Resonant Optical Excitation Research Articles

Gain Characteristics of Optically Pumped UVC Lasers with Wide AlGaN Single‐Quantum‐Well Active Regions

Herein, the lasing threshold and gain characteristics of ultraviolet‐C optically pumped edge‐emitting lasers with thick single‐quantum‐well (SQW) active regions are investigated by the variable‐stripe length method. Positive net modal gain is observed in lasers with AlGaN‐based SQWs with thicknesses up to 12 nm. The lasers show a reduction of the threshold power density with increasing SQW thickness, with the lowest threshold of 1.3 MW cm−2 achieved for a 9 nm SQW laser. The high gain and low threshold in lasers with thick quantum wells are attributed to lasing from excited states, where the polarization fields are screened by the carriers in the fundamental state and the barriers, thus recovering larger electron and hole wavefunctions overlap. These findings are supported by k · p simulations, and for a 9 nm SQW, the calculation predicts a contribution of the fundamental transition to the gain of 75% for non‐resonant optical excitation and below 1% for resonant optical or electrical excitation.

Topological edge and corner states in coupled wave lattices in nonlinear polariton condensates

Abstract Topological states have been widely investigated in different types of systems and lattices. In the present work, we report on topological edge states in double-wave (DW) chains, which can be described by a generalized Aubry-André-Harper (AAH) model. For the specific system of a driven-dissipative exciton polariton system we show that in such potential chains, different types of edge states can form. For resonant optical excitation, we further find that the optical nonlinearity leads to a multistability of different edge states. This includes topologically protected edge states evolved directly from individual linear eigenstates as well as additional edge states that originate from nonlinearity-induced localization of bulk states. Extending the system into two dimensions (2D) by stacking horizontal DW chains in the vertical direction, we also create 2D multi-wave lattices. In such 2D lattices multiple Su–Schrieffer–Heeger (SSH) chains appear along the vertical direction. The combination of DW chains in the horizonal and SSH chains in the vertical direction then results in the formation of higher-order topological insulator corner states. Multistable corner states emerge in the nonlinear regime.

Controlling Magnetism with Light in a Zero Orbital Angular Momentum Antiferromagnet.

Antiferromagnetic materials feature intrinsic ultrafast spin dynamics, making them ideal candidates for future magnonic devices operating at THz frequencies. A major focus of current research is the investigation of optical methods for the efficient generation of coherent magnons in antiferromagnetic insulators. In magnetic lattices endowed with orbital angular momentum, spin-orbit coupling enables spin dynamics through the resonant excitation of low-energy electric dipoles such as phonons and orbital resonances which interact with spins. However, in magnetic systems with zero orbital angular momentum, microscopic pathways for the resonant and low-energy optical excitation of coherent spin dynamics are lacking. Here, we consider experimentally the relative merits of electronic and vibrational excitations for the optical control of zero orbital angular momentum magnets, focusing on a limit case: the antiferromagnet manganese phosphorous trisulfide (MnPS_{3}), constituted by orbital singlet Mn^{2+} ions. We study the correlation of spins with two types of excitations within its band gap: a bound electron orbital excitation from the singlet orbital ground state of Mn^{2+} into an orbital triplet state, which causes coherent spin precession, and a vibrational excitation of the crystal field that causes thermal spin disorder. Our findings cast orbital transitions as key targets for magnetic control in insulators constituted by magnetic centers of zero orbital angular momentum.

Direct determination of the piezoelectric field using the quantum-confined Stark effect in a strained [111]-oriented zinc-blende MQW-SOA

Here, we present a method, based on Stark shifts induced by the quantum-confined Stark effect, to directly determine the piezoelectric field in strained zinc-blende quantum wells (QWs) grown along the 111 direction and embedded in the p–i–n diode structure (p-iMQW-n) of a semiconductor optical amplifier. Under short-circuit and open-circuit conditions and resonant optical excitation, we experimentally determine the energy of the 1s e-hh excitonic resonance and the potential difference across the p-iMQW-n junction. Using these parameters in an analytical expression derived in this work, we directly determined a piezoelectric field of −108.71 ± 8.51 kV/cm acting on each of the eight 12-Å-wide strained [111]-oriented In0.687Ga0.313As /In0.807Ga0.193As0.304P0.696 QWs of a semiconductor optical amplifier (SOA). The theoretical prediction of −112.24 kV/cm was in very good agreement, within the experimental error, with the value of the piezoelectric field extracted through the proposed procedure, which was also indirectly validated by comparing the experimental value of the 1s e-hh excitonic resonance Stark shift with the extracted one. Even though the implemented technique was specially designed to experimentally determine the piezoelectric field acting on each of the QWs of a multiple quantum well SOA, it can be applied to any p–i–n structure with electrodes and strained [111]-oriented zinc-blende QWs embedded in its intrinsic region. Remarkably, the method proposed here allows direct experimental determination of the piezoelectric field through an analytical expression, with which it is also possible to estimate, with high reliability, the precision of the result and how it is affected by the accuracy of each measuring instrument intervening in the procedure.

