Multiphoton Absorption Research Articles

A cyclometallated iridium(III) complex with multi-photon absorption properties as an imaging-guided photosensitizer.

Conventional photosensitizers (PSs) often have shorter excitation wavelengths and poor cancer cell targeting, resulting in a limited tissue penetration depth and increased biotoxicity, which are significant barriers to ensuring effective photodynamic therapy (PDT) in vivo. In this work, a cyclometallated iridium(III) complex (Ir-Biotin) with a long excitation wavelength and effective cancer cell targeting was designed and synthesized. The initial in vitro assessment indicated that Ir-Biotin shows excellent PDT activity with a high singlet-oxygen (1O2) generation yield (0.19) due to the facilitated intersystem crossing process. Further study shows that Ir-Biotin shows good biocompatibility, has specific selectivity for cancer cells, and can induce apoptosis under laser irradiation. Furthermore, Ir-Biotin can be applied for imaging-guided PDT using an in vivo imaging system, and showed significant anti-tumour effects (tumour growth inhibition value: 87.66%). These results reveal the importance of long excitation wavelengths of photosensitizers for efficient PDT and suggest a promising strategy for developing effective photosensitizers.

Measurement of two-photon absorption coefficient of 1030 nm ultrashort laser pulses on natural diamond color centers

An experimental study of nonlinear absorption process of ultrashort laser pulses in bulk of natural diamond has been carried out. The results of experimental studies on measuring nonlinear transmission of 1 mm thick plane-parallel plate made of diamond irradiated with focused by micro lens (NA=0.55 with focal length f'=5 mm) 0.3 and 10 ps laser pulses with 1030 nm wavelength are presented. It is shown that in this sample the main attenuation mechanism of ultrashort laser pulses with 1030 nm wavelength at intensities not exceeding 10 TW/cm2 is two-photon absorption at color centers, the absorption coefficient β2 =4.1 ± 0.3 cm/TW is determined. Keywords: femtosecond laser pulses, nonlinear absorption, natural diamond, multiphoton absorption, color centers.

Ultralow-threshold six-photon-excited upconversion lasing in a plasmonic microcavity.

Nonlinear multiphoton absorption (MPA) upconversion lasers have critical applications in fluorescence imaging probes and biological photonics. Here, we report the realization of ultralow-threshold six-photon-excited upconversion lasing through cavity quantum electrodynamics effects in a plasmonic microcavity. The value of the Purcell factor (Fp) in hybrid whisper-gallery mode (WGM) is enhanced five-fold relative to a bare microwire (MW), which enhances the nonlinear light-matter interactions dramatically. Compared with a MW, the threshold of six-photon upconversion WGM lasing is reduced by one order magnitude due to plasmonic enhancement effects. In addition, the temperature and polarization characteristics of upconversion lasing via a plasmonic-WGM approach show a distinct evolution, different from a bare MW. This work paves the way for extreme nonlinear optics, taking advantage of the processability and high Purcell factor of plasmonic microcavities.

Giant magneto field effect in up-conversion amplified spontaneous emission via spatially extended states in organic-inorganic hybrid perovskites

Up-conversion lasing actions are normally difficult to realize in light-emitting materials due to small multi-photon absorption cross section and fast dephasing of excited states during multi-photon excitation. This paper reports an easily accessible up-conversion amplified spontaneous emission (ASE) in organic-inorganic hybrid perovskites (MAPbBr₃) films by optically exciting broad gap states with sub-bandgap laser excitation. The broad absorption was optimized by adjusting the grain sizes in the MAPbBr₃ films. At low sub-bandgap pumping intensities, directly exciting the gap states leads to 2-photon, 3-photon, and 4-photon up-conversion spontaneous emission, revealing a large optical cross section of multi-photon excitation occurring in such hybrid perovskite films. At moderate pumping intensity (1.19 mJ/cm²) of 700 nm laser excitation, a significant spectral narrowing phenomenon was observed with the full width at half maximum (FWHM) decreasing from 18 nm to 4 nm at the peak wavelength of 550 nm, simultaneously with a nonlinear increase on spectral peak intensity, showing an up-conversion ASE realized at low threshold pumping fluence. More interestingly, the up-conversion ASE demonstrated a giant magnetic field effect, leading to a magneto-ASE reaching 120%. In contrast, the up-conversion photoluminescence (PL) showed a negligible magnetic field effect (< 1%). This observation provides an evidence to indicate that the light-emitting states responsible for up-conversion ASE are essentially formed as spatially extended states. The angular dependent spectrum results further verify the existence of spatially extended states which are polarized to develop coherent in-phase interaction. Clearly, using broad gap states with spatially extended light-emitting states presents a new approach to develop up-conversion ASE in organic-inorganic hybrid perovskites.

