Two-photon Resonance Enhancement Research Articles

Simulation of resonance hyper-Rayleigh scattering of molecules and metal clusters using a time-dependent density functional theory approach.

Resonance hyper-Rayleigh scattering (HRS) of molecules and metal clusters have been simulated based on a time-dependent density functional theory approach. The resonance first-order hyperpolarizability (β) is obtained by implementing damped quadratic response theory using the (2n + 1) rule. To test this implementation, the prototypical dipolar molecule para-nitroaniline (p-NA) and the octupolar molecule crystal violet are used as benchmark systems. Moreover, small silver clusters Ag 8 and Ag 20 are tested with a focus on determining the two-photon resonant enhancement arising from the strong metal transition. Our results show that, on a per atom basis, the small silver clusters possess two-photon enhanced HRS comparable to that of larger nanoparticles. This finding indicates the potential interest of using small metal clusters for designing new nonlinear optical materials.

Resonance-enhanced three-photon single ionization of Ne by ultrashort extreme-ultraviolet pulses

Time-resolved extreme-ultraviolet (EUV) fluorescence spectroscopy has been applied to study the multi-photon, single ionization of Ne irradiated by intense EUV-free-electron laser (FEL) pulses at a wavelength of 51 nm. A broad, intense peak at a wavelength of around 46 nm is observed, which is shorter than the incident FEL wavelength. The time dependence of the fluorescence reveals that the peak has two unresolved components, which we attribute to the decay of the excited ion states Ne+ 2s−1(2S) and 2p−2(3Pe)3s(2Pe) at 46.0 and 44.6 nm. From the observed intensity ratios and fluorescence lifetimes we conclude that the Ne+ 2p−2(3Pe)3s(2Pe) state is populated by two-photon resonance enhancement, via a 2p43p2 doubly excited state of Ne.

Quadratic Nonlinearity of One- and Two-Electron Oxidized Metalloporphyrins and Their Switching in Solution

We report the quadratic nonlinearity of one- and two-electron oxidation products of the first series of transition metal complexes of meso-tetraphenylporphyrin (TPP). Among many MTPP complexes, only CuTPP and ZnTPP show reversible oxidation/reduction cycles as seen from cyclic voltammetry experiments. While centrosymmetric neutral metalloporphyrins have zero first hyperpolarizability, beta, as expected, the cation radicals and dications of CuTPP and ZnTPP have very high beta values. The one- and two-electron oxidation of the MTPPs leads to symmetry-breaking of the metal-porphyrin core, resulting in a large beta value that is perhaps aided in part by contributions from the two-photon resonance enhancement. The calculated static first hyperpolarizabilities, beta0, which are evaluated in the framework of density functional theory by a coupled perturbed Hartree-Fock method, support the experimental trend. The switching of optical nonlinearity has been achieved between the neutral and the one-electron oxidation products but not between the one- and the two-electron oxidation products since dications that are electrochemically reversible are unstable due to the formation of stable isoporphyrins in the presence of nucleophiles such as halides.

Surface-Enhanced Hyper-Raman Spectra and Enhancement Factors for Three SERS Chromophores. SEHRS Spectra on Ag Films at Pulse Energies below 2 pJ

Hyper-Raman scattering, the nonlinear (two-photon excited) analogue of ordinary Raman scattering, is usually considered to be a weak process that requires high-energy pulsed laser excitation. The sensitivity can be improved greatly by making use of two-photon electronic resonance and/or electromagnetic field enhancement on metal nanoparticle surfaces. Here we demonstrate that hyper-Raman spectra can be obtained from organic dyes on silver colloidal nanoparticle films at laser powers below 1 mW of average power (pulse energies below 2 pJ). These results open the door to exploiting the potential advantages of hyper-Raman scattering for analytical applications.

Theoretical investigation on dispersion effect and two-photon resonance enhancement of molecular first hyperpolarizability

The calculations of the first hyperpolarizability frequency dispersion effect of hemicyanine derivative chromophore have been performed employing time-dependent Hartree-Fock and sum-over-states method respectively. Two-photon resonance enhancement characteristics of second harmonic generation β(-2ω；ω，ω) have also been investigated using the two-level model (TLM), which was originated by Ouder and Chemla. The results indicate that the first excited state dominates the nonlinear optical properties of hemicyanine derivatives. The first hyperpolarizability β(-2ω；ω，ω) increases with the decrease of fundamental wavelength and gives rise to two-photon resonance near the fundamental wavelength of 950 nm, in accordance with the known experimental facts. In addition, the relaxation effect of the excited states should be considered for TLM in the resonance regime. These researches provide theoretical references for experimental measurements.

