of low-energy phonon frequencies are in good agreement with our terahertz measurements. Therefore, for both energy scales (around 2 eV and 0-20 meV), our calculations reveal the importance of many-body effects and their contributions to the desirable optical properties in the cubic organometallic perovskites system.Comment: 5 pages, 4 figure

New insights into the diverse electronic phases of a novel vanadium dioxide polymorph: A terahertz spectroscopy study

10.1038/srep09182Scientific Reports

Low-frequency optical phonon modes and carrier mobility in the halide perovskite CH_3NH_3PbBr_3 using terahertz time-domain spectroscopy

As a light absorber in photovoltaic applications, hybrid organic-inorganic halide perovskites should have long and balanced diffusion lengths for both the separated electrons and holes before recombination, which necessitates high carrier mobility. In polar semiconductors, the room-temperature carrier mobility is often limited by the scattering between carriers and the lowest-frequency optical phonon modes. Using terahertz time-domain spectroscopy, we examine the temperature evolution of these phonon modes in CH_3NH_3PbBr_3 and obtained high carrier mobility values using Feynman's polaron theory. This method allows us to estimate the upper limit of carrier mobilities without the need to create photogenerated free carriers, and can be applied to other heteropolar semiconductor systems with large polarons

Phonon features in terahertz photoconductivity spectra due to data analysis artifact: A case study on organometallic halide perovskites

We propose a simple scenario where the superimposed phonon modes on the photoconductive spectra are experimental artifacts due to the invalid formula used in data analysis. By use of experimental and simulated data of CH_3NH_3PbI_3 perovskites as a case study, we demonstrate that a correction term must be included in the approximated thin-film formula used in the literature; otherwise, parts of the spectra with high background permittivity near the phonon-mode resonances might interfere with the transient photoconductivity. The implication of this work is not limited to perovskites but other materials with strong vibrational modes within the THz spectral range

Quantifying Efficiency Loss of Perovskite Solar Cells by a Modified Detailed Balance Model

A modified detailed balance model is built to understand and quantify efficiency loss of perovskite solar cells. The modified model captures the light-absorption dependent short-circuit current, contact and transport-layer modified carrier transport, as well as recombination and photon-recycling influenced open-circuit voltage. Our theoretical and experimental results show that for experimentally optimized perovskite solar cells with the power conversion efficiency of 19%, optical loss of 25%, non-radiative recombination loss of 35%, and ohmic loss of 35% are the three dominant loss factors for approaching the 31% efficiency limit of perovskite solar cells. We also find that the optical loss will climb up to 40% for a thin-active-layer design. Moreover, a misconfigured transport layer will introduce above 15% of energy loss. Finally, the perovskite-interface induced surface recombination, ohmic loss, and current leakage should be further reduced to upgrade device efficiency and eliminate hysteresis effect. The work contributes to fundamental understanding of device physics of perovskite solar cells. The developed model offers a systematic design and analysis tool to photovoltaic science and technology.Comment: 21 pages, 9 figures, 3 table

Monitoring Electron–Phonon Interactions in Lead-Halide Perovskites Using Time-Resolved THz Spectroscopy

Lead halide perovskite semiconductors have low-frequency phonon modes within the lead halide sublattice and thus are considered to be soft. The soft lattice is considered to be important in defining their interesting optoelectronic properties. Electron–phonon coupling governs hot-carrier relaxation, carrier mobilities, carrier lifetimes, among other important electronic characteristics. Directly observing the interplay between free charge carriers and phonons can provide details on how phonons impact these properties (e.g., exciton populations and other collective modes). Here, we observe a delicate interplay among carriers, phonons, and excitons in mixed-cation and mixed-halide perovskite films by simultaneously resolving the contribution of charge carriers and phonons in time-resolved terahertz photoconductivity spectra. We are able to observe directly the increase in phonon population during carrier cooling and discuss how thermal equilibrium populations of carriers and phonons modulate the carrier transport properties, as well as reduce the population of carriers within band tails. We are also able to observe directly the formation of free charge carriers when excitons interact with phonons and dissociate and to describe how free carriers and exciton populations exchange through phonon interactions. Finally, we also time-resolve how the carriers are screened via the Coulomb interaction at low and room temperatures. Our studies shed light on how charge carriers interact with the low-energy phonons and discuss implications

Low-frequency optical phonon modes and carrier mobility in the halide perovskite CH_3NH_3PbBr_3 using terahertz time-domain spectroscopy

As a light absorber in photovoltaic applications, hybrid organic-inorganic halide perovskites should have long and balanced diffusion lengths for both the separated electrons and holes before recombination, which necessitates high carrier mobility. In polar semiconductors, the room-temperature carrier mobility is often limited by the scattering between carriers and the lowest-frequency optical phonon modes. Using terahertz time-domain spectroscopy, we examine the temperature evolution of these phonon modes in CH_3NH_3PbBr_3 and obtained high carrier mobility values using Feynman's polaron theory. This method allows us to estimate the upper limit of carrier mobilities without the need to create photogenerated free carriers, and can be applied to other heteropolar semiconductor systems with large polarons

Low-frequency optical phonon modes and carrier mobility in the halide perovskite CH3NH3PbBr3 using terahertz time-domain spectroscopy

As a light absorber in photovoltaic applications, hybrid organic-inorganic halide perovskites should have long and balanced diffusion lengths for both the separated electrons and holes before recombi- nation, which necessitates high carrier mobility. In polar semiconductors, the room-temperature carrier mobility is often limited by the scattering between carriers and the lowest-frequency optical phonon modes. Using terahertz time-domain spectroscopy, we examine the temperature evolution of these phonon modes in CH 3 NH 3 PbBr 3 and obtained high carrier mobility values using Feynman’s polaron theory. This method allows us to estimate the upper limit of carrier mobilities without the need to create photogenerated free carriers, and can be applied to other heteropolar semiconductor systems with large polarons

Elucidating the role of disorder and free-carrier recombination kinetics in CH_3NH_3PbI_3 perovskite films

Apart from broadband absorption of solar radiation, the performance of photovoltaic devices is governed by the density and mobility of photogenerated charge carriers. The latter parameters indicate how many free carriers move away from their origin, and how fast, before loss mechanisms such as carrier recombination occur. However, only lower bounds of these parameters are usually obtained. Here we independently determine both density and mobility of charge carriers in a perovskite film by the use of time-resolved terahertz spectroscopy. Our data reveal the modification of the free carrier response by strong backscattering expected from these heavily disordered perovskite films. The results for different phases and different temperatures show a change of kinetics from two-body recombination at room temperature to three-body recombination at low temperatures. Our results suggest that perovskite-based solar cells can perform well even at low temperatures as long as the three-body recombination has not become predominant