Laser Dose Delivery Systems

In the other sections, we identify four important modes of potoionization: single-photon, optical breakdown, below optical breakdown threshold, and filamentary ionization, and three mixed modes. Other than ionization, laser light can also induce excitation and dissociation. The concept of dose can be adapted to laser light, and can be used to understand the application of laser light for inducing material chemical and structural changes. On this site, the section "Laser Applications" I present a body of knowledge that enables one to control the photolytic effects induced by pulsed laser beams. This body of knowledge couples controllable optical characteristics of the laser beam (such as: wavelength/spectrum, average intensity, spatial intensity distribution, temporal intensity distribution, polarization, and pulse duration) with the yields of the primary photolytic species, and their spatial distribution. It enables one to generate a great variety of dose distributions within dielectric materials, by selectively inducing a superposition of the four modes of ionization mentioned above. For example, to reproduce important aspects from the characteristic spatial dose distribution of photons (X-rays and γ-rays) or heavy charged particle radiation (protons or ions).

The most important conclusion of this study is that on can use laser light to generate in water-based solutions the same primary photolytic species as with high energy radiation, with similar spatial characteristics. This tells us that one can replace radiation with laser light at least in applications where only the radiolytic effects are considered, and not the radiation particle itself. The replacement of radiation with laser light brings along significant advantages. But the range of applications of pulsed lasers in the domain of material processing goes beyond the applications of radiation.

In this section I will present different configurations of dose delivering systems. There are two main categories of system configurations: the one pulsed beam (1PB) system and the two-beam systems (2B). The second category includes the two pulsed beams system (2PB), and the one pulsed beam and one continuous beam (1P1CB). The two-beam systems (2B) aim to control ultra-fast processes, or to probe the photolytic changes with fs time resolution.

One pulsed beam system (1PB)

All pure and mixed modes of ionization can be induced in the 1PB configuration. Moreover, the yields of primary photolytic species and their spatial distribution can be controlled as described in the previous sections.

The first idea is that the dose distribution induced by high energy radiation can be reproduced using an appropriate laser pulse. The red curve inside the sample represents the ionization density as a function of depth. The allure of the curve is the typical pick of Bragg. This particular case depicted in the figure bellow refers to a F-OB mixed photoionization mode. The sample is positioned on a xyz stage for spatial scanning. An important difference between high energy radiation and a high power laser pulse, is that in the case of the radiation ionization starts as soon as the particle enters the sample. A laser ionization trace can start inside the sample.

The body of knowledge presented on this site, which enables one to control the laser effects on the sample by controlling laser properties is encapsulated within the "Optical Adjustments" box on the image.

The figure bellow depicts a dose delivery system with life feedback. A second laser pulse, in sync with the ionizing pulse is used for analysis. The feedback can be of many sorts: spectroscopy, shadow, imaging, etc.

Two pulsed beams system (2PB)

In this configuration, the two pulses originate from a unique pulse at the source, but they are independent, in the sense that they can be modified separately. In the sample, these two pulsed-beams are brought together again, in a collinear (same direction of propagation) or a non-collinear configuration. The delay between the two pulses is adjustable, and becomes an important parameter for the control of photolytic effects. Each pulse can be individually prepared as in the single-pulse beam configuration. One reason of having two pulses is to control ultra-fast processes: to selectively induce different ionization/dissociation/excitation channels, and to control relaxation pathways. In return, that has an affect on the relative yields of primary photolytic species, and on the long-term photolytic effects. A 2PB system can be used instead of a 1PB system with pulse sleeting, giving much more control over ultra-processes. The delay between the tow pulses can be easily modified, and their wavelength can be chosen to be different. Although a 2PB system is more versatile, the 1PB system with temporal breaking offers the advantage of simplicity and is recommended whenever possible. In the SP mode, this configuration can be used to induce two color processes. The beams having different colors, one induces a given excited state population by a single-photon process, and the other ionizes it by a single-photon vertical transition. Giving the lifetime of the excited state, one can control the ion population generated by the second pulse by adjusting the delay between the two pulses. The 2PB configuration can also be used for the study of ultra-fast processes induced by photolysis. This is the pump-probe scheme. The two pulses arrive at the target with a variable delay one after the other. One is called pump; it delivers the dose to the target, and triggers photolytic changes. The other is called probe, and it measures the changes at different delays after the pump, reconstructing the dynamics of photolytic induced processes. We can also look at it as a feedback loop system: the probe pulse is used to collect feedback in effective time, which is used to readjust the pump parameters in order to converge towards the desired effect. In essence, the system operates like the 1PB system, the second pulsed beam being used only for calibration purposes. In the case where the characteristic time of the photolytic processes taken into account is much shorter then the duration of one of the pulsed beams, the later can be considered a continuous beam.

