Two-photon Absorption Process Research Articles

Characterizations of Two-photon Absorption Process Induced by Defects in Aluminum Nitride Using Z-scan Method

Characterizations of Two-photon Absorption Process Induced by Defects in Aluminum Nitride Using Z-scan Method

Two-photon absorption in quantum dots with Hellmann potential

Abstract We study the nonlinear optical absorption properties of a spherical quantum dot with Hellmann potential, focusing on the two-photon absorption (TPA) process, using GaAs/AlGaAs material as an example. { The radial Schr"{o}dinger equation is solved using the Nikiforov-Uvarov method, while the two-photon absorption coefficient is determined through second-order perturbation theory concerning the electron-photon interaction}. Our study shows that the intraband transition has a smaller energy transition than the interband transition, leading to TPA spectra for the intraband transition that is restricted within a smaller energy range and exhibits a higher peak value than those for the interband transition. The peak corresponding to $\ell=2$ consistently appears to the left of the peak corresponding to $\ell=1$ in both intraband and interband transition cases. Additionally, the dependence of the absorption peak position on the order of transition, $n$, differs between intra- and inter-band transitions. We also observe blue shift behavior in the TPA spectra as all three parameters, $r_0$, $V_{0e}$, and $\eta$, increase. Our investigation has the potential to enable the design of novel photonic devices, ultra-fast optical switches, and highly efficient solar cells through the optimization of quantum dot material properties.

Single-shot pump-probe technique by the combination of an echelon and a grating with a time window of 109 ps

In this study, using only a single pulse, pump-probe measurement with a large time window of more than 100 ps is implemented. A commercial grating is used to encode a time window of ∼56 ps in a single pulse; therefore, there is no need for machining customization. In addition, in this technique, the grating surface is accurately imaged, eliminating the image blur problem caused by phase differences in previous echelon-based techniques. Moreover, to make full use of the grating surface and obtain a larger time window, a simple reflection echelon is combined that matches the grating in the time window. This combination encoding strategy results in a total time window of ∼109 ps and maintains accurate imaging of the grating surface. This time window is an order of magnitude greater than the maximum reported values of the echelon encoding strategy and the angle beam encoding strategy. To demonstrate this single-shot pump-probe technique, the two-photon absorption process of ZnSe and the excited-state absorption process of a symmetrical phenoxazinium bromine salt were studied. The possibility of further improving the experimental setup is also discussed.

Novel Indium nanocomposites for Photonic applications

In the present communication, MgO:In2O3- 1:1 (MIO), SrO/MgO/In2O3 (1:1:1) (SMIO), SrO/Al2O3/In2O3 (1:1:1) (SAIO) and CeO2/ZnO/In2O3 (1:1:1) (CZIO) NCs are synthesized by Punicagranatum juice extract as a reducing agent followed by calcination at 600 °C for 3 h. The Bragg reflections confirm that Magnesium indate, SrO, Al2O3, ZnO crystallizes in orthorhombic, trigonal, rhombohedral and hexagonal crystalline phases whereas CeO2, In2O3 and MgO crystallizes in cubic phase. Surface morphology consists large number of irregularly sized and shaped nanoparticles along with few pores and hallows. The direct energy band gap was determined using Wood and Tauc’s plot. The direct energy band gap was found to be 3.12, 3.15, 3.18 and 3.05 eV. The optical nonlinearity of the specimens was investigated using the open aperture Z-scan technique. With the exception of SAIO, the synthesized samples exhibited reverse saturable absorption attributed to a two-photon absorption (2PA) process. The nonlinear absorption coefficient follows the order CZIO > MIO > SMIO. However, the optical limiting threshold follows the order SMIO > MIO > CZIO NCs. These findings distinctly highlight the superior potential of CZIO binary NCs for optical limiting applications. This characteristic is advantageous for shielding sensors and the human eye, providing protection against the detrimental effects of intense green laser radiation. Unexpectedly, the SAIO NCs displayed Q-switching behavior. This behavior holds potential applications in optical discriminators, spatial light filters, optical isolation, and other photonic mode-locking devices.

