Photon Absorption Process Research Articles

Above-threshold ionization with X-ray free-electron lasers

This study delves into the relatively uncharted territory of Above Threshold Ionization in atoms, triggered by intense X-ray radiation fields. At these frequencies, the energy of a single photon far exceeds the ionization potential of valence electrons in atoms and molecules. The conditions we examine are similar to those achievable with current or future free-electron laser facilities. Under such high-energy scenarios, the onset of strong field ionization requires a shift away from the traditional quasi-classical approach. Here, we present an analytical model to characterize how the field-free ionization potential, ponderomotive energy, and photon energy govern the transition to this regime, all accounted for by means of the Keldysh and Reiss parameters. We show that both of these parameters are needed to capture the onset of strong-field behavior due to both bound state and continuum state properties. At higher X-ray intensities, we find that ionization rates deviate from the linear intensity scaling expected from lowest order perturbative processes, corresponding to channel closure and higher-order photon absorption processes.

Singlet Oxygen in Photodynamic Therapy.

Photodynamic therapy (PDT) is a therapeutic modality that depends on the interaction of light, photosensitizers, and oxygen. The photon absorption and energy transfer process can lead to the Type II photochemical reaction of the photosensitizer and the production of singlet oxygen (1O2), which strongly oxidizes and reacts with biomolecules, ultimately causing oxidative damage to the target cells. Therefore, 1O2 is regarded as the key photocytotoxic species accountable for the initial photodynamic reactions for Type II photosensitizers. This article will provide a comprehensive review of 1O2 properties, 1O2 production, and 1O2 detection in the PDT process. The available 1O2 data of regulatory-approved photosensitizing drugs will also be discussed.

Kinetic modeling of the photocatalytic degradation of acetaminophen and its main transformation products

In this study, a kinetic model of the heterogeneous photocatalytic degradation of acetaminophen and its main transformation products is presented. Kinetic photocatalytic modeling and photon absorption rate modeling were included. Monte Carlo method was used to model the photon absorption process. Experiments were carried out in a reactor operated in batch mode and TiO2 nanotubes were used as photocatalyst irradiated with 254 nm UVC. Kinetic parameters were estimated from the experiments data by applying a non-linear regression procedure. Intrinsic expressions to the kinetics of acetaminophen degradation and its main transformation products were derived. Model, kinetics and photon absorption formulations and parameters proved to be affordable for describing the photocatalytic degradation of acetaminophen, but improvements should be done for better description of formation and oxidation kinetics of main transformation products. The model should be tested with other pharmaceuticals and emergent pollutants to calibrate it and evaluate its applicability in a wide range of compounds.

In-plane photoelectric effect in phosphorene nanoribbons

Recently, a new photoelectric phenomenon dubbed in-plane photoelectric (IPPE) effect has been demonstrated experimentally. In this work, we investigate the IPPE effect in a zigzag-edged or armchair-edged phosphorene nanoribbons (ZPNRs or APNRs) that is modulated by a step barrier. Based on the Floquet theory, we calculate the directed photocurrent Ī|| (Ī⊥) generated by a non-resonant linearly-polarized light with polarization vector in parallel with (perpendicular to) the nanoribbon direction. We find that such a photocurrent depends on the edge type of the nanoribbon and the polarization direction of light. At high Fermi energies, the IPPE current Ī|| in APNRs is significantly stronger than that in ZPNRs under the same conditions. In APNRs, both Ī|| and Ī⊥ always have the same direction and |Ī||:Ī⊥|>10. In contrast, in ZPNRs Ī⊥ can reverse its direction (becoming negative) and is comparable to Ī|| in amplitude. The negative photocurrent arises from both the photon-emission and photon-absorption process related to the quasi-flat-band in ZPNRs. This indicates that the mid-gap states can be detected by the IPPE effect. The energy scope and amplitude of negative photocurrent can be tuned by adjusting the height of the step barrier, the light frequency, and the light intensity.

