Characterization of Laser-Induced Plasmas with Terahertz Pulses
Creators: McDowell, Aran J. N.
Publisher: name: University of Alberta Library
Year: 2023

<p>Laser-induced breakdown spectroscopy (LIBS) is a method of material analysis used to determine the composition of a sample by ablating it with an ultrafast laser pulse. By monitoring the light emission from the resulting plasma, the elemental composition of the sample and the concentration of constituents can be determined from the spectral emission lines. This makes it suitable for industrial applications where quick-standoff detection of constituents is required, such as in agriculture. However, matrix effects arising from the water content in soils complicate LIBS analysis for agricultural applications, making it more difficult to determine constituent concentrations from the spectral lines. Proper calibration of the LIBS spectra requires information regarding the water content. A possible optical means by which to isolate water content is to use terahertz (THz) frequency band pulses. This is because water molecule vibrational and rotational modes oscillate at THz frequencies and are attenuated by the presence of water. In addition to possible applications as a probe for water content in soil samples, it could also be possible to provide a direct means to measure plasma conductivity with picosecond resolution. Thus, a hybrid THz-LIBS approach could provide both water content information, plasma conductivity with picosecond resolution, and elemental composition.</p>

<p>In this thesis, the feasibility of using terahertz (THz) pulses to study laser-induced breakdown plasmas (LIBPs) produced in LIBS is investigated. Two approaches were used in this work, the first being a transmissive approach (i.e., THz pulse is sent through the plasma plume) and the second being an emissive approach (i.e., THz emission from a plasma plume is monitored). Plasma formation in both cases was achieved using a copper (Cu) ring that was ablated using a Ti:Sapphire laser of wavelength, $\lambda=795$ nm, a pulse-width, $\tau<em>p=70$ fs, and typical pulse energies of $E</em>p\approx \SI{200}{\mu J}$. Both approaches were tested first using photoexcited (PE) semiconductors, as photoexcitation of a semiconductor results in the formation of an electron-hole plasma (EHP) which has analogous behavior to the space-charge separation in LIBPs. This benchmarked the two approaches. </p>

<p>While a measurable $\Delta \s{T}/\s{T}<em>0$ was observed for the case of a PE semiconductor EHP, there was no discernible $\Delta \s{T}/\s{T}</em>0$ detected for the Cu-LIBP plume. The reason for this is the difficulty encountered synchronizing the THz pulse arrival with Cu-LIBP evolutionary timescales. The emissive approach yielded THz emission in both the photoexcited semiconductor EHP and the femtosecond (fs) ablated Cu cases. To eliminate the possibility that the THz emission in the latter was from a bulk nonlinear process (e.g., optical rectification), the Cu sample was oriented to be at normal incidence with respect to the oncoming fs-laser pulse. This resulted in subsequent frequency components vanishing, but THz emission was still observed. This indicated that most of the THz emission arises from the rapid separation and recombination of charge carriers in the plume, like the photo-Dember effect in semiconductors. To understand this effect, a general model of the charge carrier separation is proposed.</p>

<p>With regards to the development of a hybrid THz-LIBS approach, the results here indicate that for the transmissive approach to work, proper timing between the ablation pulse arrival, the plasma plume timescales, and the THz probe needs further development. While for the emissive approach, it is yet to be determined if the water molecule vibrational and rotational mode oscillations present in the THz pulse come from the ambient environment, the sample, or both. If it solely comes from the ambient environment, the emissive approach for THz-LIBS is non-viable. However, if it is from the latter two, the emissive approach is potentially viable. Future experiments are proposed for both approaches.</p>