Single-photon Process Research Articles

Laser ablation of ceramic Al2O3 at 193 nm and 248 nm: The importance of single-photon ionization processes

The aim of this work is to demonstrate that single-photon photoionization processes make a significant difference in the expansion and temperature of the plasma produced by laser ablation of ceramic Al2O3 in vacuum as well as to show their consequences in the kinetic energy distribution of the species that eventually will impact on the film properties produced by pulsed laser deposition. This work compares results obtained by mass spectrometry and optical spectroscopy on the composition and features of the plasma produced by laser ablation at 193 nm and 248 nm, i.e., photon energies that are, respectively, above and below the ionization potential of Al, and for fluences between threshold for visible plasma and up to ≈2 times higher. The results show that the ionic composition and excitation of the plasma as well as the ion kinetic energies are much higher at 193 nm than at 248 nm and, in the latter case, the population of excited ions is even negligible. The comparison of Maxwell-Boltzmann temperature, electron temperatures, and densities of the plasmas produced with the two laser wavelengths suggests that the expansion of the plasma produced at 248 nm is dominated by a single population. Instead, the one produced at 193 nm is consistent with the existence of two populations of cold and hot species, the latter associated to Al+ ions that travel at the forefront and produced by single photon ionization as well as Al neutrals and double ionized ions produced by electron-ion impact. The results also show that the most energetic Al neutrals in the plasma produced at the two studied wavelengths are in the ground state.

Dynamic Stark effect on XUV-laser-generated photoelectron spectra: Numerical experiment on atomic hydrogen

We present the results of our numerical simulation on the photoelectron spectra of hydrogen atoms ionized by the single-photon process of intense XUV laser pulses. The dynamic interference structure in the photoelectron spectra is investigated with respect to the dynamic Stark effect, XUV laser parameters, and properties of the ionization continuum states. The research outcome is that plane waves can be readily deployed to study photoelectron spectroscopy, which is crucial to the wave-function ansatz for generic systems of atoms and molecules. The dynamic Stark effect is prominently demonstrated above certain carrier frequencies of the XUV laser pulse, which mandates a photon energy threshold for the dynamic Stark effect to be discerned in the photoelectron spectra. A one-dimensional model for the hydrogen atom is feasible, which is of fundamental significance in modeling atoms and molecules with reduced dimensionality models. We also point out that detailed characterization of the interference fringes in the photoelectron spectra will facilitate more quantitative investigations of the Stark effect contained in the photoelectron spectra created by single XUV photons.

Visible-Light Response of 4-Aminobenzenethiol and 4,4′-Dimercaptoazobenzene Silver Salts

Silver thiolate, which is an organic–inorganic heterostructured material, converts into thiol-derivatized Ag nanoparticles after irradiation with a visible laser. In this study, the visible-laser responses of 4-aminobenzenethiol (4-ABT) and 4,4′-dimercaptoazobenzene (4,4′-DMAB) silver salts were examined to verify that 4,4′-DMAB on Ag converts to 4-ABT on Ag because of the irradiation of light with a wavelength of 514.5 nm, to resolve the issues that result from the surface-enhanced Raman scattering (SERS) characteristics of 4-ABT. Due to the appearance of unpredicted, non-a1-type bands, the SERS spectrum of 4-ABT is very similar to that of 4,4′-DMAB. One explanation for the non-a1-type bands is that they are associated with the charge-transfer chemical-enhancement mechanism of SERS, but a few researchers have attributed them to photoinduced catalytic conversion of 4-ABT on Ag to 4,4′-DMAB. Using 514.5 nm radiation, we recently reported the successful conversion of 4,4′-DMAB on Ag to 4-ABT; however, the reverse reaction from 4-ABT to 4,4′-DMAB did not occur. Similarly, in this study, 4,4′-DMAB Ag salts first converted to 4,4′-DMAB-adsorbed Ag nanoparticles and then to 4-ABT on Ag nanoparticles after prolonged 514.5 nm irradiation. According to the results of a laser-power dependence study, the photoinduced reduction of 4,4′-DMAB on Ag to 4-ABT is a single-photon process. The photoinduced conversion of 4,4′-DMAB to 4-ABT was more facile as Ag salts than as being assembled on Ag films, which implies that fresh Ag nanoparticles produced in situ are more efficient photoelectron emitters than are aged ones. The non-a1-type bands that appear in the SERS of 4-ABT are thus attributed to the chemical-enhancement mechanism rather than to photoconversion.

