One-photon Process Research Articles

Time-resolved buildup of two-slit-type interference from a single atom

A photoelectron forced to pass through two atomic energy levels before receding from the residual ion shows interference fringes in its angular distribution as manifestation of a two-slit-type interference experiment in wave-vector space. This scenario was experimentally realized by irradiating a Rubidium atom by two low-intensity continuous-wave lasers [Pursehouse et al., Phys. Rev. Lett. 122, 053204 (2019)]. In a one-photon process the first laser excites the 5p level while the second uncorrelated photon elevates the excited population to the continuum. This same continuum state can also be reached when the second laser excites the 6p state and the first photon then triggers the ionization. As the two lasers are weak and their relative phases uncorrelated, the coherence needed for generating the interference stems from the atom itself. Increasing the intensity or shortening the laser pulses enhances the probability that two photons from both lasers act at the same time, and hence the coherence properties of the applied lasers are expected to affect the interference fringes. Here, this aspect is investigated in detail, and it is shown how tuning the temporal shapes of the laser pulses allows for tracing the time-dependence of the interference fringes. We also study the influence of applying a third laser field with a random amplitude, resulting in a random fluctuation of one of the ionization amplitudes and discuss how the interference fringes are affected.

Manipulating dynamical Rabi-splitting with two-color laser pulses.

We theoretically investigate strong-field ionization of hydrogen atoms by orthogonally polarized two-color (OTC) laser pulses consisting of a fundamental field that is resonant with the 1s - 2p transition and its second harmonic. Numerical simulations are performed by solving the two-dimensional time-dependent Schrödinger equation and recording the photoelectron momentum distribution. In this strong-field resonant ionization, the atom undergoes many Rabi cycles and the electron can be emitted within a completed Rabi-cycle leading to the splitting of the localized momentum distribution. Here, the splitting is attributed to dynamic Rabi-splitting as a result of the dynamic Stark effect. The employed OTC scheme is shown to be efficient for controlling the dynamic Rabi-splitting through the control of quantum-path interferences involved in one-photon and two-photon absorption processes. The control scheme is accomplished by varying the relative ratio intensity and optical phase between the two pulses, and its footprint is mapped in the momentum distribution. This is shown to lead to an asymmetric distribution and suppression of the ionization process. The obtained results suggest the OTC scheme as a tool for coherent control of dynamic Rabi-oscillations via the controlled quantum-path interferences, thus opening new directions towards designing quantum states via the control OTC scheme.

Electromagnetically induced amplification via counter-rotating-wave terms in a three-level Λ-type system

We report the remarkable phenomena caused by high-order interaction of counter-rotating-wave terms in a three-level Λ-type system coupled with two laser fields. With the low-order interaction of the rotating-wave terms, transparency induced by Autler−Townes splitting is obtained. When the high-order interaction of the counter-rotating-wave terms is considered, some sideband peaks show up and the probe field is amplified without population inversion. The stronger the coupling, the larger the amplification. The gain of probe photons comes mainly from the loss of the control field through a one-photon or two-photon process.

Electrochemically controlled energy storage in a norbornadiene-based solar fuel with 99% reversibility

Solar fuels based on molecular photoswitches hold the potential to combine solar energy conversion, storage, and release in an extremely simple one-photon one-molecule process. In this work we demonstrate electrochemically controlled solar energy storage and release with high reversibility in a tailor-made norbornadiene photoswitch. We investigated the photochemical conversion of the energy-lean 2-cyano-3-(3,4-dimethoxyphenyl)-norbornadiene (NBD’) to its energy-rich counterpart 2-cyano-3-(3,4-dimethoxyphenyl)-quadricyclane (QC’) and the electrochemically triggered reconversion. Characteristic bands of NBD’ and QC’ were identified by density functional theory (DFT) and monitored in-situ during the energy storage and release process by photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS). We identified the stable regions of both isomers at a Pt(111) electrode and tested the stability of the NBD’/QC’ couple over 1000 storage and release cycles. It is shown that reversibilities of more than 99% per cycle can be achieved in this electrochemically triggered energy storage system.

