Photon Absorption Spectra Research Articles

Anomalous spectral response of plasmon-exciton strong coupling beyond J-C model

The strong coupling between plasmons and excitons has aroused tremendous interest in recent years, due to its implications for diverse important quantum technologies. Understanding the spectral response properties of these strongly coupled systems is crucial for both the fundamental physics science and the practical applications. Here, we numerically and theoretically demonstrate the spectrum responses of plasmon-exciton strong coupling beyond the J-C model based upon the Au nanorod/dye molecules hybrid system. With increasing the number or oscillator strength of dye molecules, a third/fourth mode in the normal Rabi splitting emerges. These anomalous spectrum Rabi splitting beyond the J-C model are revealed by a unified quantum theory model, which gives the quantified contributions from the plasmon and exciton channels to the total photon absorption spectrum of the hybrid system. Our work not only enriches the understanding of the plasmon-exciton strong coupling, but also may stimulate further researches in this field.

A three-photon probe for highly selective and sensitive detection of Ag+ bearing an AIE fluorophore

It is well known that the silver ions (Ag+) are important in the human body, however, novel probes were lacked with good water solubility, biocompatibility and deep penetration to detect Ag+ in the organism. In this work, we report two new three-photon absorption AIE luminogens (AIEgen), abbreviated as Tpy and Tpy-Hex, whereby Tpy-Hex could detect intracellular and environmental Ag+ quantitatively at a micromolar level. Tpy-Hex binding with Ag+ possesses enhanced three-photon absorption cross-section under1550 nm excitation (NIR II window), which is indeed beneficial for deep penetration in vitro and in vivo. The mechanism underlying the changes in three photon absorption spectra caused by bromohexyl chain substitution was further explored. The fluorescence bioimaging results reveal that the proposed probe Tpy-Hex could be ultra-sensitive for detection of Ag+ in HeLa cells and poisoned mice.

Infrared spectra of micro-structured samples with microPhotoacoustic spectroscopy and synchrotron radiation.

Photoacoustic spectroscopy (PAS) measures the photon absorption spectrum of a sample through detection of the acoustic wave generated by the photothermal effect as one modulates the intensity of the incident radiation at each wavelength. We have recently demonstrated the implementation of PAS in a microscopy configuration with mid-infrared radiation (microPAS). In the present work, we describe the performance of microPAS using synchrotron radiation (SR) in diffraction-limited spectromicroscopy and imaging experiments. Spectra were obtained for polystyrene beads, polypropylene fibres, and single fibres of human hair. SR produced microPAS spectra of much higher intensity as compared with those obtained using conventional mid- and near-infrared sources. For hair samples, the penetration depth of mid-infrared light, even with bright SR, is significantly shorter than the probed sample thickness at very low modulation frequencies resulting in saturated PAS spectra. In contrast, microPAS spectra of polymer beads were in general of much better quality than those obtained with conventional sources. We also demonstrated the capability to collect line profiles and line spectra at diffraction limited spatial resolution. The microPAS spectra of beads appear free from appreciable bandshape distortions arising from the real part of the refractive index of the sample. This observation confirms microPAS as an absorption-only technique and establishes it as a valuable new tool in the microspectroscopic analysis of particulates and of samples with a complex topography.

Dynamic Majorana resonances and universal symmetry of nonequilibrium thermoelectric quantum noise

Nonequilibrium states of a nanoscopic system may be achieved by both applying a bias voltage $V$ to its contacts and producing a difference $\Delta T$ in their temperatures. Then the total current results from two competing flows, induced by $V$ and $\Delta T$, respectively. Here we explore finite frequency quantum noise of this thermoelectric current flowing through a quantum dot whose low-energy dynamics is governed by Majorana degrees of freedom. We demonstrate that at finite frequency $\omega$ Majorana zero modes induce a perfect universal symmetry between photon emission and absorption spectra and produce universal thermoelectric resonances in their frequency dependence in contrast to non-Majorana quantum noise which is either asymmetric or non-universal. In particular, at low temperatures the differential thermoelectric quantum noise induced by Majorana zero modes shows resonances with a nontrivial maximum $\frac{e^3}{h}\log(2^{1/4})$ at $\omega=\mp\frac{|eV|}{\hbar}$ when $k_\text{B}\Delta T\ll|eV|$. Our results challenge cutting-edge experiments using quantum noise detectors to reveal the universal spectral symmetry and resonant structure of the Majorana thermoelectric finite frequency quantum noise.

