Microchip Laser-Induced Breakdown Spectroscopy: A Preliminary Feasibility Investigation

Abstract

A commercial, 7 microJ/pulse, 550 ps microchip laser is used to induce plasma on Pb, Si, Cu, Fe, Ni, Ti, Zn, Ta, and Mo foils and a Si wafer. The measured plasma lifetime is comparable with the duration of the laser pulse (a few ns). The plasma continuum radiation is low, while some of the strong resonance lines (e.g., Zn 213.86 nm) show self-reversal. Quantitative analysis is possible using non-gated detectors but analytical lines should be chosen with care to avoid reduction in the linear dynamic range. The mass removed (0.5-20 ng/pulse) is sufficient to yield spectra that are detectable with portable grating spectrometers equipped with non-gated, non-intensified detector arrays. The spectrum of Cd is detected with a broadband portable spectrometer (200-950 nm). The combination of the broadband spectrometer and the microchip laser is very promising for material identification, especially in field applications.

Laser Induced Breakdown Spectroscopy

“…As its name indicates, this material has the property of absorbing the electromagnetic radiation in an amount that increases less and less while the radiation intensity increases, that is, the differential increase decreases until total saturation of the absorber. 55,57 In this way, it absorbs the initial laser emission, increasing the energy losses in the laser cavity, but without preventing the amplification of the laser radiation in the active medium, so that the radiation increases until reaching a threshold energy that promotes the total saturation of the absorber, decreasing the energy losses and allowing the production of a laser pulse. Passive Q-switching is much employed in Nd:YAG lasers or similar solid state lasers, using a solid state saturable absorber, such as Cr…”

“…Microchip lasers present lower pulse to pulse amplitude variation (by one order of magnitude), higher repetition rates (up to tens of kilohertz), and lower pulse durations (low to hundreds of picoseconds) than do solid state actively Q-switched lasers. 55,57,62 On the other hand, they produce pulses with lower energies (generally up to hundreds of microjoules for each pulse), which can be partially compensated by the high laser beam quality (described by the beam propagation ratio, M 2 , discussed below), allowing focusing to a micrometer sized spot (compared to spots of tens of micrometers generally obtained with solid state actively Q-switched lasers), 57,63 which increase the energy and power delivered per surface area (called fluence and irradiance, respectively).…”

“…83,125,126 The use of a microscope or just its objective lens to focus the laser beam is very common. 57,83,103,107 This allows a more precise focus and a lower focusing area, leading to a higher space resolution and, in the case of using the microscope, better observation of the sample surface, leading to a better control of the area to be ablated. Collection of the emitted radiation also can be made on the same axis as that propagating the laser beam by using a dichroic or pierced mirror.…”

“…These kinds of spectrometers, available commercially, have been used in many works. 57,138 In the last years, a kind of spectrograph already used in other atomic emission spectroscopic techniques since the 1990s, 139,140 called echelle spectrograph, which comprises a special kind of grating in a specific design, is being increasingly used in LIBS. It is also compact and, associated with appropriate detectors, constitutes a spectrometer covering a wide range of wavelengths (commonly from 200 up to 1000 nm) with a high resolving power (λ/Δλ up to above 10000).…”

“…161 In addition of providing the acquisition of emission spectra, CCD cameras have also been used in LIBS applications for plasma imaging or sample imaging, in order to monitor plasma formation, 150,162,163 or to adjust the sample position and the optical alignment in space resolved applications, 66,83,107 respectively. Studies of the temporal evolution of the plasma and the laser pulse are also carried out using fast photodiodes and PMT connected to oscilloscopes, 57,134 while the acoustic emission associated to the laser ablation process can be measured using microphones connected to an oscilloscope or, 81 in a lower cost system, a standard microcomputer sound card. 125 The variation of acoustic emission as a function of the laser energy can be used to calculate the ablation threshold energy, or the minimum energy needed to start the ablation process, for a given sample.…”