Four-wave Difference-frequency Mixing Research Articles

Ultrafast internal conversion and photodissociation of molecules excited by femtosecond 155 nm laser pulses

The dynamics of several prototypical molecular systems after excitation with femtosecond laser pulses at 155 nm has been studied in pump–probe experiments. The vacuum ultraviolet (VUV) pump pulses with a pulse width of 350–450 fs were generated by near-resonant four-wave difference frequency mixing in argon. The careful analysis of the time-dependent ion signals has allowed us to determine the lifetime of the excited molecular states down to about 30 fs. The extremely short lifetime of water molecules excited to the repulsive à state has been directly observed for the first time: τD⩽20 fs. For molecular oxygen highly excited in the Schumann–Runge band, a decay time of 40±20 fs was obtained. The lifetimes of ethylene and chloroethylenes as well as of benzene and toluene reaching from 40 up to 180 fs are primarily caused by internal conversion. The decay times τD=(1.9±0.1) and τD=(90±20) ps obtained for carbon disulfide and nitric oxide, respectively, are due to predissociation of the VUV excited states.

Femtosecond-pulse two-photon resonant difference-frequency mixing in gases: A technique for tunable vacuum-ultraviolet femtosecond-pulse generation and a spectroscopic tool for studying atoms in strong laser fields

Two-photon resonant and near-resonant four-wave difference-frequency mixing in gases in the interaction regime when laser pulse durations are comparable to or shorter than the medium polarization relaxation time ${T}_{2}^{\ensuremath{'}}$ is investigated. The results of experimental studies of the process in Ar and Kr using pump pulses from the ArF-excimer laser are presented demonstrating a generation of tunable short pulse radiation in the range 102--124 nm. The results are discussed in terms of a theoretical model based on a self-consistent solution of the Bloch equations for the atomic transitions and the Maxwell equations for the fields. This enables one to interpret specific nonstationary resonant and quasiresonant phenomena involved in the frequency conversion process. It is shown that the femtosecond-pulse four-wave frequency-mixing technique with probe pulses significantly shorter than the pump pulses makes it possible to study the coherent dynamics of an atomic transition exposed to an intense field. Using atomic Kr as the nonlinear medium, coherent Rabi oscillations and the subsequent phase relaxation of excitation were observed under the condition of two-photon interaction of Kr with femtosecond 193-nm laser pulses. The obtained information is important for controlling and optimizing processes of two-photon resonant frequency conversion and for time-resolved studies of Rydberg states in atoms and molecules.

Short-pulse high-intensity excimer lasers — A powerful tool for the generation of coherent VUV and XUV radiation

Different approaches for the generation of coherent VUV and XUV radiation with a 400 fs KrF excimer-laser system are studied. In nonlinear optical experiments it is shown that four-wave difference-frequency mixing in Xe, using a near two-photon resonance with the KrF laser radiation, is well suited for the generation of tunable VUV radiation in the range 130–200 nm. Conversion efficiencies of 2% and output energies up to 260 μJ have been demonstrated. Further prospects to achieve μJ energies are discussed. Using this VUV source and the KrF laser, powerful XUV radiation can be generated by different low-order frequency mixing processes. In first experiments on this subject, direct frequency tripling of the KrF laser pulse has resulted in 14 μJ XUV radiation at 83 nm.For the realization of soft-X-ray lasers, specific advantages of short-pulse KrF drivers are discussed. Novel scenarios based on a hybrid KrF/Ti: sapphire laser system and multiphoton resonant excitation are considered.

Generation of broadly tunable femtosecond vacuum-ultraviolet pulses

Tunable femtosecond vacuum-ultraviolet radiation in the range omega(D) = 102-124 nm has been generated by twophoton-resonant and near-resonant four-wave difference-frequency mixing (omega(D) = 2omega(p) - omega(I)) in krypton and argon by use of intense 250-fs ArF laser pulses (omega(p)) and tunable femtosecond signal and idler pulses (omega(I)) generated by an optical parametric generator.

Direct observation of coherent medium response under the condition of two-photon excitation of krypton by femtosecond UV-laser pulses.