Combination of 1 pulsed beam and 1 continuous beam (1P1CB)

This configuration can serve two main purposes. First, the continuous beam prepares and maintains the sample in a certain state, and the pulsed beam delivers the dose. Second, the pulse delivers the dose, and the continuous beam controls the evolution of the primary photolytic effects. The two beams are brought together at the location of the target inside the sample in a collinear (same or opposed direction of propagation) or non-collinear configuration.

A pulsed laser beam can be considered continuous, if the characteristic time of the photolytic processes taken into consideration is much shorter then the duration of the pulse.

Why is this important?

This page relates generally to methods of dose delivery using laser light, as well as to laser based systems used for such purposes; particularly it relates to methods of delivering a dose within a dielectric material, with a controllable spatial distribution, using laser light pulses, as well as to the laser based system used for such purpose.

Background

This page is concerned with the use of lasers for material processing. Laser light affects materials of all types through fundamental processes such as: excitation, ionization, dissociation of atoms and molecules. These processes depend on the proprieties of the light, as well as on the proprieties of the material. Using lasers for material processing necessitates being able to control these fundamental processes in any circumstance. Here we present a method to induce a great variety of ionization patterns to condensed materials (amorphous or structured). This is achieved by supperposing different modes of ionization. A mode of ionization is a class of ionization effects that share similar characteristiques, and that can be induced in all types of condensed materials.

The effects of high energy radiation on materials

The most important and fundamental effects of radiation on mater are ionization, dissociation, and excitation of atoms or molecules. These effects result from the random and discrete interaction events between the radiation particle and the atoms or molecules that compose the material. During such an event, pockets of energy are transferred from the radiation particle to the material. The energy per unit of mass (J/Kg) deposited through a combination of these effects within the material is called dose. The unit of dose is the gray (1 gray = 1 joule/kg). As we will see later, the energy is deposited at, and around the coordinates of the interaction event. In order to understand the long-term (or stable) effects of radiation on materials, one has to understand, for each type of radiation, the spatial distribution of the dose deposited, the relative importance of different channels of energy deposition (ionization channels, dissociation channels, excitation channels, heat, etc), and the immediate changes in structure or composition of the material (new chemical species generated during interaction, or modification of the structure, or the order, within the material).

The interest in the secondary effects of radiation

The understanding of the effects of radiation on materials has led to numerous applications in a variety of fields such as: biology, medicine, industry (materials, food, energy, pharmaceutical), environment, and others. Some applications are not concerned with the radiation particle itself. In such cases, the effects of the radiation particle on the material, such as changes in composition, or changes of its structure are of interest. Anything that would produce similar effects can, in principle, replace that type of radiation.

As said earlier, the possible outcomes of the stochastic interactions of the radiation particles with the atoms or molecules composing the material are: ionization, excitation, or dissociation (in the case of molecules). As a result of that, primary reactive species such as ions, excited atoms or molecules, free or solvated electrons, and charged or neutral radicals, are created. For some applications, the role of the radiation rests merely in creating these primary species. As time elapses from the initial event, new secondary compounds are formed from the combination of primary species, which can play an important role in chemical and bio-chemical processes of interest. In the case of ordered materials, such as crystals, carbon nanotubes, thin membranes, thin films, gels, etc., a change in the local structure might occur, having an impact on the mechanical, electrical, and optical properties of these materials. Some of these changes (compositional or structural) are temporal, while others are permanent.

Concrete examples of applications where only the compositional or structural changes are important are: the study of oxidative stress, the study of ultra-fast processes (in the case of pulsed radiolysis), in material processing (polymerization, grafting, degradation, crystal lattice modification), in radiochemistry (synthesis of compounds involving radiation), and others. The radiolysis of water is very importnat for applications in biology and chemistry. The most important radiolytic species generated by the irradiation of water are: e-, H2O+, H2O*, HO2•, •OH, OH-, H+, H•, O, O-, O2, O2• -. The yields and spatial distribution of these species depend on the type of radiation, water temperature and impurities.

On the other hand, some applications are concerned directly with the radiation particles. Examples are: imaging (X-ray, γ-ray, scanning electron microscopy, PET scan, etc.), doping of materials, ***.