First- and second-order photon–phonon interactions and optical parameters of ZnTe crystal: a broadband terahertz time-domain spectroscopy study

We performed broadband terahertz (THz) time-domain spectroscopy measurements to study the photon–phonon interactions and frequency-resolved optical parameters of bulk ZnTe. Using laser-induced plasma as the source of THz radiation and a GaP bulk wafer as the sensor crystal, a broadband THz response of bulk ZnTe wafer with thickness of 0.1 millimeter within the frequency range from 0.1 to 7.5 THz was obtained. Comparing the measured photon absorption spectrum with phonon dispersion curves obtained from ab initio calculations, we identified the first- and second-order photon–phonon interactions of ZnTe crystal. The frequency-resolved optical parameters including the complex refractive index, dielectric response and conductivity of ZnTe crystal within the frequency range between 0.1 and 7.0 THz were derived from the measured THz transmitted field. By fitting the frequency dependent dielectric functions with the Lorentz oscillator model, a static permittivity of 10.5 along with a transverse optical phonon frequency of 5.9 THz and a phonon lifetime of 1.25 picosecond were obtained for ZnTe crystal.

Majorana finite-frequency nonequilibrium quantum noise

Quantum finite frequency noise is one of fundamental aspects in quantum measurements performed during quantum information processing where currently Majorana bound states offer an efficient way to implement fault-tolerant quantum computation via topological protection from decoherence or unitary errors. Thus a detailed exploration of Majorana finite frequency noise spectra, preferably in a nonequilibrium device, is a timely challenge of fundamental importance. Here we present results on finite frequency differential noise that is the derivative of the noise with respect to the frequency. This quantity has universal units of $e^2$ and scans in high detail all peculiarities of the Majorana noise clearly demonstrating its universal finite frequency features. In particular, we provide photon absorption spectra on all energy scales and reveal a rich structure including universal Majorana plateaus as well as universal Majorana resonances and antiresonances at characteristic frequencies. Our results are of immediate interest to state-of-the-art experiments involving quantum noise mesoscopic detectors able to separately measure photon absorption and emission spectra.

Two-Photon Absorption and Two-Photon Circular Dichroism of a Hexahelicene Derivative with a Terminal Donor-Phenyl-Acceptor Motif.

Herein, we report on the theoretical-experimental analysis of the two photon absorption and circular dichroism spectra of 1-(2-pyridyl)-4-methoxy-carbo[6]helicene derivative (P6). The primary outcomes of our investigation on this particular helicene derivative with a donor-acceptor motif on one end led to two important conclusions: (1) the lengthening of the π-electron delocalization within the helical core of P6 predominantly increases the contribution of the magnetic dipole transition moment to the two-photon circular dichroism (TPCD) signal; and (2) the electric quadrupole transition moment contribution to the TPCD signal is enhanced by the intramolecular charge transfer (ICT) produced by the donor-acceptor combination on one end of the molecule. To corroborate our results, we performed a comparative theoretical analysis of the effect of the energy gap and ICT on TPCD on a series of P6-like helicenes with different donor-acceptor combinations. Two-photon absorption and TPCD spectra were obtained using the double L-scan technique over a broad spectral range (400-900 nm) using 90 fs pulses at a low repetition rate (2-50 Hz) produced by an amplified femtosecond system. The theoretical simulations were calculated using time dependent density functional theory at the CAM-B3LYP/6-311++G(d,p) level of theory.

