Simin Feng

Physical origin of the Gouy phase shift

We show explicitly that the well-known Gouy phase shift of any focused beam originates from transverse spatial confinement, which, through the uncertainty principle, introduces a spread in the transverse momenta and hence a shift in the expectation value of the axial propagation constant. A general expression is given for the Gouy phase shift in terms of expectation values of the squares of the transverse momenta. Our result also explains the phase shift in front of the Kirchhoff diffraction integral.

Spatiotemporal focusing of single-cycle light pulses

We investigate experimentally and numerically the properties of single-cycle terahertz pulses propagating through a focus. The experimental data clearly show changes in pulse shape resulting from the Gouy phase shift and apparent superluminal pulse propagation. The pulses are also considerably distorted by diffraction effects. A solution of the time-domain diffraction integral is necessary to explain the details of the data and leads to an excellent agreement between experiment and theory.

Spatiotemporal transformation of isodiffracting ultrashort pulses by nondispersive quadratic phase media

We show how nondispersive quadratic phase media (e.g., lenses, curved mirrors, and gradient-index rods) transform the spatial and temporal profiles of isodiffracting single-cycle pulses. Because all the frequency components of an isodiffracting pulsed beam have the same Rayleigh range, the entire pulse can be transformed directly in the time domain by use of ABCD matrices. The accumulated phase shift plays an important role in changing the spatial and temporal profiles. These pulses form natural spatiotemporal modes of a stable cavity resonator.

Cavity phase engineering for stable enhanced terahertz pulse trains.

We show that the cavity round-trip Gouy phase leads to pulse-to-pulse variation of the absolute phase and temporal profile of circulating few- or single-cycle pulses in empty resonators. This pulse-to-pulse variation can be eliminated by the proper insertion of a lens into the cavity. An application to terahertz resonators with phase-locked feedback is discussed.

Spatiotemporal transformation of single-cycle pulses by quadratic phase media

Summary form only given. Ultrawideband single-cycle pulses of subpicosecond duration are now readily available through optical rectification or photoconductive switching techniques. At optical frequencies femtosecond pulses as short as two and one-half cycles have also been generated through passive mode locking. For these ultrashort pulses, coupling between spatial and temporal variables results in significant temporal reshaping even when these pulses propagate through lossless non-dispersive media. In the paper we show that the spatiotemporal transformation of a class of single-cycle pulses by non-dispersive quadratic phase media can be analyzed in closed form by means of the well-known ABCD matrices of Gaussian beam optics. The class of pulses discussed are exact non-separable wavepacket solutions of the time-dependent paraxial wave equation. These wavepackets are isodiffracting in the sense that their Fourier spectrum is composed of a distribution of Gaussian beams all of which have the same Rayleigh range z/sub R/. Such an isodiffracting pulse can be produced, for example in a confocal resonator that forces all the modes to have the same frequency-independent confocal parameter 2z/sub R/. The spatiotemporal behavior of such a pulse can then be characterized in the time domain by a single q-parameter which contains the radius of curvature R, the radius a, and the pulse width /spl tau//sub 0/ of the pulsed beam.

Propagation effects at the focus of single-cycle terahertz pulses

Summary form only given. We investigate experimentally and numerically the temporal profiles of single-cycle pulses passing through a focus. Both experimental and numerical results show significant pulse shaping, time reversal, and superluminal propagation as a pulse evolves through the focus.

Spatiotemporal evolution of focused single-cycle electromagnetic pulses

tal seems to correspond to conical emission already observed in solid or gas. Our measurements show that the conical emission only takes place in the very first part of the filament path, which explains the well-defined shape of the conical rings observed outside the sample. The stable 3D propagation ofthe filament cannot be explained in the frame of the nonlinear Schrodinger equation and ionization plays a important role in the physics of the filament. Measurements, including 3D mapping of the filament, made on samples with different ionization and electronic recombination rates or time constants will be presented to give more insight and try to understand the filament propagation that potentially has very important applications.