Robin S. Marjoribanks

Laser shaping of photonic materials : deep-ultraviolet and ultrafast lasers

Optical materials are especially challenging to process with conventional lasers simply because of their high transparency. We are exploiting two extremes in laser technology — ultrafast lasers and very short wavelength F2 lasers — to microsculpt surfaces and to control refractive index in transparent glasses. These lasers drive fundamentally different interactions, many-photon and ‘big’ photon, respectively, that offer distinct advantages and limitations for shaping photonic devices in fused silica. Comparisons of surface morphology, shock-induced microcracking, resolution, and photosensitivity responses are presented.

Relativistic electron mirrors from nanoscale foils for coherent frequency upshift to the extreme ultraviolet

Reflecting light from a mirror moving close to the speed of light has been envisioned as a route towards producing bright X-ray pulses since Einstein’s seminal work on special relativity. For an ideal relativistic mirror, the peak power of the reflected radiation can substantially exceed that of the incident radiation due to the increase in photon energy and accompanying temporal compression. Here we demonstrate for the first time that dense relativistic electron mirrors can be created from the interaction of a high-intensity laser pulse with a freestanding, nanometre-scale thin foil. The mirror structures are shown to shift the frequency of a counter-propagating laser pulse coherently from the infrared to the extreme ultraviolet with an efficiency >104 times higher than in the case of incoherent scattering. Our results elucidate the reflection process of laser-generated electron mirrors and give clear guidance for future developments of a relativistic mirror structure.

High-contrast terawatt chirped-pulse-amplification laser that uses a 1-ps Nd:glass oscillator

We have developed a fiberless 1-TW all-Nd:glass chirped-pulse-amplification laser system that uses high-contrast 0.8-1.4-ps pulses produced directly from a Nd:glass feedback-controlled oscillator. Employing grating-only expansion and compression, the system produces clean (~10(7) contrast ratio) 1-J, 1- 1.4-ps recompressed pulses without added pulse cleaning. Clean microjoule-energy pulses from the oscillator require less subsequent amplification than cw oscillator schemes, thereby minimizing gain-bandwidth narrowing and offering improved contrast with amplified stimulated emission background.

Investigation of laser-driven proton acceleration using ultra-short, ultra-intense laser pulses

We report optimization of laser-driven proton acceleration, for a range of experimental parameters available from a single ultrafast Ti:sapphire laser system. We have characterized laser-generated protons produced at the rear and front target surfaces of thin solid targets (15 nm to 90 μm thicknesses) irradiated with an ultra-intense laser pulse (up to 1020 W⋅cm−2, pulse duration 30 to 500 fs, and pulse energy 0.1 to 1.8 J). We find an almost symmetric behaviour for protons accelerated from rear and front sides, and a linear scaling of proton energy cut-off with increasing pulse energy. At constant laser intensity, we observe that the proton cut-off energy increases with increasing laser pulse duration, then roughly constant for pulses longer than 300 fs. Finally, we demonstrate that there is an optimum target thickness and pulse duration.

Towards ultrahigh-contrast ultraintense laser pulses--complete characterization of a double plasma-mirror pulse cleaner

The effects of small amounts of energy delivered at times before the peak intensity of ultrahigh-intensity ultrafast-laser pulses have been a major obstacle to the goal of studying the interaction of ultraintense light with solids for more than two decades now. We describe implementation of a practical double-plasma-mirror pulse cleaner, built into a f=10m null telescope and added as a standard beamline feature of a 100 TW laser system for ultraintense laser-matter interaction. Our measurements allow us to infer a pulse-height contrast of 5×1011—the highest contrast generated to date—while preserving ∼50% of the laser intensity and maintaining excellent focusability of the delivered beam. We present a complete optical characterization, comparing empirical results and numerical modeling of a double-plasma-mirror system.

Ablation and thermal effects in treatment of hard and soft materials and biotissues using ultrafast-laser pulse-train bursts

