Multiphoton Processes Research Articles

Luminescence enhancement through co-sensitization in lanthanide composites for efficient photocatalysis.

Lanthanide-doped nanocrystals that convert near-infrared (NIR) irradiation into shorter wavelength emission (ultraviolet-C) offer many exciting opportunities for biomedicine, bioimaging, and environmental catalysis. However, developing lanthanide-doped nanocrystals with high UVC brightness for efficient photocatalysis is a formidable challenge due to the complexity of the multiphoton process. Here, we report a series of heterogeneous core-multishell structures based on a co-sensitization strategy with multi-band enhanced emission profiles under 980 nm excitation. Interestingly, the multiphoton processes involving two to six-photon upconversion are highly promoted via a co-sensitization strategy. More importantly, through growth layers of TiO2 and CdS photocatalysts, these lanthanide nanocomposites with efficient multi-upconverted emission show efficient photocatalytic activity. This study provides a new perspective for mechanistic understanding of multiphoton processes in heterostructures and also offers exciting opportunities for highly efficient photocatalytic applications.

Theoretical study of double oscillating fields induced electron-positron pairs creation process

<sec>We investigate an important aspect of electron-positron pair creation from vacuum in the presence of a strong background field, where the combined field plays a key role in the pair creation process. By utilizing computational quantum field theory, we explore electron-positron pair creation induced by double-located oscillating electric fields by numerically solving the Dirac equation in full spacetime dimensions. We demonstrate theoretically that computational quantum field theory is equivalent to the first-order time-dependent perturbation theory for single-photon transition pair creation in a spatially inhomogeneous and time-dependent electric field, and verify their equivalence through numerical simulations of pair creation in double-located oscillating fields. We show some interesting results about the periodic oscillation of the momentum spectrum structure of the created particle and the asymmetric multi-photon pair creation process due to the interference between two fields. By using first-order time-dependent perturbation theory, we find that the periodic oscillation in the momentum distribution of the created particle is affected by the field width, the field frequency and the distance between two fields. The period of the oscillation of momentum spectrum structure is changed by the distance between two fields, while the field width has an influence on both the difference between the peak and valley of the momentum spectra and the width of the momentum space available to the created particle. Increasing the frequency of the electric field results in larger momentum for the created particle pairs, while correspondingly reducing the coupling matrix element <inline-formula><tex-math id="M1">\begin{document}$ \langle p|V|n \rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20230432_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20230432_M1.png"/></alternatives></inline-formula> and diminishing the probability of electron-positron pair creation.</sec><sec>The interference between two fields significantly enhances the yield of pair numbers for small distances between two fields. When the distance is too large, the number of pairs created by double oscillating fields is twice that created by a single field, and the enhancement is vanished. When the distance between two fields increases, the period of oscillation decreases. In turn, the creation of electron-positron pairs can become more monochromatic in momentum (energy), while the number of pairs created remains almost constant. As the electric field broadens, the yield of the created pairs decreases for constant potential height. Increasing the field width will reduce the number of particles for each momentum and narrow the momentum space of the created particle. Increasing the field frequency leads to the reduction of the coupling matrix element <inline-formula><tex-math id="M2">\begin{document}$ \langle p|V|n \rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20230432_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20230432_M2.png"/></alternatives></inline-formula> and subsequently reduces the total number of electron-positron pairs created. The field profile parameters such as frequency, width, and distance between two fields can be utilized to select a specific momentum (energy) of particles in future electron-positron pair creation experiments.</sec>

Integrating Optical Nanofibres with Cold Rubidium Ground-State and Rydberg Atoms

In recent years, optical nanofibres have become a promising platform for trapping, manipulating and controlling atomic systems. In this work, I will highlight our recent work on the demonstration of multiphoton processes using optical nanofibres embedded in a Rb MOT for the generation of entangled photons and the excitation of Rydberg atoms for all-fibred quantum networks.