Surface-Plasmon-Mediated Alloying for Monodisperse Au-Ag Alloy Nanoparticles in Liquid.

Plasmonic noble-metal nanoparticles with broadly tunable optical properties and catalytically active surfaces offer a unique opportunity for photochemistry. Resonant optical excitation of surface-plasmon generates high-energy hot carriers, which can participate in photochemical reactions. Although the surface-plasmon-driven catalysis on molecules has been extensively studied, surface-plasmon-mediated synthesis of bimetallic nanomaterials is less reported. Herein, we perform a detailed investigation on the formation mechanism and colloidal stability of monodisperse Au-Ag alloy nanoparticles synthesized through irradiating the intermixture of Au nanochains and AgNO3 solution with a nanosecond pulsed laser. It is revealed that the Ag atoms can be extracted from AgNO3 solution by surface-plasmon-generated hot electrons and alloy with Au atoms. Particularly, the obtained Au-Ag alloy nanoparticles without any surfactants or ligands exhibit superior stability that is confirmed by experiments as well as DLVO-based theoretical simulation. Our work would provide novel insights into the synthesis of potentially useful bimetallic nanoparticles via surface-plasmon-medicated alloying.

Nonlocal nonlinear phononics

Nonlinear phononics relies on the resonant optical excitation of infrared-active lattice vibrations to induce targeted structural deformations in solids. This form of dynamical crystal structure design has been applied to control the functional properties of many complex solids, including magnetic materials, superconductors and ferroelectrics. However, phononics has so far been restricted to protocols in which structural deformations occur within the optically excited volume, sometimes resulting in unwanted heating. Here, we extend nonlinear phononics to propagating polaritons, spatially separating the functional response from the optical drive. We use mid-infrared optical pulses to resonantly drive a phonon at the surface of ferroelectric LiNbO3. Time-resolved stimulated Raman scattering reveals that the ferroelectric polarization is reduced over the entire 50 µm depth of the sample, far beyond the micrometre depth of the evanescent phonon field. We attribute this effect to the anharmonic coupling between the driven mode and a polariton that propagates into the material. For high excitation amplitudes, we reach a regime in which the ferroelectric polarization is reversed, as revealed by a sign change in the Raman tensor coefficients of all the polar modes.

Electromagnetically induced transparency in inhomogeneously broadened divacancy defect ensembles in SiC

Electromagnetically induced transparency (EIT) is a phenomenon that can provide strong and robust interfacing between optical signals and quantum coherence of electronic spins. In its archetypical form, mainly explored with atomic media, it uses a (near-)homogeneous ensemble of three-level systems, in which two low-energy spin-1/2 levels are coupled to a common optically excited state. We investigate the implementation of EIT with c-axis divacancy color centers in silicon carbide. While this material has attractive properties for quantum device technologies with near-IR optics, implementing EIT is complicated by the inhomogeneous broadening of the optical transitions throughout the ensemble and the presence of multiple ground-state levels. These may lead to darkening of the ensemble upon resonant optical excitation. Here, we show that EIT can be established with high visibility also in this material platform upon careful design of the measurement geometry. Comparison of our experimental results with a model based on the Lindblad equations indicates that we can create coherences between different sets of two levels all-optically in these systems, with potential impact for RF-free quantum sensing applications. Our work provides an understanding of EIT in multi-level systems with significant inhomogeneities, and our considerations are valid for a wide array of defects in semiconductors.

