Optical Excitation Research Articles

Direct density modulation of photo-assisted field emission from an RF cold cathode

This paper explores direct density modulation of high-current electron beam emission from an RF cold cathode using optical excitation. We theoretically study the photo-assisted field emission of periodically bunched electron beams of various pulse shapes under the combined excitation of an RF field and an optical field, using an exact quantum model. Both continuous-wave (CW) and pulsed optical fields are considered. The emission current pulse amplitude, pulse width, electron number density per pulse, as well as pulse shape and its harmonic contents are investigated in detail. For CW photon sources in the UV to NIR range (i.e., 200–1200 nm), increasing the optical intensity under an RF bias tends to change the current pulse from a Gaussian to sinusoidal-like shape, thus offering strong flexibility to control the frequency components in beam current emission. Pulsed photon sources combined with an RF field can produce sharp, high-current electron bunches with pulse duration comparable with or even less than that of the optical pulse. A contour map of the density modulation depth is constructed for different combinations of RF and laser fields. The results provide insight into unlocking new opportunities to achieve direct density modulation during electron current emission by optical means.

A Reversed Inverse Faraday Effect

The inverse Faraday effect is a magneto‐optical process allowing the magnetization of matter by an optical excitation carrying a non‐zero spin. In particular, a right circular polarization generates a magnetization in the direction of light propagation and a left circular polarization in the opposite direction to this propagation. Herein, it demonstrates that by manipulating the spin density of light, i.e., its polarization, in a plasmonic nanostructure, a reversed inverse Faraday effect is generated. A right circular polarization will generate a magnetization in the opposite direction of the light propagation, a left circular polarization in the direction of propagation. Also, it demonstrates that this new physical phenomenon is chiral, generating a strong magnetic field only for one helicity of the light, the opposite helicity producing this effect only for the mirror structure. This new optical concept opens the way to the generation of magnetic fields with unpolarized light, finding application in the ultrafast manipulation of magnetic domains and processes, such as spin precession, spin currents, and waves, magnetic skyrmion or magnetic circular dichroism, with direct applications in data storage and processing technologies.

Optical control of ultrafast structural dynamics in a fluorescent protein

The photoisomerization reaction of a fluorescent protein chromophore occurs on the ultrafast timescale. The structural dynamics that result from femtosecond optical excitation have contributions from vibrational and electronic processes and from reaction dynamics that involve the crossing through a conical intersection. The creation and progression of the ultrafast structural dynamics strongly depends on optical and molecular parameters. When using X-ray crystallography as a probe of ultrafast dynamics, the origin of the observed nuclear motions is not known. Now, high-resolution pump–probe X-ray crystallography reveals complex sub-ångström, ultrafast motions and hydrogen-bonding rearrangements in the active site of a fluorescent protein. However, we demonstrate that the measured motions are not part of the photoisomerization reaction but instead arise from impulsively driven coherent vibrational processes in the electronic ground state. A coherent-control experiment using a two-colour and two-pulse optical excitation strongly amplifies the X-ray crystallographic difference density, while it fully depletes the photoisomerization process. A coherent control mechanism was tested and confirmed the wave packets assignment.

Abstract P1102: Organic Semiconductor Photomodulation Enhances Maturation Of Pluripotent Stem Cell-derived Cardiomyocytes

