Optical Line Shapes Research Articles

Encapsulation of Poly(3-hexylthiophene) J-Aggregate Nanofibers with an Amphiphilic Block Copolymer

Poly(3-hexylthiophene) (P3HT) nanofibers (NF) displaying J-aggregate exciton coupling behavior are encapsulated with the amphiphilic block copolymer (BCP), poly(3-hexyl-thiophene)-block-poly(ethylene-glycol), (PHT(20)-b-PEG(108)). Encapsulation results in the formation of hierarchical superstructures, and the BCP coating is expected to exert a mild chemical pressure on the periphery of the NFs. Photoluminescence from encapsulated NF superstructures show line shape distortions due to self-absorption of the 0-0 transition which is consistent with preservation of J-aggregate character (intrachain order). Detailed resonance Raman spectra of encapsulated BCP-NF structures show no discernible changes in the P3HT aggregation state, and overtone and combination bands involving the symmetric stretching C═C (~1450 cm(-1)) and C-C (~1380 cm(-1)) backbone modes are observed. These features permit quantitative estimates of vibrational mode-specific excited state structural displacements using a time-dependent Raman intensity analysis which is not possible from conventional vibronic analysis of optical lineshapes.

Properties of the optical quantum transitions of ZnS and Ge for an electron-piezoeletric interaction system under two circularly oscillating fields

We study the optical quantum transition line shapes (QTLSs) and the optical quantum transition line widths (QTLWs) in relation to magnetic-field-dependent properties of the electron-deformation potential phonon interaction system. We consider two systems — one is subject to right circularly oscillating external fields and the other is subject to left circularly oscillatory external fields. The main purpose of this work is to compare QTLSs, which indicate absorption power, in the two oscillating external fields. Our results indicate that the QTLSs of right circularly oscillating external fields are larger than the QTLSs of left circularly oscillating external fields while the opposite result is obtained for the QTLWs. Through the analysis of this work, we found the properties of the QTLW and the QTLS of ZnS and Ge increased with increasing temperature and the magnetic fields. We also found the dominant scattering process to be the phonon emission transition process.

Modeling of Resonant Hole-Burning Spectra in Excitonically Coupled Systems: The Effects of Energy-Transfer Broadening

Resonant hole-burned (HB) spectra provide detailed insight into optical line shapes and single-site absorption spectra. In this work, a new method is presented for modeling low-fluence resonant HB spectra in excitonically coupled systems. Our calculations make use of the line shape theory of Renger and Marcus [J. Chem. Phys. 2002, 116, 9997−10019] to include explicitly the effects of excitonic interactions and energy transfer. We find that an accurate treatment of lifetime broadening is crucial for reproducing resonant hole shapes and zero-phonon action spectra. Representative spectra are calculated for a series of simple dimers, with a description and physical explanation of the main features of the calculated spectra. Qualitatively, the results are in excellent agreement with experimental literature data. It is anticipated that the new method will provide both more detailed insight into the excitonic information encoded in resonant HB spectra and a means of testing theoretical treatments of excitonic op...

Magnetic dependence of cyclotron transition absorption in piezoelectric materials based on the quantum transport theory

We investigated theoretically the magnetic field dependence of the quantum optical transition of quasi 2-Dimensional Landau splitting system, in CdS and ZnO. Through the analysis of the current work, we found the increasing properties of the optical Quantum Transition Line Shapes (QTLSs) which show the absorption power and the Quantum Transition Line Widths(QTLWs) with the magnetic-field in CdS and ZnO. We also found that QTLW, γ( B) total of CdS < γ( B) total of ZnO in the magnetic field region B < 25 Tesla.

Faraday rotation spectrometer with sub-second response time for detection of nitric oxide using a cw DFB quantum cascade laser at 5.33 μm

A Faraday modulation spectrometer for sensitive and fast detection of nitric oxide at 5.33 μm utilizing a room temperature continuous wave distributed feedback quantum cascade laser and a Peltier cooled MCT detector is presented. The magnetic field was modulated at 7.4 kHz whereas the laser wavelength was scanned at 20 Hz across the most favorable rotational-vibrational transition for FAMOS, Q3/2(3/2), at 5.33 μm. Using a 15 cm optical path and lineshape fitting, the spectrometer provides a detection limit of 4.5 ppb for a response time of 1 s. An Allan variance analysis demonstrates that the system has an excellent stability, up to several hours of operation.

