Spin-phonon dispersion in magnetic materials.

Microscopic coupling between the electron spin and the lattice vibration is responsible for an array of exotic properties from morphic effects in simple non magnets to magnetodielectric coupling in multiferroic spinels and hematites. Traditionally, a single spin-phonon coupling constant is used to characterize how effectively the lattice can affect the spin, but it is hardly enough to capture novel electromagnetic behaviors to the full extent. Here, we introduce a concept of spin-phonon dispersion to project the spin moment change along the phonon crystal momentum direction, so the entire spin change can be mapped out.

Lattice Vibrations and Time-Dependent Evolution of Local Phonon Modes during Exciton Formation in Conjugated Polymeric Molecules.

Based on nonadiabatic molecular dynamics that integrate electronic transitions with the time-dependent phonon spectrum, this article provides a panoramic landscape of the dynamical process during the formation of photoinduced excitons in conjugated polymers. When external optical beam/pulses with intensities of 10 µJ/cm and 20 µJ/cm are utilized to excite a conjugated polymer, it is found that the electronic transition firstly triggers local lattice vibrations, which not only locally distort alternating bonds but change the phonon spectrum as well. Within the first 60 fs, the occurrence of local distortion of alternating bonds accompanies the localization of the excited-state's electron.

Charge Accumulation of Amplified Spontaneous Emission in a Conjugated Polymer Chain and Its Dynamical Phonon Spectra.

In this article, the detailed photoexcitation dynamics which combines nonadiabatic molecular dynamics with electronic transitions shows the occurrence of amplified spontaneous emission (ASE) in conjugated polymers, accompanied by spontaneous electric polarization. The elaborate molecular dynamic process of ultrafast photoexcitation can be described as follows: Continuous external optical pumping (laser of 70 µJ/cm) not only triggers the appearance of an instantaneous four-level electronic structure but causes population inversion for ASE as well. At the same time, the phonon spectrum of the conjugated polymer changes, and five local infrared lattice vibrational modes form at the two ends, which break the original symmetry in the system and leads to charge accumulation at the ends of the polymer chain without an external electric field.

Photoexcitation-induced local phonon spectra and local hot excitons in polymer solar cells.

In this article, based on nonadiabatic molecular dynamics with electronic transitions, the elaborate ultrafast process of hot excitons in conjugated polymer solar cells is revealed. When an external optical beam/pulse with the intensity of 30 µJ/cm is utilized to excite a conjugated polymer, just within only 50 fs, the electronic transition not only redistributes the electron population in the original molecular orbital, but also starts to localize the electron cloud of excited states and to distort the alternating bonds in the polymer chain. Up to 300 fs, the lattice distortion has been stabilized.

Asymptotic dynamics of three-dimensional bipolar ultrashort electromagnetic pulses in an array of semiconductor carbon nanotubes.

We study the propagation of three-dimensional bipolar ultrashort electromagnetic pulses in an array of semiconductor carbon nanotubes at times much longer than the pulse duration, yet still shorter than the relaxation time in the system. The interaction of the electromagnetic field with the electronic subsystem of the medium is described by means of Maxwell's equations, taking into account the field inhomogeneity along the nanotube axis beyond the approximation of slowly varying amplitudes and phases. A model is proposed for the analysis of the dynamics of an electromagnetic pulse in the form of an effective equation for the vector potential of the field.

Breaking of lattice potential well-induced confinement of carriers in conjugated polymers.

Recent experimental research has reported that a surface electric field on the polymer solar cell can restrain the recombination of the resultant charged carriers [23]. Based on this, this article reveals an underlying mechanism: If a surface electric field below 4.5 × 10 V/cm is applied to the polymer layer, the electric field drives the charged polaron to transport.

Transient Aspects and Ultrafast Dynamical Processes of Amplified Spontaneous Emission in Conjugated Polymers.

Experimental research has revealed that the stimulated emission in organic semiconductors is associated with lattice vibrations. To reveal the dynamical aspects of the excited state of the conjugated polymer, the electron-transition process has been incorporated into the molecular dynamics, making it possible to simulate in detail the whole ultrafast process of amplified spontaneous emission (ASE). A typical conjugated polymer, poly( p-phenylene vinylene), is chosen as a model for the research.

Laser-induced ultrafast transport and demagnetization at the earliest time: first-principles and real-time investigation.

