Two-photon Excitation Process Research Articles

Two-Photon Planar Laser-Induced Fluorescence Of Near-Wall CO During The Interaction Between A Flame And A Cooling Air Film

Reducing the anthropogenic pollutant emissions is a major concern in the aeronautical community. Implementing high-power density core engines made of lighter materials would undoubtedly increase the thermal efficiency and reduce CO 2 emissions. Nevertheless, near-wall combustion processes will become significant, raising concerns regarding thermal management. As a result, walls in aero-engine combustors are commonly cooled, but it also introduces more complex physical phenomena associated to pollutant formation during flame-cooling air interaction (FCAI). This experimental study aims at elucidating the role of the cooling air film on CO emissions during FCAI. The experiments are conducted in an atmospheric pressure and optically-accessible lab-scale combustion test rig dedicated to near-wall combustion. It generates a lean premixed turbulent methane/air V-shaped flame, with one branch of the flame that interacts with an oil-cooled stainless-steel wall. A splash-cooling plate system enables to generate a momentum-controlled parietal cooling air film. The novelty of this study is to develop and implement planar laser-induced fluorescence of the CO molecule (CO-PLIF), complemented by planar laser-induced fluorescence of the OH radical (OH-PLIF) as well as global CO emissions in the exhaust gases. Results show that the excitation of the Hopfield-Birge system enables to detect the AN ngström bands and the third positive system. However, significant interferences with C 2 and CN emissions are highlighted, being more pronounced in rich combustion conditions, and originating from the incomplete fuel oxidation as well as the high energy density of the two-photon excitation process. A broadband collection strategy of the AN ngström bands is selected, since these interferences are limited in lean combustion regimes. The analysis of the flame dynamics indicates that the increase of the cooling air film momentum shifts the reactive flow away from the wall, but also controls the flame wrinkling through modified aerodynamics. As a result, the flame is not influenced anymore by thermal quenching processes, and can effectively burn further away from the wall. Global emission measurements indicate an increase in CO when the cooling air film momentum increases. While thermal quenching favors a faster oxidation downstream of the flame, the establishment of the cooling air film leads to a longer flame with more production areas. CO-PLIF imaging concurs this phenomenon, with a CO distribution all along the wall when the cooling air film is well established.

Cubic Nonlinearity of Graphene-Oxide Monolayer.

The cubic nonlinearity of a graphene-oxide monolayer was characterized through open and closed z-scan experiments, using a nano-second laser operating at a 10 Hz repetition rate and featuring a Gaussian spatial beam profile. The open z-scan revealed a reverse saturable absorption, indicating a positive nonlinear absorption coefficient, while the closed z-scan displayed valley-peak traces, indicative of positive nonlinear refraction. This observation suggests that, under the given excitation wavelength, a two-photon or two-step excitation process occurs due to the increased absorption in both the lower visible and upper UV wavelength regions. This finding implies that graphene oxide exhibits a higher excited-state absorption cross-section compared to its ground state. The resulting nonlinear absorption and nonlinear refraction coefficients were estimated to be approximately ~2.62 × 10-8 m/W and 3.9 × 10-15 m2/W, respectively. Additionally, this study sheds light on the interplay between nonlinear absorption and nonlinear refraction traces, providing valuable insights into the material's optical properties.

Two-Photon Excitation Photoredox Catalysis Enabled Atom Transfer Radical Polymerization.

In recent years, metal-free photoredox-catalyzed atom transfer radical polymerization (O-ATRP) has gained wide attention because of its advantages (e.g., no metal contamination and mild reaction conditions). However, this traditional one-photon excitation catalysis has thermodynamic limits. Most photocatalysts cannot effectively reduce the initiators and drive the polymerization under visible light. Herein, we investigate the two-photon excitation-catalyzed O-ATRP, in which the catalyst can absorb two photons to accumulate energy. Compared to one-photon excitation catalysis, this method not only has distinct advantages in the controllability, reaction rate, and catalyst loading but also can chemically reduce the various initiators (e.g., aryl halides) to initiate the polymerization. Density functional theory (DFT) calculation reveals that the two-photon excitation process reached a higher energy end state with stronger reduced ability via a thermodynamically more stable intermediate. We believe that this work will provide a new strategy for photoredox-catalyzed O-ATRP.

