Chao Gong

Supercontinuum generation from 2 to 20 μm in GaAs pumped by picosecond CO 2 laser pulses

We report on the generation of supercontinuum radiation from 2 to 20 μm in a 67 mm long GaAs crystal pumped by a train of 3 ps CO2 laser pulses. Temporal measurements indicate that sub-picosecond pulse splitting is involved in the production of such wide-bandwidth radiation in GaAs. The results show that the observed spectral broadening is heavily influenced by four-wave mixing and stimulated Raman scattering.

Plasma dynamics near critical density inferred from direct measurements of laser hole boring

We have used multiframe picosecond optical interferometry to make direct measurements of the hole boring velocity, v_{HB}, of the density cavity pushed forward by a train of CO_{2} laser pulses in a near critical density helium plasma. As the pulse train intensity rises, the increasing radiation pressure of each pulse pushes the density cavity forward and the plasma electrons are strongly heated. After the peak laser intensity, the plasma pressure exerted by the heated electrons strongly impedes the hole boring process and the v_{HB} falls rapidly as the laser pulse intensity falls at the back of the laser pulse train. A heuristic theory is presented that allows the estimation of the plasma electron temperature from the measurements of the hole boring velocity. The measured values of v_{HB}, and the estimated values of the heated electron temperature as a function of laser intensity are in reasonable agreement with those obtained from two-dimensional numerical simulations.

Production of Multi‐Terawatt Time‐Structured CO2 Laser Pulses for Ion Acceleration

The UCLA Neptune Laboratory CO2 laser system has been recently upgraded to produce 3ps multi‐terawatt 10μm laser pulses. The laser energy is distributed over several 3 ps pulses separated by 18 ps. These temporally structured pulses are applied for laser driven ion acceleration in an H2 gas jet at a measured plasma density of 2×1019 cm−3. Protons in excess of 20 MeV have been observed in the forward direction and with energy spreads (ΔE/E∼10%).

High throughput on-chip analysis of high-energy charged particle tracks using lensfree imaging

We demonstrate a high-throughput charged particle analysis platform, which is based on lensfree on-chip microscopy for rapid ion track analysis using allyl diglycol carbonate, i.e., CR-39 plastic polymer as the sensing medium. By adopting a wide-area opto-electronic image sensor together with a source-shifting based pixel super-resolution technique, a large CR-39 sample volume (i.e., 4 cm × 4 cm × 0.1 cm) can be imaged in less than 1 min using a compact lensfree on-chip microscope, which detects partially coherent in-line holograms of the ion tracks recorded within the CR-39 detector. After the image capture, using highly parallelized reconstruction and ion track analysis algorithms running on graphics processing units, we reconstruct and analyze the entire volume of a CR-39 detector within ∼1.5 min. This significant reduction in the entire imaging and ion track analysis time not only increases our throughput but also allows us to perform time-resolved analysis of the etching process to monitor and optimize the growth of ion tracks during etching. This computational lensfree imaging platform can provide a much higher throughput and more cost-effective alternative to traditional lens-based scanning optical microscopes for ion track analysis using CR-39 and other passive high energy particle detectors.

Four-frame picosecond interferometry system for probing near-critical density CO2 laser-produced plasmas

In this paper we describe a four-frame picosecond green laser interferometer developed at the Neptune Laboratory at UCLA. This diagnostic tool is used to measure the dynamics of CO2 laser-produced plasmas in a helium gas jet. We also present here an example of results of the modification of the plasma density profile by the radiation pressure of the laser pulse.

Ion acceleration in a gas jet using multi-terawatt CO2 laser pulses

This paper describes ongoing research in the Neptune Laboratory at UCLA on ion acceleration from plasmas formed in different gases using a 4-10TW CO2 laser. Our experiment is focused on acceleration of Helium and Nitrogen ions using collisionless shocks. The status of the experiment is presented. One dimensional particle-in-cell (PIC) simulations are carried out to study conditions under which a collisionless shock is launched in two interpenetrating plasmas with different charge densities and charge states.This paper describes ongoing research in the Neptune Laboratory at UCLA on ion acceleration from plasmas formed in different gases using a 4-10TW CO2 laser. Our experiment is focused on acceleration of Helium and Nitrogen ions using collisionless shocks. The status of the experiment is presented. One dimensional particle-in-cell (PIC) simulations are carried out to study conditions under which a collisionless shock is launched in two interpenetrating plasmas with different charge densities and charge states.

CO 2 laser driven ion acceleration in a gas jet

A train of multiterawatt, 3 ps CO2 laser pulses is used for proton acceleration in a plasma at the critical density. Maximum energy of ions is strongly affected by a temporal structure of the pulse.

Application of a Multi-Terawatt 3ps CO2 Laser for Monoenergetic Proton Beam Generation

15TW picosecond 10μm laser pulses are obtained at the UCLA Neptune laboratory achieving record CO2 laser power. This laser system is applied for laser driven ion acceleration achieving 20 MeV proton beams from a H2 gas jet.