Non-trivial dependence of spectral characteristics of excitons in quantum wells on the resonant optical excitation power

Basic exciton parameters, such as the energy of exciton transition and the radiative and nonradiative broadenings, are experimentally studied by means of reflectance spectroscopy for a heterostructure with a 14-nm GaAs/AlGaAs quantum well. Particular attention is paid to the nonradiative broadening which is sensitive to densities of free carriers and long-lived nonradiative excitons. A sublinear increase of the broadening of the heavy-hole and light-hole exciton resonances is observed when the light-hole exciton resonance is excited with increasing power. A simple model is developed, which allows one to well reproduce the observed dependence. Keywords: exciton, quantum well, nonradiative broadening.

Phase Diagram for Light-Induced Superconductivity in κ-(ET)_{2}-X.

Resonant optical excitation of certain molecular vibrations in κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Br has been shown to induce transient superconductinglike optical properties at temperatures far above equilibrium T_{c}. Here, we report experiments across the bandwidth-tuned phase diagram of this class of materials, and study the Mott insulator κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Cl and the metallic compound κ-(BEDT-TTF)_{2}Cu(NCS)_{2}. We find nonequilibrium photoinduced superconductivity only in κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Br, indicating that the proximity to the Mott insulating phase and possibly the presence of preexisting superconducting fluctuations are prerequisites for this effect.

Nonresonant coherent two-dimensional spectroscopy

Coherent electronic two-dimensional spectroscopy is nowadays a matured experimental technique that monitors the time evolution of the studied sample after its resonant optical excitation. However, the experimental experience shows that even nonresonant interactions can provide detectable spectral contributions. These are often present as a weak parasitic signals originating in the solvent and/or cuvette walls underlying the resonant spectrum of the actual sample and as such they are usually discarded from the analysis. In this work, we adapt the formalism of double-sided Feynman diagrams for the needs of coherent two-dimensional spectroscopy in the nonresonant regime. We analytically calculate the third-order polarization of a two-level and several variants of three-level systems. As a result, we demonstrate the typical appearance of the optical Kerr-effect, cross-phase modulation, excited-state coherence, two-photon absorption and stimulated Raman scattering in the 2D spectrum. This provides a framework for studying these effects by means of coherent two-dimensional spectroscopy.

Ultrafast Control of Material Optical Properties via the Infrared Resonant Raman Effect

The Raman effect -- inelastic scattering of light by lattice vibrations (phonons) -- produces an optical response closely tied to a material's crystal structure. Here we show that resonant optical excitation of IR and Raman phonons gives rise to a Raman scattering effect that can induce giant shifts to the refractive index and induce new optical constants that are forbidden in the equilibrium crystal structure. We complete the description of light-matter interactions mediated by coupled IR and Raman phonons in crystalline insulators -- currently the focus of numerous experiments aiming to dynamically control material properties -- by including a forgotten pathway through the nonlinear lattice polarizability. Our work expands the toolset for control and development of new optical technologies by revealing that the absorption of light within the terahertz gap can enable control of optical properties of materials over a broad frequency range.

Cylindrical vector beam revealing multipolar nonlinear scattering for superlocalization of silicon nanostructures

The resonant optical excitation of dielectric nanostructures offers unique opportunities for developing remarkable nanophotonic devices. Light that is structured by tailoring the vectorial characteristics of the light beam provides an additional degree of freedom in achieving flexible control of multipolar resonances at the nanoscale. Here, we investigate the nonlinear scattering of subwavelength silicon (Si) nanostructures with radially and azimuthally polarized cylindrical vector beams to show a strong dependence of the photothermal nonlinearity on the polarization state of the applied light. The resonant magnetic dipole, selectively excited by an azimuthally polarized beam, enables enhanced photothermal nonlinearity, thereby inducing large scattering saturation. In contrast, radially polarized beam illumination shows no observable nonlinearity owing to off-resonance excitation. Numerical analysis reveals a difference of more than 2 orders of magnitude in photothermal nonlinearity under two types of polarization excitations. Nonlinear scattering and the unique doughnut-shaped focal spot generated by the azimuthally polarized beam are demonstrated as enabling far-field high-resolution localization of nanostructured Si with an accuracy approaching 50 nm. Our study extends the horizons of active Si photonics and holds great potential for label-free superresolution imaging of Si nanostructures.