Since the first observation of coherent optical phenomena (e.g., self-induced transparency, optical nutation, photon echo, etc.), the resonant interaction of matter with laser pulses of duration tp short compared to the irreversible relaxation time T 0 2 of the medium polarization has become the subject of intensive studies, both theoretical and experimental. Phenomena arising from the coherent excitation of electric-dipole allowed transitions have been investigated in great detail [1]. Resonant and near-resonant two-photon excitations with ultrashort laser pulses of forbidden atomic transitions give access to an even wider class of nonlinear-optical processes such as two-photon absorption, stimulated Raman scattering, and four-wave sumand differencefrequency mixing. The latter has proved to be a powerful tool for efficient femtosecond pulse generation in the vacuum ultraviolet (VUV) spectral range [2]. Nevertheless, up to now only a few experimental results have been obtained which allow the direct observation of the coherent dynamics in two-photon induced radiationmatter interactions (see experimental papers [3–6] and monograph [7]). In this Letter we report for the first time a direct experimental observation of the coherent dynamics of an atomic two-photon excitation process using timeresolved four-wave difference-frequency mixing svs ­ 2vp 6 vid with femtosecond UV and VUV pulses. Keeping the injected pulses svid significantly shorter than the pump pulses svpd we were able to resolve Rabi oscillations and subsequent relaxation of the excitation at the probe difference frequency svsd. The results are of great importance for the understanding of coherent phenomena underlying the femtosecond VUV-pulse generation and further optimization of this process. A two-photon transition has no dipole moment and coherent emission occurs only in the presence of an external field. This specific feature is exploited in coherent two-photon spectroscopy [3,4]. An additional field of frequency vi applied to a two-photon transition which had been excited by an intense pump pulse of frequency vp > v21y2 (v21 being the frequency of the atomic tran-

Generation of picosecond VUV radiation by four-wave mixing of nanosecond and picosecond laser radiations

We use the four-wave difference frequency mixing of a nanosecond laser (UV) and a picosecond laser (visible) in xenon gas to generate a picosecond VUV laser radiation. Since the visible laser has a spectral range between 450 nm and 800 nm, we can obtain a tunable VUV laser, from 148 nm to 173 nm. The time duration of the pulses generated depends (linearly in our case) on the shortest laser, which is the picosencond laser.

Generation of high-power subpicosecond pulses at 155 nm

Subpicosecond vacuum-ultraviolet radiation at 155 nm with pulse energies above 0.2 mJ has been obtained by near-resonant four-wave difference-frequency mixing in a Xe gas jet. Laser fields for the mixing process have been generated by a short-pulse KrF dye excimer laser system and a Raman converter. The process permits tuning in a broad vacuum-ultraviolet range and can be scaled up to higher output energies.

Novel scheme for atomic hydrogen detection by double-resonant four-wave mixing

A novel laser diagnostic scheme for the detection of atomic hydrogen in high temperature plasmas is proposed. This scheme is based on double-resonant four-wave difference frequency mixing. Two waves are resonant with the n=1→n=3 two-photon transition at 2×205 nm and a third wave is slightly detuned off the Balmer-α resonance at 656 nm. A signal wave close to Lyman-α is generated and is emitted like a laser beam within a small solid angle, comparable to that of the incoming laser beams. The signal scales with the square of the ground state density. By applying spatial and spectral filtering background radiation can be efficiently suppressed. The determining physical quantities are presented. For an atomic temperature of about 10 eV a detection limit close to N=109 cm−3 results in a spatial and temporal resolution of a few cm and about 10 ns, respectively.

Observation of parametric amplification with a femtosecond vacuum-ultraviolet four-wave difference-frequency mixing source

The high conversion efficiency of the Xe-atom four-wave difference-frequency mixing (ω3 = 2ω1 − ω2) vacuum UV (VIJV) femtosecond source relies on a coincidental near two-photon resonance occurring when the ω1 light pulses are at ≃248.5 nm. It also depends on the coherence properties of the input pulses. As in previous studies of this system, high-intensity femtosecond light pulses at ω1 were produced by amplification in a KrF-excimer discharge, but the required 248.5-nm seed pulses were obtained from a new source based on near-threshold continuum generation. Nonlinear mixing in Xe of the pulses prepared at ω1 with femtosecond visible light pulses of ω2 yielded ω2 → ω3 energy conversions of ~1 and VUV pulse energies as high as 60 μJ. At low input energies the beam at ω2 was observed to be parametrically amplified by a factor ≃150.

Generation of tunable short pulse VUV radiation by four-wave mixing in xenon with femtosecond KrF-excimer laser pulses

A four-wave difference-frequency mixing scheme based on near resonant two-photon excitation of xenon with a femtosecond KrF excimer laser system is used to generate tunable short pulse radiation in the VUV and UV spectral range between 133 and 355 nm with output energies of several microjoules. Tunable nanosecond laser radiation in the range from 1905 nm to 185 nm is used. At excimer laser wavelengths of 193 nm (ArF), 308 nm (XeCl) and 350 nm (XeF), the process has been used to generate high power short pulse radiation by double-pass amplification in an additional excimer laser discharge. Output energies of 1.9, 3.0, and 2.8 mJ, respectively, have been obtained so far at pulse durations in the range of 1 ps. Nonlinear susceptibilities for the difference-frequency mixing process are calculated using a stationary susceptibility formalism and compared to experimental values.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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.

Polarization properties of the coherent radiation generated by four-wave mixing in Hg

The polarization of the generated radiation in the four-wave difference frequency mixing in Hg vapor has been investigated with respect to those of the input radiations. The observed polarization properties are explained properly by the theoretical treatment of the third order nonlinear process in isotropic media.