Investigation of Ca2Al2SiO7: Pr3+/Gd3+ UVB upconverting phosphor and its optical thermometric properties

On basis of fluorescence intensity ratio (FIR) over certain narrow coupled levels of rare-earth ions, up-conversion of RE-doped materials perform excellent temperature-sensing abilities for thermometry applications. A series of Pr3+/Gd3+ co-doped Ca2Al2SiO7 phosphors (Ca2Al2SiO7:Pr3+/Gd3+) were prepared by a conventional high-temperature solid-state reaction. Under excitation of blue ∼445 nm, the Ca2Al2SiO7:Pr3+/Gd3+ phosphor yields intense photoluminescence (PL) bands around 489, 528, 537, 580, 604, 613, and 646 nm due to electronic transition of 3P0 → 3H4, 3P0 → 3H5, 3P1 → 3H5, 3P1 → 3H6, 1D2 → 3H4, 3P0 → 3H6, and 3P0 → 3F2 of Pr3+, respectively. Their relative intensities are sensitively determined by Pr3+ contents, which, from 0.1 % to 5 %, tune the emission color from pink to warm white under excitation of 445 nm. It is of great interest that intense irradiation using a blue ∼445 laser does make efficient ultraviolet B (UVB) up-conversion at about 297, 306, and 313 nm, just stemming from the 6P3/2 → 8S7/2, 6P5/2 → 8S7/2, and 6P7/2 → 8S7/2 of Gd3+, respectively. A typical two-photon absorption process was proved to account for the UVB up-conversion, i.e. successive absorption of two blue ∼445 nm photons populating Pr3+ 4f5d excited state, which further efficiently transfer its energy to the 6IJ/6PJ states of Gd3+ neighbor for UVB emission. Moreover, the thermally coupled levels (TCLs) of Gd3+: 6P3/2-6P7/2, and Gd3+: 6P5/2-6P7/2 were thoroughly investigated for temperature sensing in 298–598 K. Maximum relative sensitivity of the 6P3/2-6P7/2 TCLs, i.e. Sr(I306/I313), is proved to be 0.96 % K−1 at 298 K, and that of the 6P5/2-6P7/2 TCLs, i.e. Sr(I297/I313), is 1 % K−1 at 373 K. The relevant thermal stabilities of the Ca2Al2SiO7:Pr3+/Gd3+ thermometric candidate was well validated with 10 cycles of heating and cooling processes in 298–598 K. Such continuous explorations of Pr3+/Gd3+ co-doping system will significantly promote the development of robust UVB up-converting phosphors as well as their practical applications in the field of high-resolution optical thermometry.

Tuning the linear optical properties and two-photon absorption on the cinnamylideneacetophenone derivatives: The effect of methoxy group by a experimental and theoretical point of view

The present work studied the effect of the methoxy group position in two cinnamylideneacetophenone (CA) derivatives on both one-photon (1PA) and two-photon absorption (2PA) processes. Our outcomes exhibit a dependence on the methoxy group position spectrally and in magnitude for both. A decrease in the lower energy 1PA band of 0.23eV was observed when the methoxy group was moved to the more conjugated part of the cinnamaldehyde molecule. The opposite was also observed for the next higher absorption band. The Quantum Chemical Calculations (QCCs), besides a good agreement between the experimental and simulated 1PA spectra, except for the magnitude of the high energy absorption band, showed that the effect behind the observed experimental red-shift of the lower absorption band is caused by the higher charge transfer when the methoxy group is bounded to the longer conjugated part of the cinnamaldehyde derivative. For experimental 2PA spectra, the higher energy band is strongly affected by the presence of the methoxy group, pointing out an increase of about 56 % in the values of the two-photon absorption cross-section (2PACS). Furthermore, the phenomenological Sum-Over-Essential States (SOS) approach and QCCs were performed for further clarification in the 2PACS experimental results. Hence, our findings demonstrate that changing the methoxy position on opposite sides of the CA may open up a novel insight for designing new organic molecules, particularly when seeking to enhance the values of 2PACS significantly of CA derivatives.

Two-Photon Absorbing Dendrimers and Their Properties-An Overview.

