Resonant Excitation Conditions Research Articles

Spectral Signatures of Ground- and Excited-State Wavepacket Interference after Impulsive Excitation.

Impulsive transient absorption spectroscopy is used to track the formation and evolution of vibrational coherences in cresyl violet perchlorate under different excitation conditions. Resonant and off-resonant pump pulses result in the selective formation of excited (S1)- and ground (S0)-state wavepackets. Partially resonant and broadband excitation conditions lead to the simultaneous formation of wavepackets in the ground and excited states. The wavepackets are characterized by the phase-flips in the coherent signal associated with wavepacket motion across the absorption and emission maxima and by a red shift of 2-10 cm-1 in the Raman features of the excited state compared to the ground-state wavepacket. We observe that, when wavepackets are simultaneously excited on the ground- and excited-state surfaces, interference on a picosecond timescale between coherent oscillations in the two wavepackets gives rise to features that cannot be attributed to the passage of a wavepacket through a conical intersection, such as shifting phase-flips and zero-amplitude nodes. Wavepacket filtering using windowed Fourier transforms highlights these interference effects and demonstrates that special care must be taken in order to properly interpret data that have been processed in this manner.

Purcell-Enhanced and Indistinguishable Single-Photon Generation from Quantum Dots Coupled to On-Chip Integrated Ring Resonators.

Integrated photonic circuits provide a versatile toolbox of functionalities for advanced quantum optics applications. Here, we demonstrate an essential component of such a system in the form of a Purcell-enhanced single-photon source based on a quantum dot coupled to a robust on-chip integrated resonator. For that, we develop GaAs monolithic ring cavities based on distributed Bragg reflector ridge waveguides. Under resonant excitation conditions, we observe an over 2-fold spontaneous emission rate enhancement using Purcell effect and gain a full coherent optical control of a QD-two-level system via Rabi oscillations. Furthermore, we demonstrate an on-demand single-photon generation with strongly suppressed multiphoton emission probability as low as 1% and two-photon interference with visibility up to 95%. This integrated single-photon source can be readily scaled up, promising a realistic pathway for scalable on-chip linear optical quantum simulation, quantum computation, and quantum networks.

Single-molecule resonance Raman effect in a plasmonic nanocavity

Tip-enhanced Raman spectroscopy (TERS) is a versatile tool for chemical analysis at the nanoscale. In earlier TERS experiments, Raman modes with components parallel to the tip were studied based on the strong electric field enhancement along the tip. Perpendicular modes were usually neglected. Here, we investigate an isolated copper naphthalocyanine molecule adsorbed on a triple-layer NaCl on Ag(111) using scanning tunnelling microscope TERS imaging. For flat-lying molecules on NaCl, the Raman images present different patterns depending on the symmetry of the vibrational mode. Our results reveal that components of the electric field perpendicular to the tip should be considered aside from the parallel components. Moreover, under resonance excitation conditions, the perpendicular components can play a substantial role in the enhancement. This single-molecule study in a well-defined environment provides insights into the Raman process at the plasmonic nanocavity, which may be useful in the nanoscale metrology of various molecular systems.

A Divide-and-Conquer Strategy for Quantification of Light Absorption, Scattering, and Emission Properties of Fluorescent Nanomaterials in Solutions.

Optical properties of fluorescent materials including their UV-vis absorption, scattering, and on-resonance fluorescence activities are strongly wavelength-dependent. Reported herein is a divide-and-conquer strategy for experimental quantification of fundamental optical constants of fluorescent nanomaterials including their UV-vis absorption, scattering, and on-resonance-fluorescence (ORF) cross-section spectra and ORF fluorescence and light scattering depolarization spectra. The fluorophore UV-vis extinction spectrum is first divided into a blue and a red wavelength region. The UV-vis extinction cross-section spectrum in the blue wavelength region is decomposed into its absorption and scattering extinction spectra straightforwardly using the established polarized resonance synchronous spectroscopic technique. In its red wavelength region, however, the fluorophores can be simultaneous photon absorbers, scatterers, and anti-Stokes-shifted, on-resonance, and Stokes-shifted fluorescence emitters under the resonance excitation and detection conditions. A polarized anti-Stokes'-shifted, on-resonance, and Stokes'-shifted spectroscopic method is developed for quantifying fluorophore absorption, scattering, one-resonance fluorescence (ORF) cross-section spectra, and scattering and ORF fluorescence depolarization spectra in this wavelength region. Example applications of the presented techniques were demonstrated with fluorescent polystyrene nanoparticles, fluorescent quantum dots, and molecular fluorophores Rhodamine 6G and Eosin Y.