(Invited) Manipulating Light Emission from Rare Earth Doped Nanoparticles for Applications in Theranostics

Luminescent nanomaterials that can be excited, as well as emit, in the near-infrared (NIR) have been investigated for use in a plethora of applications including nanomedicine, nanoelectronics, biosensing, bioimaging, photovoltaics, photocatalysis, etc. The use of NIR light for excitation mitigates some of the drawbacks associated with high-energy (UV or blue) excitation, for example, little to no background autofluorescence from the specimen under investigation as well as no incurred photodamage. Moreover, one of the biggest limitations is of course, that of penetration. As such, NIR light can penetrate tissues much better than high-energy light especially when these wavelengths lie within the three biological windows (BW-I: 700-950, BW-II: 1000-1350, BW-III: 1550-1870 nm) where tissues are optically transparent. At the forefront of NIR excited nanomaterials are rare earth doped nanoparticles, which due to their 4f electronic energy states can undergo conventional (Stokes) luminescence and emit in the three NIR biological windows. However, unlike other classes of nanoparticles, they can also undergo a multiphoton process (known as upconversion) where the NIR excitation light is converted to higher energies resulting in anti-Stokes luminescence spanning the UV-visible-NIR regions. Perhaps the biggest impact of such materials would be in the field of disease diagnostics and therapeutics, now commonly referred to as theranostics. Due to the versatility of their optical properties, it now becomes possible to generate high-energy light (UV or blue) in situ to trigger other light activated therapeutic modalities (i.e. drug release) while using the NIR emission for diagnostics (i.e. bioimaging, nanothermometry). Here, we present the synthesis of various NIR excited (and emitting) rare earth doped core/shell (and multishell) nanoparticles and demonstrate how their luminescence properties can be exploited for potential use in diverse biomedical applications.

Multiphoton Imaging of Maturation in Tissue Engineering.

Donor cell-specific tissue-engineered (TE) implants are a promising therapy for personalized treatment of cardiovascular diseases, but current development protocols lack a stable longitudinal assessment of tissue development at subcellular resolution. As a first step toward such an assessment approach, in this study we establish a generalized labeling and imaging protocol to obtain quantified maturation parameters of TE constructs in three dimensions (3D) without the need of histological slicing, thus leaving the tissue intact. Focusing on intracellular matrix (ICM) and extracellular matrix (ECM) networks, multiphoton laser scanning microscopy (MPLSM) was used to investigate TE patches of different conditioning durations of up to 21 days. We show here that with a straightforward labeling procedure of whole-mount samples (so without slicing into thin histological sections), followed by an easy-to-use multiphoton imaging process, we obtained high-quality images of the tissue in 3D at various time points during development. The stacks of images could then be further analyzed to visualize and quantify the volume of cell coverage as well as the volume fraction and network of structural proteins. We showed that collagen and alpha-smooth muscle actin (α-SMA) volume fractions increased as normalized to full tissue volume and proportional to the cell count, with a converging trend to the final density of (4.0% ± 0.6%) and (7.6% ± 0.7%), respectively. The image analysis of ICM and ECM revealed a developing and widely branched interconnected matrix. We are currently working on the second step, that is, to integrate MPLSM endoscopy into a dynamic bioreactor system to monitor the maturation of intact TE constructs over time, thus without the need to take them out.

Evolution of chirality in a multiphoton pair production process

Evolution of chirality in a multiphoton pair production process

Synthesis, optical and photocatalysis property of corn-like ZnO/ZnS heterojunction with a certain lattice defects.

In order to greatly improve the photocatalytic properties, corn-like ZnO/ZnS heterojunctions with a particle size of about 60-71nm have been synthesized by the solvothermal method and the subsequent sulfuration process. A declining trend is found for the specific surface area with increasing sulfuration time. The corn-like ZnO/ZnS heterojunctions exhibit good photocatalytic properties. With increasing sulfuration time, the degradation rate increases first and then decreases. The best degradation rate is observed for the heterojunction sulfurated for 90min. The strong broad luminescence band is extremely beneficial to the absorption of visible light by multiphoton process. In addition, the energy transfer from ZnS to ZnO contributes to charge separation, forming a type-II heterojunction mechanism. After one cycle of photocatalytic process, except that corns become more broken, variation of particle size and shape is very small. The degradation speed of RhB after a second cycle of photocatalytic process is slower than the first one except when using the sample sulfurated for 360min.