Optical phonon lineshapes and transport in metallic carbon nanotubes under high bias voltage

We calculate the current-voltage characteristic of metallic nanotubes lying on a substrate at high bias voltage showing that a bottleneck exists for short nanotubes in contrast to large ones. We attribute this to a redistribution of lower-lying acoustic phonons caused by phonon-phonon scattering with hot optical phonons. The current-voltage characteristic and the electron and phonon distribution functions are derived analytically, and serve to obtain in a self-contained way the frequency shift and line broadening of the zone-center optical phonons due to the electron-phonon coupling at high bias. We obtain a positive offset on the zero bias shift and no broadening of the optical phonon mode at very high voltages, in agreement with recent experiments.

The Temperature Dependence of the Quantum Transition Properties of the Quasi-two-dimensional System in Quasi-two-dimensional Semiconductors

We investigated theoretically the temperature dependence of the quantum optical tran- sition of quasi-2-dimensional Landau splitting, in GaN and GaAs. We apply the quantum transport theory (QTR) to a system of electrons confined in a potential. We use the projected Liouville equa- tion method with equilibrium average projection scheme (EAPS). Through the analysis of this work, we found increasing optical quantum transition line shapes (QTLSs) and the quantum tran- sition line widths (QTLWs) in GaN and GaAs with increasing temperature. We also found the temperature dependences of the scattering factor functions: (T) of GaN < (T) of GaAs in the temperature region 50 K < T and (T) of GaAs < (T) of GaN in the temperature T < 50 K and = 394 µm.

Optical line shapes for polymethine dyes and their aggregates: Novel theory of quantum transitions and its correlation with experiment

Abstract This work reports how a proper theoretical definition of electron–nuclear(–phonon) coupling in the dynamics of the transient state in quantum transitions enables an explanation for the basic experimental data on the optical line shapes for polymethine dyes and their dimers as well as H-, H*-, and J-aggregates, which defied understanding for many decades.

Excitons in Molecular Aggregates with Lévy-Type Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra

We predict the existence of exchange broadening of optical line shapes in disordered molecular aggregates and a nonuniversal disorder scaling of the localization characteristics of the collective electronic excitations (excitons). These phenomena occur for heavy-tailed Lévy disorder distributions with divergent second moments-distributions that play a role in many branches of physics. Our results sharply contrast with aggregate models commonly analyzed, where the second moment is finite. They bear a relevance for other types of collective excitations as well.

Impurity-related photoluminescence line shape asymmetry in GaAs/AlAs multiple quantum wells: Fractional-dimensional space approach

The optical transitions in 20 nm wide silicon and beryllium δ-doped GaAs/AlAs multiple quantum wells with various doping levels were investigated at different excitation intensities. A fractional dimensionality model was used to describe the free hole-donor and free electron-acceptor transitions in the quantum wells. The measured photoluminescence spectra from samples of different doping level related to donor-impurity or acceptor-impurity induced effects in the photoluminescence lineshape, were compared within the framework of these model calculations. Both experimentally and theoretically it was shown that acceptor and donor related optical transitions and photoluminescence line shapes were related to the difference in the effective masses of holes and electrons. This effect also leads to a difference in the photoluminescence spectra in which the luminescence band for the donor related spectrum is narrower in comparison to the acceptor related spectrum.

Crystal field splitting on D↔S transitions of atomic manganese isolated in solid krypton

Narrow excitation features present on the [Ar]3d64s1aD(J=9∕2−1∕2)6←[Ar]3d54s2aS1∕26 transitions of manganese atoms isolated in solid Kr are analyzed within the framework of weak crystal field splitting. Use of the Wp optical lineshape function allowed identification of multiple zero-phonon lines for individual spin-orbit J states of the a aD6←aS6 transition recorded with laser-induced excitation spectroscopy. Excellent agreement exists between the predicted crystal field splitting patterns for the J levels of the aD6 state isolated in the «red» tetravacancy site of solid Kr. The tetrahedral crystal field of the «red» trapping site splits J&amp;gt;3∕2 levels of the aDJ6 and aD7∕24 states by approximately 30cm−1. This report represents the first definitive evidence of crystal field splitting, induced by the weak van der Waals interactions between a neutral metal atom and the rare gas atoms surrounding it in a well-defined solid-state site.

