Note that these 1|]# and the techniques mentioned in the introduction do not complete the ever-growing list of possible effects and applications of ultrashort lasers for sensing and spectroscopy but provide a sampling of the various ultrashort-related effects that have been used to modify or extend existing approaches.2.?Laser-Induced Breakdown SpectroscopyIn this section we will focus on emission spectroscopy techniques that employ ultrafast, high-fluence, optical excitation. Among these, the technique that has been most widely investigated is laser-induced breakdown spectroscopy (LIBS), an analytical technique based on the spectral analysis of optical emission from laser-induced plasmas [13].

This technique has inherent stand-off detection capability, requires a very small amount of material for analysis and can perform at high detection rates making it attractive for analysis of organic and inorganic materials in a variety of circumstances [14,15]. It takes approximately one second or less to acquire a broadband spectrum for LIBS allowing for rapid materials analysis with the primary limitation being the read-out time of the spectrometer used to measure the emission spectrum [16].Pulsed lasers can easily achieve the required conditions for LIBS materials analysis since the rates of energy deposition greatly exceed those of energy redistribution and dissipation with the result that extremely high temperatures can be achieved in regions where energy absorption occurs.

Even so, the interactions of femtosecond laser pulses with materials are substantially different from those of nanosecond laser pulses since the rates of energy deposition are significantly higher.

This leads to a range of material responses that ultimately affect LIBS measurements. For example, material removal [17,18] and plasma expansion characteristics [19] can vary significantly with excitation pulse duration. For materials with complicated chemical compositions, femtosecond excitation can yield ejecta resembling bulk stoichiometry [20,21] resulting in LIBS spectra that can be used to assess composition with tighter confidence intervals than for nanosecond excitation [22].

Also, ultrafast excitation results in a smaller heated volume around the ablation region generally leading Brefeldin_A Batimastat to material removal processes that are more reproducible [23,24]. For molecular solids, ultrafast excitation has the potential to improve the analytical capability of LIBS, since mass spectrometry studies have shown high mass fragment and cluster formation under femtosecond laser irradiation [25] as well as optical emission from small molecules characteristic of the irradiated solid-phase species [26].