Impact of the Hexagonal Phase on Multiphoton‐Absorption Properties of Mixed‐Cation Halide Perovskite: FA0.8MA0.2PbI3

AbstractRecently, mixed‐cation halide perovskites have gained significant attention for developing stable, high‐performance solar cells. However, their nonlinear optical (NLO) properties are much less explored. Herein, this study reports on intriguing NLO properties observed from FA0.8MA0.2PbI3 single crystals, having an ideal bandgap of 1.52 eV for photovoltaic performance. Contrary to a simple prediction, the perovskite exhibits higher‐order multiphoton absorption (MPA) over a broad wavelength range from 1300 to 1710 nm as evidenced by wavelength‐ and intensity‐dependent photoluminescence measurements. The corresponding MPA coefficients are very large to dominate lower‐order ordinary MPA across the bandgap. The hexagonal phase present in the perovskite is responsible for the observed higher‐order transition as confirmed by the recovery of ordinary MPA in the absence of the hexagonal phase. Although the perovskite is known to crystallize into the cubic phase at room temperature, these results indicate that minor inclusion of the hexagonal phase (≈4.0% at room temperature) can drastically alter the nature of the multiphoton process by double resonance uniquely offered by the mixed phases.

Efficient vacuum pair production in frequency chirped electric fields with spatiotemporal inhomogeneity

Vacuum pair production under intense electromagnetic fields is an intriguing prediction in quantum electrodynamics (QED), and has become a topic of considerable interest with the rapid advancements in laser technologies. The pair production efficiency can be enhanced using chirped electric fields. In this study, the effects of electric field chirping on electron–positron pair production in different oscillating electric fields were investigated using the Dirac–Heisenberg–Wigner (DHW) formalism. We primarily considered quadratic and sinusoidal chirped electric fields. The results revealed that chirping significantly influences the momentum spectra of the generated particles in an electric field dominated by low-frequency tunneling effects, resulting in pronounced oscillations of the momentum spectrum, particularly in a sinusoidal chirped field. However, multiphoton processes gradually emerge as the chirp parameter increases. Consequently, in a high-frequency electric field dominated by multiphoton processes, the number density of the generated particles increases with the chirping parameter in a quadratic chirped field, whereas the momentum distribution remains relatively symmetric. However, in a sinusoidal chirped electric field, introducing a chirp parameter immediately alters the momentum distribution and enhances pair production rate. Moreover, there exists an optimal chirp parameter and the total particle number is enhanced approximately more than an order of magnitude with optimal chirping in a sinusoidal chirped electric field. These findings provide valuable insights into the intricate dynamics of pair production under the influence of chirping in different oscillating electric fields, and offer potential directions for future research in this domain.

Theoretical study of nonlinear absorption of a strong electromagnetic wave in infinite semi-parabolic Plus Semi-inverse Squared Quantum Wells by using quantum kinetic equation

General analytic expressions for the total absorption coefficient of strong electromagnetic waves caused by confined electrons in Infinite semi-parabolic plus Semi-inverse Squared Quantum Wells (ISPSISQW) are obtained by using the quantum kinetic equation for electrons in the case of electron-optical phonon scattering. A second-order multi-photon process is included in the result. The dependence of the total absorption coefficient on the intensity E0, the photon energy ħΩ of an SEMW, and the temperature T for a specific GaAs/GaAsAl ISPPSISQW is achieved by using a numerical method. The computational results demonstrate that the total absorption coefficient’s dependence on photon energy can be utilized for optically detecting the electric sub-bands in an ISPPSISQW. Besides, we also give theoretical rules on the dependence of the Full Width at Half Maximum on important external parameters such as temperature and perpendicular magnetic field. Furthermore, the obtained results are consistent with prior theoretical and experimental findings.

Effect of multiphoton processes on the asymmetric fluorescence spectrum and spectral correlations of a two-level atom driven bichromatically by two strong laser fields

Effect of multiphoton processes on the asymmetric fluorescence spectrum and spectral correlations of a two-level atom driven bichromatically by two strong laser fields

Controlling optical bistability, multistability and all-optical switching through multi-photon excitation process

We present a theoretical study on optical bistability (OB), optical multistability (OM) and tunable all-optical switching in a four-level cascade type atomic system. The density matrix formalism is used to find out the response of the probe laser field. We explore OB and OM under unidirectional optical ring cavity configuration. A comparison between the effects of electromagnetically induced transparency and electromagnetically induced absorption windows on OB is presented. The threshold of OB and the hysteresis width can be tuned effectively by adjusting the applied coherent fields. A transition from OB to OM and vice-versa can be easily achieved by tuning the field parameters and the atomic cooperation parameter. Dynamic control of the probe laser propagating through the medium has been investigated. We also demonstrate conversion of a continuous wave input probe field into output switched pulse. The ON/OFF time of the switched probe pulse can also be controlled by tuning the field parameters.