This review describes the two-photon absorption properties of dendrimers, which are arborescent three-dimensional macromolecules differing from polymers by their perfectly defined structure. The two-photon absorption process is a third order non-linear optical property that is attractive because it can be used in a wide range of applications. In this review, dendrimers that were studied for their two-photon absorption properties are first described. Then, the use of dendritic TPA chromophores for light harvesting, photopolymerization, optical power limitation, cell imaging, singlet oxygen generation, and photodynamic therapy is described. This review thus proposes an overview of the properties and possible applications of two-photon absorbing dendrimers.

β-NaBiF4:Yb3+,Er3+:Novel green synthesis, crystal structure, upconversion luminescence and optical thermometry

Non-lanthanide based NaBiF4 host have evoked remarkable interest due to unique luminescent property and potential applications. Current synthetic methods are not environment friendly and causes pollution. This is the first report mentioning green synthetic innovative route to develop upconversion, crystalline, and temperature sensing material. Here, we deigned ultrasonic assisted green approach for successful synthesis of β-NaBiF4:Yb3+, Er3+ (size ∼25–100 nm) using Tabernaemontana divaricata leaves extract without any surfactant. Characterization techniques such as XRD, SEM, TEM, XPS, EDX, and PL unveiled its properties. Green and red upconversion emissions (λex = 980 nm) were two-photon absorption process. The possible energy transfer and thermal quenching mechanism is discussed. Using thermally coupled levels (2H11/2 and 4S3/2) of Er3+, FIR based temperature sensing revealed ΔE ∼841 cm−1, linear response (R2 ∼0.99), outstanding relative sensitivity (Sr max∼ 1.4 % K−1 @294 K), good absolute sensitivity (Sa max∼ 0.29 %K−1 @ 398 K) and a low temperature resolution (δTmin = 0.35 K) over temperature range 294–398 K.

Non-linear light–matter interactions from the Bethe–Salpeter equation

A route to assess non-linear light–matter interactions from the increasingly popular GW-Bethe–Salpeter equation (GW-BSE) method is outlined. In the present work, the necessary analytic expressions within the static-screened exchange approximation of the BSE are derived. This enables a straightforward implementation of the computation of the first hyperpolarizability as well as two-photon absorption processes for molecular systems. Benchmark calculations on small molecular systems reveal that the GW-BSE method is intriguingly accurate for predicting both first hyperpolarizabilities and two-photon absorption strengths. Using state-of-the-art Kohn–Sham references as a starting point, the accuracy of the GW-BSE method rivals that of the coupled-cluster singles-and-doubles method, outperforming both second-order coupled-cluster and time-dependent density-functional theory.

Energy Spectrum and Properties of SiC: Using Two-Photon Absorption for Different Harmonics

This paper describes a methodology for studying the energy spectrum and characteristics of Silicon Carbide (SiC) semiconductor materials, utilizing various harmonics for two-photon absorption (TPA). The approach involves developing theoretical models to simulate the energy levels and transitions of SiC, based on the TPA process. By analyzing the resulting spectra obtained by varying the harmonic order, the energy spectrum, and properties of SiC are explored. In this work also includes a comparison of the energy spectrum and properties of SiC for single and two-photon absorption, providing insights into the distinctive features of SiC under these conditions. In particularly absorption co-efficient of the material was calculated from optical transmittance and reflectance measurements at room temperature (300 K) in the wavelength range of 200 -900 nm. In addition, Gaussian functions centered at different energies were modeled using TPA in SiC materials and their contribution to the Harmonic Generation (HG) signal was calculated.

Theoretical study on photoinduced charge transfer in one-photon and two-photon absorption of branching oligofluorenes

Oligofluorenes have been identified as very promising two-photon absorption (TPA) materials and present great application potential for the fabrication of nonlinear optical devices, but the TPA mechanism and corresponding electron excitation properties have not been studied. Here, the photoinduced charge transfer characteristics of V-shaped and Y-shaped branching oligofluorenes that consist of two and three fluorene units in each branch during one-photon absorption (OPA) and TPA processes are analyzed theoretically using the density functional theory and visualization sum-over-states model. The calculated results show that the OPA intensity and TPA cross-section are significantly enhanced by increasing the branch length or changing the structure from V-shaped to Y-shaped. The long-distance charge transfer only occurs on the second transition of TPA at high excited states. Compared to Y-shaped molecules, V-shaped structures exhibit a stronger cooperative effect among the different branches.