It is generally believed that there are at least two ways to use an ultrafast laser pulse to demagnetize a magnetic sample. One is to directly photo-demagnetize the system through spin-orbit coupling (SOC), and the other is to utilize ultrafast hot electron transport without SOC. The challenge is that these two processes are entangled on the same time scale.

Generating high-order optical and spin harmonics from ferromagnetic monolayers.

High-order harmonic generation (HHG) in solids has entered a new phase of intensive research, with envisioned band-structure mapping on an ultrashort time scale. This partly benefits from a flurry of new HHG materials discovered, but so far has missed an important group. HHG in magnetic materials should have profound impact on future magnetic storage technology advances.

Conjugation of Nanomaterials and Nematic Liquid Crystals for Futuristic Applications and Biosensors.

The established role of nematic liquid crystals (NLCs) in the recent rapid development of displays has motivated researchers to modulate the electro-optical properties of LCs. Furthermore, adding nanomaterials into NLCs has led to enhancements of the properties of NLCs, like reduced threshold of the operating voltage, variation in pretilt angle, reduced switching time, etc. These enhanced properties, due to interfacial dynamics, are enabling wider applications of NLCs and nanomaterials.

Ultrafast optical pulse convertor caused by oscillations of the energy level structure in the conjugated polymer poly(p-phenylenevinylene).

For a conjugated polymer irradiated by two optical pulses, the whole process of excitation, involving lattice oscillations, oscillations of the energy level structure, and evolution of the electron cloud, is investigated. Localization of the electron cloud appears in the first 100 fs of irradiation, which in turn induces vibrations of lattice of the polymer chain as well as oscillations of the band gap. These oscillations filter the absorption of the external optical field inversely and convert the original optical field to an ultrafast light field whose intensity varies with a certain period.

Is perpendicular magnetic anisotropy essential to all-optical ultrafast spin reversal in ferromagnets?

All-optical spin reversal presents a new opportunity for spin manipulations, free of a magnetic field. Most of all-optical-spin-reversal ferromagnets are found to have a perpendicular magnetic anisotropy (PMA), but it has been unknown whether PMA is necessary for spin reversal. Here we theoretically investigate magnetic thin films with either PMA or in-plane magnetic anisotropy (IMA).

Nanotherapy of cancer by photoelectrons emitted from the surface of nanoparticles exposed to nonionizing ultraviolet radiation.

Aim: We introduce a new method for selectively destroying cancer cell organelles by electrons emitted from the surface of intracellularly localized nanoparticles exposed to the nonionizing ultraviolet (UV) radiation.

Methods: We propose to target cancerous intracellular organelles by nanoparticles and expose them to UV radiation with energy density safe for healthy tissue.

Results: We simulate the number of photoelectrons produced by the nanoparticles made of various metals and radii, calculate their kinetic energy and compare it to the threshold energy for producing biological damage.

Intrinsic Delocalization during the Decay of Excitons in Polymeric Solar Cells.

In bulk heterojunction polymer solar cells, external photoexcitation results in localized excitons in the polymer chain. After hot exciton formation and subsequent relaxation, the dipole moment drives the electron to partially transfer to extended orbitals from the original localized ones, leading to self-delocalization. Based on the dynamic fluorescence spectra, the delocalization of excitons is revealed to be an intrinsic property dominated by exciton decay, acting as a bridge for the exciton to diffuse in the polymeric solar cell.

Ultrafast reduction of exchange splitting in ferromagnetic nickel.

A decade ago Rhie et al (2003 Phys. Rev. Lett.

Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: beyond the rigid band approximation.

The exchange interaction among electrons is one of the most fundamental quantum mechanical interactions in nature and underlies any magnetic phenomena from ferromagnetic ordering to magnetic storage. The current technology is built upon a thermal or magnetic field, but a frontier is emerging to directly control magnetism using ultrashort laser pulses. However, little is known about the fate of the exchange interaction.

Absorption efficiency and heating kinetics of nanoparticles in the RF range for selective nanotherapy of cancer.

Unlabelled: Radio-frequency (RF) waves have an excellent ability to penetrate into the human body, giving a great opportunity to activate/heat nanoparticles delivered inside the body as a contrast agent for diagnosis and treatment purposes. However the heating of nanoparticles in the RF range of the spectrum is controversial in the research community because of the low power load of RF waves and low absorption of nanoparticles in the RF range. This study uses a phenomenological approach to estimate the absorption efficiency of metal and dielectric nanoparticles in the RF range through a study of heating kinetics of those particles in radio wave field.