Differences of up-conversion and temperature sensing behaviors of NaYF4: 2 mol % Er3+, 20 mol % Yb3+ Phosphors with cubic and hexagonal phases

In this work, NaYF4: 2 mol % Er3+, 20 mol % Yb3+ phosphors with cubic and hexagonal phases were prepared by co-precipitation method with different reaction temperature. The multi-photon up-conversion processes and optical temperature sensing properties are discussed. The emission spectra of hexagonal phase show higher emission intensity and lower red-to-green ratio (R/G), which are attributed to the decrease of local symmetric. And the number of photons (n), participated in the up-conversion processes, declines as the phase changes from cubic to hexagonal. This can be originated to the exhaustion of electrons on ground state level: 4I15/2 in hexagonal phase. And the fitting n of green emissions decline more obviously than red emission with temperature increasing. The temperature dependent FIR is fitted and the fitted function implies that the energy gap ∆E increases as the phase changes to hexagonal. And the max relative sensitivity reaches 0.012 K−1 at 300 K. Thus, NaYF4: 2 mol % Er3+, 20 mol % Yb3+ phosphors with hexagonal phase have higher SR and can be promising in FIR-based temperature sensing.

Photoredox catalysis harvesting multiple photon or electrochemical energies.

Photoredox catalysis (PRC) is a cutting-edge frontier for single electron-transfer (SET) reactions, enabling the generation of reactive intermediates for both oxidative and reductive processes via photon activation of a catalyst. Although this represents a significant step towards chemoselective and, more generally, sustainable chemistry, its efficacy is limited by the energy of visible light photons. Nowadays, excellent alternative conditions are available to overcome these limitations, harvesting two different but correlated concepts: the use of multi-photon processes such as consecutive photoinduced electron transfer (conPET) and the combination of photo- and electrochemistry in synthetic photoelectrochemistry (PEC). Herein, we review the most recent contributions to these fields in both oxidative and reductive activations of organic functional groups. New opportunities for organic chemists are captured, such as selective reactions employing super-oxidants and super-reductants to engage unactivated chemical feedstocks, and scalability up to gram scales in continuous flow. This review provides comparisons between the two techniques (multi-photon photoredox catalysis and PEC) to help the reader to fully understand their similarities, differences and potential applications and to therefore choose which method is the most appropriate for a given reaction, scale and purpose of a project.

Multiphoton processes and higher resonances in the quantum regime of the free-electron laser

Multiphoton processes and higher resonances in the quantum regime of the free-electron laser

Modulational instability and nonlinear dynamics of femtosecond lasers in transparent materials with non-Kerr nonlinearities

Femtosecond laser inscription in transparent materials is a physical process that finds widespread applications in material engineering, particularly in laser micromachining technology. In this process, the nonlinear optical response of the transparent material can be either intrinsic or induced by multiphoton ionization processes. In this work, a generic model is considered to describe the dynamics of femtosecond laser filamentation in transparent materials characterized by non-Kerr nonlinearities, focusing on the influence of multiphoton ionization processes in the generation of an electron plasma of inhomogeneous density. The mathematical model consists of a complex Ginzburg–Landau equation with a generalized saturable nonlinearity, besides the residual nonlinearity related to multiphoton ionization processes. This generalized complex Ginzburg–Landau equation is coupled to a rate equation for time evolution of the electron plasma density, where multiphoton ionizations are assumed to be the sole processes controlling the generation of the electron plasma. Dynamical properties of the model are discussed starting from the continuous-wave regime, where a modulational-instability analysis enables us to determine the stability conditions of continuous-wave modes in the system. The analysis reveals a dominant tendency of continuous-wave stability for relatively large values of the multiphoton ionization order K, provided the femtosecond laser operates in the anomalous dispersion regime. Numerical simulations of the mathematical model feature a family of wavetrains composed of self-focused, well-separated, pulse-shaped optical filaments whose repetition rates are shortened but amplitudes are increased, with an increase in K. Simulations suggest that such nonlinear wavetrain structures do not need the transparent material to be intrinsically nonlinear and that they may also be favored solely by the nonlinearity induced by multiphoton ionization processes in a linear transparent material.

