Shaun Kerr

Optical resolution photoacoustic microscopy using novel high-repetition-rate passively Q-switched microchip and fiber lasers

Optical-resolution photoacoustic microscopy (OR-PAM) is a novel imaging technology for visualizing optically absorbing superficial structures in vivo with lateral spatial resolution determined by optical focusing rather than acoustic detection. Since scanning of the illumination spot is required, OR-PAM imaging speed is limited by both scanning speed and laser pulse repetition rate. Unfortunately, lasers with high repetition rates and suitable pulse durations and energies are not widely available and can be cost-prohibitive and bulky. We are developing compact, passively Q-switched fiber and microchip laser sources for this application. The properties of these lasers are discussed, and pulse repetition rates up to 100 kHz are demonstrated. OR-PAM imaging was conducted using a previously developed photoacoustic probe, which enabled flexible scanning of the focused output of the lasers. Phantom studies demonstrate the ability to image with lateral spatial resolution of 7±2 μm with the microchip laser system and 15±5 μm with the fiber laser system. We believe that the high pulse repetition rates and the potentially compact and fiber-coupled nature of these lasers will prove important for clinical imaging applications where real-time imaging performance is essential.

Optical-Resolution Photoacoustic Micro-Endoscopy using Image-Guide Fibers and Fiber Laser Technology

In this paper the feasibility of optical-resolution photoacoustic micro-endoscopy (OR-PAME) using image guide fibers and a unique fiber laser system is demonstrated. The image guide consists of 30,000 individual fibers in a bundle 800 μm in diameter and the diode-pumped, pulsed Ytterbium fiber laser can provide repetition rates up to 600 kHz. Phantom studies indicate 7+/-2 μm resolution. The compact, flexible nature of the image guide and the small footprint of the apparatus make it ideal for photoacoustic micro-endoscopy, as well as increasing the usability of OR-PAM for potential clinical applications.

Real-time optical-resolution photoacoustic microscopy using fiber-laser technology

Optical-resolution photoacoustic microscopy (OR-PAM) is an emerging technology providing visualization of superficial structures in vivo with optical-absorption contrast. High resolution is possible as the lateral spatial resolution is determined by the optical spot size rather than acoustic detection. The imaging speed is dictated by both the beam scanning speed and the laser pulse repetition rate. We are developing a realtime OR-PAM system that uses a high repetition rate pulsed laser and high speed XY mirror galvanometers. We have demonstrated OR-PAM imaging by employing a diode-pumped pulsed Ytterbium fiber laser with a pulse repetition rate ranging from 20 kHz - 600 kHz, second harmonic generation at a wavelength of 532 nm and average output power up to 13 W. In our study, we utilized 0.13μJ ~1-ns pulses. A photoacoustic probe consisting of a 45-degree glass prism in an optical index-matching fluid is used to transmit the focused output of the laser to the sample and also to reflect exiting photoacoustic signals to an ultrasound transducer. Phantom studies with a ~7.5-μm carbon fiber demonstrate the ability to image with ~7-μm optical lateral spatial resolution. Combined with a fast-scanning mirror oscillating at 800 (B-scan) lines per second, we demonstrate a system capable of C-scan imaging at 4 frames per second. These near-realtime frame-rates should permit clinical applications.

High Repetition Rate Passively Q-switched Fiber and Microchip Lasers for Optical Resolution Photoacoustic Imaging

Optical-resolution photoacoustic microscopy is a novel imaging technology for visualizing optically-absorbing superficial structures in vivo with lateral spatial resolution determined by optical focusing rather than acoustic detection. Since scanning of the illumination spot is required, the imaging speed is limited by the scanning speed and the laser pulse repetition rate. Unfortunately, lasers with high-repetition rate and suitable pulse durations and energies are difficult to find. We are developing compact laser sources for this application. Passively Q-switched fiber and microchip lasers with pulse repetition rates up to 300 kHz are demonstrated. Using a diode-pumped microchip laser fiber-coupled to a large mode-area Yb-doped fiber amplifier we obtained 60μJ 1-ns pulses at the frequency-doubled 532-nm wavelength. The pulse-repetition rate was determined by the power of the microchip laser pump source at 808nm and may exceed 10 kHz. Additionally, a passively Q-switched fiber laser utilizing a Yb-doped double-cladding fiber and an external saturable absorber has shown to produce 250ns pulses at repetition rates of 100-300 KHz. A photoacoustic probe enabling flexible scanning of the focused output of these lasers consisted of a 45-degree glass prism in an optical index-matching fluid. Photoacoustic signals exiting the sample are deflected by the prism to an ultrasound transducer. Phantom studies with a 7.5-micron carbon fiber demonstrate the ability to image with optical rather than acoustic resolution. We believe that the high pulse-repetition rates and the potentially compact and fiber-coupled nature of these lasers will prove important for clinical imaging applications where realtime imaging performance is essential.