Deep localized distortion of alternating bonds and reduced transport of charged carriers in conjugated polymers under photoexcitation.

In a real bulk heterojunction polymer solar cell, after exciton separation in the heterojunction, the resulting negatively-charged carrier, a polaron, moves along the polymer chain of the acceptor, which is believed to be of significance for the charged carrier transport properties in a polymer solar cell. During the negative polaron transport, due to the external light field, the polaron, which is re-excited and induces deep localization, also forms a new local distortion of the alternating bonds. It is revealed that the excited polaron moves more slowly than the ground-state polaron.

X-ray optics of gold nanoparticles.

Gold nanoparticles have been investigated as contrast agents for traditional x-ray medical procedures, utilizing the strong absorption characteristics of the nanoparticles to enhance the contrast of the detected x-ray image. Here we use the Kramers-Kronig relation for complex atomic scattering factors to find the real and imaginary parts of the index of refraction for the medium composed of single-element materials or compounds in the x-ray range of the spectrum. These complex index of refraction values are then plugged into a Lorenz-Mie theory to calculate the absorption efficiency of various size gold nanoparticles for photon energies in the 1-100 keV range.

Photon statistics of resonance fluorescence in the limit of separated spectral lines.

We have studied the statistics of fluorescent photons emitted by a two-state atom in a laser beam in the limit where either the detuning or the Rabi frequency is large. For this case, the spectrum of resonance fluorescence has three separated lines. We have obtained closed form expressions for the conditional probability density for the emission of the nth photon and for the probability for emission of n photons in a time interval [0,T].

Thermal or nonthermal? That is the question for ultrafast spin switching in GdFeCo.

GdFeCo is among the most interesting magnets for producing laser-induced femtosecond magnetism, where light can switch its spin moment from one direction to another. This paper aims to set a criterion for the thermal/nonthermal mechanism: we propose to use the Fermi-Dirac distribution function as a reliable criterion. A precise value for the thermalization time is needed, and through a two-level model, we show that since there is no direct connection between the laser helicity and the definition of thermal/nonthermal processes, the helicity is a poor criterion for differentiating a thermal from a nonthermal process.

Localization and relaxation of singlet exciton formation in conjugated polymers under photoexcitation.

This paper employs a molecular dynamics approach to uncover the time profile of exciton formation, which can be divided into two stages: localization of electron-hole pairs and relaxation process (nuclear and electronic). Under photoexcitation, an electron-hole pair is formed by an electronic transition, and the pair in turn becomes localized through the electron-lattice interaction, which triggers the total energy to shift violently and oscillate. The oscillation during the first 40 fs induces the excitation to step into the second stage, i.

Electronic Two-Transition-Induced Enhancement of Emission Efficiency in Polymer Light-Emitting Diodes.

With the development of experimental techniques, effective injection and transportation of electrons is proven as a way to obtain polymer light-emitting diodes (PLEDs) with high quantum efficiency. This paper reveals a valid mechanism for the enhancement of quantum efficiency in PLEDs. When an external electric field is applied, the interaction between a negative polaron and triplet exciton leads to an electronic two-transition process, which induces the exciton to emit light and thus improve the emission efficiency of PLEDs.

Proposed coherent trapping of a population of electrons in a C60 molecule induced by laser excitation.

This Letter demonstrates the possibility of generating coherent population trapping in C(60). Similar to a three-level Λ system, C(60) has a forbidden transition between the highest occupied molecular orbital (HOMO) (|a}) and the lowest unoccupied molecular orbital (LUMO) (|c}), but a dipole-allowed transition between HOMO and LUMO+1 (|b}) and between |b} and |c}. We employ two cw laser fields, one coupling and one probe.

Carrier-collision-induced formation of charged excitons and ultrafast dynamics fluorescence spectra.

After a hole injection layer is inserted into a polymer light-emitting material, the injection of positive charge not only easily causes distortion in the conjugated polymer chain but also produces positive polarons. The ultrafast dynamics shows that, when the positive polaron approaches and collides with the triplet exciton, that exciton will become charged, whereby the non-emissive triplet exciton becomes radiative and emits light. Furthermore, the lifetime of the charged triplet exciton is longer than the singlet exciton.