Effect of an External Magnetic Field on the Absorption Efficiency of the RF Power in a Spatially Bounded Inductive Plasma Source

The measured dependences of the equivalent plasma resistance on the external magnetic field (0–50 G) in a 46-cm-diameter RF inductive plasma source operating at frequencies of 2, 4, and 13.56 MHz and a power of 100–500 W are presented. The experiments were carried out in argon at pressures of 0.1–30 mTorr. The presence of the external magnetic field leads to the appearance of resonance domains of efficient RF power absorption corresponding to the conditions of resonance excitation of helicons coupled with Trivelpiece–Gould modes. It is shown that RF power absorption at frequencies of 2 MHz can be optimized by applying an external magnetic field corresponding to the domains of resonance absorption. The effect is enhanced with increasing operating frequency.

Resonance fluorescence from an atomic-quantum-memory compatible single photon source based on GaAs droplet quantum dots

Single photon sources, which are compatible with quantum memories, are important components of quantum networks. In this article, we show optical investigations on isolated GaAs/Al0.25Ga0.75As quantum dots grown via droplet epitaxy, which emit single photons on resonance with the Rb-87-D2 line (780 nm). Under continuous wave resonant excitation conditions, we observe bright, clean, and narrowband resonance fluorescence emission from such a droplet quantum dot. Furthermore, the second-order correlation measurement clearly demonstrates the single photon emission from this resonantly driven transition. Spectrally resolved resonance fluorescence of a similar quantum dot yields a linewidth as narrow as 660 MHz (2.7 μeV), which corresponds to a coherence time of 0.482 ns. The observed linewidth is the smallest reported so far for strain free GaAs quantum dots grown via the droplet method. We believe that this single photon source can be a prime candidate for applications in optical quantum networks.

Kinetics of Low-Temperature Microphotoluminescence of Exciton-Impurity Complexes in CdZnTe Single Crystals

Low-temperature (6.5 K) microphotoluminescence near the intrinsic absorption edge of CdZnTe alloy single crystals is studied under conditions of non-resonant and resonant excitation by picosecond pulses. Characteristic relaxation times for free excitons and exciton-impurity complexes of various types are determined. A significant (by a factor of 4–8) decrease in the photoluminescence signal decay time of exciton-impurity complexes on neutral donors during the transition to the resonant excitation mode is detected. The detected sharp decrease in the photoluminescence signal decay time can indicate the manifestation of collective effects in this system of emitters.

Multiwavelength Raman Spectroscopic Analysis of Superficial Iron-Chromium Oxides Generated Using Laser Irradiation.

In order to characterize iron-chromium oxides generated by laser irradiation on the surface of stainless steel plates, an ultraviolet-visible (UV-Vis) near-infrared (NIR) multiwavelength excitation Raman analysis has been performed using both austenitic SS304 and ferritic SS430 stainless steel samples. Raman spectra were obtained using five different excitation wavelengths from blue (455 nm) to NIR (830 nm). These measurements have allowed us to observe and identify four Raman bands, among which two have not been previously observed for iron-chromium oxides, and characterize the existence of different resonant excitation conditions for the different excitation wavelengths. For example, when using 455 nm as excitation wavelength the band at 485 cm-1 did not show up, although that when using 830 nm as excitation wavelength is a clear characteristic band for iron-chromium oxide. In addition, the dependence of the spectra profile with the excitation wavelength for films and microspheres features was observed. This experimental Raman analysis shows the importance of the excitation wavelength for the characterization of metallic oxides with different features.

Unidirectional Spin-Wave-Propagation-Induced Seebeck Voltage in a PEDOT:PSS/YIG Bilayer

We clarify the physical origin of the dc voltage generation in a bilayer of a conducting polymer film and a micrometer-thick magnetic insulator Y_{3}Fe_{5}O_{12} (YIG) film under ferromagnetic resonance and/or spin wave excitation conditions. The previous attributed mechanism, the inverse spin Hall effect in the polymer [Nat. Mater. 12, 622 (2013)NMAACR1476-112210.1038/nmat3634], is excluded by two control experiments. We find an in-plane temperature gradient in YIG which has the same angular dependence with the generated voltage. Both vanish when the YIG thickness is reduced to a few nanometers. Thus, we argue that the dc voltage is governed by the Seebeck effect in the polymer, where the temperature gradient is created by the nonreciprocal magnetostatic surface spin wave propagation in YIG.