Investigation on nonlinear absorption and optical limiting properties of Tm:YLF crystals

Nonlinear optical limiting has recently attracted significant attention, thanks to the continuous development of laser technology. In order to explore more potential functional materials, the spectral and nonlinear optical properties of yttrium lithium fluoride doped with thulium (Tm:YLF) crystals are reported. Absorption and emission spectra of Tm:YLF are measured, and the absorption peaks correspond to the energy transition of Tm3+. Nonlinear absorption measurements are performed on crystals with different thulium doping concentrations (2.0 %, 2.5 %, and 3.0 %). The Z-scan results indicate the presence of two-photon absorption processes at 1064 nm pulse laser. Additionally, we measured the nonlinear transmittance of Tm:YLF crystals with various thulium doping concentrations and lengths to evaluate their nonlinear optical limiting performance. For the 2.5 % doped Tm:YLF crystal with 20 mm length, the maximum transmittance at low laser intensity is 73.9 %, and the minimum transmittance at high laser intensity is 38.1 %. These experimental results suggest that Tm:YLF crystals may have potential as solid-state nonlinear optical limiting materials.

Two-photon Absorption and Photoionization of a Bacterial Phytochrome

Phytochromes constitute a family of photosensory proteins that are utilized by various organisms to regulate several physiological processes. Phytochromes bind a bilin pigment that switches its isomeric state upon absorption of red or far-red photons, resulting in protein conformational changes that are sensed by the organism. Previously, the ultrafast dynamics in bacterial phytochrome was resolved to atomic resolution by time-resolved serial femtosecond X-ray diffraction (TR-SFX), showing extensive changes in its molecular conformation at 1 picosecond delay time. However, the large excitation fluence of mJ/mm2 used in TR-SFX questions the validity of the observed dynamics. In this work, we present an excitation-dependent ultrafast transient absorption study to test the response of a related bacterial phytochrome to excitation fluence. We observe excitation power-dependent sub-picosecond dynamics, assigned to the population of high-lying excited state Sn through resonantly enhanced two-photon absorption, followed by rapid internal conversion to the low-lying S1 state. Inspection of the long-lived spectrum under high fluence shows that in addition to the primary intermediate Lumi-R, spectroscopic signatures of solvated electrons and ionized chromophore radicals are observed. Supported by numerical modelling, we propose that under excitation fluences of tens of μJ/mm2 and higher, bacterial phytochrome partly undergoes photoionization from the Sn state in competition with internal conversion to the S1 state in 300 fs. We suggest that the extensive structural changes of related, shorter bacterial phytochrome, lacking the PHY domain, resolved from TR-SFX may have been affected by the ionized species. We propose approaches to minimize the two-photon absorption process by tuning the excitation spectrum away from the S1 absorption or using phytochromes exhibiting minimized or shifted S1 absorption.

Tremendous enhancement of green emission from Er3+/In3+ co-doped β-Ga2O3 ultrawide bandgap semiconductors

Er3+ singly doped and Er3+/In3+ co-doped gallium oxide ceramics with intense green emission were prepared using the conventional solid-state reaction method. The Ga2-x-yErxInyO3 ceramics exhibit green and red emission at 555 and 673 nm, and follow the two-photon absorption process under the excitation of 980 nm. Systematic measurements of the upconversion luminescence spectra of Ga2-x-yErxInyO3 show that the optimal doping concentrations, sintering temperature and time are x = 0.015, y = 0.035, 1250 °C, and 8 h, respectively. For a fixed Er3+ doping concentration of x = 0.015, the green emission intensity at 555 nm can be enhanced by 60 folds through 3.5 mol% In3+ doping, which is attributed to reduced non-radiative decay, enhanced energy transfer from In3+ ions to Er3+ ions via defect states, and increased photon absorption. The introduction of 3.5 mol% In3+ also increases the fluorescence lifetime by 235–272%. These observations provide compelling evidence for the presence of effective energy transfer between In3+ and Er3+ ions.

Donor-π-Acceptor Porphyrin-Assisted Bifunctional Defect Passivation for Enhanced Temporal Domain-/Wavelength-Dependent Nonlinear Absorption Properties in Perovskite Films.