Populations and Dynamics of Guanine Radicals in DNA strands-Direct versus Indirect Generation.

Guanine radicals, known to be involved in the damage of the genetic code and aging, are studied by nanosecond transient absorption spectroscopy. They are generated in single, double and four-stranded structures (G-quadruplexes) by one and two-photon ionization at 266 nm, corresponding to a photon energy lower than the ionization potential of nucleobases. The quantum yield of the one-photon process determined for telomeric G-quadruplexes (TEL25/Na+) is (5.2 ± 0.3) × 10−3, significantly higher than that found for duplexes containing in their structure GGG and GG sequences, (2.1 ± 0.4) × 10−3. The radical population is quantified in respect of the ejected electrons. Deprotonation of radical cations gives rise to (G-H1)• and (G-H2)• radicals for duplexes and G-quadruplexes, respectively. The lifetimes of deprotonated radicals determined for a given secondary structure strongly depend on the base sequence. The multiscale non-exponential dynamics of these radicals are discussed in terms of inhomogeneity of the reaction space and continuous conformational motions. The deviation from classical kinetic models developed for homogeneous reaction conditions could also be one reason for discrepancies between the results obtained by photoionization and indirect oxidation, involving a bi-molecular reaction between an oxidant and the nucleic acid.

Quantum Optimal Control of Rovibrational Excitations of a Diatomic Alkali Halide: One-Photon vs. Two-Photon Processes

We investigated the roles of one-photon and two-photon processes in the laser-controlled rovibrational transitions of the diatomic alkali halide, 7Li37Cl. Optimal control theory calculations were carried out using the Hamiltonian, including both the one-photon and two-photon field-molecule interaction terms. Time-dependent wave packet propagation was performed with both the radial and angular motions being treated quantum mechanically. The targeted processes were pure rotational and vibrational–rotational excitations: (v = 0, J = 0) → (v = 0, J = 2); (v = 0, J = 0) → (v = 1, J = 2). Total time of the control pulse was set to 2,000,000 atomic units (48.4 ps). In each control excitation process, weak and strong optimal fields were obtained by means of giving weak and strong field amplitudes, respectively, to the initial guess for the optimal field. It was found that when the field is weak, the control mechanism is dominated exclusively by a one-photon process, as expected, in both the targeted processes. When the field is strong, we obtained two kinds of optimal fields, one causing two-photon absorption and the other causing a Raman process. It was revealed, however, that the mechanisms for strong fields are not simply characterized by one process but rather by multiple one- and two-photon processes. It was also found that in the rotational excitation, (v = 0, J = 0) → (v = 0, J = 2), the roles of one- and two-photon processes are relatively distinct but in the vibrational–rotational excitation, (v = 0, J = 0) → (v = 1, J = 2), these roles are ambiguous and the cooperative effect associated with these two processes is quite large.

Photocarrier injection by two-photon excitation in rubrene single crystal

We performed photocarrier injection by two-photon excitation in rubrene crystal using laser pulses of nanosecond-duration at various wavelengths under an external electric field. Based on the excitation spectra, it is revealed that photocarriers are injected at excitation energies corresponding to the intrinsic absorption by two-photon process, in contrast to the midgap-states-mediated injection by one-photon process. This result means that the two-photon excitation method is useful to inject photocarriers free from the surface traps.

Generation processes of superfluorescence of biexcitons in CuCl quantum dots by one- and two-photon resonant excitation

We investigated the influence of excitation processes of a biexciton on the generation of superfluorescence, which is pulsed radiation from coherently coupled two-level systems, from biexcitons in CuCl quantum dots. Two excitation processes of biexcitons were examined: resonant two-photon excitation of biexcitons, and resonant one-photon excitation of excitons. The excitation density dependence of the time-resolved photoluminescence was measured. The results showed that for one-photon excitation, a stronger peak intensity, longer delay time and wider pulse width were observed compared to resonant two-photon excitation. The high density of the excited dots in the one-photon excitation process results in the strong pulse, while an initial population of excitons results in a suppression of coherent coupling.

