Optical Photoexcitation Research Articles

Terahertz imaging through emissivity control

Electromagnetic radiation in the terahertz (THz) frequency band has unique potential for future communication and imaging applications. However, the adoption of THz technologies is hindered by the lack of cost-effective THz sources. Here we demonstrate a way to generate and control THz radiation, via spatio-temporal emissivity modulation. By patterning the optical photoexcitation of a surface-passivated silicon wafer, we locally control the free-electron density, and thereby pattern the wafer’s emissivity in the THz part of the electromagnetic spectrum. We show how this unconventional source of controllable THz radiation enables a form of incoherent computational THz imaging. We use it to image various concealed objects, demonstrating that this scheme has the penetrating capability of other THz imaging approaches, without the requirement of femtosecond pulsed laser sources. Furthermore, the incoherent nature of thermal radiation also ensures the obtained images are free of interference artifacts. Our spatio-temporal emissivity control could enable a family of long-wavelength structured illumination, imaging, and spectroscopy systems.

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe_{2}.

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe_{2} with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe_{2} layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K^{'} valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides.

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

Femtosecond control of phonon dynamics near a magnetic order critical point

The spin-phonon interaction in spin density wave (SDW) systems often determines the free energy landscape that drives the evolution of the system. When a passing energy flux, such as photoexcitation, drives a crystalline system far from equilibrium, the resulting lattice displacement generates transient vibrational states. Manipulating intermediate vibrational states in the vicinity of the critical point, where the SDW order parameter changes dramatically, would then allow dynamical control over functional properties. Here we combine double photoexcitation with an X-ray free-electron laser (XFEL) probe to control and detect the lifetime and magnitude of the intermediate vibrational state near the critical point of the SDW in chromium. We apply Landau theory to identify the mechanism of control as a repeated partial quench and sub picosecond recovery of the SDW. Our results showcase the capabilities to influence and monitor quantum states by combining multiple optical photoexcitations with an XFEL probe. They open new avenues for manipulating and researching the behaviour of photoexcited states in charge and spin order systems near the critical point.

Study of Photocatalytic Properties of Ag/AgCl-Decorated Soybean Protein Knitting Fabric Against Acid Blue 260 Dye by Factorial Design

Photocatalytic materials have been the focus of several studies due to their excellent optical photoexcitation properties for waste treatment. However, their use promotes the generation of inorganic secondary waste, due to their nanometric grain size, which can cause several environmental hazards. The optimization and immobilization of such photocatalysts for treatment after the catalytic process have thus attracted much attention. In this work, Ag/AgCl nanoparticles were immobilized by hydrothermal treatment on soybean knitted fabric. The photocatalytic activity of the resulting Ag/AgCl-functionalized soybean knitted fabric was estimated based on the reduction of the concentration of Acid Blue 260 dye. The response surface factorial design methodology was applied, varying the Ag/AgCl concentration, nanocatalyst immobilization time, and hydrothermal process temperature, to analyze the individual effect of and interaction between the variables, besides establishing a mathematical model of the process. The photocatalytic results showed that addition of Ag/AgCl to the fabric favored its use and reuse for the degradation of the Acid Blue 260 dye. The mathematical model together with the response surfaces showed that the residence time in the hydrothermal bath was the most significant variable in terms of the dye reduction, favoring this advanced treatment method.

Dense Electron-Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material Tuning.

