5 STEPS TO BUILD A HIGH-PERFORMANCE SCIENTIFIC LIBS SETUP

Brief introduction to the world of LIBS systems

2. How to arrange and configure them?

All the shopping is done, so what is the next? Here comes the part that can be amusing as well as tiring and never ending: system assembling and adjustment. Optical table with breadboard with sufficient size and robustness is suitable basement; it absorbs vibrations and enables optical and mechanical system components to be mounted with the help of threaded holes raster. Optical table (Thorlabs website, 2016) Placement of elements depends on the desired performance of the setup. Either way it is convenient to make a sketch of the system (those more enthusiastic can even make a 3D model) so that we lay out the space for intended configuration with respect to potential future extension. We assemble and mount the sample manipulator. We position the laser head and the spectrometer with detector in sufficient distance from the manipulator, providing maneuvering space in the future. Then we decide the direction, from which the primary pulse hits the sample – either from above, from side, or from below – and subsequently we assemble optical pathways. It is useful to build optomechanical assemblies for laser pathways, plasma emission collection and other elements from the so called cage system . Components of the cage system (Thorlabs website, 2016) Cage system is an extensive system of mutually compatible holders, adapters, tubes, kinematic mounts for lenses, mirrors, prisms, optical fibers, objectives, etc. They are mutually connected by four thin rods resembling a cage. This system offers great versatility and considerable number of degrees of freedom for adjustment of all present optical elements, which is fundamental for correct system setting. In the simplest case, i.e. when we want to build a setup for the basic single pulse technique (SP-LIBS) with one detection system with no supplements, it is sufficient to guide the laser beam to the sample via simple (plano-convex) focusing lens and focus it „by ear“ (i.e. on the base of the ablation noise intensity). Plasma emission is then collected by experimentally set collection optics in the form of optical fiber end or one (biconvex) UVFS lens that focuses the plasma emission into the input of the optical fiber. On the other hand, if we are really serious about LIBS, it is necessary to more optimize and especially to extend the system… The focusing lens may be exchanged for (non-cemented) multi-lens system (doublet, triplets or special microscopy objectives), with corrected optical aberration (especially spherical and coma), which are coated with a suitable antireflection layers. The outcome will be a distinctively smaller ablation crater, smaller burnt area around the crater, higher surface laser intensity and elimination of defocusing of individual wavelengths of the utilized lasers. Chosen focal length dictates the spot size, working distance and depth of field. Focusing lens/objectives (Optosigma website, 2016) Commonly utilized simple UVFS biconvex lens for plasma radiation collection may be replaced with a system that has corrected color aberration in the requested spectral range. Chosen focal length and placement of the collecting lens depends on what percentage of the plasma is to be imaged into the optical fiber input, i.e. according to the desired magnification. At this point it is usually time to employ special optical design software. Reflective systems using parabolic or elliptic mirrors with aluminum coating and a dielectric layer increasing the reflectivity in the UV range may be advantageous for light collection due to the non-existing color aberration. The LIBS system may be equipped by a camera for a sample view. Choice of right combination of common CMOS camera and suitable objective lens depends on many factors, especially on requested field of view and intended camera position with respect to the sample. Most likely the camera will be placed in a direction perpendicular to the sample, looking to the sample through the laser-focusing objective. In this configuration, it is necessary to include into consideration apertures and optical parameters of the optical elements, which are usually located between the view system and the sample – including the laser-focusing objective. Vignetation may appear in the corners and image may be negatively affected by the bad aberration corrections. The sample should be illuminated, ideally with continuously adjustable intensity. The image from the camera of such system can be utilized for an autofocusing algorithm. This has to be, however, preceded by a specific calibration of the system. Utilizing more than one illumination type is good choice to properly illuminate various types of sample surfaces (rough as well as polished). A complex experimental LIBS setup should be capable of double pulse technique (DP-LIBS); both in orthogonal and collinear arrangement. The former requires creating a secondary laser path to the sample. Gradually, many other items can be added:

• Secondary collecting system – for implementation of another spectrometer. A combination of wide range echelle system with highly sensitive Czerny-Turner is an excellent choice.
• Fast, triggered camera with an objective lens for recording of plasma shape evolution in time.
• A nozzle/jet with air or gas to create local atmosphere.
• A module with a microphone to record plasma noise intensity
• A module with a magnets to influence the induced plasma
• And broad range of other elements such as secondary sample view system, laser paths for laser fluorescence, optical tweezers, optical catapulting/injection, or other elements for shadowgraphy, laser diffraction, etc.

All these elements have to be mounted with sufficient and precise adjustment options. Moreover, many of them need to be aimed at the identical point. This multi-axis adjustment can be time consuming and troublesome. Even the unification of only two laser beams during the DP-LIBS technique is quite demanding and it is convenient to have suitable calibration tools prepared. Effort to align more than 2 optical systems is of course even harder. In advanced LIBS setups it is therefore suitable to have a mounting system around the manipulator that will enable mounting of all elements with highest possible versatility and modularity. Cage chamber by Atomtrace There are also many other useful tools which should not be absent during assembling and adjusting of any LIBS setup: Targeting lasers provide invaluable help when assembling a LIBS setup. It can be small, cheap, continuous red/green diode laser that illuminates the ablation spot on the sample for laser pathways adjustment, or it can enter the collection optical systems in the reverse direction to check the target spot. Other useful elements are neutral density filters, diffusers, CMOS camera modules, aligning target plates, fluorescing and liquid crystal alignment disks, series of basic singlet lenses, etc.