Investigation of laser annealing mechanisms in thin film coatings by photothermal microscopy.

We study the evolution of the absorptance of amorphous metal oxide thin films when exposed to intense CW laser radiation measured using a photothermal microscope. The evolution of the absorptance is characterized by a nonexponential decay. Different models that incorporate linear and nonlinear absorption, free carrier absorption, and defect diffusion are used to fit the results, with constraints imposed on the fit parameters to scale with power and intensity. The model that best fits is that two types of interband defects are passivated independently, one by a one-photon process and the other one by a two-photon process.

Impact of heat treatment on NBOHC luminescence of OH-containing and H2-impregnated fused silica for deep-ultraviolet applications

OH-containing and H2-impregnated fused silica is the optical materials of choice for deep-ultraviolet (DUV) applications. Intrinsic and UV laser irradiation induced non-bridging oxygen hole center (NBOHC) defects play a central role in the optical properties of fused silica in the DUV spectra range. Understanding the NBOHC generation mechanism is of great importance to the performance improvement. In this paper, NBOHC-related 650 nm photoluminescence (PL) band is investigated in details by heat-treating the fused silica at different temperature and exciting the PL band at different wavelength. From the 650 nm PL characteristics of un-treated and heat-treated fused silica samples excited at 193 nm, 266 nm, 355 nm, and 532 nm, it is found that the 650 nm PL band of un-treated fused silica is excited at 193 nm and 266 nm mainly via a two-photon process, and cannot be excited with 355 nm and 532 nm. The un-treated fused silica also presented a reversible time delay (annealing) in 650 nm PL intensity following an abrupt change of excitation fluence at 193 nm, due to the recombination of NBOHCs with impregnated H2. On the other hand, for the heat-treated fused silica, the 650 nm PL band is excited by a combination of one- and two-photon processes at 193 nm and 266 nm with the one-photon process dominating, and by the one-photon excitation process at 355 nm and 532 nm. The quantum yield of 650 nm PL band decreases with the decreasing excitation photon energy. From these experimental results it is confirmed with no doubt that in un-treated fused silica NBOHCs are produced by 193 nm and 266 nm laser irradiation induced photolysis of an Si‒OH group via a two-photon process, with a minimum photolysis energy between 7.0 eV and 7.9 eV. In heat-treated fused silica NBOHCs are produced by heat treatment induced breakage of Si‒OH bonds. Heat treatment also removed totally the impregnated H2 in fused silica, that makes the heat-treatment produced NBOHCs become stable and show one-photon excited 650 nm PL band when excited at 193 nm, 266 nm, 355 nm, and 532 nm. In contrary to the un-treated fused silica, the heat-treated fused silica presented no reversible time delay in 650 nm PL intensity following the abrupt change of excitation fluence at 193 nm, due to the total removal of H2 in fused silica and the absence of recombination of NBOHCs with H2.

Photoluminescence Upconversion by Defects in Hexagonal Boron Nitride.

Hexagonal boron nitride (h-BN) was recently reported to display single photon emission from ultraviolet to near-infrared range due to the existence of defects. Single photon emission has potential applications in quantum information processing and optoelectronics. These findings trigger increasing research interests in h-BN defects, such as revealing the nature of the defects. Here, we report another intriguing defect property in h-BN, namely photoluminescence (PL) upconversion (anti-Stokes process). The energy gain by the PL upconversion is about 162 meV. The anomalous PL upconversion is attributed to optical phonon absorption in the one-photon excitation process, based on excitation power, excitation wavelength, and temperature-dependence investigations. Possible constitutions of the defects are discussed from the results of scanning transmission electron microscopy (STEM) studies and theoretical calculations. These findings show that defects in h-BN exhibit strong defect-phonon coupling. The results from STEM and theoretical calculations are beneficial for understanding the constitution of the h-BN defects.

