Resonant Four-wave Mixing Research Articles

Enhancement of phase-conjugate reflectivity using Zeeman coherence in highly degenerate molecular systems

A comprehensive theoretical analysis is developed for the vectorial phase conjugation using resonant four-wave mixing (FWM) in a highly degenerate rotational vibrational molecular system. The dynamic Stark shifts, saturation, and Doppler broadening are included for a realistic analysis. It is shown that the electromagnetically induced multilevel coherence controls the nonlinear wave mixing yielding interesting results for the phase conjugate (PC) reflectivity. It turns out that the efficiency of the PC reflectivity is decided by the relative phase of the Zeeman coherence and the population grating. When these two contributions are aligned in phase by a small detuning of the pump frequency, a large PC reflectivity ($~$20%) is obtained with moderate pump intensity ($~$500 mW/cm${}^{2}$).

Vibrational specificity of proton-transfer dynamics in ground-state tropolone

The vibrational dependence of large-amplitude proton transfer taking place in the ground electronic state (X1A1) of tropolone has been explored by implementing a coherent variant of the stimulated emission pumping (SEP) technique within the framework of two-color resonant four-wave mixing (TC-RFWM) spectroscopy. The lowest 1700 cm(-1) portion of this potential surface has been interrogated under ambient bulk-gas conditions, enabling rotationless term energies (Tv+) and tunneling-induced bifurcations Delta(v)X to be extracted for 43 assigned vibrational features of a1 and b2 symmetry. The resulting values of Delta(v)X reflect the state-specificity long attributed to the hydron-migration pathways of tropolone and range in magnitude from 0.0 cm(-1) to 17.8 cm(-1), where the former implies essentially complete quenching of unimolecular dynamics whilst the latter represents nearly a twenty-fold increase in reaction rate over that of the zero-point level. This vibrational mediation of tunneling behavior is discussed in terms of attendant atomic displacements and permutation-inversion symmetries, with choreographed motion of the five-member reaction site (C-O-H...O=C) found to exert the most significant influence on the efficacy of proton transfer.

Rotationally Resolved Ground State Vibrational Levels of HC2S Studied by Two-Color Resonant Four-Wave Mixing

A high-resolution study of the X(2)Pi(3/2) ground state rovibronic energy levels of jet-cooled HC(2)S radical using the double-resonance two-color four-wave mixing technique (TC-RFWM) is reported. The rotational structure of the bands is observed by excitation of selected R-branch lines in the origin band of the A(2)Pi(3/2)-X(2)Pi(3/2) electronic system. The second laser frequency is scanned to transfer the population from the rotational level(s) of the upper state to selected vibronic levels of the ground state. Fourteen rotationally resolved vibrational bands have been recorded for energies up to 1800 cm(-1) above the v'' = 0 X(2)Pi(3/2) electronic ground state. Effective rotational constants and origins are determined for levels that involve fundamental and overtone combinations of the nu(3) (CS stretch), nu(4) (CCH bend), and nu(5) (CCS bend) vibrations. This illustrates the power and advantages of the TC-RFWM approach for the study of the ground state manifold of reactive intermediates produced in low concentrations with high resolution, good signal-to-noise and wide dynamic range.

Dissection of Rovibronic Structure by Polarization-Resolved Two-Color Resonant Four-Wave Mixing Spectroscopy

A synergistic theoretical and experimental investigation of stimulated emission pumping (SEP) as implemented in the coherent framework of two-color resonant four-wave mixing (TC-RFWM) spectroscopy is presented, with special emphasis directed toward the identification of polarization geometries that can distinguish spectral features according to their attendant changes in rotational quantum numbers. A vector-recoupling formalism built upon a perturbative treatment of matter-field interactions and a state-multipole expansion of the density operator allowed the weak-field signal intensity to be cast in terms of a TC-RFWM response tensor, RQ(K)(epsilon4*epsilon3epsilon2*epsilon1;Jg,Je,Jh,Jf), which separates the transverse characteristics of the incident and generated electromagnetic waves (epsilon4*epsilon3epsilon2*epsilon1) from the angular momentum properties of the PUMP and DUMP resonances (Jg,Je,Jh,Jf). For an isolated SEP process induced in an isotropic medium, the criteria needed to discriminate against subsets of rovibronic structure were encoded in the roots of a single tensor element, R0(0)(epsilon4*epsilon3epsilon2*epsilon1;Jg,Je,Jh,Je). By assuming all optical fields to be polarized linearly and invoking the limit of high quantum numbers, specific angles of polarization for the detected signal field were found to suppress DUMP resonances selectively according to the nature of their rotational branch and the rotational branch of the meshing PUMP line. These predictions were corroborated by performing SEP measurements on the ground electronic potential energy surface of tropolone in two distinct regimes of vibrational excitation, with the near-ultraviolet 1B2-1A1 (pi*<--pi) absorption system affording the requisite PUMP and DUMP transitions.