Lateral Distribution of Optical Excitation at Boundaries around Rubrene Islands Visualized by Microspot Two-Photon Photoemission Spectroscopy

The lateral distribution of optical excitation occurring at the edge part of rubrene (5,6,11,12-tetraphenyltetracene) molecular islands formed on highly oriented pyrolytic graphite (HOPG) substrate has been visualized with microspot two-photon photoemission (2PPE) spectroscopy of 0.4 µm resolution. As we reported previously [T. Ueba, et al., J. Phys. Chem. C 117 (2013) 20098–20103], an unoccupied peak labeled Ln is strongly enhanced in 2PPE spectroscopy by a resonant optical excitation between a diffuse unoccupied Ln molecular orbital and the HOMO. The strong enhancement is due to the hybridization of Ln with the image potential state (IPS) on the HOPG substrate. In this study, using the spatially-resolved microspot 2PPE method, we obtained the real space image of the area where the optical excitation occurs. One-photon photoelectron emission microscope (1PPE-PEEM) with 50 nm resolution is used to compare with the microspot 2PPE. The 1PPE-PEEM images are homogeneous for films below and above 1 monolayer (ML) coverage, indicating that the molecular density of rubrene is homogeneous within the lateral resolution. On the other hand, microspot 2PPE images for sub-ML films show inhomogeneous areas of several µm sizes. The images depend on the detected unoccupied levels of interest. The inhomogeneity is very significant when the image is recorded at the energy of Ln peak. The different appearance between 1PPE-PEEM and microspot 2PPE images is explained on the basis that the Ln excitation occurs at the edges of islands smaller than the resolution of the PEEM. Though the microspot 2PPE achieves only moderate lateral resolution, it is sensitive to the resonant excitation occurring at nm-scale boundaries of the molecular islands. The distribution of the nm-scale islands is the origin of the µm-scale lateral inhomogeneity. The microspot 2PPE images become homogeneous for films thicker than 1 ML, reflecting the quenching of the resonant excitation. These results demonstrate a possibility that spatially- and energy-resolved microspot 2PPE can be a powerful tool to clarify nanoscale geometric and electronic structures at the molecule/substrate interfaces.

Circularly polarized coherent light-induced boosting of polymer solar cells photovoltaic performance

We report a microscopic quantum theory which can be used to study circularly polarized light (CPL) induced singlet to triplet conversions in polymer solar cells (PSCs). We demonstrate that by properly adjusting CPL irradiation parameters, the photo-created singlet excitons can be efficiently converted into non radiative triplet excitons. The maximum singlet–triplet conversion ratio might be reached when the incident CPL frequency is chosen in such a way that the resonant optical excitation occurs between singlet and triplet states. In addition, we also illustrate that with varying CPL oscillating strength, the singlet–triplet conversion patterns behave as the Rabi oscillations and the steadily maintained triplet excitons. The electron-lattice coupling leads to the self-trapping of the excitons, which enhances the coherence between the electron’s and hole’s spin oscillations. This study opens up a novel approach for enhancing and controlling the photovoltaic response of PSCs by an effective all-optical route.

On-chip deterministic operation of quantum dots in dual-mode waveguides for a plug-and-play single-photon source

A deterministic source of coherent single photons is an enabling device for quantum information processing. Quantum dots in nanophotonic structures have been employed as excellent sources of single photons with the promise of scaling up towards multiple photons and emitters. It remains a challenge to implement deterministic resonant optical excitation of the quantum dot required for generating coherent single photons, since residual light from the excitation laser should be suppressed without compromising source efficiency and scalability. Here, we present a planar nanophotonic circuit that enables deterministic pulsed resonant excitation of quantum dots using two orthogonal waveguide modes for separating the laser and the emitted photons. We report a coherent and stable single-photon source that simultaneously achieves high-purity (g(2)(0) = 0.020 ± 0.005), high-indistinguishability (V = 96 ± 2%), and >80% coupling efficiency into the waveguide. Such ‘plug-and-play’ single-photon source can be integrated with on-chip optical networks implementing photonic quantum processors.