Rydberg and ion-pair state mixing in the 1 1Σ+u and 2 3Πu vibronic states of Cl2 as studied by vacuum ultraviolet laser spectroscopy

Fluorescence excitation and resonantly enhanced photoionization spectra of jet-cooled Cl2 in the 71 000–77 500 cm−1 region are measured using a tunable vacuum ultraviolet light generated by four-wave difference frequency mixing of two lasers in Kr. For the 1 1Σ+u state, vibrational term values for 35Cl2, 35Cl37Cl, and 37Cl2 and rotational constants for 35Cl2 and 35Cl37Cl are determined with high precision by the measurement of the 1 1Σ+u –X 1Σ+g transitions. It is shown clearly that the v′=39 vibrational wave functions of the 1 1Σ+u state of 35Cl2 and 35Cl37Cl are localized in the Rydberg well while the vibrational wave functions for the v′&amp;lt;39 levels are delocalized in the ion-pair well and those for the v′&amp;gt;39 levels are between the inner Rydberg wall and the outer ion-pair wall. A double minimum potential composed of the Rydberg and the ion-pair states explains well the observed vibrational and rotational structures. The molecular constants for the Rydberg well of 2 3Π(0+u) determined from the rovibrational structures of the 2 3Π(0+u)–X 1Σ+g transitions are ωe=647.3(33) cm−1, ωexe=3.37(59) cm−1 and re=1.833(20) Å, respectively.

Interatomic potentials of the C1 and D+ states of XeNe, XeAr, and XeKr as studied by tunable vacuum ultraviolet laser spectroscopy

Fluorescence excitation and resonantly enhanced photoionization spectra of the C–X and D–X bands of XeNe, XeAr, and XeKr, formed in a supersonic free jet are measured by use of a tunable vacuum ultraviolet light generated by four-wave difference frequency mixing of two lasers in Kr. The potential parameters of the heteronuclear rare gas dimers for the two excited state potentials, C1 and D0+, correlated with the Xe6s′(1/2)1 state are derived by an analysis of rovibronic structures of the C–X and D–X bands. The dissociation energies De for the C and D states are, respectively, determined to be 244 and 112 cm−1 for XeAr and are 1445 and 54 cm−1 for XeKr.

Four-wave mixing in CO via the C 1?+ (??=0) state

Coherent vacuum ultraviolet (VUV) radiation was generated by four-wave difference frequency mixing (ωVUV=2ω1−ω2) of pulsed dye laser radiation in carbon monoxide (CO). The frequency ω1 was tuned to the C 1Σ+(υ′=0)←X 1Σ+(υ″=0) two-photon transition, while the dye laser frequency ω2 was scaned around 17650 cm−1 which corresponds to the A 1Π(υ=7)«C 1Σ+(υ′=0) transition energy. The VUV intensity was found to be strongly wavelength dependent. The analysis of the spectrum revealed (i) that the VUV intensity was enhanced by the rotational levels of the A 1Π(υ=7) state and (ii) that the off-resonance excitation in the C 1Σ+(υ′=0)←X 1Σ+(υ″=0) two-photon transition greatly contributed to the present four-wave mixing process. The effects of pumping laser detuning, saturation and foreign gases are briefly discussed.

VUV laser spectroscopy of Xe 2 in the C′ 1 u state

Xe 2 molecules in a supersonic free jet have been excited to the C′ 1 u state with tunable and monochromatic VUV laser radiation generated by four-wave difference frequency mixing in Kr. From the vibrationally resolved excitation spectrum, the following spectroscopy constants are derived for the C′ 1 u state: T′ e = 77240.6, ω′ e = 16.2, ω eχ′ e = 1.1, and D′ e = 140 cm −1. It was also found that the radiative lifetimes of the ν′ = 0 and 1 states of the C′ 1 u state were 4.3±0.5 and 3.8±0.6 ns, respectively.

Enhancement of VUV Generation in Resonant Four-Wave Mixing in Kr Diluted with Xe

Tunable vacuum ultraviolet radiation was generated by two-photon resonant four-wave difference frequency mixing in Kr. Improvement in conversion efficiency was obtained by a diluted gas mixture with Xe whose dispersion was negative.

Coherent vacuum ultraviolet generation by frequency mixing in glass hollow waveguides

Efficiency for vuv generation in a glass hollow waveguide has been compared with that obtained by tight focussing of the primary radiation. Coherent vuv radiation is generated by resonant four-wave difference frequency mixing and four-wave parametric processes in xenon. At vuv wavelengths close to the 5p-6s resonance, the efficiency of generation was found to be enhanced by a factor of up to 2.5 under phase-matched conditions. The measured enhancement is limited by losses due to absorption of the generated radiation and by propagation losses of the primary beams along the waveguide.

Tunable and coherent radiation in the VUV: high efficiency four-wave difference-frequency mixing in xenon

We report a novel source of tunable, coherent radiation in the vacuum ultraviolet, which use resonantly enhanced difference-frequency mixing in xenon gas Efficiencies are high (o.4%) at modest input laser intensities. With existing dye lasers the range of tunability will span the 152–200 nm region.