A Newly Developed Method of Direct Hydrogen Production from Seawater Using Femtosecond Pulse Laser

The direct production method of H2 gas from seawater using femtosecond pulse laser was developed. This approach uses the photolysis triggered by multiphoton absorption of H2O molecules and does not emit greenhouse or harmful gas such as CO2 and Cl2. In this study, direct H2 production from seawater without desalination was demonstrated, and influences of focusing lens and salinity on H2 production were experimentally revealed with a gas chromatograph. Aqueous solutions with moderate salinity (0.1-4%) showed a positive effect on H2 production owing to nonlinear optical effects by ultrashort laser pulses, while excessive salt (the salinity of 7.0%) interfered with the ionization of H2O.

Recent advances of multiphoton absorption in metal–organic frameworks

Inorganic–organic hybrid materials such as metal–organic frameworks (MOFs) or coordination polymers (CPs) are of high interest in chemistry and materials science due to their modular design and versatile applicability, for example in gas storage, catalysis and sensor systems.

Предельно короткие импульсы в оптически анизотропной среде с углеродными нанотрубками с учетом многофотонного поглощения

In this paper, we study the influence of multiphoton absorption on the propagation of electromagnetic waves in a nonlinear optical anisotropic medium with carbon nanotubes. The effective equation for the vector potential of the electromagnetic field of the pulse is obtained, taking into account the second component of the polarization of the field, as well as two- and three-photon absorption processes. The dependence of the components of the pulse field on the parameters of the problem is revealed.

Crystallographic Studies of Rhodopsins: Structure and Dynamics.

Crystal structures have provided detailed insight in the architecture of rhodopsin photoreceptors. Of particular interest are the protein-chromophore interactions that govern the light-induced retinal isomerization and ultimately induce the large structural changes important for the various biological functions of the family. The reaction intermediates occurring along the rhodopsin photocycle have vastly differing lifetimes, from hundreds of femtoseconds to milliseconds. Detailed insight at high spatial and temporal resolution can be obtained by time-resolved crystallography using pump-probe approaches at X-ray free-electron lasers. Alternatively, cryotrapping approaches can be used. Both the approaches are described, including illumination and sample delivery. The importance of appropriate photoexcitation avoiding multiphoton absorption is stressed.

Difference Squeezing in Degenerate Three-Photon Absorption Six-Wave Interaction Process

: In this paper, difference squeezing in the degenerate three-photon absorption six-wave interaction process is examined. The difference squeezing in fields between the pump and Stokes modes can be converted to normal squeezing in the signal mode, and vice versa. It is demonstrated that in the pump mode, amplitude-cubed squeezing is directly turned into normal squeezing in the signal mode. The squeezing in the stimulated interaction, which reduces the depth of classicality of the field amplitude, is seen to be larger than the corresponding squeezing in the spontaneous interaction, despite having the same number of pump photons. Difference squeezing is observable only in certain domain values of pump photons. It is deduced that the multi-photon absorption nonlinear optical approach is the best way to generate optimal squeezed laser light in any optical system.