Examples of atoms absorbing photon via Schrödinger equation and vacuum fluctuations

The absorption of photons by atoms encompasses fundamental quantum mechanical aspects, particularly the emergence of randomness to account for the inherent unpredictability in absorption outcomes. We demonstrate that vacuum fluctuations can be the origin of this randomness. An illustrative example of this is the absorption of a single photon by two symmetrically arranged atoms. In the absence of a mechanism to introduce randomness, the Schrödinger equation alone can govern the time evolution of the process initially. Then, it becomes stuck, and an entangled state of the two atoms emerges. This entangled state consists of two components: in one, the first atom is excited by the photon while the second is in the ground state, and in the other, the second atom is excited while the first remains in the ground state. These components form a superposition state characterized by an unbreakable symmetry in the absence of external influences. Consequently, the absorption process remains incomplete. When vacuum fluctuations come into play, they can induce fluctuations in the weights of these components, akin to Brownian motion. Over time, one component diminishes, thereby breaking the entanglement between the two atoms and allowing the photon absorption process to conclude. The remaining component shows which atom completes the photon absorption. Vacuum fluctuations not only introduce randomness but also have the potential to give rise to the Born rule in this context. Furthermore, the Casimir effect, which is closely tied to vacuum fluctuations, presents a promising experimental avenue for validating this mechanism. Similar studies can also be conducted with varying numbers of atoms.

Tuning quantum pathway interference in two-color laser photoemission using DC bias

Coherent control of quantum systems depends on the manipulation of quantum interference through external fields. In this work, we investigate the effects of DC bias field on coherent control of quantum pathways in two-color laser photoemission using exact analytical solutions of the one-dimensional time dependent Schrödinger equation. Increasing DC bias lowers and narrows the surface potential barrier, shifting the dominant emission to lower order multiphoton photoemission, photo-assisted tunneling and then direct tunneling. Those lower order photon absorption processes result in fewer possible pathways, and therefore modulation of photoemission current can be suppressed as DC field increases. It is shown that a maximum modulation depth of 99.4% can be achieved for a gold emitter at local DC bias F 0 = 0.5 V nm−1, fundamental (800 nm) laser field F 1 = 2.6 V nm−1 and second harmonic laser field F 2 = 0.25 V nm−1 . For a given set of input parameters, the total photoemission consists of different k-photon processes, each of which has their own different multiple possible pathways and interference effects. However, the quantum pathways and their interference for the dominant k-photon process and for the total photoemission probability show the same trends. This study demonstrates strong flexibility in tuning two-color lasers induced photoemission using a DC bias and provides insights into coherent control schemes of general quantum systems.

Laser-induced breakdown spectroscopy using modulated ns-laser pulses for multi-property measurements in high-pressure combustion environments

The present work proposes a novel method to improve the calibration accuracy of laser-induced breakdown spectroscopy (LIBS) in high-pressure combustion environments. Typically, optical plasma breakdown suffers from instability in highly pressurized environments, and the plasma emits photons of noisy and broadened spectra that are unusable for extracting quantitative property information. An improved plasma emission spectrum signal is obtained by modulating or chopping the pulse width of a fundamental 10 Hz Nd:YAG laser pulse by limiting the stochastic inverse-Bremsstrahlung (IB) photon absorption process. The modulated pulse duration is decreased from 9.2 ns to 5 ns utilizing a simple optical setup prior to being focused into a high-pressure combustor. In order to evaluate the impact of the modulated temporal laser pulse profile on the plasma emission, Proper Orthogonal Decomposition (POD) is applied to the emission spectra collected over a customized flat flame burner with varied composition and pressure. POD is capable of decomposing property-sensitive spectrum components that determine the accuracy of the spectrum calibration for property measurements using LIBS. The POD-analyzed database comprises 50 flame cases with pressure and equivalence ratios ranging from 1 to 10 bars and 0.7 to 1.1, respectively. Two Reduced-Order Models (ROMs) are trained with the collected spectra; modulated and original pulse, to predict three distinct flame properties; pressure, equivalence ratio, and adiabatic flame temperature. The output model estimates pressure, equivalence ratio and adiabatic flame temperature with an improved accuracy of 8%, 7% and 55%, respectively when the ROM is trained with chopped spectra over the calibrated pressure range. This is because the reduced pulse width in general lowers the signal level of broadband plasma emission that is sensitive to pressure. The POD-aided spectrum analyses suggest that flexible pulse modulation helps to improve the calibration accuracy in a broad gas density range with varied pressure and temperature.