Absorption spectrum of a two-level system subjected to a periodic pulse sequence

We investigate how the quantum control of a two-level system (TLS) coupled to photons can modify and tune the TLS's photon absorption spectrum. Tuning and controlling the emission and the absorption is of much interest e.g.\ for the development of efficient interfaces between stationary and flying qubits in modern architectures for quantum computation and quantum communication. We consider the periodic pulse control, where the TLS is subjected to a periodic sequence of the near-resonant Rabi driving pulses, each pulse implementing a 180$^\circ$ rotation. For small inter-pulse delays, the absorption spectrum features a pronounced peak of stimulated emission at the pulse frequency, as well as equidistant satellite peaks with smaller spectral weights. As long as the detuning between the carrier frequency of the driving and the TLS transition frequency remains moderate, this spectral shape shows little change. Therefore, the quantum control allows shifting the absorption peak to a desired position, and locks the absorption peak to the carrier frequency of the driving pulses. Detailed description of the spectrum, and its evolution as a function time, the inter-pulse spacing and the detuning, is presented.

Photon Absorption Enhancement of TiO2 Nanotube Arrays Decorated with Aluminum Nanoparticles

In this paper, well-aligned TiO2 nanotubes (TNTs) decorated with aluminum nanoparticles have been fabricated by electrochemical anodization and subsequently magnetron sputtering technique. UV-vis spectroscopy analysis indicate that photon absorption spectrum of crystallized TNTs with Aluminum nanoparticles is extended to visible light band with strong absorption (wavelength 400 – 800 nm). Photocurrent characteristics of the TNTs/Al show that the plasmonic nanoparticles Al contribute extra electron-hole pairs to enhance photocurrent under visible light illumination, which further indicate electrons generation, separation and transportation in this system. This paper gives a systematic study of the subject for photon absorption enhancement of TiO2 nanotube arrays decorated with aluminum nanoparticles.

Threshold response using modulated continuous wave illumination for multilayer 3D optical data storage

In order to achieve a high capacity 3D optical data storage medium, a nonlinear or threshold writing process is necessary to localize data in the axial dimension. To this end, commercial multilayer discs use thermal ablation of metal films or phase change materials to realize such a threshold process. This paper addresses a threshold writing mechanism relevant to recently reported fluorescence-based data storage in dye-doped co-extruded multilayer films. To gain understanding of the essential physics, single layer spun coat films were used so that the data is easily accessible by analytical techniques. Data were written by attenuating the fluorescence using nanosecond-range exposure times from a 488 nm continuous wave laser overlapping with the single photon absorption spectrum. The threshold writing process was studied over a range of exposure times and intensities, and with different fluorescent dyes. It was found that all of the dyes have a common temperature threshold where fluorescence begins to attenuate, and the physical nature of the thermal process was investigated.

Tunable Properties of Surface Plasmon Resonance of Metal Nanospheroid: Graphene Plasmon Interaction

Graphene plasmonic resonances play a significant role for enhancing the photon absorption inside thin film solar devices. We investigate the field rising at the intersection of graphene physics and plasmonics followed by analytical methods in visible domain. The electromagnetic signature of oblate-shaped silver and gold nanoparticles (smaller size) with graphene-supported silicon substrate are analyzed under the quasi-static approximation. The results show that the magnitude of scattering and surface plasmon resonances depend on the aspect ratio, semi major axis length, and thickness of graphene monolayer. We also analyzed the wavelength-dependent photon absorption spectrum for optimized thickness (1 nm) of graphene monolayer with aspect ratio (0.328) of silver and gold nanoparticles under the AM1.5 solar spectrum.

Enhanced photon absorption of organic solar cells by three-dimensional metallic structures

The photon absorption enhancement of thin organic solar cells with periodic three-dimensional (3D) metal structures in a photoactive medium is theoretically investigated in this study. The suggested metal structures have a match form, a gold sphere on a thin cylindrical post. With a finite difference time method tool, the optimized sizes and positions of the metal structure are obtained in the scattered radiation pattern of the incident light, and the photon absorption spectra are calculated and analyzed. The organic solar cell equipped with the 3D metal structures shows approximately a 10% enhanced photon absorption compared to the value obtained by same solar cells without the 3D metal structures. The reason for the enhanced absorption can be understood based on the electric field distributions and is attributed to the fact that the 3D metal structure can lead to improved absorption at long wavelengths region where almost negligible absorption occurs in the same solar cells without the metal structures.

Graphdiyne as a promising material for detecting amino acids.