Cardiomyocytes differentiated from pluripotent stem cells (PSC-CMs) hold a great potential for the study and the cure of cardiovascular disease; indeed they have been largely used as platform for disease modeling and drugs testing, and represent a promising source of cells for regenerative therapies. However, their immature phenotype represents a major hurdle for their full application; indeed, PSC-CMs differ from adult ones for molecular, metabolic and morpho-functional properties. Recently, a new technology based on optical excitation of organic semiconductors (OS) has been shown to be able to modulate the behavior of many cell types by targeting different cellular processes, as proliferation, angiogenesis and calcium signalling. Here, we adopted a multidisciplinary approach based on morpho-functional, metabolic and transcriptional analyses to study the effect of OS-photoexcitation on PSC-CMs, by the use of a red-light sensitive OS polymer, namely Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), in the form of a thin film. Our results revealed a significant modulation of markers of maturation in PSC-CMs exposed to PCPDTBT and photomodulation, showing increased size, a shift of their maximal diastolic potential toward more negative values, and augmented Ca 2+ transient amplitude. Moreover, by Scanning ElectroChemical Microscopy (SECM) we found a decrease in glucose uptake and lactate release upon PCPDTBT light stimulation, potentially indicating a switch toward a more adult-like metabolism in stimulated PSC-CMs. Recently obtained RNA sequencing data are in line with these results, and will provide us with hints on the underlying molecular mechanisms. In conclusion, although not definitive, our data are in support of a potential effect of polymer-mediated optical photoexcitation in boosting PSC-CMs toward a more mature phenotype. The demonstration of a relevant effect of OS-photomodulation on PSC-CM maturation and functionality will significantly promote their full applications toward personalized medicine.This research was supported by EU Horizon 2020 FETOPEN- 2018-2020 Programme ‘LION- HEARTED’-GA #828984

Fabrication of pyramid-shaped gold tip for adiabatic nanofocusing of surface plasmon polaritons

Scattering-type scanning near-field optical microscopy based on adiabatic nanofocusing of surface plasmon polaritons (SPPs) is a powerful technique that can achieve free-background nanoscale optical resolution of material. However, the performance of the propagated confined modes strongly depends on the characteristic structure of the probe. Although the metallic pyramid structure provides excellent tightly confined modes, however, it is challenging to realize the required pyramid geometry. Here, we propose a simple method for fabricating a reproducible and controllable gold pyramid-shaped tip. The produced pyramid-shaped tips were made by electrochemical etching and by applying a pulse wave to the system. From a systematic study, we found that the key factor of fabrication of desired tip geometry is based on the platinum (Pt) wire shapes. Traditional circular-shaped platinum ring electrodes are used for gold tip fabrication in an electrochemical etching. In our method, we bent the Pt wire into a triangular shape as the electrode for the etching process. The influence of the geometrical ring shapes on the fabrication of the Au tip structure is investigated. The gold tip structure was optimized by controlling the Pt ring shape, and the desired pyramid-shaped gold tip was achieved with a yield of 70%. The obtained etched pyramid-shaped tips were then mounted along the side of one of the arms of a quartz tuning fork force sensor to test their performance for shear-force topographical image and for guiding SPPs along the pyramid wedge based on adiabatic nanofocusing microscopy. The result shows topographical images of indium tin oxide with a spatial resolution smaller than 20 nm. Furthermore, we experimentally demonstrate the generation of the SPPs that propagated adiabatically along the wedge of an appropriate fabricated pyramid-shaped tip toward a nanometer-size spot at the tip apex. The demonstration of this method strongly suggests that the obtained pyramid-shaped tip will enable new experiments probing the dynamics of optical excitations of individual metallic, semiconducting, and magnetic nanostructures.

Controllable Resistive Switching in ReS2 /WS2 Heterostructure for Nonvolatile Memory and Synaptic Simulation.

Memristors with nonvolatile storage performance and simulated synaptic functions are regarded as one of the critical devices to overcome the bottleneck in traditional von Neumann computer architecture. 2D van der Waals heterostructures have paved a new way for the development of advanced memristors by integrating the intriguing features of different materials and offering additional controllability over their optoelectronic properties. Herein, planar memristors with both electrical and optical tunability based on ReS2 /WS2 van der Waals heterostructure are demonstrated. The devices show unique unipolar nonvolatile behavior with high Roff /Ron ratio of up to 106 , desirable endurance, and retention, which are superior to pure ReS2 andWS2 devices. When decreasing the channel length, the set voltage can be notably reduced while the high Roff /Ron ratios are retained. By introducing electrostatic doping through the gate control, the set voltage can be tailored in a wide range from 4.50 to 0.40V. Furthermore, biological synaptic functions and plasticity, including spike rate-dependent plasticity and paired-pulse facilitation, are successfully realized. By employing optical illumination, resistive switching can also be modulated, which is dependent on the illumination energy and power. A mechanism related to the interlayer charge transfer controlled by optical excitation is revealed.