Second harmonic generation and two-photon luminescence upconversion in glasses doped with ZnSe nanocrystalline quantum dots

We report two-photon excited emission in borosilicate glasses doped with ZnSe nanocrystalline quantum dots. The emission, predominantly near the two-photon energy and detected in the direction of the excitation beam, is in the visible, and the fundamental excitation is the near-infrared output of a tunable femtosecond laser. Depending on the two-photon energy, time- and frequency-resolved measurements at room temperature reveal that the emission largely consists of second harmonic generation (SHG) and two-photon luminescence upconversion, and a much smaller luminescence from redshifted, low-lying trap states and other trap levels residing near the semiconductor band edge. We discuss the SHG origin in terms of bulk-like and surface contributions from the nanocrystals and the two-photon resonant enhancement near the excitonic absorption.

Heterodyne detected fifth-order Raman response of liquid CS 2: ‘Dutch Cross’ polarization

The heterodyne detected fifth-order Raman response of liquid CS 2 was measured using the novel ‘Dutch Cross’ polarization configuration which provides suppression of lower order processes by several orders of magnitude. The experimental Dutch Cross results show solid agreement with recent molecular dynamics simulations. A three-color modification was also employed to address the possibility of two-photon resonance enhancement and it was found that such effects are negligible. The principle finding is that liquid CS 2 quickly loses its rephasing ability (nuclear coherence peaks at 100 fs with a 1/ e decay of 320 fs), and reflects the rapid loss in nuclear correlations and modal description of simple liquids.

Two-photon resonant third-harmonic generation in La2CuO4.

Combining linear absorption and nonlinear third harmonic generation (THG) experiments, we investigate details of the electronic structure of the highly correlated electronic system in La2CuO4. We demonstrate strong THG mainly due to the charge transfer excitation from O (2p(sigma)) to Cu (3d(x2-y2)). The THG spectrum shows pronounced features due to three-photon and two-photon resonance enhancement as well as quantum interference effects. We obtain excellent agreement with a THG spectrum calculated in terms of the excitonic cluster model and can identify both odd and even symmetry excitation modes.

Optical and nonlinear optical properties of surface-modified CdS nanoparticles

Hyper-Rayleigh scattering (HRS) technique was used to measure the first-order hyperpolarizability, β, of CdS nanoparticles with different surface-modifying agents (anion of surfactant bis (2-ethylhexyl) sulfosuccinate, disodium salt (named as AOT-SO 3 −) and pyridine). The ‘per particle’ (4 nm mean diameter) β-values are estimated to fall in the range of 10 −27 esu, which is among the largest values reported for solution species and the β-value of pyridine-modified CdS is 1.4-times smaller than AOT-SO 3 −-modified CdS. As a possible explanation, this difference in the β-values is attributed to the weakening of the two-photon resonance enhancement effect in the pyridine-modified CdS nanoparticles due to the σ-donation formation of pyridine to the Cd 2+-rich surface of CdS. These were demonstrated by two-photon fluorescence measurements, one-photon fluorescence quenching experiments and photolysis experiments.

Two-photon resonance enhanced multiphoton ionization photoelectron spectroscopy of the SH (SD) radical below and above the lowest ionization threshold

A two-photon resonance enhanced multiphoton ionization spectroscopic study on the mercapto radical is carried out in the one-photon energy region between 258 and 208 nm. Thirteen previously unobserved Rydberg states converging upon the a 1Δ or b 1Σ+ excited ionic states are reported. Identification and characterization of several states with excitation energies below the lowest ionization limit are performed by analyses of the rotationally resolved two-photon excitation spectra, polarization studies, and photoelectron spectroscopy after excitation of the states. Detailed comparisons between the experimental and simulated excitation spectra, which show for three states clear indications of intensity anomalies, as well as the observation of multiple ionization channels demonstrate that for all states Rydberg–Rydberg and/or Rydberg–valence interactions determine the finer details of their electronic wave functions. Above the lowest ionization limit two-photon resonance enhancement is observed for six Rydberg states. These states are found to ionize by autoionization, rather than by absorption of a third photon. Remarkably, rotationally well-resolved excitation spectra can still be obtained for two of these states. Comparison of the dominant electronic configuration of the six Rydberg states allows us to explain these differences in terms of the possible autoionization channels.