Many-body interactions in photoexcited semiconductors can bring about strongly interacting electronic states, culminating in the fully ionized matter of electron-hole plasma (EHP) and electron-hole liquid (EHL). These exotic phases exhibit unique electronic properties, such as metallic conductivity and metastable high photoexcitation density, which can be the basis for future transformative applications. However, the cryogenic condition required for its formation has limited the study of dense plasma phases to a purely academic pursuit in a restricted parameter space. This paradigm can potentially change with the recent experimental observation of these phases in atomically thin MoS2 and MoTe2 at room temperature. A fundamental understanding of EHP and EHL dynamics is critical for developing novel applications on this versatile layered platform. In this work, we studied the formation and dissipation of EHP in monolayer MoS2. Unlike previous results in bulk semiconductors, our results reveal that electromechanical material changes in monolayer MoS2 during photoexcitation play a significant role in dense EHP formation. Within the free-standing geometry, photoexcitation is accompanied by an unconstrained thermal expansion, resulting in a direct-to-indirect gap electronic transition at a critical lattice spacing and fluence. This dramatic altering of the material's energetic landscape extends carrier lifetimes by 2 orders of magnitude and allows the density required for EHP formation. The result is a stable dense plasma state that is sustained with modest optical photoexcitation. Our findings pave the way for novel applications based on dense plasma states in two-dimensional semiconductors.

Excited-state lifetime of the NV− infrared transition in diamond

The negatively charged nitrogen vacancy ($\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$) defect in diamond serves as a popular platform for manipulating and exploiting long-lived coherent spin dynamics at room temperature combined with optical readout. The required spin polarization of the spin triplet $^{3}A_{2}$ electronic ground state occurs through a cycle of repetitious optical photoexcitation events to the $^{3}E$ electronic excited state that is accompanied by a series of electronic transitions to a $^{1}A_{1}$ and a $^{1}E$ electronic state, and back to the $^{3}A_{2}$ state. The timescales of these transitions are largely known, yet for the relaxation time of the $^{1}A_{1}\ensuremath{\rightarrow}^{1}E$ infrared transition, which predominantly occurs via nonradiative recombination, only an upper limit of 1 ns could be determined so far. Here, we employ ultrafast transient absorption spectroscopy to probe the dynamics of the nonradiative relaxation from the $^{1}A_{1}$ to the $^{1}E$ state after photoexcitation of the $^{3}E$ state and find a relaxation time of 100 ps at a temperature of 78 K.

Using ultrashort terahertz pulses to directly probe spin dynamics in insulating antiferromagnets

Terahertz pulses are a direct and general probe of ultrafast spin dynamics in insulating antiferromagnets (AFM). This is shown by using optical-pump, THz-probe spectroscopy to directly track AFM spin dynamics in the hexagonal multiferroic HoMnO3 and the orthorhombic multiferroic TbMnO3. Our studies show that despite the different structural and spin orders in these materials, THz pulses can unambiguously resolve spin dynamics after optical photoexcitation. We believe that this approach is quite general and can be applied to a broad range of materials with different AFM spin alignments, providing a novel non-contact approach for probing AFM order with femtosecond temporal resolution.

Spatiotemporal Evolution of Coherent Elastic Strain Waves in a Single MoS2 Flake.

We use bright-field imaging in an ultrafast electron microscope to spatiotemporally map the evolution of photoexcited coherent strain waves in a single, micrometer-size flake of MoS2. Following in situ femtosecond photoexcitation, we observe individual wave trains emerge from discrete nanoscale morphological features and propagate in-plane along specific wave vectors at approximately the speed of sound (7 nm/ps). Over the span of several hundred picoseconds, the 50 GHz wave trains (20 ps periods) are observed to undergo phonon-phonon scattering and wave-train interference, resulting in a transition to larger-scale, incoherent structural dynamics. This incoherent motion further evolves into coherent nanomechanical oscillations over a few nanoseconds, ultimately leading to megahertz, whole-flake multimode resonances having microsecond lifetimes. These results provide insight into the low-frequency structural response of MoS2 to relatively coherent optical photoexcitation by elucidating the origin and the evolution of high-velocity, gigahertz strain waves.