Examining the origins of observed terahertz modes from an optically pumped atomistic model protein in aqueous solution.

The microscopic origins of terahertz (THz) vibrational modes in biological systems are an active and open area of current research. Recent experiments [Phys Rev X. 8, 031061 (2018)] have revealed the presence of a pronounced mode at ∼0.3 THz in fluorophore-decorated bovine serum albumin (BSA) protein in aqueous solution under nonequilibrium conditions induced by optical pumping. This result was heuristically interpreted as a collective elastic fluctuation originating from the activation of a low-frequency phonon mode. In this work, we show that the sub-THz spectroscopic response emerges in a statistically significant manner () from such collective behavior, illustrating how photoexcitation can alter specific THz vibrational modes. We revisit the theoretical analysis with proof-of-concept molecular dynamics that introduce optical excitations into the simulations. Using information theory techniques, we show that these excitations can give rise to a multiscale response involving two optically excited chromophores (tryptophans), other amino acids in the protein, ions, and water. Our results motivate new experiments and fully nonequilibrium simulations to probe these phenomena, as well as the refinement of atomistic models of Fröhlich condensates that are fundamentally determined by nonlinear interactions in biology.

Valley Polarized Holes Induced Exciton Polaron Valley Splitting.

Monolayer transition metal dichalcogenide semiconductors are promising valleytronic materials. Among various quasi-particle excitations hosted by the system, the valley polarized holes are particularly interesting due to their long valley lifetime preserved by the large spin-orbit splitting and spin-valley locking in the valence band. Here we report that in the absence of any magnetic field a surprising valley splitting of exciton polarons can be induced by such valley polarized holes in monolayer WSe2. The size of the splitting is comparable to that of the Zeeman effect in a magnetic field as high as 7 T and offers a quantitative approach to extract the hole density imbalance between the two valleys. We find that the density difference can easily achieve more than 1011 per cm2, and it is tunable by gate voltage as well as optical excitation power. Our study highlights the response of exciton polarons to optical pumping and advances understanding of valley dependent phenomena in monolayer transition metal dichalcogenide.

Pixelated vertical-cavity surface-emitting laser arrays from colloidal quantum dot films

Pixelated arrays of vertical-cavity surface-emitting lasers from colloidal quantum dot thin films have been demonstrated. CdSe/ZnCdS core/shell structured colloidal quantum dots have been used as laser gain medium. 10 pairs of alternating layers between GaN and nanoporous GaN consisted highly reflective distributed Bragg reflectors, exhibiting more than 99% of reflectivity in the spectral range of 600 nm–650 nm. Close-packed colloidal quantum dot solid thin films were sandwiched between the two GaN/nanoporous GaN distributed Bragg reflectors to achieve well-defined lasing outputs under pulsed optical excitation with the lasing threshold as low as 11.0 μJ/cm2. The resulting CQD-VCSEL devices exhibited a lasing output power of 1.5 μW and a conversion efficiency of 11.4%. Finally, a two-axis scanning micro-mirror was used to complete the 25 (5 by 5) pixelated laser arrays system.

