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-2 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. During the formation of hot excitons, the prominent self-trapping effect of conjugated polymer triggers the occurrence of local infrared active phonon modes, with five peaks in the phonon spectrum as the hot excitons relax. The characteristic phonon spectrum and infrared modes hence form the fingerprint of the hot excitons of a conjugated polymer, which are readily distinguished from other excitation states in the polymer.

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/cm2) 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. This novel phenomenon gives a brand-new avenue to explain how the lattice vibrations play a role in the evolution of the stimulated emission, which leads to an ultrafast effect in solid conjugated polymers.

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 × 104 V/cm is applied to the polymer layer, the electric field drives the charged polaron to transport. Once the polaron approaches and collides with the exciton, it is easily trapped by the potential well produced by the exciton and then forms a charged exciton. The decay of the resultant charged exciton rapidly reduces the number of excitons. However, once the external field surpasses the threshold value of 4.5 × 104 V/cm, the charged polaron absorbs momentum from the external electric field and shakes off the trapping of the exciton. It finally steps out of the original lattice potential well, where the appropriate electric field magnitude ranges from 5.5 × 104 V/cm to 8 × 105 V/cm. After a collision of 300 fs, apart from a phase shift, the exciton still exists. Then, the originally carriers is dissociated when the electric field reaches 0.8 MV/cm. The applied surface field is able to effectively keep the excitons from fusion with the transporting charged polarons, which provides a valid and easily manufactured approach to yield higher efficiency of polymer solar cells.

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. Our numerical analysis demonstrates the possibility of a satisfactory description of the evolution of the pulse field at large times by means of a three-dimensional generalization of the sine-Gordon and double sine-Gordon equations.

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. Based on the mechanism, oscillations of the energy level structure, induced by the external excitation, can be designed as an ultrafast response optical convertor that is able to change the external optical pulse into a new effective light field with a certain oscillation period. This helps provide new insight into designing nanostructures for polymeric optoelectronics.

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

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. We also discuss the specific features of heating kinetics of nanoparticles, such as a short time scale for heating and cooling of nanoparticles in a liquid biological environment, and the effect of the radiation field structure on the heating kinetics by single-pulse and multipulse RF radiation. FROM THE CLINICAL EDITOR: In this study a phenomenological approach was applied to estimate the absorption efficiency of radiofrequency radiation (RF) by metal and dielectric nanoparticles. Such nanoparticles can be designed and used for therapeutic purposes, like for localized heating and to activate nanoparticles by RF. The authors also discuss the differences in heating kinetics using single-pulse and multi-pulse RF radiation.

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]. Our solutions are complementary to the known solutions for the case of perfect resonance.

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. Since the output from most medical diagnostic x-ray devices follows a wide and filtered spectrum of photon energies, we introduce and compute the effective intensity-absorption-efficiency values for gold nanoparticles of radii varying from 5 to 50 nm, where we use the TASMIP model to integrate over all spectral energies generated by typical tungsten anode x-ray tubes with kilovolt potentials ranging from 50 to 150 kVp.

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. The strong coupling field is switched on first to resonantly excite the transition between |b} and |c}. After a delay, the probe is switched on; the coherent interaction between the coupling and probe fields traps the population in |a} and |c}. This forms a partially dark state in C(60), analogous to that in atomic vapors. Turning off the coupling field restores C(60)'s absorption. Pulsed lasers work as well. We use two pulses to steer the system into a dark state; when we send in a cw probe field, the electric polarization of C(60) plunges by five times, in comparison with the noncontrol case. This should be detectable experimentally.

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. This paper explicitly depicts the dynamic fluorescence spectra of the radiative transition of the charged triplet exciton occurring during the decay of the charged exciton, and also exhibits the difference between traditional adiabatic dynamics and non-adiabatic dynamics.