Probing ultrafast spin dynamics through a magnon resonance in the antiferromagnetic multiferroicHoMnO3

We demonstrate a new approach for directly measuring the ultrafast energy transfer between elec- trons and magnons, enabling us to track spin dynamics in an antiferromagnet (AFM). In multiferroic HoMnO3, optical photoexcitation creates hot electrons, after which changes in the spin order are probed with a THz pulse tuned to a magnon resonance. This reveals a photoinduced transparency, which builds up over several picoseconds as the spins heat up due to energy transfer from hot elec- trons via phonons. This spin-lattice thermalization time is ?10 times faster than that of typical ferromagnetic (FM) manganites. We qualitatively explain the fundamental differences in spin-lattice thermalization between FM and AFM systems and apply a Boltzmann equation model for treating AFMs. Our work gives new insight into spin-lattice thermalization in AFMs and demonstrates a new approach for directly monitoring the ultrafast dynamics of spin order in these systems.

TeraHertz electronic noise in field-effect transistors

We present a theoretical investigation of high-frequency electronic noise in field-effect transistors used as detectors of TeraHertz radiation. Calculations are performed using the hydrodynamic-Langevin approach and specialized to the case of InGaAs high-electron mobility transistors. The main physical phenomena associated with the effect of branching of the total current between channel and gate and the appearance of two-dimensional plasma waves are discussed. We demonstrate that thermally excited standing plasma waves originate series of resonant peaks in the corresponding noise spectral densities whose presence can be controlled by the embedding circuit. A significant damping of the high-frequency excess noise is found when the transistor is submitted to a two-lasers optical photo-excitation presenting a beating frequency in the TeraHertz range. Finally, we discuss the dependence of the damping effect, as well as a shift of the resonance peaks from the presence of channel ungated regions.

Current response and gain in plasmonic vertical diodes in the presence of electrical and optical THz excitations

An analysis of generated Terahertz (THz) current oscillations taking place in one dimensional n+nn+ InGaAs diodes due to an optical photoexcitation and due to a dc operating bias are presented at room temperature. These oscillations have been investigated, photoresponse resonance frequency appears clearly at the THz spectrum. We study the current gain and the differential mobility in both n and n+ regions according to the optical photoexcitation as well as with respect to dc operating bias. The obtained results demonstrate the possibility to use vertical diodes as THz resonant detectors.

Terahertz single pixel imaging with an optically controlled dynamic spatial light modulator

We present a single pixel terahertz (THz) imaging technique using optical photoexcitation of semiconductors to dynamically and spatially control the electromagnetic properties of a semiconductor mask to collectively form a THz spatial light modulator (SLM). By co-propagating a THz and collimated optical laser beam through a high-resistivity silicon wafer, we are able to modify the THz transmission in real-time. By further encoding a spatial pattern on the optical beam with a digital micro-mirror device (DMD), we may write masks for THz radiation. We use masks of varying complexities ranging from 63 to 1023 pixels and are able to acquire images at speeds up to 1/2 Hz. Our results demonstrate the viability of obtaining real-time and high-fidelity THz images using an optically controlled SLM with a single pixel detector.

Hydrodynamic Study of Terahertz Three-Dimensional Plasma Resonances in InGaAs Diodes

We investigate the presence of plasma resonances in InGaAs n+−n−n+ diodes under different optical excitation conditions. In particular, we study the case of diodes submitted to an optical photoexcitation presenting a beating in the terahertz frequency domain. For this purpose, we calculate the electric field response in the middle of the n and n+ regions using a hydrodynamic approach self-consistently coupled to a one-dimensional Poisson solver. In particular, the analysis of the electric field response to an optical beating as a function of the doping and the geometry of the devices allows us to evidence in all the considered cases the presence of resonances in both n and n+ regions. However, while the observed resonances agree with the theoretical 3D plasma frequency in the n+ external regions, we point out a shift towards higher frequencies in the n region. We show that this shift towards the n+ 3D plasma frequency is due to the strong coupling between the two region modes, and tends to disappear when the n region lengthens, whereas the influence of the n+ regions length on the resonance frequency is negligible. Moreover, we show that the amplitude of the plasma resonances can be enhanced at high doping levels and by increasing the level of the optical photoexcitation. The obtained results show clearly that the resonances associated with 3D plasma waves in InGaAs diodes lie in the THz domain for typical values of dopings and lengths, thus opening new possibilities to exploit not only field effect transistors but also diodes as solid-state terahertz devices operating at room temperature.