High Q reflection filter using a gradient-index membrane with a grating surface

High Q reflection filter using a gradient-index (GI) membrane with a grating surface is proposed. The thickness of GI membrane is very small comparing with the traditional multilayer reflection filter or the GI reflection filter, and the GI membrane can also break the restriction of the resonant excitation condition of the conventional guided-mode resonance (GMR) filter. High Q filtering features can be maintained even on the high-index substrate. The grating thickness of the GI membrane filter can be used to select the resonance wavelength with different quality factors (QFs), the reflection peak is blue-shifted, and the QF is decreased from 554.4 to 207.8 as the grating thickness is increased from 50 nm to 150 nm. The gradient coefficient of the GI membrane filter can be used to tailor the number of the reflection channels. The resonant excitations of high order waveguide modes confined in the GI membrane are responsible for the high Q filtering properties with multiple channels.

Silver Chloride Encapsulation-Induced Modifications of Raman Modes of Metallicity-Sorted Semiconducting Single-Walled Carbon Nanotubes

The internal channels of semiconducting single-walled carbon nanotubes (SWCNTs) were filled with silver chloride. The filling was confirmed by high-resolution scanning transmission electron microscopy. The filling-induced modifications of Raman modes of SWCNTs were analyzed. The fitting of the radial breathing mode (RBM) and G-bands of Raman spectra of the pristine and filled nanotubes with individual components allowed analyzing in detail the influence of encapsulated silver chloride on the electronic properties of different diameter nanotubes. The analysis of the RBM-band allowed revealing the changes in resonance excitation conditions of SWCNTs upon filling. The analysis of the G-band allowed concluding about p-doping of nanotubes by incorporated silver chloride accompanied by charge transfer from nanotubes to the inserted salt.

Mechanisms of resonant low frequency Raman scattering from metallic nanoparticle Lamb modes.

The low frequency Raman scattering from gold nanoparticle bimodal assemblies with controlled size distributions has been studied. Special care has been paid to determining the size dependence of the Raman intensity corresponding to the quadrupolar Lamb mode. Existing models based on a microscopic description of the scattering mechanism in small particles (bond polarizability, dipole induced dipole models) predict, for any Raman-active Lamb modes, an inelastic intensity scaling as the volume of the nanoparticle. Surprisingly experimental intensity ratios are found to be anomalously much greater than theoretical ones, calling into question this scaling law. To explain these discrepancies, a simple mechanism of Raman scattering, based on the density fluctuations in the nanoparticles induced by the Lamb modes, is introduced. This modeling, in which the nanoparticle is described as an elastic isotropic continuous medium-as in Lamb theory, successfully explains the major features exhibited by low frequency Raman modes. Moreover this model provides a unified picture for any material, suitable for handling both small and large size ranges, as well as non-resonant and resonant excitation conditions in the case of metallic species.

Operando Resonant Inelastic X-ray Scattering: An Appropriate Tool to Characterize Sulfur in Li–S Batteries

Application of new analytical tools providing insight into the mechanism of lithium–polysulfide formation in the cathode and their interaction with the host matrix and electrolyte is essential for further development of Li–S batteries. In this work, resonant inelastic X-ray scattering (RIXS) measurements at the sulfur K-edge were performed on the Li–S battery in operando mode to study the lithium-polysulfide formation during the discharge process. The relative amounts of each sulfur compound in the cathode during the discharge process were determined from the linear combination fit using reference spectra measured on standard sulfur compounds. Because of resonant excitation condition the sensitivity for the lithium–polysulfide detection was significantly enhanced, while the sulfate signal from the electrolyte was heavily suppressed. In addition, the influence of self-absorption effects in the analysis was minimized due to fixed excitation energy. Consequently, a full quantitative analysis could be perform...

Physical origin of Davydov splitting and resonant Raman spectroscopy of Davydov components in multilayerMoTe2

We systematically study the high-resolution and polarized Raman spectra of multilayer (ML) MoTe2. The layer-breathing (LB) and shear (C) modes are observed in the ultralow-frequency region, which are used to quantitatively evaluate the interlayer coupling in ML MoTe2 based on the linear chain model, in which only the nearest interlayer coupling is considered. The Raman spectra on three different substrates verify the negligible substrate effect on the phonon frequencies of ML MoTe2. Ten excitation energies are used to measure the high-frequency modes of N-layer MoTe2 (NL MoTe2; N is an integer). Under the resonant excitation condition, we observe N-dependent Davydov components in ML MoTe2 , originating from the Raman-active A'1(A21g) modes at ~172 cm-1. More than two Davydov components are observed in NL MoTe2 for N larger than 4 by Raman spectroscopy. The N-dependent Davydov components are further investigated based on the symmetry analysis. A van der Waals model only considering the nearest interlayer coupling has been proposed to well understand the Davydov splitting of high-frequency A'1(A21g) modes. The different resonant profiles for the two Davydov components in 3L MoTe2 indicate that proper excitation energy of ~1.8-2.2 eV must be chosen to observe the Davydov splitting in ML MoTe2 . Our work presents a simple way to identify layer number of ultrathin MoTe2 flakes by the corresponding number and peak position of Davydov components. Our work also provides a direct evidence from Raman spectroscopy of how the nearest van der Waals interactions significantly affect the frequency of the high-frequency intralayer phonon modes in multilayer MoTe2 and expands the understanding on the lattice vibrations and interlayer coupling of transition metal dichalcogenides and other two-dimensional materials.