Selective Detection of Radicals and Ions in a Slit-Jet Discharge by Degenerate and Two-Color Four-Wave Mixing

Degenerate four-wave mixing (DFWM) was used to record the spectra of charged and neutral carbon-containing radicals generated in a pulsed discharge source within a supersonic slit-jet expansion. Detection limits of approximately 10(9) molecules cm(-3) are achieved. The DFWM method allows a selective molecular detection by varying the discharge timings. Increased spectral selectivity is obtained by applying the two-color, doubly resonant four-wave mixing variant. This shows the potential of the techniques for sensitive and selective spectral analysis of radicals in discharges. The methods are successfully used for the detection of C(4)H, HC(2)S, and HC(4)H(+) with signal-to-noise in the range of 10(2)-10(4).

Resonant four-wave mixing of gold nanoparticles for three-dimensional cell microscopy

By detecting the transient four-wave mixing from gold nanoparticles in resonance with their surface plasmon, we demonstrate a multiphoton imaging modality suited for cell microscopy. Four-wave mixing is measured free from background using a three-beam excitation geometry and heterodyne detection. We achieve a spatial resolution of 140 nm in-plane and 470 nm in the axial direction, surpassing the one-photon diffraction limit. With this technique, high-contrast photostable imaging of Golgi structures is demonstrated in HepG2 cells labeled with gold nanoparticles of 10 nm and 5 nm diameter.

Mid-infrared efficient generation by resonant four-wave mixing in a three-coupled-quantum-well nanostructure

We present a cascade configuration for the realization of highly efficient four-wave mixing (FWM) process in an asymmetric semiconductor three-coupled-quantum-well (TCQW) structure based on intersubband transitions (ISBTs). In the proposed TCQW scheme, the efficiency of the generated FWM mid-infrared (MIR) signal is significantly enhanced and the obtained maximum efficiency is greater than 50%. The corresponding explicit analytical expressions for the input probe and generated FWM pulsed fields are derived by use of the coupled Schrödinger–Maxwell approach and the FWM efficiency versus several variables is also discussed in details. Such a semiconductor system is much more practical than its atomic counterpart because of its flexible design and the wide adjustable parameters. This nonlinear optical process in the TCQW solid-state material can be used for efficiently generating coherent short-wavelength radiation.

Slow light propagation and amplification via electromagnetically induced transparency and four-wave mixing in an optically dense atomic vapor

We experimentally and theoretically analyze the propagation of weak-signal field pulses under the conditions of electromagnetically induced transparency (EIT) in hot Rb vapor, and study the effects of resonant four-wave mixing (FWM). In particular, we demonstrate that in a double-Λ system, formed by the strong control field with the weak resonant signal and a far-detuned Stokes field, both continuous-wave spectra and pulse propagation dynamics for the signal field depend strongly on the amplitude of the seeded Stokes field, and the effect is enhanced in optically dense atomic medium. We also show that the theory describing the coupled propagation of the signal and Stokes fields is in good agreement with the experimental observations.

Resonant nonlinear optics of backward waves in negative-index metamaterials

The extraordinary properties of resonant four-wave mixing of backward and ordinary electromagnetic waves in doped negative-index materials are investigated. The feasibility of independent engineering of the negative refractive index and the nonlinear optical response as well as quantum control of the nonlinear propagation process in such composites is shown due to the coherent energy transfer from a control field to a signal field. Laser-induced transparency, quantum switching, frequency-tunable narrow-band filtering, amplification, and realizing a miniature mirrorless optical parametric generator of the entangled backward and ordinary waves are among the possible applications of the investigated processes.