Simulation of the photoionization of nonmetal crystals by few-cycle femtosecond laser pulses

Modeling of laser-induced photoionization is a key component in simulations of ultrafast laser ablation of transparent solids. Available analytical and phenomenological models consider the ionization by promoting valence band electrons to a conduction band via simultaneous absorption of several laser photons of the same energy defined at central wavelength of laser-pulse spectrum. That assumption corresponds to nearly zero spectrum bandwidth and is not true for ultrashort few-cycle pulses. Treatment of the few-cycle-pulse photoionization by numerical methods, e.g., time-dependent density-functional theory, meets substantial difficulties when modeling of ionization-rate scaling with multiple laser-pulse and material parameters. To address those gaps, we report an analytical approach to evaluation of the photoionization rate by a few-cycle laser pulse with nonnegligible spectral bandwidth. We assume that the pulse spectrum supports the few-cycle pulse duration, but is narrow enough to prevent quantum interference between multiphoton transitions of different orders. We outline the calculation procedure, report examples of simulations with the proposed model, and discuss some novel physics of the photoionization. Our model includes dependence of the photoionization rate on carrier-envelope phase and delivers significantly higher rates compared with the Keldysh model. We interpret those results in terms of laser-driven variations of effective band gap, opening of extra channels of the multiphoton absorption, and involvement of a continuous range of electron states determined by pulse spectrum width. Based on the reported model, we discuss new options to control threshold of ultrafast laser ablation via carrier-envelope and pulse-shape scaling of the photoionization rate.

Multiphoton Absorption Simulation of Sapphire Substrate under the Action of Femtosecond Laser for Larger Density of Pattern-Related Process Windows.

It is essential to develop pattern-related process windows on substrate surface for reducing the dislocation density of wide bandgap semiconductor film growth. For extremely high instantaneous intensity and excellent photon absorption rate, femtosecond lasers are currently being increasingly adopted. However, the mechanism of the femtosecond laser developing pattern-related process windows on the substrate remains to be further revealed. In this paper, a model is established based on the Fokker–Planck equation and the two-temperature model (TTM) equation to simulate the ablation of a sapphire substrate under the action of a femtosecond laser. The transient nonlinear evolutions such as free electron density, absorption coefficient, and electron–lattice temperature are obtained. This paper focuses on simulating the multiphoton absorption of sapphire under femtosecond lasers of different wavelengths. The results show that within the range of 400 to 1030 nm, when the wavelength is large, the number of multiphoton required for ionization is larger, and wider and shallower ablation pits can be obtained. When the wavelength is smaller, the number of multiphoton is smaller, narrower and deeper ablation pits can be obtained. Under the simulation conditions presented in this paper, the minimum ablation pit depth can reach 0.11 μm and the minimum radius can reach 0.6 μm. In the range of 400 to 1030 nm, selecting a laser with a shorter wavelength can achieve pattern-related process windows with a smaller diameter, which is beneficial to increase the density of pattern-related process windows on the substrate surface. The simulation is consistent with existing theories and experimental results, and further reveals the transient nonlinear mechanism of the femtosecond laser developing the pattern-related process windows on the sapphire substrate.

Bloch-electron dynamics in homogeneous electric fields: Application to multiphoton absorption in semiconductors and insulators

The theory of Bloch-electron dynamics for carriers in a homogeneous electric field of arbitrary time dependence is developed in consideration of the properties of multiphoton absorption (MPA) in semiconductors and insulators. The general approach is to utilize the accelerated Bloch-state representation (ABR) as a basis thereby treating the electric field exactly; also, the electric field is described in the vector potential gauge. In developing the ABR, the instantaneous eigenstates for the central Hamiltonian are obtained as the accelerated Bloch states and utilized to find the time-dependent solution to the Schr\"odinger equation. In introducing the Wigner-Weisskopf approximation (WWA), the transition probability amplitude between the states of the system is obtained and used in the application to MPA in semiconductors and insulators. The probability amplitude for MPA is derived for a plane-polarized radiation field. The periodicity of the time-dependent electric field serves as a principle time constant in developing the probability amplitude per period. Utilizing the WWA, the general MPA transition probability is derived for transitions between an arbitrary set of valence and conduction bands for a time-varying electric field in the $x$ direction. Within the WWA, the MPA transition probability, equivalent to the well-known Keldysh transition amplitude, is derived and expressed in terms of infinite-variable generalized Bessel functions and modified Bessel functions. The exact MPA result is analyzed in the small ($\ensuremath{\lesssim}{10}^{6}$ V/cm) and large ($\ensuremath{\gtrsim}{10}^{8}$ V/cm) electric field amplitude limits.

