Graeme J. Hirst

Nanoscale spatial induction of ultraviolet photoproducts in cellular DNA by three-photon near-infrared absorption.

The high‐resolution spatial induction of ultraviolet (UV) photoproducts in mammalian cellular DNA is a goal of many scientists who study UV damage and repair. Here we describe how UV photoproducts can be induced in cellular DNA within nanometre dimensions by near‐diffraction‐limited 750 nm infrared laser radiation. The use of multiphoton excitation to induce highly localized DNA damage in an individual cell nucleus or mitochondrion will provide much greater resolution for studies of DNA repair dynamics and intracellular localization as well as intracellular signalling processes and cell–cell communication. The technique offers an advantage over the masking method for localized irradiation of cells, as the laser radiation can specifically target a single cell and subnuclear structures such as nucleoli, nuclear membranes or any structure that can be labelled and visualized by a fluorescent tag. It also increases the time resolution with which migration of DNA repair proteins to damage sites can be monitored. We define the characteristics of localized DNA damage induction by near‐infrared radiation and suggest how it may be used for new biological investigations.

A high performance excimer pumped Raman laser

An electron beam pumped large aperture KrF laser operating in a short pulse multiplexed mode has been used to pump a methane Raman laser to produce a single high intensity pulse at 268 nm. With an output beam divergence of 20 μrad and final amplifier conversion efficiency of greater than 50%, intensity at the focus of an F/3 lens was greater than 1017 W/cm2. Prepulse intensity was less than 10−10 of peak intensity.

A 1 TW KrF laser using chirped pulse amplification

Abstract Chirped pulse amplification (CPA) and recompression have been used in a large aperture KrF laser system. The power focused onto target in a 300 fs pulse reached 1 TW with an irradiance of ≈ 10 19 W/cm 2 .

Gap junction communication dynamics and bystander effects from ultrasoft X-rays.

Gap junctions provide a route for small molecules to pass directly between cells. Toxic species may spread through junctions into ‘bystander’ cells, which may be exploited in chemotherapy and radiotherapy. However, this may be prevented by junction closure, and therefore an understanding of the dose-dependency of inhibition of communication and bystander effects is important. Low-energy ionising radiation (ultrasoft X-rays) provides a tool for the study of bystander effects because the area of exposure may be carefully controlled, and thus target cells may be clearly defined. Loss of gap junction-mediated intercellular communication between irradiated cells was dose-dependent, indicating that closure of junctions is proportional to dose. Closure was associated with hyperphosphorylation of connexin43. Inhibition of communication occurred in bystander cells but was not proportional to dose. Inhibition of communication at higher radiation doses may restrict the spread of inhibitory factors, thus protecting bystander cells. The reduction in communication that takes place in bystander cells was dependent on cells being in physical contact, and not on the release of signalling factors into the medium.

Titania—a 1020 W cm−2 ultraviolet laser

The Titania laser system, based around a 42 cm e-beam pumped KrF amplifier, is currently being installed at the Rutherford Appleton Laboratory and will come on line as a user facility in 1996. Like Sprite, its predecessor, it will operate in both CPA (249 nm) and Raman (268 nm) short-pulse modes, delivering up to 10 TW to target in high-quality beams. With brightness expected to reach 10 21 W cm -2 sterad -1 , it will be the worlds brightest ultraviolet laser. The design of the Titania system includes a number of novel features. The multi-pass Ti :sapphire front-end amplifier uses an unusual beam-folding scheme. The Raman system will involve the first application of Raman multiplexing, combining high KrF efficiency with low transport cost. Reflective coatings with very high damage thresholds are being developed for the CPA compressor gratings and the UV transport optics. A windowless configuration for the final Raman amplifier is presently under analysis, to allow the performance of this maximally stressed component to be extended substantially. Finally the design of the Titania e-beam machine, featuring novel split-cathode diodes, has resulted in unusually high efficiency of electron transport into the laser gas. The lasers infrastructure has involved sophisticated mechanical and electrical design, and a computerized diagnostic, control and safety package is being developed to allow one-man operation of the whole 1000 m 2 installation.

Ultrahigh-brightness KrF laser system for fast ignition studies

The main requirements for a fast igniter laser beam are reviewed and shown to favour short wavelength and ultrahigh brightness. These requirements are met by the new KrF laser system at Rutherford Appleton Laboratory called TITANIA. TITANIA uses two schemes to enhance the laser beam brightness. The first is chirped pulse amplification which is used to enhance brightness by compressing the pulse into the femtosecond region. In this mode TITANIA produces in the region of 250 mJ on target in 700 fs. The second mode of operation uses a Raman technique for beam combining and beam clean-up which is designed to give a single beam of 80 joules on target in a pulselength of 60 ps. In this scheme the KrF wavelength is Raman shifted to 268 nm. The Raman amplifiers will use gaseous rather than solid windows and experiments which demonstrate their feasibility will be described. A concept for a reactor scale fast igniter beam using the Raman technique will be discussed.

Ultrahigh-brightness laser beams with low prepulse obtained by stimulated Raman scattering.

A unique high-power laser system is described that is based on stimulated Raman scattering in methane. KrF-laser pump beams of 10-ps duration are confined in a square-section light guide and amplify a 268-nm Stokes-shifted beam to 0.5 TW with extremely low prepulse. The near-diffraction-limited beam quality gives a peak brightness of >1020 W cm−2 sr−1.

On the origin of the dip in the KrF laser gain spectrum. II. The short-pulse gain saturation experiment

The wavelength resolved spectra recorded during subnanosecond depletion of the gain in a KrF laser amplifier have been used to specify the state responsible for the dip in the KrF(B→X) emission spectra. Self-absorption of the KrF laser emission at 248.9 nm has been found to be due to the phototransition from the KrF(C) state to a 2Π Rydberg state.

ON THE ORIGIN OF THE DIP IN THE KRF LASER GAIN SPECTRUM

High‐resolution spectra of KrF (B–X) amplified spontaneous emission from various discharge‐pumped and electron‐beam‐pumped KrF lasers have been analyzed. An underlying structured absorption spectrum has been discovered with a well‐resolved peak at 248.91 nm. The absorption coefficient of this peak was found to vary in exact proportion to the peak laser gain coefficient but was independent of laser gas purity. We suggest that the absorption arises internally within the KrF molecule and is due to transitions from the B state to a higher‐lying Rydberg state. This hypothesis was tested by simulating the absorption spectrum from KrF*(B) to a weakly repulsive state dissociating to Kr*(3P1)+F(2P3/2). A good agreement was obtained between simulated and experimental absorption spectra.

Vacuum-ultraviolet resonant photoabsorption imaging of laser produced plasmas

We present results from a vacuum-ultraviolet (VUV) “photoabsorption imaging” technique based on the measurement of the time and space resolved absorption of a quasimonochromatic VUV beam from a laser plasma light source. The use of VUV radiation as a probe beam permits direct access to resonance lines of (singly and more highly charged) ions and also to the resonant and nonresonant continua of atoms and ions. In this experiment we have confined ourselves to measurements using the 3p–3d resonances of Ca, Ca+, and Ca2+ as markers of the temporal and spatial distribution of ground state atoms and ions in an expanding laser plasma plume. We show how time resolved column density maps may be extracted from such images. In addition we have extracted plasma plume velocities from the data, which compare well with an analytical laser ablation model.