Study of fluorescent nanodiamonds concentrations in aqueous solutions for biological applications

Commercial fluorescent nanodiamonds constitute a new class of nanoprobes with excellent perspectives for biomedical applications. They produce a broad emission spectrum partially matching the biological window for high tissue penetration, which can be excited in the visible region (380–780 nm) and in the biological window (1000–2500 nm) using two-photon excitation processes, giving a chance for high-resolution multi-photon fluorescence imaging at a sub-cellular resolution scale. Because of their carbon-based structure, nanodiamonds are highly biocompatible and display low toxicity if adequately coated. The nanodiamonds fluorescence spectrum arises due to the so-called Nitrogen-Vacancy (NV) color centers, which are highly photostable and are formed by local charge density compensating defects because of the typical nitrogen presence impurity ions in the host nanocrystals. In this work, a study of different nanodiamonds’ concentrations in terms of their fluorescent activity was investigated in the range of 0.1–10 mg/mL by means of fluorescence spectroscopy with the purpose of determining which concentration of nanodiamonds suspended in milliQ water achieves the maximum intensity, in which it was found that the 5 mg/mL was the most intense in our experimental observations. Additionally, a non-linear fit of the experimental observations of intensity was performed to predict the fluorescence intensity following an expression of a fourth order polynomial with respect to concentration, with a figure of merit, FOM = 0.03–0.04 (3–4%). Based on this effect, the optimal nanodiamonds/aqueous media solution has been determined by means of cell viability assays, finding that concentrations ≤0.1 mg/mL are as biocompatible as milliQ water.

Single Temporal-Pulse-Modulated Parameterized Controlled-Phase Gate for Rydberg Atoms

We propose an adiabatic protocol for implementing a controlled-phase gate CZ$_{\theta}$ with continuous $\theta$ of neutral atoms through a symmetrical two-photon excitation process via the second resonance line, $6P$ in $^{87}$Rb, with a single-temporal-modulation-coupling of the ground state and intermediate state. Relying on different adiabatic paths, the phase factor $\theta$ of CZ$_{\theta}$ gate can be accumulated on the logic qubit state $|11\rangle$ alone by calibrating the shape of the temporal pulse where strict zero amplitudes at the start and end of the pulse are not needed. For a wide range of $\theta$, we can obtain the fidelity of CZ$_{\theta}$ gate over $99.7\%$ in less than $1~\mu$s, in the presence of spontaneous emission from intermediate and Rydberg states. And in particular for $\theta=\pi$, we benchmark the performance of the CZ gate by taking into account various experimental imperfections, such as Doppler shifts, fluctuation of Rydberg-Rydberg interaction strength, inhomogeneous Rabi frequency, and noise of driving fields, etc, and show that the predicted fidelity is able to maintain at about $98.4\%$ after correcting the measurement error. This gate protocol provides a robustness against the fluctuation of pulse amplitude and a flexible way for adjusting the entangling phase, which may contribute to the experimental implementation of near-term noisy intermediate-scale quantum (NISQ) computation and algorithm with neutral-atom systems.

Study on the mechanism of blue light generation by ultra-broadband absorption in Rb–He mixture

In the ultra-broadband range, by continuously tuning the wavelength of the pump laser to excite the rubidium-helium (Rb–He) mixture, an obvious blue fluorescence signal can always be detected. Through the experimental study of the difference between resonant excitation and non-resonant excitation to produce blue fluorescence, it is verified that Rb–He excimer plays an important role in ultra-broadband absorption, and five mechanisms for producing blue fluorescence are summarized: (1) excimer + one-photon resonant excitation process from 5P; (2) two-photon resonant excitation process from 5S; (3) one-photon resonant excitation process from 5S + energy pooling process; (4) excimer + one-photon resonant excitation process from 4D; (5) excimer + energy pooling process. The influence of ionization on the generation of blue fluorescence is discussed. Under several typical pump wavelengths (based on different mechanisms of generating 420 nm fluorescence), the variation of 420 nm fluorescence with the Rb vapor atom concentration, the characteristics of 420 nm collimated light, and its competition with the fluorescence were discussed. The ultra-broadband absorption properties of the alkali metal-noble gas system can be applied to the detection of infrared signals.