Abstract Ultrafast laser pulses (≤1 ps) are qualitatively different in the nature of their interaction with materials, including biotissues, as compared to nanosecond or longer pulses. This can confer pronounced advantages in outcomes for tissue therapy or laser surgery. At the same time, there are distinct limitations of their strong-field mode of interaction. As an alternative, it is shown here that ultrafast laser pulses delivered in a pulse-train burst mode of radiant exposure can access new degrees of control of the interaction process and of the heat left behind in tissues. Using a laser system that delivers 1 ps pulses in 20 μs pulse-train bursts at 133 MHz repetition rates, a range of heat and energy-transfer effects on hard and soft tissue have been studied. The ablation of tooth dentin and enamel under various conditions, to assess the ablation rate and characterize chemical changes that occur, are reported. This is compared to ablation in agar gels, useful live-cell-culture phantom of soft tissues, and presenting different mechanical strength. Study of aspects of the optical science of laser-tissue interaction promises to make qualitative improvements to medical treatments using lasers as cutting and ablative tools. Zusammenfassung Ultraschnelle Pulse (≤1 ps) unterscheiden sich von Nanosekunden- oder noch längeren Pulsen qualitativ in der Art ihrer Wechselwirkung mit Materialien, einschließlich Biogewebe. Dies kann in der Gewebetherapie oder in der Laserchirurgie von Vorteil sein. Andererseits gibt es klare Einschränkungen hinsichtlich ihrer Starkfeld-Effekte. Als Alternative wird in der vorliegenden Arbeit gezeigt, dass ultraschnelle Pulse, die im sogenannten Burst-Modus der Bestrahlung abgeben werden (d.h. in einer schnellen Folge stoßweise ausgesendeter Impulse oder Pulszüge) dazu beitragen können, den Wechselwirkungsprozess und die dabei erzeugte Wärme besser zu kontrollieren. Dazu wurden die Wärme- und Energietransfereffekte an Hart- und Weichgewebe untersucht, die mittels eines Lasersystems erzeugt wurden, mit dem 1 ps-Pulse in 20 μs-Impulsfolgen mit einer Wiederholrate von 133 MHz abgegeben wurden. Es wird über Ablationsversuche an Dentin und Zahnschmelz unter verschiedenen Bedingungen berichtet, die mit dem Ziel durchgeführt wurden, die Ablationsrate zu evaluieren und auftretende chemische Veränderungen zu charakterisieren. Die Ergebnisse wurden mit der Ablation in Agargels verglichen, die gut als Weichgewebephantome geeignet sind und eine unterschiedliche mechanische Festigkeit aufweisen. Insgesamt verspricht die Untersuchung der optischen Aspekte der Laser-Gewebe-Wechselwirkung eine qualitative Verbesserung von medizinischen Laseranwendungen.

Thomson scattering measurements of heat flow in a laser-produced plasma

Measurements of the electron distribution and heat flow between the critical and ablation surfaces in a laser-produced plasma have been obtained using Thomson scattering. A frequency-quadrupled probe beam was used to obtain Thomson spectra at above-critical densities in a plasma produced by irradiation of solid targets with the fundamental laser light at irradiances of 3 × 1014 W cm−2. Comparison of Thomson spectra at the ion acoustic frequency (sensitive to the cold return current) with simulated spectra shows that the data are consistent with Fokker–Planck simulations of the electron distribution function, providing the first direct information on the electron distribution function.

Picosecond pumping of extreme-ultraviolet lasers using preformed laser plasmas

Weak laser prepulses were used for the first time with picosecond-duration laser light to enhance laser-target absorption for efficient excitation of extreme-ultraviolet lasers. A traveling-wave excitation geometry and a self-healing mercury-wetted target were used with 300-ps prepulses to pump the photoionization Xe III laser at 109-nm wavelength. Fully saturated laser gain was demonstrated for both 32-ps and 1.4-ps pump pulses with use of only 150-mJ pulse energy: small-signal gain coefficients exceeded 2 cm−1 for on-target laser fluences of only 4 J/cm2.

Pulsetrain-burst mode, ultrafast-laser interactions with 3D viable cell cultures as a model for soft biological tissues.

A 3D living-cell culture in hydrogel has been developed as a standardized low-tensile-strength tissue proxy for study of ultrafast, pulsetrain-burst laser-tissue interactions. The hydrogel is permeable to fluorescent biomarkers and optically transparent, allowing viable and necrotic cells to be imaged in 3D by confocal microscopy. Good cell-viability allowed us to distinguish between typical cell mortality and delayed subcellular tissue damage (e.g., apoptosis and DNA repair complex formation), caused by laser irradiation. The range of necrosis depended on laser intensity, but not on pulsetrain-burst duration. DNA double-strand breaks were quantified, giving a preliminary upper limit for genetic damage following laser treatment.

Effects of heat transfer and energy absorption in the ablation of biological tissues by pulsetrain-burst (>100 MHz) ultrafast laser processing

Energy absorption and heat transfer are important factors for regulating the effects of ablation of biological tissues. Heat transfer to surrounding material may be desirable when ablating hard tissue, such as teeth or bone, since melting can produce helpful material modifications. However, when ablating soft tissue it is important to minimize heat transfer to avoid damage to healthy tissue - for example, in eye refractive surgery (e.g., Lasik), nanosecond pulses produce gross absorption and heating in tissue, leading to shockwaves, which kill and thin the non-replicating epithelial cells on the inside of the cornea; ultrafast pulses are recognized to reduce this effect. Using a laser system that delivers 1ps pulses in 10μs pulsetrains at 133MHz we have studied a range of heat- and energy-transfer effects on hard and soft tissue. We describe the ablation of tooth dentin and enamel under various conditions to determine the ablation rate and chemical changes that occur. Furthermore, we characterize the impact of pulsetrain-burst treatment of collagen-based tissue to determine more efficient methods of energy transfer to soft tissues. By studying the optical science of laser tissue interaction we hope to be able to make qualitative improvements to medical treatments using lasers.