The Shack-Hartmann Electron Densitometer (SHED) is a novel method to diagnose ultrashort pulse laser–produced plasmas by measuring the phase change to a probe laser beam. Using the Shack-Hartmann method, the phasefront of a probe laser is measured through a lenslet array onto a camera. Small changes in the location of the individual focal points on the camera plane are translated into changes in the probe beam’s wavefront. These wavefront distortions arise from refractive index variations caused by the free electrons. This method allows for superior performance in measuring minute variations in the electron density in 2 dimensions, single-shot, with sub-picosecond time resolution. The data taken with SHED demonstrated the ability to diagnose plasmas with densities of approximately 5x10<sup>17</sup> cm<sup>-3</sup> and show the temporal evolution of the plasma long after the driving laser pulse has left. The method can be further improved by enclosing the probe beam and adding a second axis to allow for tomographic reconstruction of the electron density in 3 dimensions. By using SHED in a pump/probe setup, we are able to obtain temporally synchronized, ultrashort snap shots of the evolution of free electrons in laser-induced ionization of air and other transparent media.
We demonstrate the performance of an optical differentiation wavefront sensor (ODWS) relying on an optical system that images the pupil to a camera. A binary pixelated transmission filter with a linear amplitude-transmission gradient is located in a far field of the pupil. The ODWS uses the fluence data measured in the detection plane for two orthogonal orientations of the filter to determine wavefront-slope data along the two corresponding directions in the pupil plane. This technique allows for acquisition in real-time without moving parts, providing high resolution, high dynamic range, and achromatic wavefront sensing for astronomical imaging or metrology applications.
We describe the design and fabrication of an integrated optical disk resonator and demonstrate its ability to perform as an environmental sensor. The device consists of a 500 micron radius disk side-coupled to a straight bus waveguide, fabricated in silicon oxy-nitride (SiON). The guiding layer has a refractive index of 1.8 and is 350 nm thick. Since the devices require few processing steps and can be fabricated using the well-established techniques of plasma enhanced chemical vapor deposition and optical lithography, they are reasonably easy to produce. By monitoring the transmission of 1550 nm light through the resonators, we can measure changes in the refractive index at the surface. We determine a sensitivity of 1.0 x 10-5 to changes in the surface index with experiments using sucrose solutions of varying concentrations.
Practical applications of slow light methods require that one be able to controllably delay a pulse of light by many pulse lengths. In this contribution we analyze the possible limitations to the maximum achievable time delay and suggest methods for overcoming these limits.
Microring resonators can serve as key elements in the realization
of engineerable photonic media. A sequence of resonators coupled
to an optical waveguide can be viewed as an optical transmission
line with highly controllable dispersive and nonlinear properties,
similar to those of photonic crystals or gratings. We have
constructed and characterized several optical micro-ring
resonators with scale sizes of the order of 10 microns. These
devices are intended to serve as building blocks for engineerable
linear and nonlinear photonic media. Light is guided vertically by
an epitaxially grown structure and transversely by deeply etched
air-clad sidewalls. In this work, we chose to construct ring
resonators in AlGaAs and probe them at a photon energy below the
half-gap of the material. Our motivation for this choice was to
maximize the ultrafast bound (Kerr) nonlinearities resulting from
virtual transitions while minimizing the two-photon contribution
to carrier generation. We report on the spectral phase transfer
characteristics of such resonators. We also report the observation
of a pi-radian Kerr nonlinear phase shift accumulated in a single
compact ring resonator evidenced by all-optical switching between
output ports of a resonator-enhanced Mach-Zehnder interferometer.