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. Different from the phonon dispersion, the spin-phonon dispersion has both positive and negative frequency branches even in the equilibrium ground state, which correspond to the spin enhancement and spin reduction, respectively. Our study of bcc Fe and hcp Co reveals that the spin force matrix, that is, the second-order spatial derivative of spin moment, is similar to the vibrational force matrix, but its diagonal elements are smaller than the off-diagonal ones. This leads to the distinctive spin-phonon dispersion. The concept of spin-phonon dispersion expands the traditional Elliott-Yafet theory in nonmagnetic materials to the entire Brillouin zone in magnetic materials, thus opening the door to excited states in systems such as CoF2and NiO, where a strong spin-lattice coupling is detected in the THz regime.

Modeling nanophotothermal therapy: kinetics of thermal ablation of healthy and cancerous cell organelles and gold nanoparticles.

Nanoparticles are being researched as a noninvasive method for selectively killing cancer cells. With particular antibody coatings on nanoparticles, they attach to the abnormal cells of interest (cancer or otherwise). Once attached, nanoparticles can be heated with ultraviolet-visible/infrared or radiofrequency pulses, heating the surrounding area of the cell to its point of death. Researchers often use single-pulse or multi-pulse modes of laser heating when conducting nanoparticle ablation research. In this article, time-dependent simulations and detailed analyses are carried out for different nonstationary pulsed laser-nanoparticle interaction modes, and the advantages and disadvantages of single-pulse and multi-pulse (set of short pulses) laser heating of nanoparticles are shown. Simulations are performed for the metal nanoparticles in the biological surrounding medium as well as for healthy and cancerous cell organelles. FROM THE CLINICAL EDITOR: External laser pulses can be used to generate heating of targeted metal nanoparticles for thermal ablation therapy of cancers, however the approach used in individual studies is idiosyncratic. In this manuscript, time-dependent simulations and analyses are used to determine the pros and cons of single versus multiple laser pulses for differential impact of healthy versus cancerous cell organelles.

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/cm2 and 20 µJ/cm2 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. Up to 100 fs, both alternating bonds and the excited electronic state are well localized in the middle of the polymer chain. In the first ~200 fs, the strong lattice vibration makes a local phonon mode at 1097.7 cm-1 appear in the phonon spectrum. The change of electron states then induces the self-trapping effect to act on the following photoexcitation process of 1.2 ps. During the following relaxation of 1.0 ps, new local infrared phonon modes begin to occur. All of this, incorporated with the occurrence of local infrared phonon modes and localized electronic states at the end of the relaxation, results in completed exciton formation.

Remote sensing image registration approach based on a retrofitted SIFT algorithm and Lissajous-curve trajectories.

Through retrofitting the descriptor of a scale-invariant feature transform (SIFT) and developing a new similarity measure function based on trajectories generated from Lissajous curves, a new remote sensing image registration approach is constructed, which is more robust and accurate than prior approaches. In complex cases where the correct rate of feature matching is below 20%, the retrofitted SIFT descriptor improves the correct rate to nearly 100%. Mostly, the similarity measure function makes it possible to quantitatively analyze the temporary change of the same geographic position.

Optically-controlled spin valves in conjugated polymers.

In this article, two optically-controlled spin transfer effects are proposed for pi-conjugated polymers. When such a polymeric molecule undergoes two-photon excitation, the charge of a spin carrier can be reversed, and simultaneously an applied external electric field drives the charge-reversed spin carrier to move in the opposite direction. As for a spinless carrier, the photoexcitation dissociates it into two spin carriers, forming entanglement. The coupling between the newly produced spin carriers and a ferromagnet will change the magnetoresistance. Both the fissions of spinless and spin carriers are ultrafast dynamical processes. By combining an electric field, magnetic field, and photoexcitation, two generic optically-controlled ultrafast response organic spin valves are designed.

Nonlinear optical response and ultrafast dynamics in C60.