Controlling the electro-optic response of a semiconducting perovskite coupled to a phonon-resonant cavity

Optical cavities, resonant with vibrational or electronic transitions of material within the cavity, enable control of light-matter interaction. Previous studies have reported cavity-induced modifications of chemical reactivity, fluorescence, phase behavior, and charge transport. Here, we explore the effect of resonant cavity-phonon coupling on the transient photoconductivity in a hybrid organic-inorganic perovskite. To this end, we measure the ultrafast photoconductivity response of perovskite in a tunable Fabry–Pérot terahertz cavity, designed to be transparent for optical excitation. The terahertz-cavity field-phonon interaction causes apparent Rabi splitting between the perovskite phonon mode and the cavity mode. We explore whether the cavity-phonon interaction affects the material’s electron-phonon interaction by determining the charge-carrier mobility through photoconductivity. Despite the apparent hybridization of cavity and phonon modes, we show that the perovskite properties in both ground (phonon response) and excited (photoconductive response) states remain unaffected by the tunable light-matter interaction. Yet the response of the integral perovskite-terahertz optical cavity system depends critically on the interaction strength of the cavity with the phonon: the transient terahertz response to optical excitation can be increased up to threefold by tuning the cavity-perovskite interaction strength. These results enable tunable switches and frequency-controlled induced transparency devices.

Enhanced photoconductivity of hybrid 2D-QD MoS2-AgInS2 structures.

This study describes the fabrication of hybrid two-dimensional (2D)-quantum dot (QD) MoS2-AgInS2 photoconductive devices through the mechanical pressing of a MoS2 flake onto an AgInS2 QD film. The devices exhibit an enhanced photoresponse at both continuous and modulated optical excitations, compared with the bare MoS2 or AgInS2 layer, due to the formation of a built-in electric field near the MoS2/AgInS2 interface. The continuous wave photoresponse is significantly higher due to the effective photoconductive gain when electrons flow freely through the MoS2 flake, whereas holes are effectively trapped in AgInS2 QDs. The study highlights the potential of hybrid 2D-QD MoS2-AgInS2 devices for photovoltaic and optoelectronic applications.

Adsorption Structures Affecting the Electronic Properties and Photoinduced Charge Transfer at Perylene-Based Molecular Interfaces.

Perylene-based organic semiconductors are widely used in organic electronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) diindenoperylene (DIP) and the electron acceptor (A) dicyano-perylene-bis(dicarboximide) (PDIR-CN2 ) using femtosecond time-resolved second harmonic generation (SHG) in combination with large scale quantum chemical calculations. Thereby, we varied in bilayer structures of DIP and PDIR-CN2 the interfacial molecular geometry. For an interfacial configuration which contains a edge-on geometry but also additional face-on domains an optically induced charge transfer (CT) is observed, which leads to a pronounced increase of the SHG signal intensity due to electric field induced second harmonic generation. The interfacial CT state decays within 7.5±0.7 ps, while the creation of hot CT states leads to a faster decay (5.3±0.2 ps). For the bilayer structures with mainly edge-on geometries interfacial CT formation is suppressed since π-π overlap perpendicular to the interface is missing. Our combined experimental and theoretical study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of these molecules.

Electron Energy-gain Spectroscopy of Optical Excitations in an Integrated Photonics Microresonator.

Electron Energy-gain Spectroscopy of Optical Excitations in an Integrated Photonics Microresonator.

Ultrafast THz spectroscopy of carbon nanotube-graphene composites

Mixed nanomaterial composites can combine the excellent properties of well-known low-dimensional nanomaterials. Here we highlight the potential of one-dimensional single-walled carbon nanotubes interfaced with two-dimensional graphene by exploring the composite’s ac conductivity and photoconductivity, and the influence of HAuCl4 doping. In the composite, the equilibrium terahertz conductivity from free carrier motion was boosted, while the localised plasmon peak shifted towards higher frequencies, which we attribute to shorter conductivity pathways in the composite. A negative terahertz photoconductivity was observed for all samples under 410 nm optical excitation and was reproduced by a simple model, where the Drude spectral weight and the momentum scattering rate were both lowered under photoexcitation. The composite had an enhanced modulation depth in comparison to reference carbon nanotube films, while retaining their characteristically fast (picosecond) response time. The results show that carbon nanotube-graphene composites offer new opportunities in devices by controlling charge carrier transport and tuning their optoelectronic properties.