Elucidating complex triplet-state dynamics in the model system isopropylthioxanthone

SummaryWe introduce techniques for probing the dynamics of triplet states. We employ these tools, along with conventional techniques, to develop a detailed understanding of a complex chemical system: a negative-tone, radical photoresist for multiphoton absorption polymerization in which isopropylthioxanthone (ITX) is the photoinitiator. This work reveals that the same color of light used for the 2-photon excitation of ITX, leading to population of the triplet manifold through intersystem crossing, also depletes this triplet population via linear absorption followed by reverse intersystem crossing (RISC). Using spectroscopic tools and kinetic modeling, we identify the reactive triplet state and a non-reactive reservoir triplet state. We present compelling evidence that the deactivation channel involves RISC from an excited triplet state to a highly vibrationally excited level of the electronic ground state. The work described here offers the enticing possibility of understanding, and ultimately controlling, the photochemistry and photophysics of a broad range of triplet processes.

Second Harmonic Generation in Lithiated Silicon Nanowires: Derivations and Computational Methods

This research will examine the computational methods to calculate the nonlinear optical process of second harmonic generation (SHG) that will be hypothesized to be present during lithium ion insertion into silicon nanowires. First it will be determined whether the medium in which SHG is conveyed is non-centrosymmetric or whether the medium is inversion symmetric where SHG as a part of the second-order nonlinear optical phenomenon does not exist. It will be demonstrated that the main interaction that determines SHG is multiphoton absorption on lithium ions. The quantum harmonic oscillator (QHO) is used as the background that generates coherent states for electrons and photons that transverse the length of the silicon nanowire. The matrix elements of the Hamiltonian which represents the energy of the system will be used to calculate the probability density of second-order nonlinear optical interactions which includes collectively SHG, sum-frequency generation (SFG) and difference-frequency generation (DFG). As a result, it will be seen that at varies concentrations of lithium ions (Li+) within the crystallized silicon (c-Si) matrix the second-order nonlinear optical process has probabilities substantial enough to create second harmonic generation that could possibly be used for such applications as second harmonic imaging microscopy.

Optical Properties and Applications of Crystalline Materials

Optical Properties and Applications of Crystalline Materials

Photo-exfoliation of MoS2 quantum dots from nanosheets: an in situ transmission electron microscopy study

Fabrication of transition metal dichalcogenide quantum dots (QDs) is complex and requires submerging powders in binary solvents and constant tuning of wavelength and pulsed frequency of light to achieve a desired reaction. Instead of liquid state photoexfoliation, we utilize infrared laser irradiation of free-standing MoS2 flakes in transmission electron microscope (TEM) to achieve solid-state multi-level photoexfoliation of QDs. By investigating the steps involved in photochemical reaction between the surface of MoS2 and the laser beam, we gain insight into each step of the photoexfoliation mechanism and observe high yield production of QDs, led by an inhomogeneous crystalline size distribution. Additionally, by using a laser with a lower energy than the indirect optical transition of bulk MoS2, we conclude that the underlying phenomena behind the photoexfoliation is from multi-photon absorption achieved at high optical outputs from the laser source. These findings provide an environmentally friendly synthesis method to fabricate QDs for potential applications in biomedicine, optoelectronics, and fluorescence sensing.