Microwave-Assisted Thermoelectricity in S - I - S′ Tunnel Junctions

Asymmetric superconducting tunnel junctions with gaps ${\mathrm{\ensuremath{\Delta}}}_{1}>{\mathrm{\ensuremath{\Delta}}}_{2}$ have been proven to show a peculiar nonlinear bipolar thermoelectric effect. This arises due to the spontaneous breaking of electron-hole symmetry in the system, and it is maximized at the matching-peak bias $|V|={V}_{p}=({\mathrm{\ensuremath{\Delta}}}_{1}\ensuremath{-}{\mathrm{\ensuremath{\Delta}}}_{2})/e$. In this paper, we investigate the interplay of photon-assisted tunneling (PAT) and bipolar thermoelectric generation. In particular, we show how thermoelectricity, at the matching peak, is supported by photon absorption and emission processes at the frequency-shifted sidebands $V=\ifmmode\pm\else\textpm\fi{}{V}_{p}+n\ensuremath{\hbar}\ensuremath{\omega}$, $n\ensuremath{\in}\mathbb{Z}$. This represents a sort of microwave-assisted thermoelectricity. We show the existence of multiple stable solutions, being associated with different photon sidebands, when a load is connected to the junction. Finally, we discuss how the nonlinear cooling effects are modified by the PAT. The proposed device can detect millimeter-wavelength signals by converting a temperature gradient into a thermoelectric current or voltage.

AgScP2S6 van der Waals Layered Crystal: A Material with a Unique Combination of Extreme Nonlinear Optical Properties

Research in two-dimensional layered materials (2DLMs) has exploded over the past several years for a variety of applications in photonics and optoelectronics. The 2D nature of these materials allows for a very local electronic probe of material as well as flexible integration with other functional components. Herein, using the femtosecond Z-scan technique, we report a giant two photon absorption (TPA) process and its saturation in the van der Waals gapped silver scandium thiophosphate (AgScP2S6) crystal. We have found a TPA coefficient of the order of 104 cm/GW which is orders of magnitude larger compared to many existing semiconductors and nonlinear crystals. Furthermore, we found a TPA cross-section of 103 GM and characterized the optical limiting (OL) response (0.2 mJ/cm2) and the multipulse laser damage threshold (1.09 ± 0.19 J/cm2). The combination of giant TPA, extremely low OL, and very high damage threshold suggests that this material could be extremely useful in applications like optical limiters or switches.

Influence of holmium ions on the structural and optical properties of barium tellurite glasses

Barium tellurite glasses doped with holmium ions [60TeO2 + 20ZnO + 4BaF2 + (16-x) BaCO3 + xHo2O3 (0.25 ≥ x ≤ 2)] synthesized using melt quenching technique have been explored for their optical applications. Compositional tuning led to a maxima and minima in their respective optical bandgap (4.079 eV) and refractive index (2.154) values around 0.75 mol%. Green emission corresponding to the 5I8 → 5F4 (5S2) transition was found to be dominant in the visible region, with a reduction in intensity beyond 0.75 mol.%. This indicated the occurrence of concentration quenching effects and the limit of doping for obtaining high emission intensities. On exposure to nanosecond pulses of 532 nm wavelength, all samples showed good optical non-linearity when investigated using Z-scan technique. Open aperture scans showed reverse saturable absorption behavior owing to two photon absorption process. With reduced limiting threshold values, all compositions in this glass tie-line were found to act as good optical limiters, leading to the possibility of forming efficient limiting devices.