Magnetic-field-induced sum-frequency mixing in sodium vapor.

An experimental and theoretical study of magnetic-field-induced sum-frequency mixing in sodium vapor has been carried out using two single-frequency, continuous-wave dye lasers as the sources of the fundamental radiation. In addition to the usual two-photon resonant enhancement of the three-wave-mixing nonlinear-optical process, the use of near- or on-resonant single-photon transitions for further resonant enhancement was investigated. The macroscopic phase-matching behavior of the sum-frequency mixing was examined with the intermediate states off resonance. Experimental and theoretical results are presented to illustrate the differences that occur between the coherent quadrupole emission and the incoherent cascade fluorescence when the intermediate states in the two-photon absorption are resonant. Control over the single-photon resonant enhancement and the sum-frequency-mixing phase matching was achieved to increase the power conversion efficiency to 1.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$.

Broadly tunable difference-frequency generation of VUV using two-photon resonances in H_2 and Kr

We report generation of tunable VUV radiation in the range 117-150 nm by four-wave difference-frequency mixing of a tunable ArF excimer laser with an Nd:YAG-pumped dye laser. The four-wave mixing occurs through two-photon resonant enhancement of X((3)) through the E,F(1)Sigma(g)+ (E,' = 2, J' = 0-3) state(s) of H(2) or the 4p(5)6p(3/2)(2) state of Kr in the process (2omega(193) - omega(dye) = omega(VUV)). An estimate of energy efficiency yields 3 x 10(-5) near 133 nm for both H(2) and Kr. Even though several rotational resonances fall within the natural 193-nm bandwidth, mixing with broadband ArF output in H(2) yields single-line VUV output with a FWHM of 3.5 cm(-1) at half the efficiency compared with that of line-narrowed operation.

Resonance-enhanced multiphoton ionisation spectroscopy of the NH(ND) radical. Part 1—The d1Σ+state

The d 1Σ+ state, previously identified in emission, is confirmed as being the state responsible for the two-photon resonance enhancement in the MPI spectrum of a 1Δ state imidogen radicals at excitation wavelengths around 285 nm. We present the first reliable estimate of the bond-dissociation energy of this excited state. Two-photon resonant MPI via the d state is observed to yield both parent NH+(ND+) and atomic, N+ and H+(D+), ions. Three possible mechanisms for fragment ion formation are considered. Four-photon ionisation doubly resonance-enhanced, first (at the two-photon energy) by levels of the d 1Σ+ state and then (at the three-photon energy) by autoionising members of the Rydberg series converging to one or more of the excited ionisation limits, is considered best able to explain the substantially enhanced N+ ion yields observed when MPI occurs via a select subset of the d–a resonances. Parallels between the ionisation behaviour found in this present investigation and that seen in previous studies of two-photon resonant MPI of b 1Σ+ state PH(PD) radicals suggest the need for a reassignment of the resonance-enhancing excited state of PH(PD).

Multiphoton nonlinear interactions in conjugated organic polymers

The optical nonlinearity at a wavelength of 1.064 μm has been studied in a range of soluble conjugated organic polymers using the technique of degenerate forward four-wave mixing. This technique enables determination of the third-order nonlinear susceptibility | χ (3)| of the material. In all cases, determination of the response time of the material nonlinearity is limited by the experimental resolution (≈ 70 ps). Measurements of | χ (3)| as a function of polymer solution concentration were performed and contributions from both real and imaginary components were observed and estimated. The origin of the imaginary contribution in these ‘off-resonant’ conditions was investigated by intensity-dependent measurements of the four-wave mixing signal. In solutions of poly-3 and -4(butoxycarbonylmethylurethane diacetylene), (BCMU) in both chloroform and chlorobenzene, a dependence characteristic of a fifth-order nonlinear interaction is observed. Furthermore, in chloroform solutions of poly(3-butylthiophene) a seventh-order nonlinearity is observed. These high-order nonlinearities are associated with two-photon and three-photon resonant enhancement of the four-photon process and are characterized by fifth- and seventh-order susceptibilities, χ (5) and χ (7) respectively. The presence of imaginary contributions to the optical nonlinearity in all materials investigated implies that this nonlinearity is dominated in all cases by the nearest resonance. This conclusions renders redundant such simplistic models as the free-electron model.