The adsorption of glycine, glutamic acid, histidine and phenylalanine on single-layer graphdiyne/ graphene is investigated by ab initio calculations. The results show that for each amino acid molecule, the adsorption energy on graphdiyne is larger than the adsorption energy on graphene and dispersion interactions predominate in the adsorption. Molecular dynamics simulations reveal that at room temperature the amino acid molecules keep migrating and rotating on graphdiyne surface and induce fluctuation in graphdiyne bandgap. Additionally, the photon absorption spectra of graphdiyne-amino-acid systems are investigated. We uncover that the presence of amino acid molecules makes the photon absorption peaks of graphdiyne significantly depressed and shifted. Finally, quantum electronic transport properties of graphdiyne-amino-acid systems are compared with the transport properties of pure graphdiyne. We reveal that the amino acid molecules induce distinct changes in the electronic conductivity of graphdiyne. The results in this paper reveal that graphdiyne is a promising two-dimensional material for sensitively detecting amino acids and may potentially be used in biosensors.

The Changing Colors of a Quantum-Confined Topological Insulator

Bismuth selenide (Bi2Se3) is a 3D topological insulator, its strong spin-orbit coupling resulting in the well-known topologically protected coexistence of gapless metallic surface states and semiconducting bulk states with a band gap, Eg ≃ 300 meV. A fundamental question of considerable importance is how the electronic properties of this material evolve under nanoscale confinement. We report on catalyst-free, high-quality single-crystalline Bi2Se3 with controlled lateral sizes and layer thicknesses that could be tailored down to a few nanometers and a few quintuple layers (QLs), respectively. Energy-resolved photoabsorption spectroscopy (1.5 eV < E(photon) < 6 eV) of these samples reveals a dramatic evolution of the photon absorption spectra as a function of size, transitioning from a featureless metal-like spectrum in the bulk (corresponding to a visually gray color), to one with a remarkably large band gap (Eg ≥ 2.5 eV) and a spectral shape that correspond to orange-red colorations in the smallest samples, similar to those seen in semiconductor nanostructures. We analyze this colorful transition using ab initio density functional theory and tight-binding calculations which corroborate our experimental findings and further suggest that while purely 2D sheets of few QL-thick Bi2Se3 do exhibit small band gaps that are consistent with previous ARPES results, the presently observed large gaps of a few electronvolts can only result from a combined effect of confinement in all three directions.

Computational Prediction of Structures and Optical Excitations for Nanoscale Ultrasmall ZnS and CdSe Clusters.

Small semiconductor nanoclusters are important for understanding the initial formation and growth of quantum dots and also for application, for example in the tunability provided by size. However, electronic structures and effects of capping ligands have not been systematically characterized. Thus, ground and excited state calculations using coupled-cluster methods were carried out to provide benchmarks for evaluating the applicability of density functional theory (DFT) and time-dependent DFT (TDDFT) with different functionals for the ground and excited states, respectively. Our computed data suggests that the popular B3LYP functional does not deliver optimal results for the ground and excited state. While the PBE0 functional was found to provide a good description for both the ground and excited states for small bare (ZnS)n and bare and ligated (CdSe)n clusters, the results for the hydrated (ZnS)n clusters were found to deteriorate significantly. However, the errors appear to decrease with increasing cluster size. Excitation energies obtained with the long-range hybrid CAM-B3LYP and CA-B3LYP were found to provide more consistent results for both anhydrous and hydrated (ZnS)n clusters. However, their performance in spectral predictions for larger clusters requires further study. Using PBE0, electronic structures of the ground and excited states for (ZnS)n and (CdSe)n up to n = 37 using DFT and TDDFT, respectively, were re-examined. With the exception of the cage-core (ZnS)13, (CdSe)13, and (CdSe)14, small (ZnS)n and (CdSe)n are predicted to be spheroids and tubular structures (6, 8-12, 15-19) with squares and hexagons, similar to the structures of carbon single-wall nanotubes. Wurtzite (n = 23-27, 36, 37) and cage-core (n = 29-35) structures are energetically more favorable for larger clusters. We find that water and amines increase the intensities and blue shift the excitations of bare clusters. One photon absorption spectra predicted by TDDFT with the PCM solvation model for (CdSe-methylamine)13 and the larger ligated (CdSe)33 are consistent with the experimental spectra.