Implications of morphology on excited state absorption of α-MnO2 nanostructures

Nanoclusters, nanorods and nanowires of α-MnO2 were synthesized by reduction method at different annealing temperature (100 °C–500 °C). X-ray diffraction and Raman studies confirms the formation of pure tetragonal phase of α-MnO2. Surface morphology using SEM and HRTEM reveals the gradual transformation of α-MnO2 nanoclusters (100 °C–300 °C) to nanorods (400 °C) to well defined nanowires (500 °C). While increasing the annealing temperature, Ostwald ripening process initially withholds the agglomerated clusters and its size get reduced and fused that leads to orientation along particular direction. HRTEM clearly displays the influence of temperature on favoured growth plane where (411) of nanoclusters changed to (211) plane of nanorods and nanowires. Optical band edge absorption at 268 nm arising from d–d transitions of Mn4+ in MnO2 got blue shifted due to variations in morphology. Here with increase in temperature, aspect ratio increases which alters the number of surface atoms and consequently the cohesive energy put on effect in the widening of bandgap of the material from 4.31 eV (nanoclusters) to 4.63 eV (nanowires). Open aperture Z-scan technique using Nd: YAG laser (532 nm, 10 Hz & 9 ns) revealed all MnO2 nanostructures involve reverse saturable absorption ascribed due to excited state absorption and its strength varies with morphology. Significant absorption in the visible region (532 nm) provoked excited state absorption and intensity dependent variation of nonlinear absorption coefficient confirms the existence of sequential two photon absorption process. α-MnO2 nanowires with extended interconnected network and larger aspect ratio exhibits wider excited state cross-section (6.64 × 10−2 GM), higher two-photon absorption coefficient (2.01 × 10−10 m/W) and lower onset optical limiting threshold (2.13 × 1012 W/m2). This stronger optical limiting performance of α-MnO2 nanowires symbolizes the scope of utilizing optimal α-MnO2 nanostructure for optical limiting applications.

Functional Separation of Energy Transfer and Photon Absorption of Excitons Formed in Circular Nanoantennae

Functional Separation of Energy Transfer and Photon Absorption of Excitons Formed in Circular Nanoantennae

Enhanced photochemical production of reactive intermediates at the wetland soil-water interface

Photochemically produced reactive intermediates (PPRIs) formed by sunlight-irradiation of natural photosensitizers play critical roles in accelerating biogeochemical cycles on earth surface. Existing PPRI studies mostly focus on bulk phase reactions (e.g., bulk water), with PPRI processes at the environmental interfaces largely unexplored. Here, we report the wetland soil-water interface (SWI) as a widespread but previously unappreciated hotspot for PPRI productions. Massive productions of four important PPRI species (i.e., triplet-state excited organic matter (3OM*), singlet oxygen (1O2), hydrogen peroxide (H2O2), and hydroxyl radical (•OH)) were observed at the SWI. All four PPRI species exhibited higher productions at the SWI than those in bulk water, where •OH production was largely elevated by up to one order of magnitude. The enhanced PPRI productions at the SWI were caused by intensified photon absorption and vibrant Fe-mediated redox processes, where the light absorption by less- or non-photoactive soil substances partially offset the enhancement on PPRI productions. Nationwide wetland investigations demonstrate that the SWI was a ubiquitous hotspot for PPRI productions. Simulations on PPRIs-mediated reactions suggest that the enhanced PPRI productions could greatly affect the kinetics and transformation pathways of nutrients and pollutants. Given that the SWI also acts a hotspot for nutrient and pollutant accumulation, incorporating the SWI enhanced PPRI productions into biogeochemical process assessments is pivotal for advancing our understandings on the element cycles and pollutant dynamics in wetlands.

Impact of morphology on the nonlinear optical absorption of pristine molybdenum disulfide (MoS2) nanostructures

Tuning of nonlinear absorption and optical limiting action of MoS2 with different morphologies was demonstrated by open aperture Z-scan technique under Nd: YAG laser (532 nm, 7 ns, 10 Hz, 1–5 GW/cm2) excitation. The morphology of pristine MoS2 prepared by hydrothermal method at various reaction times gets altered from nanoclusters (24 h) to nanoplatelets (36 h) and further to nanosheets (48 and 60 h). Earlier XRD confirms the formation of honeycomb lattice of hexagonal crystal phase MoS2 (2H) and Raman ascertains the arrangement of S–Mo–S as a multilayer system. Because of alteration of band structure with morphology, hyper and hypochromic effect occurred in the absorbance of MoS2 nanoparticles. Photoluminescence spectra of MoS2 nanoparticles show two excitonic prominent peaks at 624 nm and 540 nm. All MoS2 nanoparticles prepared at varying reaction time exhibits reverse saturable absorption (RSA) pattern ascribed to two photon absorption process. Among the samples, variation in two photon absorption coefficients with different on-axis peak intensities shows zero slope value for MoS2 nanoclusters (24 h) and thus it exhibits genuine 2 PA. While non-zero slope value of MoS2 nanoplatelets (36 h) and nanosheets (48 and 60 h) display sequential 2 PA mechanism. MoS2 nanoplatelet possess high nonlinear absorption coefficient (5.9 × 10−10 m/W) with a low onset optical limiting threshold (0.73 × 1012 W/m2), which suggests its superiority for developing nonlinear absorption induced optical limiting devices.