Perovskite layer defects are a primary inhibiting factor for their optical nonlinearity, which restricts their use in nonlinear photonics devices. Nevertheless, due to the variety of defect types, the passivation and repair of these defects remain challenging. Herein, a novel bifunctional passivation strategy was proposed, and the porphyrin with a donor-π-acceptor structure was designed to bifunctionally repair perovskite defects by linking different types of functional groups via acetylenic π-conjugated linkage bridges on both sides, thus improving the nonlinear optical (NLO) absorption properties of porphyrin-perovskite hybrid materials. Research results indicate that the amino and carboxyl groups of porphyrins endow the ability to bifunctionally passivate charged defects via effective coordination interactions. The nonlinear absorption properties of all porphyrin-passivated MAPbI3 films were remarkably enhanced compared to that of the MAPbI3 film across multiple wavelengths and temporal domains. Particularly, the Por3-passivated perovskite film (MAPbI3/Por3) exhibited optimized strongest NLO performance, including reverse saturable absorption (RSA) under 800 nm femtosecond (fs) and 1064 nm nanosecond (ns) laser irradiations, as well as saturable absorption (SA) with 515 and 532 nm ns laser excitations. The value of the NLO absorption coefficient (β = 266.23 cm GW-1) is 1 order of magnitude higher than that of the pristine perovskite film (β = 12.93 cm GW-1), also outperforming other porphyrin-passivated perovskite films and some reported materials. The bifunctional passivation mechanism of porphyrin not only intensifies the perovskite's photoinduced ground-state dipole moment in the two-photon absorption (TPA) process and the free carrier absorption ability to deepen the RSA properties under 800 nm fs and 1064 nm ns lasers, respectively, but also enables the improvement of SA responses under 515 nm fs and 532 nm ns lasers by expediting the Pauli blocking effect of perovskite. Our study offers a viable paradigm, which aims at exploiting high-performance NLO perovskite materials across wide spectral regions and time scales.

Optical traps induce fluorophore photobleaching by two-photon excitation

Techniques combining optical tweezers with fluorescence microscopy have become increasingly popular. Unfortunately, the high-power, infrared lasers used to create optical traps can have a deleterious effect on dye stability. Previous studies have shown that dye photobleaching is enhanced by absorption of visible fluorescence excitation plus infrared trap photons, a process that can be significantly reduced by minimizing simultaneous exposure to both light sources. Here, we report another photobleaching pathway that results from direct excitation by the trapping laser alone. Our results show that this trap-induced fluorescence loss is a two-photon absorption process, as demonstrated by a quadratic dependence on the intensity of the trapping laser. We further show that, under conditions typical of many trap-based experiments, fluorescence emission of certain fluorophores near the trap focus can drop by 90% within 1 min. We investigate how photostability is affected by the choice of dye molecule, excitation and emission wavelength, and labeled molecule. Finally, we discuss the different photobleaching pathways in combined trap-fluorescence measurements, which guide the selection of optimal dyes and conditions for more robust experimental protocols.

Tailoring of visible light driven photocatalytic activities of Bi2MoO6 flower-like microspheres via synergistic effect of doping and surface Plasmon resonance

Series of Er3+/Yb3+-codoped Bi2MoO6 (Bi2MoO6:Er3+/xYb3+) and Bi2MoO6:Er3+/0.03Yb3+@Biy flower-like microspheres were designed to improve the visible light driven photocatalytic activities of Bi2MoO6 flower-like microspheres. The electronic structure of Bi2MoO6 host was analyzed through the first-principle density functional theory. Excited at 980 nm, glaring UC emissions are gained in Bi2MoO6:Er3+/xYb3+ compound and their strongest intensities are obtained at × = 0.05, where the corresponding mechanism is a two-photon absorption process. Through studying the tetracycline (TC) degradation under visible light irradiation, the photocatalytic activities of final products were explored. Compared with Bi2MoO6 flower-like microspheres, these samples doped with Er3+ and Yb3+ can remove TC more efficiently and the optimal performance is achieved when a = 0.03. Furthermore, via using the surface plasmon resonance (SPR) effect caused by metallic Bi, the photocatalytic activities of the designed samples were further manipulated, where the TC removal rate of Bi2MoO6:Er3+/0.03Yb3+@Bi0.10 flower-like microspheres is 91.0% after irradiating by visible light for 24 min. Moreover, the included photocatalytic processes are triggered by ·O2–, ·OH and h+ active species. Additionally, the TC solution treated by the designed photocatalysts shows non-toxicity to plants. These results imply that Bi2MoO6:Er3+/0.03Yb3+@Biy flower-like microspheres are potential visible light driven photocatalysts for wastewater purification and our finding may also provide guidelines for tailoring of the photocatalytic activities of semiconductors by synergistic effect of doping and SPR engineering.