High-resolution nanosecond spectroscopy of even-parity Rydberg excitons in Cu2O

We present a study of even parity Rydberg exciton states in cuprous oxide using time-resolved second harmonic generation (SHG). Excitonic states with principal quantum number n = 5 - 12 were excited by nanosecond pulses around 1143 nm. Using time-resolved single-photon counting, the coherently generated second harmonic was isolated both temporally and spectroscopically from inelastic emission due to lower-lying free and bound excitonic states, which included narrow resonances at 1.99 eV associated with an exceptional lifetime of 641 $\pm$ 7 $\mu$s. The near transform-limited excitation bandwidth enabled detailed measurements of the exciton lineshape and position, from which we obtained values for the quantum defects of the S and D excitonic states associated with the appropriate crystal symmetries. Odd parity P and F excitonic states were also observed, in accordance with predicted quadrupole-allowed two-photon excitation processes. We compared our measurements to conventional one-photon spectroscopy in the same sample, and find that the SHG spectrum is cut off at a lower principal quantum number (n = 12 vs n = 15). We attribute this effect to a combination of spatial inhomogeneities and local heating, and discuss the prospects for observing higher principal quantum number even parity states in future experiments.

Up-conversion and Tunable Up-conversion Emission from Nitrogen Functionalized Graphene Quantum Dots

In amidst of conflicts of opinion regarding the presence of up-conversion in carbonaceous nanostructure. We have critically and precisely asserted that there is a real up-conversion in the nitrogen functionalized graphene quantum dots (NGQDs), which is found to be its inherent feature and not a measurement artefact. We have demonstrated that NGQDs exhibit real up-conversion in the blue-green region. To define the perspective of the study a variant of GQDs, NGQDs was synthesized using facile, one-pot synthetic methods. We have critically examined the up-conversion emission in NGQDs, interestingly, N-GQDs exhibited very superior optical properties such as high quantum yield of 97%, up-conversion emission at 535 nm and tunable up-conversion that exhibited a red shift of 12 nm. The two-photon excitation process is the governing phenomenon behind the real up-conversion in NGQDs which is suggested by the quadratic dependence of the integrated intensity on excitation power. Up-conversion can enhance the efficiency of the photovoltaic solar cell by even converting infrared region of sunlight to electricity. Moreover, tunable up-conversion open the door for promising applications of NGQDs in tunable fibre lasers, multicolour displays and multiphoton optical data storage.

Highly Sensitive Dual-Mode Optical Thermometry of Er3+/Yb3+ Codoped Lead-Free Double Perovskite Microcrystal.

In this Letter, erbium (Er3+) and ytterbium (Yb3+) codoped perovskite Cs2Ag0.6Na0.4In0.9Bi0.1Cl6 microcrystal (MC) is synthesized and demonstrated systematically to the most prospective optical temperature sensing materials. A dual-mode thermometry based on fluorescence intensity ratio and fluorescence lifetime provides a self-reference and highly sensitive temperature measurement under dual wavelength excitation at a temperature from 300 to 470 K. Combined with the white-light emission derived from self-trapped excitons (STEs), the characteristic emission peak of Er3+ ions can be observed under 405 nm laser excitation. The fluorescence intensity ratio (FIR) between perovskite and Er3+ is used as temperature-dependent probe signal, of which maximum value for relative and absolute sensitivities reaches to 1.40% K-1 and 8.20 × 10-2 K-1. Moreover, Er3+ luminescence becomes stronger with the feeding Yb3+ increasing under 980 nm laser excitation. The energy transfer of Er3+ and Yb3+ is revealed by power-dependent photoluminescence (PL) spectroscopy, and the involved upconversion mechanism pertains to the two-photon excitation process. The results reveal that the Er3+/Yb3+ codoped lead-free double perovskite MC is a good candidate for a thermometric material for the novel dual-mode design.