Radiation based dose delivery systems

Different systems are used to deliver radiation to a target material. Common radiation sources are: particle accelerators (emit charged particles such as electrons, protons, or heavy ions), synchrotrons and X-ray tubes (emit X-rays), nuclear reactors, isotopes, radio-labeled bio-active molecules, and others. A common characteristic of the ionizing radiation is that it interacts not only with the target, but also with everything that it encounters in its path: the air or any other type of material in witch the target is immersed or contained. This factor can be a major inconvenient. In some cases, the sample must be maintained in vacuum or at very low pressures. When it is impossible to separate the target from its ambient milieu, one has to live with the inconvenience of irradiating also everything around the target.

Effects of laser light on materials

We will now look at similar effects produced on materials by laser light. A broad range of laser light sources is available nowadays. We can produce coherent light up to the UV domain, and even X-ray more recently. The ionizing power of UV light is well known. The energy of the photons is greater then the ionizing energy of any atom and molecule. Ionization, dissociation and excitation processes resemble that of X-rays. In essence, an atom or a molecule absorbs one UV photon and an electron is ejected. The dose distribution is similar to that produced by X-rays and γ –rays. In reality, the distinction between UV and X-ray is only artificial, since both types of radiation are of the same nature. There is only a smooth passage from one domain to the other, and the only difference resides in the energy of the photon. As we descend on the scale of energy (going from X-ray to UV), the electrons affected are these situated on higher orbitals. If inner shell ionization is possible with X-rays, valence electrons are affected by UV light. As we continue to descend towards the visible domain, single-photon ionization becomes impossible, dissociation and electronic transition remain likely in the visible, and vibration, librations, and rotations are excited in the IR. This is where the properties of laser light come into play. Coherence makes the ionization possible even in the IR domain, if the light’s intensity is sufficiently high. Different mechanisms of photoionization, and\or dissociation, and/or excitation are incited, depending on the frequency of the light, the intensity, the time of irradiation, and the polarization. In a given experimental condition, a group of these processes concur for the final photolytic effects.

Photoionization is a very important process, giving rise to a variety of characteristic photolytic effects. There are many types of photoionization processes depending on the laser light’s characteristics, and on the type of material. In a given setting, a few photoionization processes can operate simultaneously, forming larger photoionization patterns called modes of photoionization. We can distinguish four modes of photoionization: the single-photon (SP) mode, optical breakdown (OB), below OB threshold (B/OB), and filamentary (F) ionization. The first is possible only in the UV, the last from the visible to the IR, and the two in between from the UV to the IR. Their specific ionization spatial distribution, density of ionization, as well as the nature and yield of primary photolytic species characterize these modes of photoionization.

The existence of the filamentary ionization mode was demonstrated only recently, and only a few applications have been developed. A comprehensive study of the filamentary ionization mode was published as a master’s thesis under the mane of “Description de la photoionization de l’eau au moyen d’impulsions laser fs à 790 nm dans le régime d’autofocalisation. Applications possibles dans le domaine de la Radiobiologie”, at Sherbrooke University (Canada/Quebec), Department of Nuclear Medicine and Radiobiology, in September 2004, by myself. The method, and the device that embodies it, that are claimed here are largely dependent on this newly discovered mode of photoionization.

Give the parameters of the laser pulses of interest here, the intensity, the pulse duration, wavelength, according to the applications. The Fondamental processes depend also on these parameters, and a limitation of these parameters limits the variety of ptocesses that must be discussed.

The interest in the secondary effects of laser light

Along with these four modes of ionization, the laser light also induces excitation and dissociation. The concept of dose can be adapted to laser light. The same types of primary species can be generated by photolysis, as in the case of radiolysis (using X-ray, γ-rays, neutrons, or charged particle radiation- electrons, protons or ions). Since we are able to generate the same primary photolytic species with a similar spatial distribution, we deduce that radiation can be replaced by laser light at least in some applications, where only the radiolytic effects are considered, and not the radiation particle itself. The replacement of radiation by laser light brings along significant advantages, which we will illustrate shortly.

The method and the device that embodies it, that are claimed here are based on a body of knowledge that enables one to control the yields of the primary photolytic species generated, and their spatial distribution. This body of knowledge couples controllable optical characteristics of the laser beam (such as: wavelength/spectrum, average intensity, spatial and temporal intensity and wavelength distribution, polarization, and pulse duration) with the desirable photolytic effects. It enables one to generate a great variety of dose distributions within dielectric materials, by selectively inducing a superposition of the fore modes of ionization mentioned above. For example, to reproduce important aspects from the characteristic spatial dose distribution of photon radiation (X-rays and γ-rays) or heavy charged particle radiation (protons or ions).