Spectral X-ray imaging with single photon processing detectors

Spectral X-ray imaging with single photon processingdetectors gains substantial interest for many applications. In this paper wediscuss fundamental parameters as contrast to noise ratio (CNR) and spectralresponse as a function of the material in the object. Image properties havebeen simulated for different photon energies using MCNP5, assuming an idealdetector with 32 × 32 pixels. Simulations are supported by experimentalresults obtained with detectors from the MEDIPIX family.The CNR is strongly dependent on the number of incident photons and thenumber of photons absorbed in the object. The requirement for substantialabsorption in the object limits the range of useful photon energies. In mostcases the CNR is improved when high energy photons are removed from thespectrum.Materials can be uniquely identified or layers of different materials can beseparated provided that there is a substantial difference in their spectralX-ray absorption. In most cases an absorption edge in the spectrum is neededto obtain good results. Several examples of material identification andmaterial separation are discussed.

Two-photon excitation in H2X (X=O, S, Se, Te)

The present work is mainly to study two-photon process in H2X (X=O, S, Se, Te) by using the full relativistic theory. For comparison, we also study the single-photon process by SAC-CI method. The transition probability of two-photon excitation is 10-2–10-5 times of the single-photon process; the relativistic effects become more and more obvious with the increase of atomic number. In addition, every molecule observes the selection principles; dipole transition component and oscillator strength of individual symmetrical states are greater than those of other individual states. This is due to the symmetry of molecule and it should be an important basis for selecting transition energy.

Fabrication of periodic micro-structures by holographic lithography

The principles of the holographic lithography technique are reviewed, and the ability in the formation of structures with different periods by the holographic lithography technique in SZ2080 is demonstrated. The influence of laser exposure, phase shift between interfering laser beams, and the used laser wavelength on the structures fabricated by the four-beam interference technique has been studied. It is shown that by using the − interfering beams with a different phase is it possible to reduce the period of the structure √2 times compared to the four interfering beams of the same phase. Fabrication of micro-structures by the holographic lithography technique can be realized via multi-photon and single-photon polymerization processes. The structures created by these two techniques are compared. Finally, in the experiments with the five-beam interference, the doubleperiod effect was observed by adding the zero-order beam with a low intensity. The fabricated periodic microstructures have the potential to be used as a scaffold for cell growth and micro-optics.

Atomic and photonic entanglement concentration via photonic Faraday rotation

We propose two alternative entanglement concentration protocols (ECPs) using the Faraday rotation of photonic polarization. Through the single-photon input-output process in cavity QED, it is shown that the maximally entangled atomic (photonic) state can be extracted from two partially entangled states. The distinct feature of our protocols is that we can concentrate both atomic and photonic entangled states via photonic Faraday rotation, and thus they may be universal and useful for entanglement concentration in the experiment. Furthermore, as photonic Faraday rotation works in low-$Q$ cavities and only involves virtual excitation of atoms, our ECPs are insensitive to both cavity decay and atomic spontaneous emission.

Linear up-conversion of orbital angular momentum

We experimentally demonstrated that infrared light imprinted with orbital angular momentum (OAM) was linearly converted into visible light using four-wave mixing (FWM) via a ladder-type configuration in 85Rb atoms. Simultaneously, we theoretically simulated this linear conversion process, and the theoretical analysis was in reasonable agreement with the experimental results. A large single-photon detuning process was used to reduce the absorption of the atoms to the up-converted light and to avoid pattern formation in the FWM process. The multi-mode image linear conversion process is important for applications including image communications, astrophysics, and quantum information.

Photolytic-interference-free, femtosecond two-photon fluorescence imaging of atomic hydrogen

We discuss photolytic-interference-free, high-repetition-rate imaging of reaction intermediates in flames and plasmas using femtosecond (fs) multiphoton excitation. The high peak power of fs pulses enables efficient nonlinear excitation, while the low energy nearly eliminates interfering single-photon photodissociation processes. We demonstrate proof-of-principle, interference-free, two-photon laser-induced fluorescence line imaging of atomic hydrogen in hydrocarbon flames and discuss the method's implications for certain other atomic and molecular species.