Control of the Two-photon Visual Process in ex vivo Retinas and in Living Mice

Palcewska et al. first demonstrated near infrared (NIR) visual response in human volunteers upon two-photon absorption (TPA), in a seminal work of 2014, and assessed the process in terms of wavelength- and power-dependence on murine ex-vivo retinas. In the present study, ex-vivo electroretinography (ERG) is further developed to perform a complete characterization of the effect of NIR pulse duration, energy, and focal spot size on the response. The same set of measurements is successively tested on living mice. We discuss how the nonlinear intensity dependence of the photon absorption process is transferred to the amplitude of the visual response acquired by ERG. Finally, we show that the manipulation of the spectral phase of NIR pulses can be translated to predictable change in the two-photon induced response under physiological excitation conditions.

Simulation of Radiative Transfer Within X-ray Microcalorimeter Absorbers

We present Monte Carlo simulations of radiative transfer within the absorbers of X-ray microcalorimeters, utilizing a numerical model for the photon propagation and photon absorption process within the absorber structure. In our model, we include effects of Compton scattering off bound electrons and fluorescence. Scattered or fluorescence photons as well as Auger and photoelectrons escaping the absorber can result in partial energy depositions. By implementing a simplified description of the physical processes compared to existing comprehensive particle transport software frameworks, our model aims to provide representative results at a small computational effort. This approach makes it possible to use our model for quick assessments, parametric studies, and application in other Monte Carlo-based instrument simulators like SIXTE, a software package for X-ray astronomical instrumentation. To study the impact of the energy loss effects on the spectral response of a microcalorimeter, we apply our model to the sensors of the cryogenic X-ray spectrometer X-IFU onboard the future Athena X-ray observatory.

Unique Electronic Excitations at Highly Localized Plasmonic Field.

ConspectusUnder visible light illuminations, noble metal nanostructures can condense photon energy into the nanoscale region. By precisely tuning the metal nanostructures, the ultimate confinement of photoenergy at the molecular scale can be obtained. At such a confined photon energy field, various unique photoresponses of molecules, such as efficient visible light energy conversion processes or efficient multielectron transfer reactions, can be observed. Light-matter interactions also increase with the condensation of photons with nanoscale regions, leading to efficient light energy utilizations. Moreover, the strong field confinement can often modulate electronic excitations beyond normal selection rules. Such unique electronic excitations could realize innovative photoenergy conversion systems. On the other hand, such interactions lead to changes in the optical absorption property of the system via the formation of hybridized electronic energy states. This hybridized state is expected to have the potential to modulate the chemical reaction pathways. Taking these facts into consideration, a probe for the molecular absorption process with high sensitivity allows us to find novel ways for further precise tuning of light-matter interactions. In this Account, we review phenomena of unique electronic excitations from the perspective of our previous investigations using surface-enhanced Raman scattering (SERS) spectroscopy at electrified interfaces. Because the enhancement mechanism of Raman scattering at interfaces is deeply correlated with the photon absorption process accompanied by the electronic excitations between molecules and electrode surfaces, the detailed SERS investigations of the well-defined system can provide information on the electronic excitation processes. Through SERS observations of single-molecule junctions at electrodes or well-defined low-dimensional carbon materials, we have observed the characteristic Raman bands containing additional polarization tensors, indicating the occurrence of electronic polarization induced by electronic excitations based on a distinct selection rule. The origins for the observed facts were attributed to the highly condensed electric field producing the huge intensity gradient at the nano scale. The electrochemical potential control of the system would be valuable for the control of the excitation process. Additionally, from Raman spectra of dye molecules coupled to the plasmonic field, the changes in the Raman scattering intensity depending on the strength of interactions suggested the modulation of the absorption characteristics of the system. In addition, we have proved that the electrochemical potential control method can be a powerful tool for the active tuning of the light-matter interaction, leading to the change in the light absorption property. The molecular behaviors of dyes in the strong-coupling regime were reversibly tuned to show intense SERS. The current descriptions provide novel insights for these unique electronic excitations, realized by the plasmon excitation, that lead to advanced photoenergy conversions beyond the limits of present systems.