Two-dimensional time-resolved ultra-high speed imaging of K-alpha emission from short-pulse-laser interactions to observe electron recirculation

Time resolved x-ray images with 7 ps resolution are recorded on relativistic short-pulse laser-plasma experiments using the dilation x-ray imager, a high-speed x-ray framing camera, sensitive to x-rays in the range of ≈1−17 keV. This capability enables a series of 2D x-ray images to be recorded at picosecond scales, which allows for the investigation of fast electron transport within the target with unprecedented temporal resolution. An increase in the Kα-emission spot size over time was found for targets thinner than the recirculation limit and is absent for thicker targets. Together with the observed polarization dependence of the spot size increase, this indicates that electron recirculation is relevant for the x-ray production in thin targets.

Effect of preplasma on double pulse irradiation of targets for proton acceleration

Summary form only given. We report on the experimental and simulated characterization of proton acceleration from double-pulse irradiation of um-scale foil targets with varying preplasma conditions. Temporally separated pulses of less than a picosecond in duration have been shown to increase the conversion efficiency of laser energy to MeV protons1. The experiment utilized two 700 fs, 1054 nm pulses, separated by 1 to 5 ps; total beam energy was 100 J, with 5-20% of the total energy contained within the first pulse. In contrast to the ultraclean beams used in previous experiments1, prepulse energies on the order of 10 mJ were present. The resulting significant preplasma appears to have a moderating effect on the double pulse enhancement. Proton beam measurements were made with radiochromic film stacks and magnetic spectrometers.LSP 2D PIC simulations2 have been performed to better understand the double pulse enhancement mechanism, as well as the role of preplasma in modifying this effect. Simulation results will be shown for various target conditions, and compared to experimental data.

Deflection of laser accelerated protons from cryogenic hydrogen jets due to self-generated magnetic fields

Summary form only given. Laser-driven ion acceleration is of great interest across a range of disciplines with potential applications including the fast ignition approach to inertial confinement fusion and proton therapy. The most robust acceleration mechanisms studied to date however, based on target normal sheath acceleration (TNSA), do not satisfy the emittance, flux and ion energy requirements for direct applications. In this talk, we will first discuss alternative acceleration mechanisms utilizing cryogenic hydrogen jets to work towards a high-repetition rate proton source with suitable beam parameters for various applications. We will then show a study of the spatial distribution of the energetic protons produced from a high-intensity laser-plasma interaction in cylindrical geometry. In the laser forward direction, we will show that the proton beam is highly structured with a bubble-net pattern. In addition, we observe two well-defined bands, offset ±8-15° vertically from the laser plane and surrounding the target azimuthally. We will introduce the interpretation of these structures as caustics in linear proton radiography theory where the energetic protons are deflected due to self-generated magnetic fields. Finally, these results will be compared with 2D and 3D Particle-in-cell (PIC) simulations which confirm the role of the Weibel Instability in the formation of the bubble-net structure and qualitatively reproduce the observed bands due to Biermann Battery magnetic fields.

Inverse faraday effect magnetic field generation in laser induced plasma

Summary form only given. Laser plasma interactions with high intensity laser pulses can produce high magnetic fields in the 10s of MG range. One technique for generating high magnetic fields in under dense plasmas is via the Inverse Faraday Effect (IFE) which has been shown to induce axial magnetic fields in the MG range using circularly polarized light [1]. It is also proposed that similar fields could be induced using linearly polarized light where higher order angular momentum modes [2] are employed to couple the required angular momentum to the electrons. We wish to study this magnetic field generation in under dense plasma and are carrying out a simulation study of the expected fields.IFE is a phenomenon in which the circularly polarized light propagates through a non-linear medium, transmits angular momentum to the electrons and induces an axial magnetic field [3]. This picture gets more complicated with hot electron generation by the propagation of the intense laser pulse through the plasma, which in addition generates an axial current and solenoidal magnetic field. We have carried out initial Large Scale PIC (LSP) simulations to predict the scaling law for this hot electron generation and hence the expected magnetic field levels. Data will be shown for various parameters of laser plasma interaction with different densities and intensities in the threshold relativistic intensity range of 1017 to 1019 Wcm-2.