Single-photon frequency conversion by exploiting quantum interference

We present a scheme to achieve efficient single-photon frequency conversion by exploiting quantum interference between different transition pathways for single-photon states. As an example, we discuss the single-photon frequency-conversion process in the configuration wherein a $\ensuremath{\Lambda}$-type (Lambda-type) three-level quantum emitter is coupled to a Sagnac interferometer. It is shown that the efficiency of the frequency conversion approaches unity in the ideal case. A real-space theoretical approach and a pseudospectral numerical method are developed, which facilitate the computation for the full spatiotemporal dynamics of the scattering process.

Submicron-dot chains at and beneath surfaces of glasses written by a picojoule 12-fs laser scanning microscope

A near-infrared 12-fs laser scanning microscope was employed for structuring glass surfaces. A Ti–sapphire laser operated at 85 MHz and emitted light pulses at wavelengths around 800 nm. A focused laser beam with a mean power of less than 27 mW, corresponding to a maximal pulse energy of 318 pJ, was applied for line scanning at and beneath the surfaces of cover slips as well as of a filter glass BG39. Periodic arrangements of dots along the processed lines were produced through digital control of the scanner. Depending on the pulse energy and the scan speed, the diameter of the dots ranged from 550 nm down to 100 nm. For the cover slips, the dots occur as cavities after wet chemical etching. For BG39, which exhibits strong near-infrared absorption, both chains of cavities and bumps can be generated without any etching process. The result shows that structures with a size down to 1/8λ can be generated, probably through nonlinear single-photon processes confined within the focal volume.

Photon correlation in single-photon frequency upconversion

We experimentally investigated the intensity cross-correlation between the upconverted photons and the unconverted photons in the single-photon frequency upconversion process with multi-longitudinal mode pump and signal sources. In theoretical analysis, with this multi-longitudinal mode of both signal and pump sources system, the properties of the signal photons could also be maintained as in the single-mode frequency upconversion system. Experimentally, based on the conversion efficiency of 80.5%, the joint probability of simultaneously detecting at upconverted and unconverted photons showed an anti-correlation as a function of conversion efficiency which indicated the upconverted photons were one-to-one from the signal photons. While due to the coherent state of the signal photons, the intensity cross-correlation function g(2)(0) was shown to be equal to unity at any conversion efficiency, agreeing with the theoretical prediction. This study will benefit the high-speed wavelength-tunable quantum state translation or photonic quantum interface together with the mature frequency tuning or longitudinal mode selection techniques.

Pronounced enhancement of exciton Rabi oscillation for a two-photon transition based on quantum dot coupling control

We theoretically investigate how to control the Rabi oscillation of excitons of the coupling quantum dots by manipulating static electric fields. Our results show that, for a single-photon process, when direct excitons change into indirect excitons with a bias applied on the sample, the Rabi oscillation rarely alters. However, for the two-photon process, a pronounced enhancement of Rabi oscillation is observed, which can be utilized as the logic gate in quantum information.

Two-photon excitation for C<sub>2V</sub> molecules based on the full relativistic theory

The present work explores a new phenomenon that not all the transition probability of two photon processes is negligible at low irradiance. The irreducible representation 2B2 of C2v is unexpected, for there is no much deviation in oscillator strength for two-photon and single-photon process A1 to 2B2. This new phenomenon is only possible to be explored by the symmetrical consideration: the necessary and sufficient condition is molecular plane coincident with yz plane or the operation σ ’v(yz) for group C2v. It is only possible to be evaluated out by use of the full relativistic quantum mechanical theory.

Investigation of the Utility of Laser-Secondary Neutral Mass Spectrometry for the Detection of Polyaromatic Hydrocarbons in Individual Atmospheric Aerosol Particles

The distribution of polyaromatic hydrocarbons (PAHs) in ambient aerosol particles is of importance to both human health and climate forcing. Although time-of-flight secondary ion mass spectrometry (ToF-SIMS) has proven useful for studying the distribution of organic compounds in individual aerosol particles, it is difficult to detect PAHs at relevant concentrations in individual aerosol particles because of their low ion yield. In this study, we explore the potential of using laser secondary neutral mass spectrometry (Laser-SNMS) to study three PAHs: pyrene, anthracene, and naphthalene. Because of the high volatility of PAHs, a cryostage was required for the analysis to prevent sublimation of the molecules into the vacuum chamber. We studied two laser systems, a 157 nm excimer laser, which is capable of single-photon ionization of the PAHs, and a 193 nm laser, which requires multiphoton ionization. Under optimized conditions for laser power density and primary ion pulse length, 193 nm postionization resulted in a 2-50-fold increase in ion yield over ToF-SIMS. Using the 157 nm laser, the yield was increased by more than 3 orders of magnitude for all 3 PAHs studied. The single-photon postionization process proved superior in terms of both yield enhancement and reduced fragmentation. By using the optimized 157 nm laser system and a cryostage, we were able to detect PAHs on the surface of 2 μm diameter ambient aerosol particles.