C(60) is a symbol of nanoscience. Its impact is far beyond the initial interest in this nanometer molecule itself. This paper reviews the current status of the nonlinear optical and ultrafast dynamics investigations of C(60). The review starts with the nonlinear optical response, in particular the dispersion spectra of harmonic generation and two-photon absorption. Both the experimental and theoretical challenges are highlighted. The main focus is on femtosecond and picosecond degenerate and nondegenerate four-wave mixing and pump-probe techniques as a tool to investigate ultrafast electron and nuclear dynamics, charge transfer and photoexcitation in C(60). Theoretical investigations are essential to understand these processes. Theory predicts a longer relaxation time for charge transfer than photoexcitation and reveals the underlying reasons for the normal mode excitations and changes in the electron density of states, which is directly linked to the time-resolved photoemission spectra. Theory also resolves a long-time puzzle about the dependence of normal mode excitations on the laser pulse duration and predicts that time-resolved pump-probe spectroscopy is able to probe electron correlation effects. Finally, high-harmonic generation in C(60) is reviewed. The review concludes with prospectives and possible applications.

Activation processes with memory.

We propose a mathematical treatment of the activated processes governed by stochastic Langevin dynamics with a colored random force, corresponding to a noise generated by an Ornstein-Uhlenbeck process. Such non-Markovian dynamics take place in a variety of chemical and biological systems. Using the path integral approach, we constructed the conditional probability for passing between two stationary states in configurational space. Our relations can be used for Monte Carlo sampling of evolution trajectories for systems with many degrees of freedom as well as for determining the reaction coordinate used in transition state theory. On the basis of our relation for a conditional probability, we generalize the method of determining the most probable path to the case of colored random force. Using the simple three-hole potential, we examine numerically the effect of nonzero correlation time (memory) on the evolution of the most probable path for a finite temperature.

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. The recent literature of nanomaterial-doped NLCs is rich with various kinds of nanomaterials in a variety of NLCs. The light has been focused on the most widely used and studied gold nanoparticles in NLCs. The intrinsic inherent property of easy excitation of surface plasmons polaritons (SPP) is the mediating interaction of NLC electric dipoles and the polarization of charges in the GNP surface. The concepts and methods for the application of metal nanomaterials as dopants in NLCs are discussed for future applications, especially biosensors. The biosensing application of NLCs alone has already been proven in the literature. However, it is always desirable to further enhance the detection efficiency and selectivity, which have been achieved by the conjugation of GNPs and nickel nanoparticles with NLCs and their compatibility with biological materials. This aspect of future application of nanoparticles and NLC makes the point more selective to be included in the present manuscript.

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. When an external laser beam of 60 µJ/cm2 is applied to photoexcite the polymer, the energy levels begin to oscillate within 100 fs. Thanks to the prominent self-trapping effect of the conjugated polymer, the laser beam also drives the lattice of the polymer chain to strongly vibrate. Until about 200 fs, four energy levels are pulled to the center of the energy gap, resulting in a transient discrete four-energy-level electronic structure for ASE. Simultaneously, in this ultrafast dynamical process, within the first 1 ps, inversion of the electron population occurs along with the appearance of a localized electronic cloud and locally distorted alternating bonds of the polymer chain. This detailed description of the whole ultrafast process of ASE helps in the understanding of the microscopic dynamical evolution of excitation in conjugated polymers.

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. While the experimental results have been inconclusive, theoretical investigations are even scarcer, beyond those earlier studies based on spin superdiffusion. For instance, we do not even know how fast electrons move under laser excitation and how far they move. Here we carry out a first-principles time-dependent calculation to investigate how fast electrons actually move under laser excitation and how large the electron transport affects demagnetization on the shortest time scale. To take into account the transport effect, we implement the intraband transition in our theory. In the bulk fcc Ni, we find the effect of the spin transport on the demagnetization is extremely small, no more than 1%. The collective electron velocity in Ni is 0.4 Å fs-1, much smaller than the Fermi velocity, and the collective displacement is no more than 0.1 Å. But this does not mean that electrons do not travel fast; instead we find that electron velocities at two opposite crystal momenta cancel each other. We follow the Γ-X line and find a huge dispersion in the velocities in the crystal momentum space. In the Fe/W(1 1 0) thin film, the overall demagnetization is larger than Ni, and the Fermi velocity is higher than Ni. However, the effect of the spin transport is still small in the Fe/W(1 1 0) thin film. Based on our numerical results and existing experimental findings, we propose a different mechanism that can explain two latest experimental results. Our finding sheds new light on the effect of ballistic transport on demagnetization.