Advanced Diagnostic Development

The Plasma & Ultrafast Physics Group (PUPG) develops advanced diagnostic for laser-plasma experiments. These diagnostics are often the basics for new physics understanding and provide an opportunity for hands-on experimental learning. The group utilizes state-of-the-art computational design tools, including FRED, OSLO, SIMION, to help understand the subtleties of the diagnostic performance. Many of the diagnostic projects have undergraduate and graduate student participation. Below are a few of the PUPG advanced diagnostic projects that have been fielded at the University of Rochester's Laboratory for Laser Energetics.

Grating Efficiency Calculator

As a summer undergraduate project, Jeremy Hassett developed a grating efficiency calculator. The Plane Reflectance Diffraction Grating Efficiency Calculator is available for download. It is an application structured for use in optical system design and analysis. The application allows the user to calculate diffracted efficiency for optical diffraction gratings for a wide range of wavelengths, diffracted orders, and nearly arbitrary groove shapes. The efficiency data can then be saved and exported for use in system analysis. A number of default gratings and setups have been provided along with the application. The installation package also contains several helpful documents demonstrating how the application is used.

Optical Thomson Scattering

Thomson scattering is a valuable technique for measuring the plasma conditions in laser-produced plasmas [1]. Simultaneous measurements of the ion-acoustic and electron plasma wave spectrum provide the first-principle measurements of electron temperature, ion temperature, electron density, and plasma flow velocity [8]. Thomson-scattering measurements on OMEGA use a 2ω (527-nm) or 4ω (263-nm) optical laser as a source that is scattered by electron density fluctuations in the plasma [9]. The Thomson-scattering diagnostic is routinely used on the OMEGA Laser System to measure the collective Thomson-scattering spectrum [2]. The development of a fully reflective system allows measurements to be made over a wide spectral range (190 nm to 700 nm) [3].

4ω Probe Development

A 10-ps, 263-nm (4ω) laser and a suite of optical diagnostics (schlieren, interferometry, and angular filter refractometry [3] was being built to probe plasmas produced on the OMEGA EP Laser System [4]. Light scattered by the probe beam is collected by an f/4 catadioptric telescope (figure below) and a transport system images with a near-diffraction-limited resolution (~3-µm full width at half maximum) over a 5-mm field of view to a diagnostic table. The transport system provides a contrast greater than 1:104 with respect to all wavelengths outside of the 263±2-nm measurement range.

Mechanical rendering of the f/4 catadioptric relay system located inside the target chamber and used to image a 5-mm field of view at target chamber center to a location just outside the chamber.

This system was designed to characterize long-scale-length plasmas over a field of view that is several millimeters in diameter and to detect localized channels (figure below) with high spatial and temporal resolution [5]. Long-scale-length plasmas are diagnosed using angular filter refractometry and channels within the plasmas using schlieren. Another important application for the probe beam is the diagnosis of preformed plasmas in the interaction of high-intensity, high-power, short-pulse OMEGA EP beams with solid targets. The shadowgraphy and interferometry designs are optimized for this purpose. Interferometry can provide accurate measurements of underdense plasma profiles, typically with electron densities below 1020 cm–3, in which the probe beam is not significantly refracted.

Streak-Camera Development

Streak cameras are a work horse instrument for ultrafast measurements. They measure the temporal pulse shape of optical and x-ray pulses by converting the photon pulse into an electron pulse that is "streaked" by a temporally ramped electric field. To extend the resolution of streak cameras to shorter times (≤1 ps), with one dimension of spatial resolution, a series of custom-designed x-ray streak tubes are modeled, fabricated, tested, and deployed. A cross-sectional view of a streak tube is shown below. The custom-designed streak-camera tubes allow our team to optimize for individual application by maximizing the performance for the most-demanding aspect of the measurement and relaxing the performance for the noncritical aspects if necessary.

A cross-sectional view, in the time direction, of a streak tube. The blue lines are the electron trajectories, moving from left to right.
The red curves are lines of equal potential.

Ultrafast Optical Spectrometer

A high-throughput, broadband optical spectrometer coupled to the Rochester Optical Streak System equipped with a Photonis P820 streak tube was designed to record time-resolved spectra with 1-ps time resolution [6]. Spectral resolution of 0.8 nm is achieved over a wavelength coverage range of 480 to 580 nm, using a 300-groove/mm diffraction grating in conjunction with a pair of 225-mm-focal-length doublets operating at an f/2.9 aperture. Overall pulse-front tilt across the beam diameter generated by the diffraction grating is reduced by preferentially delaying discrete segments of the collimated input beam using a 34-element reflective echelon optic. The introduced delay temporally aligns the beam segments and the net pulse-front tilt is limited to the accumulation across an individual sub-element. The resulting spectrometer design balances resolving power and pulse-front tilt while maintaining high throughput.

CAD model of the spectrometer layout. A pair of f/2.9 doublets is used to collimate and focus the dispersed input from a 50-µm-core fiber optic. The pulse-front tilt from the 300-g/mm transmission grating is reduced using a 34-element reflective echelon optic.

Fast UV Photodiode Development

Currently the PUPG is investigating commercially available materials, from a materials science prospective, with the aim of producing high-quality, ultrafast UV (ℓ < 360 nm) photodiode devices. Devices are being fabricated at the Microfabrication Facility at RIT and at the University of Rochester's Nano-Fabrication Facility. A number of possible avenues for improving device performance, including device geometry, contact metallization, and buried and trenched electrodes, are being investigated. In addition a number of defect mapping techniques using optics and x-ray diffraction have been used to pre-characterize the substrate material. In addition to GaN, various alloys of AlxGa(1-x)N will be studied. Varying the concentration, x, will tune the adsorption band edge, allowing a series of devices to perform time-resolved UV spectroscopy. There are many instances where it would be advantageous to measure ultrafast UV pulses on OMEGA or OMEGA EP with a photodiode. A few example applications would be characterizing and timing the 4ω probe, measuring the time-resolved spectrum of backscattered light, measuring the UV pulse shape, and improving the 3ω Beam-Timing System. In general such photodiodes are not commercially available. Previous work at LLE has demonstrated that UV photodiodes fabricated with GaN have picosecond response times. However, the poor quality of the substrate material resulted in devices with a large dark current. Recent research, promoted by the GaN LED lighting industry, has resulted in significantly better starting material.

Related Publications

(UR graduate students are underlined)

Related DOE Monthly Reports