Roger J. Carlson

Molecular, multiresonant coherent four-wave mixing spectroscopy

Abstract Recent research has expanded the capabilities of four-wave mixing by providing it with component selectivity, site selectivity, and mode selectivity. The selectivity is achieved by taking advantage of the three resonance enhancements that occur in a four-wave mixing process. New spectral scanning strategies allow one to scan a single resonance while maintaining the other two resonances at constant values. The constant resonances can be used to select a specific component, a specific site within an inhomogeneously broadened envelope of a component, and/or a specific vibrational or vibronic mode of that site. The scanned resonance will then contain enhanced features corresponding to the particular component, site, and/or mode that was chosen by the constant resonances. These component and site selective capabilities of the four-wave mixing complement the single vibronic level fluorescence methods. The relative transition intensities from a specific component or site reflect the mode coupling betwee...

Analysis of vibrational correlations and couplings in the lowest two singlet states of pentacene by high resolution, fully resonant, coherent four-wave mixing spectroscopy

Coherent, fully resonant four‐wave mixing (FRFWM) spectroscopy is used to probe the vibrational and vibronic state of the pentacene S0 and S1 electronic states. High resolution is obtained using mixed molecular crystals at cryogenic temperatures. The FRFWM spectra combined with conventional absorption and fluorescence spectra allow correlation of the S1 fundamentals to the corresponding ground state normal coordinates. Couplings between modes are indicated and may be related to Fermi resonance of the pentacene vibrations by means of host librational phonons.

Absorption and coherent interference effects in multiply resonant four-wave mixing spectroscopy

The effects of coherent interference between resonant and nonresonant signals and the effects of absorption on multiresonant four-wave mixing spectra are investigated both theoretically and experimentally in azu-lene-doped naphthalene crystals at 2 K. Both effects can strongly alter line shapes and intensities. Line splittings and negative spectral features are demonstrated. Coherent interference, although measurable in this system, is weak. Absorption, however, plays an important role, and its effects on phase matching, peak shapes, and peak intensities are predicted theoretically and are confirmed by the experimental measurements. A new expression for the four-wave mixing is derived in terms of sample absorption coefficients and absorption cross sections, and general conditions for maximum efficiency are determined.

Perturbative description of extra resonances in CARS

Abstract Resonance coherent anti-Stokes Raman scattering (CARS) spectra contain peaks that have been interpreted as originating from populated excited states induced by the applied field. Fifth-order perturbation theory is used here to show that the induced dipole contribution to these extra resonances exactly cancels that from induced excited-state populations in the absence of pure dephasing. The theory is also used to describe CARS line narrowing in inhomogeneously broadened systems and the experimentally observed extra resonances are interpreted as line narrowed vibronic transitions.

Line narrowing in multiresonant third order molecular spectroscopies

Abstract A complete analysis of line narrowing in third order, steady-state spectroscopies is given in terms of a recently developed perturbative theory for the nonlinear susceptibility. This approach rigorously includes realistic population dynamics and feeding without significantly complicating the analytic solutions. Unrelaxed resonant emission (i.e., resonant fluorescence and Raman emission) and coherent anti-Stokes Raman scattering (CARS) are treated as examples. Inhomogeneous broadening is included by an analytic contour integration over an inhomogeneous strain distribution. Unlike most existing approaches to line narrowing, both coherent and incoherent spectroscopies are treated by this same formalism, allowing the capabilities of each to be compared. A novel pictorial representation wich facilitates rationalization of line narrowing and its absence in certain coherent spectroscopies is introduced. Feeding effects are shown to alter line-narrowing behavior in third order for incoherent resonant emission but only in fifth order for CARS.

Effects of saturation on the elimination of inhomogeneous broadening by nondegenerate fully resonant four-wave mixing

Abstract Fifth order perturbation theory for nonlinear four-wave is used to understand line narrowing in fully resonant, nondegenerate, parametric methods of nonlinear spectroscopy. Cancellation effects between alternative coherence pathways cause some resonances to disappear unexpectedly. Inhomogeneous broadening causes the return of some of the resonances as line narrowed peaks.

Comparisons between 2D doubly vibrationally enhanced four wave mixing and site selective spectroscopy

Abstract We have constructed a nonlinear spectroscopic system for performing multiresonant four-wave mixing with infrared lasers. The system consists of three coherent sources, two of which are tunable in the infrared region of the spectrum. The sources are tuned to different vibrational resonances and the four-wave mixing output is monitored as a function of the two infrared frequencies. When the frequencies match direct infrared absorption or Raman transitions, the four-wave mixing output is enhanced. A two-dimensional display of the data shows the output intensity as a function of the two infrared frequencies. We observe that cross-peaks appear in the 2D spectra when multiple resonances are excited. We have named the method “doubly vibrationally enhanced four-wave mixing (DOVE-FWM)”. This method represents the long sought optical analogue to 2D nmr. It should provide a method that is complementary to nmr because of the difference in the time scales of the dephasing processes. Spin-lattice interactions fix the dephasing times for NMR measurements at millisecond time scales so nmr senses the ensemble average of a materials structure. Vibrational dephasing times occur on the picosecond time scale so the DOVE–FWM measurement represents a more instantaneous measurement of material structure.