Concentration dependence of upconversion luminescence of Er3+/Yb3+ in the fluorophosphate glasses with small phosphates content

Abstract Frequency upconversion into green and red luminescence in fluorophosphate glasses doped with Er3+ ions (1021 to 1016 ion/cm3) and Yb3+ ions (1021 ion/cm3) while pumped with a 970 nm diode laser is presented. The red upconversion emission results from a two-photon excitation process. The upconversion emission intensity dependence on the incident pump power shows that the green upconversion luminescence arising from 2H11/2 level is also contributed by the two-photon processes. The values of n-parameter for the 4S3/2 level change from 2.7 to 2.0 during low power excitation and continue decreasing at higher pumping energy. It has been shown that the green upconversion luminescence intensity dependence on the erbium ions content has a sharp maximum at the concentration of 2* 1019 ion/cm3. We attribute the high upconversion luminescence in case of low Er3+ and Tm3+ ions concentrations (about 1018–1016 ion/cm3) to the cooperative energy transfer from Yb–Yb* dimers. The formation of excited dimers is confirmed experimentally by cooperative luminescence band at 495 nm.

High-peak-power 918-nm laser light source based two-photon spinning-disk microscopy for green fluorophores

High-speed imaging of living specimen was performed using two-photon microscopy equipped with a spinning-disk scanning unit. Typically, a high-peak-power laser light source is needed to simultaneously induce two-photon excitation processes at several hundred focal points, generating the limitations of excitable fluorophores. Therefore, a high-peak-power neodymium-based 918-nm laser light source was used for intravital imaging of the most popular fluorophores, green fluorescent proteins. As a result, the proposed system obtained approximately 30 times brighter fluorescent signal than that obtained using a conventional mode-locked titanium:sapphire laser light source. Furthermore, the system visualized four-dimensional (xyz−t) calcium responses of pancreatic acinar cells agonist stimulations in the living G-CaMP7-expressing mouse with 60 million μm3 volume.

Symmetric Rydberg controlled- Z gates with adiabatic pulses

We analyze neutral atom Rydberg $C_Z$ gates based on adiabatic pulses applied symmetrically to both atoms. Analysis with smooth pulse shapes and Cs atom parameters predicts the gates can create Bell states with fidelity ${\mathcal F}>0.999$ using adiabatic rapid passage (ARP) pulses. With globally optimized adiabatic pulse shapes, in a two-photon excitation process, we generate Bell states with fidelity ${\mathcal F}=0.997$. The analysis fully accounts for spontaneous emission from intermediate and Rydberg states, including the Rydberg lifetime in a room temperature environment, but does not include errors arising from laser noise. The gate protocols do not require individual addressing and are shown to be robust against Doppler shifts due to atomic motion.

Luminescence and up-conversion properties in La2Ti2O7:Eu3+,Er3+ oxides under UV and NIR radiations towards a two-color sensor

