Conventional laser induced breakdown spectroscopy (LIBS) mostly uses silicon-based detectors and measures the atomic emission in the UV-Vis-NIR (UVN) region of the spectrum. It can be used to detect the elements in the sample under test, such as the presence of lead in the solder for electronics during RoHS compliance verification. This wavelength region, however, does not provide sufficient information on the bonding between the elements, because the molecular vibration modes emit at longer wavelength region. Measuring long-wave infrared spectrum (LWIR) in a LIBS setup can instead reveal molecular composition of the sample, which is the information sought in applications including chemical and explosive detection and identification. This paper will present the work and results from the collaboration of several institutions to develop the methods of LWIR LIBS for chemical/explosive/pharmaceutical material detection/identification, such as DMMP and RDX, as fast as using a single excitation laser pulse. In our latest LIBS setup, both UVN and LWIR spectra can be collected at the same time, allowing more accurate detection and identification of materials.
The infrared (IR) absorption and emission properties of Ho:KPC, Ho:KPB, and Ho:YAG were compared for possible applications in 2 µm laser cooling. Ho:KPC and Ho:KPB crystals were grown by vertical Bridgman technique using purified starting materials. A commercial Ho:YAG crystal was included in this study for comparison. Under resonant pumping at ~1.907 µm, the Ho-doped KPC/KPB crystals exhibited broad IR emission centered at ~2 µm based on the Ho<sup>3+</sup> intra-4f transition <sup>5</sup>I<sub>7</sub> → <sup>5</sup>I<sub>8</sub>. Under similar experimental conditions, Ho:YAG showed a narrow-structured emission band reflective of individual Stark levels. The average emission wavelength for Ho:YAG was determined to be ~2.03 µm. Initial heat loading/cooling experiments under ambient air were performed using a fiber laser operating at ~2.036 µm with an output power of 2 W. The Ho:KPC/KPB crystals exhibited small temperature increases of ~1.0 ºC. A significantly larger temperature increase of ~5 ºC was observed for Ho:YAG. IR transmission studies revealed the existence of OH impurities in the Ho-doped halides, which possibly lead to non-radiative decay channels.
Tm<sup>3+</sup> doped solids have shown promising results for laser cooling applications at IR wavelengths of ~2 μm. The extended IR fluorescence of the involved Tm<sup>3+</sup> transition (<sup>3</sup>H<sub>6</sub> → <sup>3</sup>F<sub>4</sub>), however, requires low-phonon energy hosts reducing the detrimental effect of non-radiative decay through multiphonon relaxation. In this work the temperature dependent absorption and emission properties of Tm doped KPC (hν<sub>max</sub><200 cm<sup>-1</sup>) and KPB (hν<sub>max</sub><140 cm<sup>-1</sup>) crystals were evaluated for applications in laser cooling. Under laser pumping both crystals exhibited broad IR fluorescence at room temperature with a mean fluorescence wavelength of 1.82 μm and bandwidth of 0.14 μm (FWHM). Initial experiments on laser-induced heating and cooling were performed using a combined IR imaging and fluorescence thermometry setup. Employing a continuous-wave laser operating at 1.907 μm, Tm: KPC and Tm: KPB crystals revealed a very small heat load resulting in a temperature increase of ~0.3 (±0.1) °C compared to undoped reference samples. Further work on material improvement will be necessary to identify possible non-radiative loss mechanisms and to improve the crystal quality.
Laser-induced breakdown spectroscopy (LIBS) is a powerful analytical technique to detect the elemental composition of solids, liquids, and gases in real time. For example, recent advances in UV-VIS LIBS have shown great promise for applications in chemical, biological, and explosive sensing. The extension of conventional UVVIS LIBS to the near-IR (NIR), mid-IR (MIR) and long wave infrared (LWIR) regions (~1-12 μm) offers the potential to provide additional information due to IR atomic and molecular signatures. In this work, a Q-switched Nd: YAG laser operating at 1064 nm was employed as the excitation source and focused onto several chlorate and nitrate compounds including KClO<sub>3</sub>, NaClO<sub>3</sub>, KNO<sub>3</sub>, and NaNO<sub>3</sub> to produce intense plasma at the target surface. IR LIBS studies on background air, KCl , and NaCl were also included for comparison. All potassium and sodium containing samples revealed narrow-band, atomic-like emissions assigned to transitions of neutral alkali-metal atoms in accordance with the NIST atomic spectra database. In addition, first evidence of broad-band molecular LIBS signatures from chlorate and nitrate compounds were observed at ~10 μm and ~7.3 μm, respectively. The observed molecular emissions showed strong correlation with FTIR absorption spectra of the investigated materials.