Probe of I( 2P j) atoms using two-photon resonant four-wave mixing spectroscopy following the 266-nm photodissociations of various alkyl and perfluoroalkyl iodides

Two-photon resonant four-wave mixing spectroscopy has been utilized successfully for probing I and I ∗ nascent from the 266 nm photodissociations of various alkyl and perfluoroalkyl iodides. The relative quantum yields for I ∗, as well as the recoil anisotropy parameters, were extracted. Recoil anisotropy parameters close to the limit value for parallel transition indicates that the ground-state I( 2P 3/2) originates from the curve crossing from the initially prepared 3Q 0 to 1Q 1 state. The curve-crossing probabilities for alkyl and perfluoroalkyl iodides were obtained and tried to explain using the well-known Landau–Zener model. In the cases of perfluoroalkyl iodides, it is necessary to consider the fluorination effect to describe the detailed dynamics observed in this study and has been revealed that the electronic effect due to fluorine atom substitution is dominant, leading to decrease in the curve-crossing probability.

Collision induced and resonantly enhanced extra resonances in four-wave mixing in inhomogeneously broadened four level systems

Role of collisions in inducing and enhancing extra resonances in non-linear generation by four-wave mixing in inhomogeneously broadened four level systems is studied. It is shown that collision induced three and two-photon resonances can be enhanced further by tuning to exact one photon (non-absorbing) transitions between unpopulated upper levels. In addition we also find existence of collision induced interference dip at the three-photon resonance that arises due to complete cancellation of signal at the line center.

Electronic spectra of radicals in a supersonic slit-jet discharge by degenerate and two-color four-wave mixing

Four-wave mixing techniques have been used for the measurement of electronic transitions of cold transient species generated in a supersonic slit-jet discharge expansion. The origin band of the d(3)Pi(g)-a(3)Pi(u) system of C(2) and A(2)Pi(3/2)-X[combining tilde](2)Pi(3/2) electronic transition of HC(4)S were recorded. A signal-to-noise ratio of 10(4) in the spectra was achieved, resulting in detection limits of 10(10) cm(-3) for these two molecules. Application of selective two-color resonant four-wave mixing is used for the spectral assignment utilizing the double-resonance nature of the method. The combination of these techniques with a slit source proves to be a sensitive approach for the detection of transient molecules in a molecular beam discharge.

Enhanced generation of vacuum-ultraviolet radiation by four-wave mixing in mercury using pulsed laser vaporization

The efficiency of a coherent vacuum ultraviolet (VUV) source at 125 nm, based on two-photon resonant four-wave mixing in mercury vapor, has been enhanced by up to two orders of magnitude. This enhancement was obtained by locally heating a liquid mercury surface with a pulsed excimer laser, resulting in a high-density vapor plume in which the nonlinear interaction occurred. Energies up to 5 μJ (1 kW peak power) have been achieved while keeping the overall mercury cell at room temperature, avoiding the use of a complex heat pipe. We have observed a strong saturation of the VUV yield when peak power densities of the fundamental beams exceeded the GW/cm2 range, as well as a large intensity-dependent broadening (up to ∼ 30 cm-1) of the two-photon resonance. The source has potential applications for high-resolution interference lithography and photochemistry.

SUR/FIN 2007: A RousingSuccess!

The calculation of laser-induced radiative transition rates for electric-dipole transitions is reviewed. The effects of laser polarization can only be understood by considering the Zeeman structure of energy levels. In particular, the case of laser coupling of degenerate levels is considered by including the Zeeman structure of the upper and lower energy levels. The interaction of the laser radiation with the atomic or molecular resonance is formulated in terms of the density matrix. The electric-dipole matrix elements are written as the product of a scalar quantity, the reduced density matrix element, and a vector quantity that is a function of the angular momentum quantum numbers J and magnetic quantum numbers M for the upper and lower Zeeman states. The calculation of absorption and stimulated emission rates, absorption coefficients, and susceptibilities for the limiting case of an isotropic medium is reviewed. Polarization effects in laser-induced fluorescence are reviewed, and the theoretical framework for the analysis of these effects is developed. The calculation of polarization spectroscopy signals and of polarization effects in resonant four-wave mixing are discussed. The effects of hyperfine structure in diatomic molecules are discussed.