Hydrodynamic study of terahertz three-dimensional plasma resonances in InGaAs diodes

Using a hydrodynamic model self-consistently coupled to a Poisson solver, we investigate the time and frequency response of InGaAs diodes excited at room temperature by an optical photoexcitation presenting a beating in the terahertz frequency domain. The analysis of the main physical quantities, such as the local electric field and the conduction current density, evidences the presence of strong resonances that are interpreted as three-dimensional plasma oscillations excited by the optical beating. By studying the influence of the geometry and doping of the diode, it is shown that, in most cases, the highly doped contacts mainly control the frequency of the plasma mode while the diode length is a crucial parameter to evidence a second resonance related to the diode active region. Moreover, the amplitude of the plasma resonances can be enhanced at high doping levels and by increasing the level of the optical photoexcitation.

Hydrodynamic study of terahertz three-dimensional plasma resonances in InGaAs diodes

We investigate the presence of plasma resonances in InGaAs diodes under different optical excitation conditions. In particular, we study the case of diodes submitted to an optical photoexcitation presenting a beating in the terahertz frequency domain. The responses of the diodes are calculated using a hydrodynamic approach coupled to a one-dimensional Poisson solver. The results show clearly the presence of three-dimensional plasma resonances in the terahertz frequency domain. We also show that the resonances frequency and amplitude can be tuned by modifying the diode geometry and doping profile.

Optical rectification at metal surfaces

The emission of freely propagating terahertz (THz) radiation coming from optical rectification at metallic surfaces has been detected and characterized for the first time to the authors' knowledge. The observed THz transients are induced through nonlinear electronic processes at gold and silver surfaces on intense pulsed optical photoexcitation and exhibit a peak electric field of as much as 200 V/cm. This finding opens a qualitatively new way to investigate nonlinear phenomena at metal surfaces and also can be exploited for the development of new THz emitters.

Coherent phonons, nanoseismology and THz radiation in InGaN/GaN heterostructures

The ultrafast optical photoexcitation of hot electrons and holes in semiconductors by femtosecond laser pulses can trigger coherent phonon oscillations. We discuss the huge coherent acoustic phonons which have been generated in InGaN/GaN heterostructures and epilayers and how they might be used in imaging of surfaces and interfaces in nanostructures. We also discuss the THz radiation emitted from these phonons.

Localization of charged excitations in polyaniline

Photogenerated polarons in the emeraldine base form of polyaniline have been extensively studied by using the infrared and optical photoexcitation, photoconductivity, and light-induced electron spin resonance (LESR) techniques. The photogenerated polarons in polyaniline are strongly localized and stable for the experimental time scales (≥ 1 msec.) at both 2.0 eV and 4.0 eV optical pumping energies as proved by infrared and LESR studies. The dynamic mass of the charged excitations in polyaniline is found to be two orders of magnitude larger than the electron band mass. Steady state photoconductivity measurements demonstrate that the photogenerated charges are highly immobile for both 2.0 eV and 4.0 eV pumping, consistent with the infrared studies. Significantly different results were obtained in the fast time scale (≤ 800 psec.) photoconductivity measurements. Photoexcitation at 4.0 eV leads to a measured photoconductivity one order of magnitude less than that of other conjugated polymers. The 2.0 eV optical pumping yields a factor of 30 smaller photoconductivity than that of the 4.0 eV pumping for equal number of incident photons. We propose that photogenerated holes are responsible for the transport, and the photogenerated electrons are mainly localized on the benzene ring (as monitored by the softening of the ring deformation mode caused by losing the aromaticity of the benzenoid structure).