Carrier dynamics analysis for efficiency droop in GaN-based light-emitting diodes with different defect densities using time-resolved electroluminescence

We developed a direct experimental approach for investigating the correlation between efficiency droop and recombination rate variation under current injection conditions by using time-resolved electroluminescence (EL) technique. We applied this approach to understand the droop phenomenon of GaN-based light-emitting diodes grown on patterned sapphire substrates (LED-on-PAT) and planar sapphire substrates (LED-on-PLA). Because of lower dislocation density and current leakage in LED-on-PAT compared to LED-on-PLA, it was found that the effective carrier density injected into quantum wells (QWs) in LED-on-PAT was higher than that of the LED-on-PLA under the same current injection conditions, based on the analysis of spectral broadening of EL spectra with varying current injection and photoluminescence experiments under resonant and non-resonant excitation conditions. The efficiency droop in LED-on-PAT was found to be much more severe than that of LED-on-PLA, despite the higher overall quantum efficiency of LED-on-PAT. From the time-resolved EL analysis, we could separate radiative and non-radiative recombination contributions and directly observe (i) the decrease and saturation of radiative recombination time and (ii) the increase and following decrease in behavior of non-radiative recombination time with increasing current injection level, showing a strong correlation between efficiency droop and recombination rate variation.

Excitation intensity dependence of photoluminescence from monolayers of MoS2 and WS2/MoS2 heterostructures

A detailed study of the excitation dependence of the photoluminescence (PL) from monolayers of MoS2 and WS2/MoS2 heterostructures grown by chemical vapor deposition on Si substrates has revealed that the luminescence from band edge excitons from MoS2 monolayers shows a linear dependence on excitation intensity for both above band gap and resonant excitation conditions. In particular, a band separated by ∼55 meV from the A exciton, referred to as the C band, shows the same linear dependence on excitation intensity as the band edge excitons. A band similar to the C band has been previously ascribed to a trion, a charged, three-particle exciton. However, in our study the C band does not show the 3/2 power dependence on excitation intensity as would be expected for a three-particle exciton. Further, the PL from the MoS2 monolayer in a bilayer WS2/MoS2 heterostructure, under resonant excitation conditions where only the MoS2 absorbs the laser energy, also revealed a linear dependence on excitation intensity for the C band, confirming that its origin is not due to a trion but instead a bound exciton, presumably of an unintentional impurity or a native point defect such as a sulfur vacancy. The PL from the WS2/MoS2 heterostructure, under resonant excitation conditions also showed additional features which are suggested to arise from the interface states at the heteroboundary. Further studies are required to clearly identify the origin of these features.

Optical methodology for process monitoring of chalcopyrite photovoltaic technologies: Application to low cost Cu(In,Ga)(S,Se)2 electrodeposition based processes

Non-destructive characterization of both single layers and completed devices are important issues for the development of efficient and low cost Cu(In,Ga)(S,Se)2 (CIGS) modules at high yields. This implies for the need of methodologies suitable for the assessment of optical, electrical, and physico-chemical parameters that are relevant for the final device efficiency and that can be used for quality control and process monitoring at different process steps. In these applications, detection of in-homogeneities in the different layers from large area modules is especially relevant, being the presence of these inhomogeneities responsible for the existing gap between the efficiencies achieved in these technologies at cell and module levels. In this context, this work reviews the different optical methodologies that have been developed in the framework of the SCALENANO European project for the advanced assessment of the different layers in high efficiency electrodeposited – based CIGS devices. This has includes different strategies as those based on Raman scattering, Photoluminescence/Electroluminescence (PL/EL) based techniques and new photoelectrochemical based tools and firstly Raman spectroscopy is very sensitive to both composition and crystal quality parameters that are determining for device efficiency. Use of resonant Raman excitation strategies allows achieving a high sensitivity of the Raman spectra to the analysed features in the different regions of the device. This involves selection of the suitable excitation wavelength (in the broad spectral region from UV to IR) for the resonant Raman excitation of the required layer in the device. The strong increase in the intensity of the Raman peaks related to the use of resonant excitation conditions allows also decreasing the measuring time to times compatible with the implementation of these techniques at online process monitoring level. Analysed parameters include the electrical conductivity of the Al-doped ZnO window layer, the thickness of the CdS buffer layer and the chemical composition (S/(S+Se) relative content) and presence of relevant secondary phases as Cu-poor ordered vacancy compounds in the surface region of the absorbers. In addition PL/EL imaging are powerful techniques that provide direct access to the optoelectronic properties of the materials and devices. Whereas EL is performed using complete devices by injecting current in analogy to the operation of a light emitting diode, PL allows the characterization of bare absorber materials without the need for any functional or contacting layers. Moreover, semiconductor photo-electrochemistry (PEC) is a versatile technique that enables many opto-electronic properties of semiconductors to be determined. Essentially, a semiconductor on a conducting substrate placed in a solution containing redox species forms a Schottky barrier junction. The formation of such a diode enables basic semiconductor properties to be measured such as doping type, doping density, band gap and the flat band position versus the vacuum energy scale. In all these cases, quality control indicators suitable for the advanced assessment of these processes have been identified and validated for the electrodeposition-based processes developed at Nexcis Company.