First-principles simulations of two photon absorption spectra of dynamic structural chromophores in green fluorescent protein

The bridge photoisomerization of the chromophores of fluorescent proteins has been suggested as the possible mechanism of radiationless decay in fluorescent proteins. It indicates that this internal structure changing of chromophores great influence the optical properties of fluorescent proteins. The two-photon absorption (TPA) properties of fluorescent proteins might also be influenced by the bridge photoisomerization of the chromophores. In this work, we simulate the dynamic conformations through rotating the bridge bond of chromophore of green fluorescent protein, and employ the time dependent density functional theory combining with the sum-over-states method to study their TPA characters. With our study, we find that the TPA characters of chromophore will be improved through controlling rotation of the bridge bond of chromophore. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Non-Condon Effects in the One- and Two-Photon Absorption Spectra of the Green Fluorescent Protein

We report calculations of one and two photon absorption (OPA and TPA, respectively) spectra of the green fluorescent protein (GFP) chromophore using the double-harmonic parallel-mode approximation and including explicit dependence of the electronic transition moments on nuclear geometry. The non-Condon effects are found to be more significant for TPA resulting in a different shape of the spectra and a blue shift of 500 cm−1 of the TPA peak absorption relative to OPA. The computed shift is in excellent agreement with the experimentally observed 700 cm−1.

Dimers of polar chromophores in solution: role of excitonic interactions in one- and two-photon absorption properties

The possibility to exploit a bottom-up approach to design and synthesize multichromophoric structures from a single molecular unit is strategic for the targeted synthesis of molecular compounds with well defined linear and nonlinear absorption properties. In this view, it is important to be able to predict the properties of multichromophoric units, based on the knowledge of the properties of the individual chromophores and their mutual arrangement. To this end, we present a combined experimental and theoretical study on 4-(para-di-n-butylaminostyryl)-pyridine, a push-pull molecule, and its dimer, 4,4'-bis(para-di-n-butylaminostyryl)-2,2'-bipyridine, formed by connecting the two pyridine groups into a bipyridine structure. One photon absorption and fluorescence spectra are measured in solvents of different polarity, and two-photon absorption spectra are recorded in dichloromethane. Experimental results are compared with results of TDDFT (Time-Dependent Density Functional Theory) and CIS (Configuration Interaction with Single excitation) methods implemented in the Gaussian03 program suite. An essential-state analysis of optical spectra is used to rationalize the observed behavior.

Phonon-coupling enhanced absorption of alloyed amorphous silicon for solar photovoltaics

Using the observed temperature dependence of $a\text{-Si}:\text{H}$ photon absorption spectrum and the weak-phonon interaction second-order transition theory, phonon-coupling enhanced photon absorption is predicted for $a\text{-Si-Ge}$ and $a\text{-Si-Sn}$ alloys. The ab initio calculated electron and phonon properties of the alloyed amorphous phase show minimally altered electronic states and significant redshifted phonon energies. This phonon shift enhances the optical phonon-coupled photon absorption resulting in increased current generation near the optical band edge. We find that this enhancement favors low-energy optical-phonon modes, thus making the soft-bond forming Sn the choice alloying element.

Validation of the DFT/N07D computational model on the magnetic, vibrational and electronic properties of vinyl radical

The DFT/N07D computational model has been applied to the study of molecular structure, vibrational frequencies and hyperfine coupling constants of vinyl radical in the ground electronic state, taking into account vibrational effects beyond harmonic approximation. Moreover, the TD-DFT/N07D approach has been applied to the study of structures and properties of the title radical in excited electronic states. Vertical and adiabatic electronic excitation energies have been computed and the vibrationally resolved one photon absorption spectrum of the A(2)A''<--X[combining tilde](2)A(1) electronic transition has been simulated. Computational results have been compared with the available experimental data and theoretical studies at the coupled-cluster and MRCI levels, showing remarkable accuracy of the TD-DFT/DFT//N07D model. This finding paves the route toward reliable studies of magnetic, vibrational and electronic properties for quite large unstable open-shell species of current biological and/or technological interest in their ground and excited electronic states.