Physical and Luminescence Studies of Er3+-Doped into Borate Glass for IR Lighting Application

The gadolinium tungsten borate glasses doped with Er3+ were prepared by melt quenching technique for study in physical, absorption and luminescence properties. The glass composition (30-x) B2O3 − 27.5 Gd2O3 − 42.5 WO3 – x Er2O3, where x = 0.1, 0.5, 1.0 and 2.0 mol%. The results show that doping Er2O3 in higher concentration make the glass density and molar volume increases. The absorption spectra indicate the photon absorbing of Er3+ in visible light and near-infrared region. The emission spectra of these glasses have been recorded under 275 nm (Gd3+) and 380 nm (Er3+) excitation wavelength and it is observed that these glasses show green 550 nm and red 577 nm. NIR photoluminescence under 311 nm (Gd3+), 524 nm (Er3+) and 978 nm (Er3+), excitation was measured of glass in the region 1400–1700 nm. NIR emissions show a peak centered at 1543 nm. The luminescence of an erbium-doped glass was investigated at room temperature. In addition, mechanisms of the luminescence by multiphoton absorption and energy transfer processes are discussed based on the energy-level diagram of Er3+ and luminescence spectra. These results benefit for efficient short and conventional-length optical amplifiers and tunable lasers.

Femtosecond nonlinear losses in multimode optical fibers

Multimode optical fibers are attracting a growing interest for their capability to transport high-power laser beams, coupled with novel nonlinear optics-based applications. However, optical fiber breakdown occurs when beam intensities exceed a certain critical value. Optical breakdown associated with irreversible modifications of the refractive index, triggered by multiphoton absorption, has been largely exploited for fiber material micro-structuration. Here we show that, for light beam intensities slightly below the breakdown threshold, nonlinear absorption strongly affects the dynamics of a propagating beam as well. We experimentally analyze this subthreshold regime and highlight the key role played by spatial self-imaging in graded-index fibers for enhancing nonlinear optical losses. We characterize the nonlinear power transmission properties of multimode fibers for femtosecond pulses propagating in the near-infrared spectral range. We show that an effective N-photon absorption analytical model is able to describe the experimental data well.

Propagation of high-power optical flat-topped beams in strongly nonlinear media

We performed numerical experiments to investigate the propagation of high-power optical flat-topped beams through a strongly nonlinear medium, accounting for the diffraction, nonlinearity, and multiphoton absorption capabilities of the beams. We found that the uniform irradiance distribution of flat-topped beams can remain constant near the centre axis for a given propagation distance. A series of comparisons were conducted for flat-topped beams with different orders, which demonstrates that increasing the order reduces change of beam characteristic. Furthermore, we investigated the influence of input power on the propagation of flat-topped beams. Moreover, we numerically analysed the dynamics of the inner energy transfer and uniform irradiance decay of the flat-topped beam.

Determining femtosecond laser fluence for surface engineering of transparent conductive thin films by single shot irradiation.

In recent years, there has been increasing interest in optoelectronic applications of transparent conductive oxide (TCO) thin-film-based materials and devices fabricated using patterning techniques. Meanwhile, femtosecond laser processing is a convenient method that further improves the performance of TCO-based functional devices and expands their application prospects. In this study, we proposed a simple and effective strategy to determine the fluences required for laser processing TCOs. We investigated the modification of an indium tin oxide (ITO) film induced by a femtosecond laser (45/150 fs, 800 nm) at different pulse fluences. The results reveal that the laser modification of ITO films is highly dependent on the irradiated pulse fluences. Several distinct types of final micro/nanostructures were observed and may be attributed to superficial amorphization, spallation ablation, stress-assisted delamination, boiling evaporation, and phase explosion. The final micro/nanostructures were studied in detail using optical microscopy, scanning electron microscopy, transmission electron microscopy and a surface profiler. At a lower fluence above the melting but below the ablation threshold, a laterally parabolic amorphous layer profiled with maximum thicknesses of several tens of nanometers was quantitatively attained. At a higher fluence, stress-assisted delamination and superheated liquid-induced micro-honeycomb structures emerged. Furthermore, the electron and lattice temperature evolutions were also obtained using a two-temperature model to prove the ablation mechanism and ascertain the micro/nanostructure formation principle. The predicted surface temperatures confirmed film amorphization without ablation below 0.23 J/cm2. These results reveal the interaction mechanism between femtosecond laser pulse and ITO film including the competition between the free electron heating of intraband transition and the multiphoton absorption of the interband transition, which promotes the potential applications for femtosecond laser processing TCO films and other wide-band-gap semiconductors such as photodetectors, solar cells, UV-light-emitting diodes, and flat-panel displays.