Microscopic theory of multiple-phonon-mediated dephasing and relaxation of quantum dots near a photonic band gap

We derive a quantum theory of the role of acoustic and optical phonons in modifying the optical absorption line shape, polarization dynamics, and population dynamics of a two-level atom (quantum dot) in the ``colored'' electromagnetic vacuum of a photonic band-gap (PBG) material. This is based on a microscopic Hamiltonian describing both radiative and vibrational processes quantum mechanically. We elucidate the extent to which phonon-assisted decay limits the lifetime of a single photon-atom bound state and derive the modified spontaneous emission dynamics due to coupling to various phonon baths. We demonstrate that coherent interaction with undamped phonons can lead to an enhanced lifetime of a photon-atom bound state in a PBG. This results in reduction of the steady-state atomic polarization but an increase in the fractionalized upper state population in the photon-atom bound state. We demonstrate, on the other hand, that the lifetime of the photon-atom bound state in a PBG is limited by the lifetime of phonons due to lattice anharmonicities (breakup of phonons into lower energy phonons) and purely nonradiative decay. We also derive the modified polarization decay and dephasing rates in the presence of such damping. This leads to a microscopic, quantum theory of the optical absorption line shapes. Our model and formalism provide a starting point for describing dephasing and relaxation in the presence of external coherent fields and multiple quantum dot interactions in electromagnetic reservoirs with radiative memory effects.

Structure-Based Calculations of Optical Spectra of Photosystem I Suggest an Asymmetric Light-Harvesting Process

Optical line shape theory is combined with a quantum-chemical/electrostatic calculation of the site energies of the 96 chlorophyll a pigments and their excitonic couplings to simulate optical spectra of photosystem I core complexes from Thermosynechococcus elongatus. The absorbance, linear dichroism and circular dichroism spectra, calculated on the basis of the 2.5 A crystal structure, match the experimental data semiquantitatively allowing for a detailed analysis of the pigment-protein interaction. The majority of site energies are determined by multiple interactions with a large number (>20) of amino acid residues, a result which demonstrates the importance of long-range electrostatic interactions. The low-energy exciton states of the antenna are found to be located at a nearest distance of about 25 A from the special pair of the reaction center. The intermediate pigments form a high-energy bridge, the site energies of which are stabilized by a particularly large number (>100) of amino acid residues. The concentration of low energy exciton states in the antenna is larger on the side of the A-branch of the reaction center, implying an asymmetric delivery of excitation energy to the latter. This asymmetry in light-harvesting may provide the key for understanding the asymmetric use of the two branches in primary electron transfer reactions. Experiments are suggested to check for this possibility.

THE THERMAL OPTICAL QUANTUM TRANSITION PROPERTIES OF PIEZOELECTRIC MATERIALS UNDER OSCILLATING EXTERNAL FIELD

ABSTRACT We consider two systems—the one system is subject to circularly polarized oscillatory external fields and the other system is subject to linearly polarized oscillatory external field. The main purpose of this work is that comparisons of the optical quantum transition line shapes (QTLS) which show the absorption power and the quantum transition line widths (QTLW) in both directional polarized oscillatory external field. Our results indicate that the QTLS of circularly polarized oscillatory external field is larger than the QTLS of linearly polarized oscillatory external field, while the opposite result is obtained for the QTLW. In both directional polarized oscillatory external field, we analyze the temperature dependence of the QTLS and the QTLW in various cases. In order to analyze the quantum transition, we compare the temperature dependence of the QTLW and QTLS of two transition process, the intra Landau level transition process and the inter Landau level transition process. We study the thermal properties of electron-piezoelectric phonon interacting system in the confinement of electrons by square well confinement potential. We use the projected Liouville equation method with Equilibrium Average ProjectionScheme (EAPS).

THE MAGNETIC FIELD DEPENDENCE PROPERTIES OF ELECTRON-PIEZOELECTRIC PHONON INTERACTING MATERIALS IN SQUARE WELL CONFINEMENT POTENTIAL

ABSTRACT We consider the system is subject to the linearly polarized oscillatory external field. We study the optical quantum transition line shapes (QTLS) which show the absorption power and the quantum transition line widths (QTLW) of electron-piezoelectric phonon interacting system. We analyze the magnetic field dependence of the QTLS and the QTLW in various cases. In order to analyze the quantum transition, we compare the magnetic field dependence of the QTLW and QTLS of two transition process, the intra-Landau level transition process and the inter-Landau level transition process.