The Functions and Mechanisms of Low-Level Laser Therapy in Tendon Repair (Review).

Tendon injury is a common disease of the musculoskeletal system, accounting for roughly 30%–40% of sports system disorder injuries. In recent years, its incidence is increasing. Many studies have shown that low-level laser therapy (LLLT) has a significant effect on tendon repair by firstly activating cytochrome C oxidase and thus carrying out the photon absorption process, secondly acting in all the three phases of tendon repair, and finally improving tendon recovery. The repair mechanisms of LLLT are different in the three phases of tendon repair. In the inflammatory phase, LLLT mainly activates a large number of VEGF and promotes angiogenesis under hypoxia. During the proliferation phase, LLLT increases the amount of collagen type III by promoting the proliferation of fibroblasts. Throughout the remodeling phase, LLLT mainly activates M2 macrophages and downregulates inflammatory factors, thus reducing inflammatory responses. However, it should also be noted that in the final phase of tendon repair, the use of LLLT causes excessive upregulation of some growth factors, which will lead to tendon fibrosis. In summary, we need to further investigate the functions and mechanisms of LLLT in the treatment of tendon injury and to clarify the nature of LLLT for the treatment of diverse tendon injury diseases.

Nanosecond laser pulse modulation using seed electrons from cascade ionization induced by inverse-Bremsstrahlung photon absorption.

Nanosecond (ns) laser pulses are modulated by seeding electrons on the laser beam path. The seed-electrons are from auxiliary ns-laser-induced breakdown (ALIB), and the ALIB is induced by a focused 1064-nm pulse, which is split after the frequency-doubling that generates the 532-nm pulse; therefore, the 532-nm and 1064-nm pulses are synchronized. The slowly converging (focal length = 500 mm) 532-nm pulse is re-directed to transmit through the region in where the ALIB-generated electrons reside. The seed-electrons from the ALIB then absorb the 532-nm photons via the inverse-Bremsstrahlung photon absorption (IBPA) process. The number density of the seed-electrons on the 532-nm beam path (ne,ALIB) is controlled by varying 1) the 532-nm pulse arrival time at the ALIB region (ΔPAT) after the 1064-nm pulse triggers the ALIB and 2) the location of the 532-nm beam relative to the core of the ALIB; the electron number density in ALIB is highly non-uniform and evolves in time. Electron-seeded laser-induced breakdown (ESLIB) occurs when ne,ALIB is sufficiently high. The 532-nm beam convergence (controlled by the focusing lens) is adjusted so that the breakdown does not occur without the electron seeding. The ESLIB immediately stops the transmission of the trailing edge of the laser pulse acting as a fast shutter, and ne,ALIB above a threshold can cut the pulse leading edge to modulate the 532-nm laser pulse.

Investigation and Optimization of Mxene Functionalized Mesoporous Titania Films as Efficient Photoelectrodes.

Three-dimensional mesoporous TiO2 scaffolds of anatase phase possess inherent eximious optical behavior that is beneficial for photoelectrodes used for solar energy conversion applications. In this regard; substantial efforts have been devoted to maximizing the UV and/or visible light absorption efficiency; and suppressing the annihilation of photogenerated charged species; in pristine mesoporous TiO2 structures for improved solar illumination conversion efficiency. This study provides fundamental insights into the use of Mxene functionalized mesoporous TiO2 as a photoelectrode. This novel combination of Mxene functionalized TiO2 electrodes with and without TiCl4 treatment was successfully optimized to intensify the process of photon absorption; charge segregation and photocurrent; resulting in superior photoelectrode performance. The photocurrent measurements of the prepared photoelectrodes were significantly enhanced with increased contents of Mxene due to improved absorption efficiency within the visible region; as verified by UV–Vis absorption spectroscopy. The anatase phase of TiO2 was significantly augmented due to increased contents of Mxene and postdeposition heat treatments; as evidenced by structural analysis. Consequently; an appreciable coverage of well-developed grains on the FTO surface was observed in SEM images. As such; these newly fabricated conductive mesoporous TiO2 photoelectrodes are potential candidates for photoinduced energy conversion and storage applications.