Three-photon Excitation Spectrum Research Articles

Measurement of three-photon excitation cross-sections of fluorescein from 1154 nm to 1500 nm.

Measurements of three-photon action cross-sections for fluorescein (dissolved in water, pH ∼11.5) are presented in the excitation wavelength range from 1154 to 1500 nm in ∼50 nm steps. The excitation source is a femtosecond wavelength tunable non-collinear optical parametric amplifier, which has been spectrally filtered with 50 nm full width at half maximum band pass filters. Cube-law power dependance is confirmed at the measurement wavelengths. The three-photon excitation spectrum is found to differ from both the one- and two-photon excitation spectra. The three-photon action cross-section at 1154 nm is more than an order of magnitude larger than those at 1450 and 1500 nm (approximately three times the wavelength of the one-photon excitation peak), which possibly indicates the presence of resonance enhancement.

Three-photon laser excitation of mesoscopic ensembles of cold rubidium Rydberg atoms

The spectra of three-photon laser excitation (according to the 5S1/2 → 5P3/2 → 6S1/2 → 37P3/2 scheme) of mesoscopic ensembles of cold Rb Rydberg atoms in a magneto-optical trap, using cw single-frequency lasers in each stage, have been investigated. The ensembles consisted of 1 to 5 atoms, and their presence was recorded by the selective field ionisation method with postselection according to the number of atoms. The dependence of the spectral shape on the excitation laser pulse duration has been studied. For 2-μs pulses, the minimum spectral width was found to be 1.3 MHz; it was determined by the laser linewidth and the spectral width of laser pulse. For pulses shorter than 0.5 μs, additional spectral broadening and occurrence of Rabi oscillations in the wings of three-photon resonances have been observed, a fact indicative of implementation of coherent three-photon laser excitation of Rydberg atoms. An analysis of the spectra within the four-level theoretical model based on optical Bloch equations showed good agreement between the experimental and theoretical results. The dependence of the shape of three-photon excitation spectrum on the number of recorded atoms was also studied. An increase in the average number of atoms leads to the occurrence of a dip in the single-atom excitation spectra; this dip is due to the specificity of the excitation and detection statistics of Rydberg atoms. The results obtained are important for applications of Rydberg atoms in quantum informatics.

Resonantly enhanced three-photon excitation spectra of lithium

We present the first measurement on the resonantly enhanced three-photon excitation spectra of natural lithium using a Nd:YAG laser pumped dye laser in conjunction with a thermionic diode ion detector. Exploiting the linear and circular polarizations, the np2P3/2(8⩽n⩽11) and nf 2F7/2 (8⩽n⩽38) series have been observed via three-photon excitation from the ground state. The measured level energies reveal a dynamic shift from calculated values, which increases with an increase of the principal quantum number n. The ac stark shift and line broadening mechanisms are studied as a function of laser intensity. It is noted that the width increases and the line center shifts towards the higher energy side as the laser intensity is increased. The maximum observed shift for the 12f2F7/2 line is 0.33cm−1 corresponding to the laser intensity variation from 1.34×1012W/m2 to 1.03×1013W/m2, whereas its width increases from 0.36cm−1 to 0.82cm−1.

Three-photon excitation of strontium Rydberg levels

We report here the first measurements of the three-photon excitation spectrum of strontium using a Nd:YAG pumped dye laser in conjunction with an atomic beam apparatus as well as with a thermionic diode detector. The members of the 5s np 1 P 1 and 5s nf 1 F 3 Rydberg series converging to the first ionization threshold are observed. A dominating two-photon resonant three-photon ionization through the 5s6s 1 S 0 level of strontium lying adjacent to the three photon ionization threshold restricted the observation of the higher Rydberg members.

Multiphoton fluorescence excitation: new spectral windows for biological nonlinear microscopy.

Intrinsic, three-dimensionally resolved, microscopic imaging of dynamical structures and biochemical processes in living preparations has been realized by nonlinear laser scanning fluorescence microscopy. The search for useful two-photon and three-photon excitation spectra, motivated by the emergence of nonlinear microscopy as a powerful biophysical instrument, has now discovered a virtual artist's palette of chemical indicators, fluorescent markers, and native biological fluorophores, including NADH, flavins, and green fluorescent proteins, that are applicable to living biological preparations. More than 25 two-photon excitation spectra of ultraviolet and visible absorbing molecules reveal useful cross sections, some conveniently blue-shifted, for near-infrared absorption. Measurements of three-photon fluorophore excitation spectra now define alternative windows at relatively benign wavelengths to excite deeper ultraviolet fluorophores. The inherent optical sectioning capability of nonlinear excitation provides three-dimensional resolution for imaging and avoids out-of-focus background and photodamage. Here, the measured nonlinear excitation spectra and their photophysical characteristics that empower nonlinear laser microscopy for biological imaging are described.

Exciton energy and its pressure dependence in alkali halides.

Three-photon excitation spectra of the lowest exciton of NaCl and KBr have been measured up to 6 kbar at low temperature. The experimental results on NaCl near 8.45 eV represent the deepest uv state studied to date under hydrostatic pressure. The exciton energy in both materials shows a linear blue shift with increasing pressure. In order to better understand the experimental results, the exciton energies were computed using the potential induced breathing model on the basis of the charge-transfer exciton model. We find evidence supporting the charge-transfer exciton model and show that the crystal-field stabilization of the anions and overlap energies which were neglected in previous models are quantitatively significant.

The rotational structure of three-photon resonances of polyatomic molecules

Abstract A general formulation of the selection rules and line strengths of three-photon excitation spectra is presented for molecules of arbitrary symmetry. For symmetric-top molecules the paper extends the discussion by Nieman in the context of rovibronic transitions of NH3. For asymmetric-top molecules it is shown that the rotational line strengths can be sensitive to quantum-mechanical interference between contributions from the various components of the three-photon vibronic transition tensor. This transition tensor is discussed within a perturbation-theory framework in terms of several models of intermediate-state participation.

Multiphoton excitation stimulated hyper-Raman

Three- and four-photon excitation resonant hyper-Raman spectra have been studied using master equation formalism. It is found that the three-photon excitation spectra consist of a quartet of sidebands, whereas four-photon excitation hyper-Raman has five lines: a central line accompanied by two pairs of sidebands symmetrically placed on both sides of it. An important generalization of multiphoton resonant hyper-Raman spectra with even or odd number of photons is also discussed.

Resonance enhanced three-photon absorption of molecular iodine

D ← X resonance enhanced three-photon excitation spectrum of iodine was observed by a cw intracavity absorption technique. Vibrational quantum numbers of D ← X transitions are given for every major spectral feature. The corresponding one-photon enhancement from the B state is evident from the one-photon vibrational assignment. The spectroscopic constants for the D state are v 00 = 40 998 cm −1 ω′ 0 = 113 cm −1 and ω′ 0χ′ 0 = 0.045 cm −.