Theoretical and experimental study of the 6S-8S two-photon absorption cross-section in cesium atoms.

The probability of a two-photon absorption (TPA) process is quantified by means of the TPA cross-section. In this work the TPA cross-section for the 6S1/2 → 8S1/2 transition in cesium (Cs) atoms is obtained from a theoretical and experimental approach. It is calculated using second order perturbation theory including hyperfine structure and Doppler broadening for a thermal atomic vapor. The theoretical values for the TPA cross-sections of the two allowed hyperfine transitions are σ~3,3=3.10×10-24 cm4/W and σ~4,4=3.65×10-24 cm4/W. An experiment is set up to measure these cross-sections based on the quadratic dependence between the TPA transition rate and the intensity of the laser light. Values of σ~3,3=(3.6±1.1)×10-24 cm4/W and σ~4,4=(4.6±1.1)×10-24 cm4/W are obtained from the measurements. The results show an agreement between theory and experiment. To the best of our knowledge, this is the first time that these cross-sections are reported.

Fiber Optical Temperature Sensor Based on 980nm Excitation of CaWO4: Yb3+, Er3+ with Up-conversion Fluorescence Intensity Ratio

The Yb3+, Er3+ co-doped calcium tungstate materials were prepared. The temperature-sensitive fiber optic probes made from CaWO4:Yb3+, Er3+ were prepared by the high-temperature solid-phase sintering method. The XRD lattice structure of CaWO4: Yb3+, Er3+ was tested. The fluorescence spectra of the probe under 980nm excitation were tested, and the variable temperature profile of the probe was analyzed to derive its two-photon absorption process. The temperature characteristics of the up-conversion fluorescence intensity ratio (FIR) of the probe in the temperature range of 303-673K were investigated, and the results showed that this optical fiber probe can be used as a fiber optic temperature sensor with a linearity of 5.8% and a temperature measurement return difference of only 1.5%; it was found that the temperature characteristic curve of FIR of this fiber optic probe was reproducible. These properties show that CaWO4: Yb3+, Er3+ is feasible to be made into a fiber optic sensor.

Effect of weak doping with cobalt over the optical magnitudes of tin oxide thin films

This paper aims to investigate the different properties of cobalt-doped and undoped tin oxide thin films. The tin oxide thin films were doped with different atomic percentages of cobalt, ranging from 0% (undoped) to 2% and 4%, using a cost-effective and easily accessible technique known as spray pyrolysis with a moving nozzle at 490 °C. The phase identification of the different thin layers was carried out using X-ray diffraction, which indicated that the compound is homogeneous has a polycrystalline structure and is, with preferential orientation growth at the (211) level. The optical properties of the synthesized thin films were investigated using UV-visible spectroscopy in the range from 300 to 900 nm, which allowed the estimation of both linear and nonlinear optical properties. The results of the investigations into the linear optical properties indicated that the samples have a direct band gap energy between 3.75 to 3.81 eV, permittivity between 2.67 to 3.266, photoelectrons between 0.2 to 3.35 = 1025, and refractive index between 1.778 to 1.875. The results also indicated that the absorption cross section changes with photon energy and the number of photoelectrons, where it has an approximately constant value at low energies. Moreover, the nonlinear optical properties were determined using the Sheik-Bahae model, which revealed that the samples have the same maximum two-photon absorption (TPA) process coefficient of 3.63 cm/W and the same maximum nonlinear refractive index (NRI) value of 6.91 × 10−8 cm2/W.