Length dependent optical characteristics analysis for semiconductor nanowires

Resonant optical mode excitations in semiconductor nanowires result in enhanced absorptions. Nominally, only the diameter dependent radial mode excitations have been considered for the increased absorption. In this paper, we try to understand how the length of the nanowires affects the resonant wavelength and peak absorption wavelengths. We answer two questions viz (1) at what minimum length are radial optical modes stabilized and dominate the absorption characteristics and (2) do longitudinal modes play a role in absorption characteristics especially in determining the resonant wavelength. Two different semiconductors are studied viz silicon and gallium arsenide. We find that even nanowires as short as 200 nm exhibit absorption characteristics dominated by the radial mode excitation. However, for lengths smaller than 200 nm, the optical characteristics are dominated by scattering. Further, we observe that longitudinal modes are excited in low absorption semiconductor materials like silicon for lengths up to 700 nm and the absorption peak depends both on the diameter and the wavelength. Further, shorter length nanowires may have higher absorption than the longer ones in this regime. We also observed that scattering from the nanowires is less than 2% of the incident light. For higher absorption semiconductor like GaAs, absorption characteristics are mainly determined by the radial mode excitations even for shorter lengths. The results provide further insight into the radial mode excitations in semiconductor nanowires.

Ultrafast acoustic switching of an optically pumped cavity polariton system in the bistable regime

Nonequilibrium transitions are investigated in a plane exciton-polariton system with complex acousto-optical excitation: stationary resonant optical excitation of polaritons along the normal to the surface of the resonator and picosecond strain pulses causing reversible exciton energy perturbations. It is shown that acoustic pulses can be used for ultrafast switching of the optical response of a bistable polariton system. Switching is experimentally implemented in a high-Q GaAs microcavity.

Quantum information processing with closely-spaced diamond color centers in strain and magnetic fields [Invited

Electron and nuclear spins of diamond nitrogen-vacancy (NV) centers are good candidates for quantum information processing as they have long coherence time and can be initialized and read out optically. However, creating a large number of coherently coupled and individually addressable NV centers for quantum computing has been a big challenge. Here we propose methods to use high-density diamond NV centers coupled by spin-spin interaction with an average separation on the order of 10 nm for quantum computing. We propose to use a strain gradient to encode the position information of each NV center in the energy level of its excited electron orbital state, which causes a shift of its optical transition frequency. With such strain encoding, more than 100 closely-packed NV centers below optical diffraction limit can be read out individually by resonant optical excitation. A magnetic gradient will be used to shift the electron spin resonant (ESR) frequencies of NV centers. Therefore, the spin state of each NV center can be individually manipulated and different NV centers can be selectively coupled. A universal set of quantum operations for two-qubit and three-qubit system is introduced by careful design of external drives. Moreover, entangled states with multiple qubits can be created by this protocol, which is a major step towards quantum information processing with solid-state spins.

Origins of overshoots in the exciton spin dynamics in semiconductors

We investigate the origin of overshoots in the exciton spin dynamics after resonant optical excitation. As a material system, we focus on diluted magnetic semiconductor quantum wells as they provide a strong spin-flip scattering for the carriers. Our study shows that overshoots can appear as a consequence of radiative decay even on the single-particle level in a theory without any memory. The magnitude of the overshoots in this case depends strongly on the temperature as well as the doping fraction of the material. If many-body effects beyond the single-particle level become important so that a quantum-kinetic description is required, a spin overshoot appears already without radiative decay and is much more robust against variations of system parameters. We show that the origin of the spin overshoot can be determined either via its temperature dependence or via its behavior for different doping fractions. The results can be expected to apply to a wide range of semiconductors as long as radiative decay and a spin-flip mechanism are present.

Active Control of Coherent Dynamics in Hybrid Plasmonic MoS2 Monolayers with Dressed Phonons

The near-field interaction due to a strong electromagnetic field induced by resonant-localized plasmons can result in a strong coupling of excitonic states or the formation of hybrid exciton–plasmon modes in quantum confined structures. The strength of this coupling can be increased by designing a system with its vibronic states resonant to the energy of the driving field induced by the localized plasmon excitation. Silver (Ag) nanoparticles (NPs) nucleated on molybdenum disulfide (MoS2) is an ideal platform for such an interaction. The influence of localized plasmons (LSP) on the formation and dissociation of excitons due to resonant and off-resonant optical excitation of carriers to excitonic states is studied using ultrafast optical spectroscopy. The presence of Ag-NPs generates a local field that enhances the magnitude of the Raman modes in MoS2 under the resonant plasmon excitation. An ultrashort pulsed optical excitation at ∼2.3 eV resonantly excites the LSP modes and the optical near-field resonant...