La2-x-yEuxEryTi2O7 oxides have an emission color which depends on the excitation wavelength. Under the UV excitation (310–393 nm), they emit in the red spectral region and, under NIR excitation (785–980 nm), they exhibit a green emission induced by upconversion luminescence from Er3+ ions. The structural and luminescence properties of these oxides synthesized by the solid state reaction method were studied according to the x and y values. The co-substituted oxides are obtained over a composition zone between x ≤ 0.2 and y ≤ 0.05. Beyond these values, a phase mixture consisting of a monoclinic phase (layered-perovskite) and cubic phase (pyrochlore) is observed. Under the UV excitation, the reddish emission color is mainly induced by the Eu3+ ions via an exchange interaction mechanism. The quenching concentration of Eu3+ ions is dependent on the excitation wavelength: x = 0.3 and 0.6, at 310 and 377 nm, respectively. With the measurements of photoluminescence decay curves, the lifetime kinetics was linked to the Eu3+ ions concentration. The CIE 1931 color coordinates were also determined according to the x and y values. This one shows linear evolutions when x is fixed and y is variable and vice versa. The good chemical and thermal stability of these oxides also make it possible to disperse them effectively in a glass of lithium tetraborate. Only a 2% transmittance drop is observed for the insertion of 2 wt. % of the oxide. The upconversion luminescence properties were also measured under NIR excitation and shown an emission in the green spectral region mainly due to Er3+ ions. The study of emission intensity dependence on the laser power led to the conclusion of a two-photon excitation process. Temperature-dependent upconversion emission follows a Mott-Seitz model. The activation energy for 4F9/2 → 4I15/2 quenching was found to be 52.8 meV.

One- and two-photon absorption properties of quadrupolar A-π-D-π-A dyes with donors of varying strengths.

In this work, the one- and two-photon absorption properties of two quadrupolar A-π-D-π-A dyes, featuring carbazole and triphenylamine as donors of varying strengths, are calculated using density functional theory. The characteristics of two-photon excitation are analysed by using the three-state model closest to the real physical process. By taking the intermediate state as a bridge, the charge transfer characteristics in the two-photon excitation process are studied in detail by using various visualisation methods. Comparing the intramolecular charge transfer (ICT) characteristics shows that increasing the ability of the push-pull electrons of the central substituents, thus increasing the intramolecular charge transfer, is a more efficient method to enhance the molecular σtp. Our study promotes a deeper understanding of the design of A-π-D-π-A molecules with large cross-sections in two-photon absorption.

Enhanced green up-conversion luminescence in In2O3:Yb3+/Er3+ by tri-doping Zn2+

By co-doping Yb3+/Er3+ into ordinary material In2O3, the up-conversion glow behavior will be triggered up. On this basis, the luminescence intensity can be significantly improved by tri-doping Zn2+ ions. Compared with In2O3:Yb3+/Er3+, the luminescence intensity of In2O3:Yb3+/Er3+/Zn2+ is significantly enhanced and the emitting light changes from faint red to bright green. This can be attributed to the reduction of site symmetry of the luminescence center and the promotion of crystallinity because of lattice distortion caused by Zn2+ ions. The up-conversion luminescence of the In2O3:Yb3+/Er3+/Zn2+ in the red and green light regions are two-photon excitation process, and the quantum yield (QY) is calculated to be 0.1%. The luminescence at the 524 nm emission peak corresponds to the transition Er3+:2H11/2 → 4I15/2. Other two emission peaks in green area (550 nm, 564 nm) correspond to the transition Er3+:4S3/2 → 4I15/2. And the emission peaks in red area (659 nm, 683 nm) root in the transition Er3+:4F9/2 → 4I15/2.

Theoretical modeling and analysis on the absorption cross section of the two-photon excitation in Rb.

The cross-section is crucial for quantitative characterization and analysis of the absorption process. A model on the absorption cross-section of the simultaneous two-photon excitation in Rb-vapor four-wave mixing process is established by using the coupled-wave equation. Taken into account of the hyperfine structures for 85Rb and 87Rb, the third-order susceptibility and hyperfine line strength are calculated respectively. Then, the influences of hyperfine transition on cross section are investigated and simulation results agree well with the experiment results. The calculated results suggest that high pumping power intensity is essential in Rb two-photon excitation, while narrow linewidth is the limiting factor of high absorption efficiency by comparing normalized absorption profile between pumping beam and two-photon excitation process. Additionally, two approaches to improving absorption efficiency, linewidth narrowness of the pumping beam and absorption linewidth broadening, are proposed.