Multiphoton Absorption in Gallium Nitride and Silicon Carbide Photodiodes: Applications for Single-Event Effects Tests

We report on the investigation of multiphoton absorption processes in gallium nitride and silicon carbide photodiodes. Up to three-photon absorption mechanisms were evidenced on both materials, thanks to a wavelength-tunable femtosecond laser chain. These results are applied to single event effect investigations on a commercial gallium nitride high electron mobility transistor. We could trigger single event transients by focusing the beam throughout the silicon substrate and collect charge that was photogenerated in the gallium nitride layer by a three-photon absorption process.

Ultrafast Infrared-to-Visible Photon Upconversion on Plasmon/TiO2 Solid Films.

Optical upconversion via a multiphoton absorption process converts incoherent low-energy photons to shorter wavelengths. In this contribution, we report a solid-state thin film for infrared-to-visible upconversion composed of plasmonic/TiO2 interfaces. When excited at λ = 800 nm, three photons are absorbed, leading to the excitation of TiO2 trap states into an emissive state in the visible domain. The plasmonic nanoparticle enhances the light absorption capabilities of the semiconductor, increasing emission efficiency by 20 times. We demonstrate that the plasmonic nanoparticle only changes the optical absorption of the semiconductor; i.e., the process is purely photonic. The process occurs in the ultrafast domain (<10 ps), contrasting with molecular triplet-triplet exciton annihilation, the commonly used method in photon upconversion, in the nano- to microsecond time scales. The process utilizes pre-existing trap states within the semiconductor bandgap and involves three-photon absorption.

Dual Emission Bands of a 2D Perovskite Single Crystal with Charge Transfer State Characteristics.

Several hybrid halide 2D-perovskite species emit light with an emergent and controversial broadband emission Stokes-shifted down from the narrow band emission. This paper uncovers the sub- and above-bandgap emission and absorption characteristics of PEA2PbI4 prepared with gap states introduced during single crystal growth. Here, gap states led to coexistent intrinsic and heterostructured electronic frameworks that are selectively accessible with ultraviolet (UV) and infrared (IR) light, respectively, resulting in the phenomenon of photoluminescence (PL) switching from narrowband green to broadband red. Electron-energy dependent cathodoluminescence shows a relative increase in the broadband red PL intensity as the electron penetration depth increases from 30 nm to 2 μm, confirming the heterostructured framework is formed in the bulk of the crystal. Excitation-emission power slope of 2.5 and up-conversion pump transient absorption (TA) spectra suggest that the IR up-conversion excitation with red photoluminescence, peaked at 655 nm, is a multiphoton process occurring in the heterostructured framework through a nonlinear optical response. The energetic pathways toward the dual emission bands are revealed by pump-probe transient absorption spectroscopy, showing energetically broad gap states with high sensitivity to an IR pump are upconverted and subsequently quickly relax from high to low energy levels within 4 ps. Furthermore, the up-conversion red PL demonstrates a linear polarization with magnetic field effects, thus affirming that the band-like heterostructured framework is crystallographically aligned with characteristics of spatially extended charge-transfer states.

Ionization transition rates in the intermediate regime of the Keldysh parameter for a (0,1)*LG spiral amplitude modulated laser field

The mechanisms of the tunnel and multiphoton ionization transitions of hydrogen-like atoms and noble gas atoms are discussed. Atoms potassium and argon, with ionization energy of 4.34 and 15.76 eV, were chosen as the target. The atoms are exposed to Ti:Sapphire, (0,1)*LG, spiral amplitude modulated, laser beam at λ = 800 nm wavelength in a broad intensity range 1012 to 1015 W/cm2. The computational approach to describe tunnel and multiphoton processes was based on using the ADK theory. Stark and ponderomotive effects are also included to study their influence on the transition rate. Obtained results show that, for the lower γ values, the contribution of multiphoton ionization was less significant than the tunnel ionization contribution. In comparison, for higher γ values, multiphoton ionization dominated over tunnel ionization in a total transition rate. It is found that, in this particular case of spiral amplitude modulated mode, the intermediate regime, where both processes equally contribute, strongly depends on the atom selection and laser field intensity. Ionization in the intermediate regime occurs for γ ≈ 10 and 12 for low laser intensities, as γ ≈ 2 and 2.5 for the higher values, in the case of potassium and argon respectively. Our analysis indicated that the Stark and ponderomotive effects have a significant influence on the total transition rate. It is shown that these effects decrease the transition rate value and move the intermediate regime’s position toward lower values of the γ parameter, mainly in the case of higher laser field intensity.