Two-photon isomerization triggers two-photon-excited fluorescence of an azobenzene derivative

Photoisomerization provides a general means for photo-controlling molecular structure and function, and two-photon induced isomerization has some advantages over one-photon processes. Here, we report the two-photon absorption (TPA) induced optical properties and characteristics of an azobenzene derivative, namely N-(3,4,5-octanoxyphenyl)-N′-4-[(4-hydroxyphenyl)azophenyl]1,3,4-oxadiazole (AOB-t8). With the activation of red light, the photoproduct of cis-AOB-t8 isomers has been unambiguously identified, and the two-photon induced isomerization process of AOB-t8 in THF is characterized with the relevant rate constants of forward and backward reactions. The TPA coefficients and cross-sections of AOB-t8 at different wavelengths, and nonlinear optical refractive index are determined from femtosecond nonlinear optical measurements. Particularly, the results indicate that the two-photon excited emission can be effectively triggered by the two-photon-induced isomerization from trans- to cis-AOB-t8, demonstrating an on-switch from non-fluorescence to visible blue emission. Consistent with the observations from one-photon excitation, we suggest that the enhanced two-photon excited fluorescence of AOB-t8 solution originates from the combined contribution of cis-AOB-t8 monomer and its aggregates, i.e., the conformation transition from trans- to cis-AOB-t8 opens the emission channel and the self-assembly aggregation of cis-AOB-t8 isomers further enhances the fluorescence upon red light excitation.

Contrast improvement by using tailored laser pulses to circumvent undesired excitations

We report on fluorescence contrast improvement by using phase, amplitude, and polarization shaped laser pulses. The measurements were conducted by applying phase functions at different spectral amplitudes for excitations of dyes and agree very well with calculations. In particular, undesired one-photon excitations are circumvented with phase and amplitude tailored pulses for two-photon transition. This is realized by cutting out the laser spectrum at the wavelength of the one-photon process while utilizing an antisymmetric phase function that allows for constructive interference of the remaining outer spectral contributions for two-photon absorption. Moreover, polarization enhanced contrast between dyes is demonstrated where the two-photon dye is predominantly excited in one polarization direction and simultaneously the one-photon dye in the other polarization direction. The presented methods of shaping ultrashort laser pulses have a high potential for imaging applications.

Multicolor-tunable upconversion emission of lanthanide doped 12CaO·7Al2O3 polycrystals

A novel multicolor-tunable lanthanide doped 12CaO·7Al2O3 (Ln3+:C12A7; Ln3+ = Yb3+, Tm3+ and Er3+) synthesized by solid-state reaction method would greatly enhance the scope of its applications ranging from infrared solar cells to field emission display. The scanning electron microscopy (SEM) was used to characterize the morphology of Ln3+:C12A7. Under 980 nm excitation, the emission color was tuned from green-yellow to red region through adjusting the concentrations of Ln3+ ions. The obtained near infrared upconversion (UC) emissions in Ln3+:C12A7 was corresponded to “window of optical transparency”. All blue, green and red UC emissions were populated by one-photon process, and the proposed UC mechanisms were studied in Ln3+:C12A7.

Highly uniform and monodisperse β-NaYF4 : Sm3+ nanoparticles for a nanoscale optical thermometer.

Monodisperse β-NaYF4:1%Sm3+ nanoparticles were fabricated successfully via the thermal decomposition technique. Strong temperature dependence of the Sm3+ emission was observed when its thermally populated state H7/26 was directly excited to the G5/24 level. This strategy not only can eliminate laser heating and background Stokes-type scattering noise but also has a high quantum yield as a result of one-photon excitation process. Under 594.0nm laser excitation, the emission intensity of G5/24-H5/26 enhances monotonously with rising temperature from 300K to 430K, including a physiological temperature range (27°C-60°C). The relative temperature sensitivity can reach 1.1% K-1 and 0.91% K-1 at 300K and 330K, respectively. In addition, the repeatability of temperature sensing was evaluated under several heating-cooling cycles, and the decay curves of the emission at 560.0nm (G5/24-H5/26) at different temperatures were also investigated. These results raise the prospects of monodisperse β-NaYF4:1%Sm3+ nanoparticles for optical temperature sensing in biomedicine fields.