On-demand generation of background-free single photons from a solid-state source

True on-demand high-repetition-rate single-photon sources are highly sought after for quantum information processing applications. However, any coherently driven two-level quantum system suffers from a finite re-excitation probability under pulsed excitation, causing undesirable multi-photon emission. Here, we present a solid-state source of on-demand single photons yielding a raw second-order coherence of g(2)(0)=(7.5±1.6)×10−5 without any background subtraction or data processing. To this date, this is the lowest value of g(2)(0) reported for any single-photon source even compared to the previously reported best background subtracted values. We achieve this result on GaAs/AlGaAs quantum dots embedded in a low-Q planar cavity by employing (i) a two-photon excitation process and (ii) a filtering and detection setup featuring two superconducting single-photon detectors with ultralow dark-count rates of (0.0056±0.0007) s−1 and (0.017±0.001) s−1, respectively. Re-excitation processes are dramatically suppressed by (i), while (ii) removes false coincidences resulting in a negligibly low noise floor.

Up-conversion photoluminescence emissions of CaMoO4:Pr3+/Yb3+ powder

CaMoO4:Pr3+/Yb3+ powder was successfully synthesized by a facile hydrothermal method. X-ray diffraction (XRD) patterns of samples confirmed tetragonal structure and morphology and sizes were confirmed by scanning electron microscopy (SEM) analyses. Particles consisted of regular micro-spheres with uniform sizes, the diameter of each sphere lay in the range of 3 to 4 μm. The up-conversion photoluminescence emission and its concentration dependence were investigated under infrared excitation at 980 nm. All the UC micro-particles exhibited the typical blue, green and red emissions. Dominant blue emissions originated from 3P0→3H4 and intense red emissions originated from 3P0→3F2 transitions, and they both belonged to two-photon excitation processes in CaMoO4: Yb3+/Pr3+ powder. The optimum doping concentrations of Pr3+ and Yb3+ for the highest UC luminescence were 0.1 mol.% and 16 mol.%, respectively. The possible up-conversion mechanisms were discussed in detail. It was found that the UC emission could be well controlled from blue to green to white color by adjusting the concentration of Pr3+ ions in CaMoO4:Pr3+/Yb3+ microcrystal. So it is a candidate material for solid-state lasers, biological imaging, solar cells, and optical communications.

Phase mask-based multimodal superresolution microscopy.

We demonstrate a multimodal superresolution microscopy technique based on a phase masked excitation beam in combination with spatially filtered detection. The theoretical foundation for calculating the focus from a non-paraxial beam with an arbitrary azimuthally symmetric phase mask is presented for linear and two-photon excitation processes as well as the theoretical resolution limitations. Experimentally this technique is demonstrated using two-photon luminescence from 80 nm gold particle as well as two-photon fluorescence lifetime imaging of fluorescent polystyrene beads. Finally to illustrate the versatility of this technique we acquire two-photon fluorescence lifetime, two-photon luminescence, and second harmonic images of a mixture of fluorescent molecules and 80 nm gold particles with > 120 nm resolution (λ/7). Since this approach exclusively relies on engineering the excitation and collection volumes, it is suitable for a wide range of scanning-based microscopies.

Optical control of nonlinearly dressed states in an individual quantum dot

We report two-photon resonance fluorescence of an individual semiconductor artificial atom. By non-linearly driving a single quantum dot via a two-photon transition, we probe the linewidth of the two-photon processes and show that, similar to their single-photon counterparts, they are close to being Fourier limited at low temperatures. The evolution of the population of excitonic states with the Rabi frequency exhibits a clear s-shaped behavior, indicative of the non-linear response via the two-photon excitation process. We model the non-linear response using a 4-level atomic system representing the manifold of excitonic and biexcitonic states of the quantum dot and show that quantitative agreement is obtained only by including the interaction with LA-phonons in the solid state environment. Finally, we demonstrate the formation of dressed states emerging from a two-photon interaction between the artificial atom and the excitation field. The non-linear optical dressing induces a mixing of all four excitonic states that facilitates the tuning of the polarization selection rules of the artificial atom.