Large transverse momentum lepton pair production at the Large Hadron Collider

Abstract We study the large transverse momentum distribution of lepton pairs produced in heavy-ion collisions, making use of the perturbative QCD. Referring to the calculation of the parton-parton production process into lepton pairs at the Relativistic Heavy Ion Collider (RHIC), the production of lepton pairs at large transverse momentum is extended to the Large Hadron Collider (LHC). The contribution of the parton-parton production process into lepton pairs in Pb-Pb collisions at the LHC is calculated, including the complete processes at large transverse momentum. Lepton pair production with the direct single photon process and the resolved single photon process are considered and confirmed to be significant at the LHC.

Linear photon up-conversion of 450 meV in InGaN/GaN multiple quantum wells via Mn-doped GaN intermediate band photodetection

Up-converted heterostructures with a Mn-doped GaN intermediate band photodetection layer and an InGaN/GaN multiple quantum well (MQW) luminescence layer grown by metal-organic vapor-phase epitaxy are demonstrated. The up-converters exhibit a significant up-converted photoluminescence (UPL) signal. Power-dependent UPL and spectral responses indicate that the UPL emission is due to photo-carrier injection from the Mn-doped GaN layer into InGaN/GaN MQWs. Photons convert from 2.54 to 2.99 eV via a single-photon absorption process to exhibit a linear up-conversion photon energy of ~450 meV without applying bias voltage. Therefore, the up-conversion process could be interpreted within the uncomplicated energy level model.

Photodissociation and Density Functional Calculations of Small VmOn+ Clusters

Oxygen-poor vanadium oxide clusters, V2On+ (n = 1, 2), V3On+ (n = 1, 2, 3), and V4O3+, were produced by laser vaporization and were mass-selected and photodissociated with 532 and 266 nm photons. The geometric structures and possible dissociation channels of these clusters were determined based on the comparison of density functional calculations and photodissociation experiments. The experiments show that the dissociation of V2O+, V2O2+, and V3O3+ mainly occurs by loss of VO, while the dissociation of V3O+ and V4O3+ mainly occurs by loss of V atom. For the dissociation of V3O2+, the VO loss channel is slightly dominant compared to the V loss channel. The combination of experimental results and theoretical calculations suggests that the V loss channels of V3O+ and V4O3+ are single photon processes at both 532 and 266 nm. The VO loss channels of V2O2+ and V3O3+ are multiple-photon processes at both 532 and 266 nm.

Photofragmentation of and electron photodetachment from a GFP model chromophore in a quadrupole ion trap

Although green fluorescent protein (GFP) is widely used in the biological sciences, the photophysics controlling GFP fluorescence versus non-radiative decay pathways are not well understood. In previous work (Forbes and Jockusch, J. Am. Chem. Soc. 2009), we reported that a gaseous anionic model chromophore of GFP, p-hydroxybenzylidene-2,3-dimethylimidazolone (HBDI −), deactivates via electron photodetachment ( ePD) as well as photofragmentation. Distinct electronic action spectra were measured for these two pathways upon activation of HBDI − in a quadrupole ion trap (QIT) mass spectrometer. Here, we explore the mechanisms of HBDI − dissociation following photoexcitation at two characteristic wavelengths: 410 nm, at which the dominant pathway is ePD and at 480 nm, at which the branching ratio between ePD and fragmentation depends strongly on the experimental conditions employed. The results indicate that ePD from HBDI − at 410 nm is a single photon process with a rate that is unaffected by changing the pressure of the helium bath gas. This is consistent with a prompt electron detachment process at 410 nm. At 480 nm, photofragmentation in the QIT requires absorption of more than one photon and is suppressed by collisions, while ePD results from both single and multiple photon absorption. A model that accounts for all these observations is discussed. ePD and effective absorption cross sections are estimated as is the rate of collisional cooling within the QIT. This work explores the connection between absorption and action spectroscopy as applied to the photophysics of a biologically important chromophore.

Single photon background toνeappearance at MiniBooNE

Neglected single photon processes are fit to an excess of electron-like events observed in a predominantly nu_mu beam at MiniBooNE. Predictions are given for analogous events in antineutrino mode.