Dynamical downfolding for localized quantum states

We introduce an approach to treat localized correlated electronic states in the otherwise weakly correlated host medium. Here, the environment is dynamically downfolded on the correlated subspace. It is captured via renormalization of one and two quasiparticle interaction terms which are evaluated using many-body perturbation theory. We outline the strategy on how to take the dynamical effects into account by going beyond the static limit approximation. Further, we introduce an efficient stochastic implementation that enables treating the host environment with a large number of electrons at a minimal computational cost. For a small explicitly correlated subspace, the dynamical effects are critical. We demonstrate the methodology by reproducing optical excitations in the negatively charged NV center defect in diamond, that agree with experimental results.

Modulation of the Transient Magnetization of an EuO/Co Bilayer Tuned by Optical Excitation

AbstractThe magnetic proximity effect provides a promising way to increase the low Curie temperature (TC) of europium monoxide (EuO) toward or even above room temperature, while keeping its stoichiometry and insulating properties. This work studies EuO/Co bilayers using static and time‐resolved magneto‐optical Kerr effect measurements, and explores the influence of magnetic proximity on TC and on the spin dynamics in EuO. Excitation above the EuO bandgap results in an ultrafast enhancement of the EuO magnetization followed by a demagnetization within nanoseconds. This behaviour is also visible upon selectively photoexciting Co in the EuO/Co bilayer placed in an out‐of‐plane magnetic field, which is attributed to propagation of a superdiffusive spin current from Co into EuO. As the spin dynamics of Co shows a transient thermal demagnetization, the bilayer provides a system where the transient magneto‐optical signal can be tuned in amplitude and sign by varying external parameters such as the sample temperature or pump fluence. Moreover, in a strong excitation regime it is possible to measure the magnetic hysteresis of the underlying EuO, which is present up to room temperature – giving experimental evidence for the presence of a tuneable magnetic proximity coupling between Co and EuO.

Regulating Terahertz Photoconductivity in Two-Dimensional Materials

Two-dimensional materials represented by graphene have attracted extensive interest owing to the unique layer-dependent physical properties that are tunable with various external fields. In addition, by stacking two or more 2D materials together, a new material with the desired properties can be tailored and designed. Fully understanding the dynamical photoconductive response in 2D materials is uttermost important to design and develop the advanced optoelectronic devices. Terahertz (THz) time-domain spectroscopy (TDS) and time-resolved THz spectroscopy are powerful spectroscopic tools with the advantages of being contact-free and noninvasive, which have been widely used to study the photoconductivity (PC) of 2D materials. In this review, firstly, we provide a short introduction of the 2D materials and THz spectroscopy, and then a brief introduction of the experimental setup and experimental data analysis based on time-resolved THz spectroscopy are presented. After that, we overview the latest progress on the regulation of the THz PC that includes: (1) regulating the THz PC of graphene (Gr) and transition metal dichalcogenide (TMD) thin films with oxygen adsorption; (2) regulating the THz PC of Gr and Gr/TMDs heterostructures by electric gating and a built-in field introduced by a substrate; (3) regulating the THz PC of Gr/TMD heterostructures via optical gating; and (4) we overview the latest progress on the observation of elementary excitations in 2D materials with THz PC spectra following optical excitation and THz PC regulation via the photoexcitation of quasi-particles. Finally, we conclude the review and present a short overview of future research directions.