TUNABLE VACUUM-ULTRA VIOLET RADIATION GENERATED BY TWO-PHTON RESONANTFOUR-WAVE DIFFERENCE MIXING IN Hg VAPOR

Vacuum-ultra violet radiation is generated by 61S0-61D2 or 61S0-63D2 two photon resonant four-wave difference mixing (ω4=2ω1-ω3) in Hg vapor. VUV radiation is tunable when one of the input wave (ω3) is tuned, the tuning range is 184.9-187.5 nm. The mechanism of this frenquency conversion, for example, the two-photon and single-photon resonant enhancement and phase matching have been investigeted. The competing processes and saturation effect on conversion efficiency have been analyzed. Using the generated tunable VUV radiation, we have recorded the absorption spetrum of O2 in air.

2+1 MPI spectroscopy of B̃ 1E'' state NH 3 and ND 3: rotational analysis of the origin bands

The origin bands of the B̃ 1E''- X̃ 1A' 1 transition of both NH 3 and ND 3 have been observed via the two-photon resonance enhancement they provide to the multiphoton ionisation spectra of these molecules. Spectral analysis, which was greatly aided by the use of controlled molecular beam expansion techniques, yields rotational constants for both origin levels.

A 1Π ← χ 1Σ + resonance-enhanced multiphoton ionization of jet-cooled CO

We report the observation of two-photon resonance enhancement in the four-photon tunable dye laser-induced multiphoton ionization spectrum of CO. The observed structure is readily assigned to the rotational bands of the A 1Π ← χ 1Σ + (3,0) transition of CO.

Resonantly enhanced second-harmonic generation in zinc vapor

Coherent second-harmonic radiation has been generated in atomic zinc vapor at 1792.5 and 1601 A from the 5(1)S(0)and 4(1)D(2) states, respectively, by two-photon resonance enhancement. Measurements of the ratio of second-to third-harmonic generation suggest the occurrence of static electric fields of ~5 kV/cm. These fields are caused by ionization of ~2% of the atoms in the focal volume of the incident laser beam.

INVESTIGATION OF DEGENERATE FOUR-WAVE MIXING AND PHASE CONJUGATION IN ORGANIC DYE SOLUTIONS

Phase conjugation of backward-and forward-wave produced by degenerate four-wave mixing using a nearly resonantly enhanced nonlinearity in four-level organic dye solutions was investigated. The conversion efficiency of 22% had been obtained in a 2.5 X 10-5 - molar solution of Chlorophyll A in methylalcohol with a 5 mm interaction length and a pumping energy of ～ 8 mJ from SH of a Q-switched Nd: YAG. Degene-rate four-wave mixing by two-photon resonant-enhancement in the solution of rhodamine 6G in alcohol was studied. Measurements were performed which described the dependence of nonlinear reflectivities on the molar concentrations of the media and the intensities of the pumping and object waves. Phase conjugation characteristics of forward-wave in four-wave mixing process were theoretically analysed. Experiments have cofirmed the theoretical prediction. The method of quartet degenerate four-wave mixing had been ex-tended to the doublet degenerate case. Using this method image conversion from 1.06 μm to 5320 ? had been realized with a conversion efficiency of 25% in a 5 mm long satu-rable absorber made of organic dye 9740 solution in dichloroethane. Theoretical analy-sis showed that using a medium, which is a near resonant absorber for the infrared object wave but is transparent for the visible reconstruction wave, the conversion efficiency may be well in excess of unity.

Nonlinear laser spectroscopy in nitric oxide studied through VUV harmonic generation

Two and three-photon spectra have been obtained by single and double resonance enhancements of the elements of the third-order nonlinear susceptibility (χ(3)), which describe four-wave sum mixing into the vacuum ultraviolet (VUV). Results are reported for nitric oxide, in which the A 2Σ+−X 2Π transition is utilized for two-photon resonant enhancement in χ3ω(3). Relative intensities of the recorded rotational structure agreed well with the predictions of theory based on multiphoton transition probabilities, with the exception of cases where there was accidental double resonant enhancement of χ(3), occurring through both two and three-photon transition probabilities. Finally, in studies designed to explore double resonance effects in χ(3)(2ω1+ω2), two lasers were used to probe simultaneous contributions from the A 2Σ+ and M 2Σ+ states of NO. Signals were further enhanced by a factor of 103 to values as large as 10−4 of the input dye laser intensity.