W.P. de Boeij

Generation of 13-fs, 5-MW pulses from a cavity-dumped Ti : sapphire laser

We report on a cavity-dumped self-mode-locked Ti:sapphire laser operating on a 10-fs time scale. Pulses with energies exceeding 60 nJ and peak powers of 5 MW were generated at repetition rates as high as 200 kHz. On injection of the output of this laser into a fiber, a white-light continuum is produced that shows great potential for compression.

PHASE-LOCKED HETERODYNE-DETECTED STIMULATED PHOTON-ECHO - A UNIQUE TOOL TO STUDY SOLUTE-SOLVENT INTERACTIONS

Abstract Heterodyne-detected phase-locked femtosecond stimulated photon echo (HSPE) and pump—probe (PLPP) experiments were accomplished on a dye dissolved in ethylene glycol. It is shown that all four Liouville pathways are needed in a proper description of the observed phenomena. By measurement of the real and imaginary part of the PLPP signal the short-time (⩽ 60 fs) solvation dynamics can be probed, but none was found in ethylene glycol. Time-resolved and conventional stimulated photon echo experiments were also performed. The fastest solvation step in ethylene glycol seems to occur on a time scale of 300 fs.

Heterodyne-detected stimulated photon echo: applications to optical dynamics in solution

Heterodyne detection of the stimulated photon echo (HSPE) is discussed and applied to explore molecular solvation dynamics. With this technique the in-phase and in-quadrature parts of the induced transient nonlinear polarization can be time-gated. A third-order perturbative description of the HSPE is presented, which shows that the transient polarization provides direct information on the real and imaginary components of the nonlinear optical response functions of the system. In the spectral diffusion limit and for impulsive excitation conditions, analytical expressions for the HSPE signal are derived and model calculations are presented. This novel technique is applied to a nonlinear optical study of the dye molecule DTTCI dissolved in ethylene glycol. Several effects are analyzed in detail as, for instance, the intriguing interference effect between the conventional and virtual echo contributions to the total HSPE signal. Furthermore, it is shown that from HSPE measurements the instantaneous frequency of the emitted (nonlinear polarization induced) signal can be derived and that its time-dependent shift projects directly onto the dynamical Stokes shift and the energy reorganization process of the coupled solute–solvent complex. It will also be shown that the experimental data can be simulated using the multimode Brownian oscillator model.

Femtosecond Dynamics of Intracellular Water Probed with Nonlinear Optical Kerr Effect Microspectroscopy

A nonlinear optical Kerr effect (OKE) microscope was developed and used to elucidate the ultra-fast diffusive motions of intracellular water molecules. In the OKE microscope, a pump-induced birefringence is sensed by a delayed probe pulse within a spatially confined volume that measures 0.5 microm in the lateral direction and 4.0 microm along the axial coordinate. This microscope allows the recording of time-resolved Kerr signals, which reflect the ultra-fast structural relaxation of the liquid, exclusively from intracellular aqueous domains. Because relaxation occurs on a picosecond time scale, only local diffusive motions are probed. The microscopic OKE signal is therefore insensitive to long-time-scale hindered translational motions enforced by intracellular mechanical barriers but probes the intrinsic orientational mobility of water molecules in cells instead. The Kerr response as determined from single intact mammalian cells under physiological conditions shows a structural relaxation time of 1.35 ps, which is 1.7 times slower than the Kerr decay observed in pure water. The data indicate that the mobility of water molecules in cellular domains is moderately restricted due to the high intracellular content of proteins and solutes.

Nonlinear coherent four-wave-mixing in optical microscopy

An analysis of the imaging properties of nonlinear coherent four-wave mixing optical microscopes is presented. The generation and propagation of coherent signals are considered under conditions of high numerical aperture with a model that circumvents the need to use the slowly varying envelope approximation. Calculations of coherent anti-Stokes Raman scattering signals show that diffraction effects play a prominent role in the spatial distribution of the coherent signal intensity. It is emphasized that, unlike for fluorescence microscopy, the detected signal is not a straightforward convolution of a point-spread function and the object but is shaped by the complex interplay of object size and coherent buildup dynamics.

SPECTRAL INTERFEROMETRY AS AN ALTERNATIVE TO TIME-DOMAIN HETERODYNING

A novel method of time resolving the amplitude and phase of a transient nonlinear-optical polarization, based on Fourier transformation of a spectral interference pattern, is discussed. We show that with this technique the delay at which the polarization is generated can be retrieved. The method is demonstrated in a transient four-wave-mixing experiment on a dye solution.

Mode suppression in the non‐Markovian limit by time‐gated stimulated photon echo

It is demonstrated that enhanced mode suppression in stimulated photon echo experiments can be obtained by diagonal time gating of the echo. This technique is especially important when the optical dynamics of the system is non‐Markovian. A two‐mode Brownian oscillator model is used to analyze the effect of time gating on the stimulated photon echo. The method is demonstrated on a dye solution of DTTCI in ethylene glycol at room temperature. Experimentally, time gating of the echo is accomplished by means of femtosecond phase‐locked heterodyne detected stimulated photon echo. The vibrational dynamics in this system are explored by conventional stimulated photon echo experiments. Especially stimulated photon echo‐maximum shift measurements are found to be particularly useful.

Short-time solvation dynamics probed by phase-locked heterodyne detected pump-probe

Abstract Phase-locked heterodyne detected pump—probe experiments are reported on solutions of a dye molecule in ethylene glycol, methanol and acetonitrile. By performing experiments at different phase-lock wavelengths, the real and imaginary parts of the line broadening function g ( t ) could be mapped out. The imaginary part of g ( t ) is directly related to dissipative dynamics. Comparison of the experimental data with calculations based on the Brownian oscillator model yields solvation frequencies that are substantial higher than those reported using MD simulations on similar systems.

30-fs, cavity-dumped optical parametric oscillator

We demonstrate cavity dumping of a synchronously pumped optical parametric oscillator (OPO). The OPO delivers 30-fs pulses with energies as high as 13 nJ at up to 400-kHz repetition rates. The OPOs tunability ranges from 570 to 600 nm and is limited by the resonator mirror coatings. The time jitter between pulses of a Ti:sapphire laser and the cavity-dumped OPO pulses is smaller than 30 fs, permitting multicolor time-resolved spectroscopic experiments.

High repetition rate femtosecond lightsource for CARS microscopy

The use of femtosecond pulsed excitation in microscopy permits the application of nonlinear optical techniques to microscopic studies of biological samples. Among the these techniques, the method of coherent anti-Stokes Raman scattering (CARS) is particularly promising for biological imaging since it allows spectrally selective probing for specific biochemical species without the need for extrinsic markers. In order to perform spectrally resolved CARS microscopy, a multicolour tunable coherent lightsource is required. Due to the nonlinear nature of the signal generating process, the signal yield would benefit from high peak powers whereas at the sample a low dose (low average power) is wanted. Furthermore, high repetition rates would significantly enhance the image acquisition times. We have constructed a high repetition rate femtosecond multicolour lightsource based on a cavity-dumped visible optical parametric oscillator. The OPO is pumped by the second harmonic (400 nm) of a mode-locked femtosecond Ti:sapphire laser.