Mode-specific tunneling dynamics in the ground electronic state of tropolone

The mode specificity of proton-transfer dynamics in the ground electronic state (X (1)A(1)) of tropolone has been explored at near-rotational resolution by implementing a fully coherent variant of stimulated emission pumping within the framework of two-color resonant four-wave mixing spectroscopy. Three low-lying (E(vib) approximately 550-750 cm(-1)) vibrational features, assigned to nu(30)(a(1)), nu(32)(b(2)), and nu(31)nu(38)(a(1)), have been interrogated under ambient, bulk-gas conditions, with term energies determined for the symmetric and antisymmetric (tunneling) components of each enabling the attendant tunneling-induced bifurcations of 1.070(9), 0.61(3), and 0.07(2) cm(-1) to be extracted. The dependence of tunneling rate (or hydron migration efficiency) on vibrational motion is discussed in terms of corresponding atomic displacements and permutation-inversion symmetries for the tropolone skeleton.

Atomic coherence in nondegenerate four-wave mixing

Two-photon resonant nondegenerate four-wave mixing (NFWM) with the addition of a coupling field in Ba atomic vapour has been studied. We find that coherence of the atomic level transitions leads to suppression of the NFWM signal, giving rise to a dip with a linewidth that is linearly proportional to the intensity of the coupling field.

Two-photon resonant four-wave mixing in a dressed atomic system: Polarization interference in a Doppler-broadened system

We study dressed-atom two-photon resonant nondegenerate four-wave mixing (NFWM) in a Doppler-broadened system. It is found that there exists interference between the macroscopic field polarizations from different ensembles of atoms within the atomic velocity distribution. This polarization interference can cause significant modification of the NFWM spectra. We also show that two-photon resonant NFWM in dressed atoms can be employed as a new type of Doppler-free high-resolution Autler-Townes spectroscopy for measuring transition dipole moments between two highly excited atomic states.

Fast optical switching using cyanine dye-doped nematic liquid crystal

A cyanine dye, 1,1′,3,3,3′,3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′-indotricarbocyanine perchlorate (NK-2014) was doped in 4-cyano-4′-n-pentylbiphenyl (5CB), and the mixture was sandwiched between two pieces of rubbed glass plates. The third-order nonlinear optical responses of oriented NK-2014-doped 5CB were measured by the resonant femtosecond degenerate four-wave mixing (DFWM) technique at 820 nm. The time resolution of the system was ca. 0.3 ps (FWHM). The electronic component of third-order nonlinear optical susceptibility of one of the present samples was 1.3×10 −8 esu (NK-2014 concentration: 1.0 wt%). The slow DFWM response of NK-2014-doped 5CB was accelerated with increasing laser power due to amplified spontaneous emission (ASE). On the other hand, we did not observe a similar phenomenon for NK-2014-doped polyethylene glycol (PEG-400). Oriented NK-2014 molecules in nematic liquid crystal must have very high ASE efficiency. Hence the population grating disappears in ca. 3 ps. NK-2014-doped 5CB can be used as a material for very fast all-optical switching.

Control of Polarization Interference in Doppler BroadenedTwo-Photon Resonant Four-Wave Mixing System

We study two-photon resonant nondegenerate four-wave mixing (NFWM) in aDoppler broadened system. It is found that because the inducedpolarization is sensitive to the atomic velocity, there existsinterference between polarizations of atoms with different velocities,leading to significant modification of the NFWM spectra. Thispolarization interference can be controlled through applying anadditional coupling field.

Observation of Rydberg series of neutral barium by two-photon resonent nondegenerate four-wave mixing

We have applied two-photon resonant nondegenerate four-wave mixing (NFWM) with a resonant intermediate state to measure the energy levels of the Ba even-parity J=0 and J=2 series. Differing from other experiment techniques, our method is a pure optical technique. Instead of detecting electrons or ions, the signal here is a coherent light. Two-photon resonant NFWM is a Doppler-free spectroscopy when the incident laser beams have narrow bandwidths.