The oxidation of borohydrides by photoexcited [UO2(CO3)3]4- .

The carbonate ion is an effective quencher of uranyl(vi) luminescence and makes uranyl(vi) tricarbonate barely luminescent and photochemically inactive. We demonstrate here that photoexcited uranyl(vi) tricarbonate, *[UVIO2(CO3)3]4-, can however oxidize borohydrides (BH3X-, X = H, CN) to give boric acid and H2 gas, reducing itself to [UVO2(CO3)3]5-. This hypothesis was supported by UV-vis and NMR spectroscopy as well as quantum chemical calculations. The charge transfer states associated with photoreduction processes were modelled by density functional theory calculations. These results suggest that the mechanism of photoreduction of [UVIO2(CO3)3]4- is similar to that of [UVIO2(H2O)5]2+ and that it occurs through a one-photon reduction process.

Making two-photon processes dominate one-photon processes using mid-IR phonon polaritons

Phonon polaritons are guided hybrid modes of photons and optical phonons that can propagate on the surface of a polar dielectric. In this work, we show that the precise combination of confinement and bandwidth offered by phonon polaritons allows for the ability to create highly efficient sources of polariton pairs in the mid-IR/terahertz frequency ranges. Specifically, these polar dielectrics can cause emitters to preferentially decay by the emission of pairs of phonon polaritons, instead of the previously dominant single-photon emission. We show that such two-photon emission processes can occur on nanosecond time scales and can be nearly 2 orders of magnitude faster than competing single-photon transitions, as opposed to being as much as 8-10 orders of magnitude slower in free space. These results are robust to the choice of polar dielectric, allowing potentially versatile implementation in a host of materials such as hexagonal boron nitride, silicon carbide, and others. Our results suggest a design strategy for quantum light sources in the mid-IR/terahertz: ones that prefer to emit a relatively broad spectrum of photon pairs, potentially allowing for new sources of both single and multiple photons.

Near-IR Photoluminescence of C60.

We have observed that C60+ ions isolated in cryogenic matrices show distinct near-IR photoluminescence upon excitation in the near-IR range. By contrast, UV photoexcitation does not lead to measurable luminescence. Near-IR C60+ photoluminescence is a one-photon process. The emission is mainly concentrated in one band and corresponds to 2A1u ← 2E1g relaxation. We present experimental data for the Stokes shift, power, and temperature dependencies as well as the quantum efficiency of the photoluminescence. Our findings may be relevant for astronomy, considering recent unequivocal assignment of five diffuse interstellar bands to near-IR absorption bands of C60+.

Continuous wave near-infrared phonon-assisted upconversion in single Nd3+-doped yttria nanoparticles

We report on Nd3+-doped yttrium oxide polycrystalline nanoparticles synthesis, their basic structural characterization and the observation of phonon-assisted upconversion in single nanoparticles with different Nd3+ doping levels. For upconversion studies, a frequency-locked CW Ti: Sapphire laser operating @ 820nm, resonant with a Stark level of the Nd3+ ions 4F5/2 manifold, was used to excite single nanoparticles on a home-made confocal microscope. To the best of our knowledge, luminescence ranging from 630 to 760nm was observed for the first time from single Nd3+-doped yttria nanoparticles under such excitation conditions. Upconversion studies on different single nanoparticles revealed a good doping homogeneity and the upconversion emissions were found to be one-photon phonon-assisted processes.

State-Population Narrowing Effect in Two-Photon Absorption for Intense Hard X-ray Pulses

We report on studies of state-populations during the two-photon absorption process using intense X-ray pulses. The calculations were performed in a time-dependent manner using a simple three-level model expressed by coupled rate equations. We show that the proposed approach describes well the measured rates of X-rays excited in the one-photon and two-photon absorption processes, and allows detailed investigation of the state population dynamics during the course of the incident X-ray pulse. Finally, we demonstrate that the nonlinear interaction of X-ray pulses with atoms leads to a time-narrowing of state populations. This narrowing-effect is attributed to a quadratic incidence X-ray intensity dependence characteristic for nonlinear interactions of photons with matter.