Radiation from Wavepackets

When photon-counting detectors are calibrated in the presence of a background signal, deadtime effects can be significant and must be carefully accounted for to achieve high accuracy. We present a method for separating the correlated signal from the background signal that appropriately handles deadtime effects. This method includes consideration of pulse timing and afterpulsing issues that arise in typical avalanche photodiode (APD) detectors. We illustrate how these effects should be accounted for in the calibration process. We also discuss detector timing issues that should be considered in detector calibration.

We discuss the recoverable and irrecoverable energy densities associated with a pulse at a point in the propagation medium and derive easily computed expressions to calculate these quantities. Specific types of fields are required to retrieve the recoverable portion of the energy density from the point in the medium, and we discuss the properties that these fields must have. Several examples are given to illustrate these concepts.

We investigate the spatial evolution of a laser pulse used to generate high-order harmonies (orders ranging from 45 to 91) in a semi-infinite helium-filled gas cell. The 5 mJ, 30 fs laser pulses experience elongated focusing with two distinct waists when focused with f/125 optics in 80 Torr of helium. Extended phase matching for the generation of harmonics occurs in the region between the double foci of the laser, where the laser beam changes from diverging to converging. (c) 2006 Optical Society of America.