Multiwavelength excitation Raman scattering analysis of bulk and two-dimensional MoS2: vibrational properties of atomically thin MoS2 layers

In order to deepen the knowledge of the vibrational properties of two-dimensional (2D) MoS2 atomic layers, a complete and systematic Raman scattering analysis has been performed using both bulk single-crystal MoS2 samples and atomically thin MoS2 layers. Raman spectra have been measured under non-resonant and resonant conditions using seven different excitation wavelengths from near-infrared (NIR) to ultraviolet (UV). These measurements have allowed us to observe and identify 41 peaks, among which 22 have not been previously experimentally observed for this compound, and characterize the existence of different resonant excitation conditions for the different excitation wavelengths. This has also included the first analysis of resonant Raman spectra that are achieved using UV excitation conditions. In addition, the analysis of atomically thin MoS2 layers has corroborated the higher potential of UV resonant Raman scattering measurements for the non-destructive assessment of 2D MoS2 samples. Analysis of the relative integral intensity of the additional first- and second-order peaks measured under UV resonant excitation conditions is proposed for the non-destructive characterization of the thickness of the layers, complementing previous studies based on the changes of the peak frequencies.

An incompressible state of a photo-excited electron gas.

Two-dimensional electrons in a magnetic field can form new states of matter characterized by topological properties and strong electronic correlations as displayed in the integer and fractional quantum Hall states. In these states, the electron liquid displays several spectacular characteristics, which manifest themselves in transport experiments with the quantization of the Hall resistance and a vanishing longitudinal conductivity or in thermodynamic equilibrium when the electron fluid becomes incompressible. Several experiments have reported that dissipationless transport can be achieved even at weak, non-quantizing magnetic fields when the electrons absorb photons at specific energies related to their cyclotron frequency. Here we perform compressibility measurements on electrons on liquid helium demonstrating the formation of an incompressible electronic state under these resonant excitation conditions. This new state provides a striking example of irradiation-induced self-organization in a quantum system.

Laser-driven electron dynamics for circular dichroism in mass spectrometry: from one-photon excitations to multiphoton ionization.

The distinction of enantiomers is a key aspect of chemical analysis. In mass spectrometry the distinction of enantiomers has been achieved by ionizing the sample with circularly polarized laser pulses and comparing the ion yields for light of opposite handedness. While resonant excitation conditions are expected to be most efficient, they are not required for the detection of a circular dichroism (CD) in the ion yield. However, the prediction of the size and sign of the circular dichroism becomes challenging if non-resonant multiphoton excitations are used to ionize the sample. Employing femtosecond laser pulses to drive electron wavepacket dynamics based on ab initio calculations, we attempt to reveal underlying mechanisms that determine the CD under non-resonant excitation conditions. Simulations were done for (R)-1,2-propylene oxide, using time-dependent configuration interaction singles with perturbative doubles (TD-CIS(D)) and the aug-cc-pVTZ basis set. Interactions between the electric field and the electric dipole and quadrupole as well as between the magnetic field and the magnetic dipole were explicitly accounted for. The ion yield was determined by treating states above the ionization potential as either stationary or non-stationary with energy-dependent lifetimes based on an approved heuristic approach. The observed population dynamics do not allow for a simple interpretation, because of highly non-linear interactions. Still, the various transition pathways are governed by resonant enantiospecific n-photon excitation, with preferably high transition dipole moments, which eventually dominate the CD in the ionized population.