Two-Dimensional Electronic Spectroscopy of β-Carotene

Two-dimensional electronic spectroscopy (2D) has been applied to beta-carotene in solution to shine new light on the ultrafast energy dissipation network in carotenoids. The ability of 2D to relieve spectral congestion provides new experimental grounds for resolving the rise of the excited state absorption signal between 18,000 and 19,000 cm(-1). In this spectral region, the pump-probe signals from ground state bleach and stimulated emission overlap strongly. Combined modeling of the time-evolution of 2D spectra as well as comparison to published pump-probe data allow us to draw conclusions on both the electronic structure of beta-carotene as well as the spectral densities giving rise to the observed optical lineshapes. To account for the experimental observations on all time scales, we need to include a transition in the visible spectral range from the first optically allowed excited state (S(2)-->S(n2)). We present data from frequency resolved transient grating and pump-probe experiments confirming the importance of this transition. Furthermore, we investigate the role and nature of the S* state, controversially debated in numerous previous studies. On the basis of the analysis of Feynman diagrams, we show that the properties of S*-related signals in chi(3) techniques like pump-probe and 2D can only be accounted for if S* is an excited electronic state. Against this background, we discuss a new interpretation of pump-deplete-probe and intensity-dependent pump-probe experiments.

Phonon Confinement in Stressed Silicon Nanocluster

Confined acoustic and optical phonons in Si nanoclusters embedded in sapphire, synthesized using ion-beam implantation are investigated using Raman spectroscopy. The l=0 and l=2 confined acoustic phonons, found at low Raman shift, are analyzed using complex frequency model and the size of the nanoparticles are estimated as 4 and 6 nm. For the confined optical phonon, in contrast to expected red shift, the Raman line shape shows a substantial blue shift, which is attributed to size dependent compressive stress in the nanoparticles. The calculated Raman line shape for the stressed nanoparticles fits well to data. The sizes of Si nanoparticles obtained using complex frequency model are consistent with the size estimated from the fitting of confined optical phonon line shapes and those found from X-ray diffraction and TEM.

Optical line shapes of molecular aggregates: Hierarchical equations of motion method

The absorption line shapes of model molecular aggregates are investigated using the recently developed Liouville space hierarchical equations of motion (HEOM) method. The exact results are further exploited for the assessment of several approximation schemes, including the high temperature approximation of HEOM, the stochastic Liouville equation approach, and the perturbative time-local and time-nonlocal quantum master equations (QMEs). The calculations on dimers, larger ring-shaped aggregates, and a model of the B850 ring in the LH2 of purple bacteria show that while the other approximate methods can give reasonable absorption line shapes over a wide range of parameter regimes, the second-order time-nonlocal QME is generally inaccurate and may give spurious peaks in the absorption spectra.

Wavelength-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signal line shapes in the Doppler limit

A thorough analysis of the shape and strength of Doppler-broadened wavelength-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signals is presented, and their dependence on modulation frequency, modulation amplitude, and detection phase is investigated in detail. The conditions that maximize the on-resonance signal are identified. The analysis is based on standard frequency modulation spectroscopy formalism and the Fourier description of wavelength modulation spectroscopy and is verified by fits to experimental signals from C2H2 and CO2 measured at 1531 nm. In addition, the line strengths of two CO2 transitions in the ν2→3ν1+ν2+ν3 hot band [Pe(7) and Pe(9)] were found to differ by ~20% from those given in the HITRAN database.

Asymmetrical shapes of optical line profiles in individual quantum dots

The experimental data on individual quantum dots show that the optical line can have the form of a very narrow spike accompanied by a shoulder. So far the shoulder has been found at the lower energy side of the narrow peak. In the present work, we study theoretically the origin of such a lineshape. We shall use a simple model of quantum dot and a simple approximation to the electronic excitation. The electronic system will be assumed to be coupled to the longitudinal optical phonons. We will show that the electronic multiple scattering on the optical phonons can then give us an explanation of the observed optical lineshape.