Pamela Bowlan

Crossed-beam spectral interferometry: a simple, high-spectral-resolution method for completely characterizing complex ultrashort pulses in real time

We present a high-spectral-resolution and experimentally simple version of spectral interferometry using optical fibers and crossed beams, which we call SEA TADPOLE. Rather than using collinear unknown and reference pulses separated in time to yield spectral fringes-and reduced spectral resolution-as in current versions, we use time-coincident pulses crossed at a small angle to generate spatial fringes. This allows the extraction of the spectral phase with the full spectrometer resolution, which allows the measurement of much longer and more complex pulses. In fact, SEA TADPOLE achieves spectral super-resolution, yielding the pulse spectrum with even better resolution. Avoiding collinear beams and using fiber coupling also vastly simplify alignment. We demonstrate SEA TADPOLE by measuring a chirped pulse, a double pulse separated by 14 ps, and a complex pulse comprising two trains of pulses with a time-bandwidth product of ~400.

Directly measuring the spatio-temporal electric field of focusing ultrashort pulses

We present the first technique for directly measuring (without assumptions) the spatio-temporal intensity and phase of a train of ultrashort pulses at and near a focus. Our method uses an experimentally simple and high-spectral resolution variant of spectral interferometry (SEA TADPOLE). To illustrate our technique, we measured the spatio-temporal electric field in and around the foci of several different types of lenses. To confirm our results, we also simulated these measurements by numerically propagating a pulse through each of the lenses used. From one set of measurements, we made a movie showing a focusing pulse with severe chromatic aberration.

Spatio-temporal couplings in ultrashort laser pulses

The electric field of an ultrashort laser pulse often fails to separate into a product of purely temporal and purely spatial factors. These so-called spatio-temporal couplings constitute a broad range of physical effects, which often become important in applications. In this review, we compile some recent experimental and theoretical work on the understanding, avoidance and applications of these effects. We first present a discussion of the characteristics of pulses containing spatio-temporal couplings, including their sources, a mathematical description and the interdependence of different couplings. We then review different experimental methods for their characterization. Finally, we describe different applications of spatio-temporal couplings and suggest further schemes for their exploitation and avoidance.

Measuring the spatiotemporal field of ultrashort Bessel-X pulses

Using SEA TADPOLE with μm-range spatial and fs-range temporal resolution, we report the first direct spatiotemporal measurements of ultrashort Bessel-X pulses. We demonstrate their propagation invariance and superluminal velocity and verify our results with simulations.

Measuring the spatiotemporal electric field of ultrashort pulses with high spatial and spectral resolution

We demonstrate an experimentally simple and high-spectral-resolution version of spectral interferometry (SEA TADPOLE) that can measure complicated pulses (in time) at video rates. Additionally, SEA TADPOLE can measure spatial information about a pulse, and it is the first technique that can directly measure the spatiotemporal electric field [E(x,y,z,λ)] of a focusing ultrashort pulse. To illustrate and test SEA TADPOLE, we measured E(λ) of a shaped pulse that had a time-bandwidth product of approximately 100. To demonstrate that SEA TADPOLE can measure focusing pulses, we measured E(x,λ) at and around the focus produced by a plano-convex lens. We also measured the focus of a beam that had angular dispersion present before the lens. We have found that SEA TADPOLE can achieve better spectral resolution than an equivalent spectrometer, and here we discuss this in detail, giving both experimental and simulated examples. We also discuss the angular acceptance and spatial resolution of SEA TADPOLE when measuring the spatiotemporal field of a focusing pulse.

Ultrafast two-dimensional terahertz spectroscopy of elementary excitations in solids

Recent experimental progress has allowed for the implementation of nonlinear two-dimensional (2D) terahertz (THz) spectroscopy in the ultrafast time domain. We discuss the principles of this technique based on multiple phase-locked electric field transients interacting in a collinear geometry with a solid and the phase-resolved detection of the THz fields after interaction with the sample. To illustrate the potential of this new method, 2D correlation spectra of coupled intersubband-longitudinal optical phonon excitations in a double quantum well system and a study of ultrafast carrier dynamics in graphene are presented.

Direct spatiotemporal measurements of accelerating ultrashort Bessel-type light bullets

We measure the spatiotemporal field of ultrashort pulses with complex spatiotemporal profiles using the linear-optical, interferometric pulse-measurement technique SEA TADPOLE. Accelerating and decelerating ultrashort, localized, nonspreading Bessel-X wavepackets were generated from a approximately 27 fs duration Ti:Sapphire oscillator pulse using a combination of an axicon and a convex or concave lens. The wavefields are measured with approximately 5 microm spatial and approximately 15 fs temporal resolutions. Our experimental results are in good agreement with theoretical calculations and numerical simulations.

Single-diffraction-grating and grism pulse compressors

We introduce and demonstrate a simple, compact, and automatically aligned ultrashort-pulse compressor that uses only a single diffraction element—a grating or a grism (a grating on a prism). This design automatically has unity beam magnification and automatically contributes zero spatiotemporal distortions to the pulse, thus avoiding spatial chirp, angular dispersion, pulse-front tilt, and all other first-order spatiotemporal distortions. It is comprised of only three elements: a diffraction element, a corner cube, and a roof mirror. Half the size of comparable two-grating compressors, it can provide large amounts of negative group-delay dispersion with small translations of the corner cube. The device can operate on pulses with both large and small bandwidths by varying the corner-cube position. Using a grism as the diffraction element, material dispersion up to the third order can be compensated, and we demonstrated compensation for 10 m of optical fiber for 800 nm pulses.

Complete single-shot measurement of arbitrary nanosecond laser pulses in time.

For essentially all applications, laser pulses must avoid variations in their intensity and phase within a pulse and from pulse to pulse. Currently available devices work very well for both long (>10ns) and short (<100ps) pulses. But intermediate (~ns) pulses remain difficult to measure and, not surprisingly, are the least stable. Here we describe a simple, elegant, complete, all-optical, single-shot device that measures ~ns pulses and that does not require a reference pulse or assumptions about the pulse shape. It simultaneously achieves a very high spectral resolution of <1pm and a very large delay range of ~10ns (several meters of light travel). It accomplishes both goals using high-efficiency, high-finesse etalons: one to generate high angular dispersion for a high-resolution spectrometer, and another to tilt the pulse front by ~89.9° without distorting it in time. Using this device, we completely measure microchip and fiber-amplifier pulses.

Terahertz radiative coupling and damping in multilayer graphene

The nonlinear interaction between intense terahertz (THz) pulses and epitaxial multilayer graphene is studied by field-resolved THz pump?probe spectroscopy. THz excitation results in a transient induced absorption with decay times of a few picoseconds, much faster than carrier recombination in single graphene layers. The decay times increase with decreasing temperature and increasing amplitude of the excitation. This behaviour originates from the predominant coupling of electrons to the electromagnetic field via the very strong interband dipole moment while scattering processes with phonons and impurities play a minor role. The nonlinear response at field amplitudes above 1?kV?cm?1 is in the carrier-wave Rabi flopping regime with a pronounced coupling of the graphene layers via the radiation field. Theoretical calculations account for the experimental results.