Exciton dissociation in 2D layered metal-halide perovskites

Layered 2D perovskites are making inroads as materials for photovoltaics and light emitting diodes, but their photophysics is still lively debated. Although their large exciton binding energies should hinder charge separation, significant evidence has been uncovered for an abundance of free carriers among optical excitations. Several explanations have been proposed, like exciton dissociation at grain boundaries or polaron formation, without clarifying yet if excitons form and then dissociate, or if the formation is prevented by competing relaxation processes. Here we address exciton stability in layered Ruddlesden-Popper PEA2PbI4 (PEA stands for phenethylammonium) both in form of thin film and single crystal, by resonant injection of cold excitons, whose dissociation is then probed with femtosecond differential transmission. We show the intrinsic nature of exciton dissociation in 2D layered perovskites, demonstrating that both 2D and 3D perovskites are free carrier semiconductors and their photophysics is described by a unique and universal framework.

Quantum control of substitutional effects on the S-involved optoelectronic properties of BeSxSe1-X ternary alloys

The research on the quantum control of substitutional effects in BeSxSe1-x ternary alloys opens up exciting opportunities in semiconductor device engineering, photovoltaic, optoelectronic integration, and quantum information processing. By understanding and manipulating the optoelectronic properties of these alloys, researchers can pave the way for advanced technologies with enhanced performance, efficiency, and functionality in various fields. We are investigating how S alloying affects the structural, electronic and optical properties of tin chalcogenide materials. We are assessing the energy band structure and optical properties of BeSxSe1-x (x = 0%, 25%, and 50%) materials using density functional theory (DFT) within a full potential linearized augmented plane wave method (FP-LAPW) framework. The theoretically expected band energy gaps of BeSxSe1-x compounds indicate that S alloying improves their electrical and optical performances, based on the assumptions used in our calculations. The majority of the bonds in the BeSxSe1-x materials (x = 0%, 25%, and 50%) are covalent. All ternary alloys and binary compounds with (x = 0%, 25%, and 50%) exhibit direct (R-Γ) band gap semiconductor behavior, with a band gap energy (Eg). Each alloy system shows a nonlinear increase in the band gap (Eg) with increasing x. As BeSxSe1-x compound belongs to strongly correlated systems, therefore modified Becke–Johnson potential is used to calculate formation enthalpy (ΔHf), and optical properties of the compounds. The intense peaks in the imaginary part of the dielectric function εi(ω) in each spectrum are contributed by chalcogen Se, Be, and S optical excitations. Similar to the trend of band gap variation (Eg) with x, the zero-frequency limit of each imaginary dielectric function as well as the extinction coefficient, refractive index, and real dielectric function, exhibit a corresponding trend of change.

A‐site coordinating cation engineering in zero‐dimensional antimony halide perovskites for strong self‐trapped exciton emission

AbstractLow‐dimensional hybrid halide perovskites represent a promising class of materials in optoelectronic applications because of strong broad self‐trapped exciton (STE) emissions. However, there exists a limitation in designing the ideal A‐site cation that makes the material satisfy the structure tolerance and exhibit STE emission raised by the appropriate electron–phonon coupling effect. To overcome this dilemma, we developed an inorganic metal‐organic dimethyl sulfoxide (DMSO) coordinating strategy to synthesize a series of zero‐dimensional (0D) Sb‐based halide perovskites including Na3SbBr6·DMSO6 (1), AlSbBr6·DMSO6 (2), AlSbCl6·DMSO6 (3), GaSbCl6·DMSO6 (4), Mn2Sb2Br10·DMSO13 (5) and MgSbBr5·DMSO7 (6), in which the distinctive coordinating A‐site cation [Am‐DMSO6]n+ efficiently separate the [SbXz] polyhedrons. Advantageously, these materials all exhibit broadband‐emissions with full widths at half maxima (FWHM) of 95–184 nm, and the highest photoluminescent quantum yield (PLQY) of 3 reaches 92%. Notably, compounds 2–4 are able to remain stable after storage of more than 120 d. First‐principles calculations indicate that the origin of the efficient STE emission can be attributed to the localized distortion in [SbXz] polyhedron upon optical excitation. Experimental and calculational results demonstrate that the proposed coordinating strategy provides a way to efficiently expand the variety of novel high‐performance STE emitters and continuously regulate their emission behaviors.