Ultrafast Laser Technology and Spectroscopy

Abstract

Ultrafast laser technology and spectroscopy involves the use of femtosecond (10 −15 s) laser and other (particle) sources to study the properties of matter. The extremely short pulse duration allows one to create, detect and study very short‐lived transient chemical reaction intermediates and transition states. Ultrafast lasers can also be used to produce laser pulses with enormous peak powers and power densities. This leads to applications such as laser machining and ablation, generation of electromagnetic radiation at unusual wavelengths (such as millimeter waves and X‐rays), and multiphoton imaging. The difficulty in applying femtosecond laser pulses is that the broad frequency spectrum can lead to temporal broadening of the pulse on propagation through the experimental set‐up. In this article, we describe the generation and amplification of femtosecond laser pulses and the various techniques that have been developed to characterize and manipulate the pulses.

Laser Induced Breakdown Spectroscopy

“…3, 2007 very useful for understanding the principles of operation of some devices used in LIBS instrumentation. [46][47][48][49][50][51][52][53][54] As a typical LIBS instrument is made up of well defined parts, including a laser source for sample ablation and excitation, an optical system for driving and collecting the laser and plasma radiations, a wavelength analyzer, and a detection system, the following sections will describe the instrumentation in this sequence, highlighting the technological advances of recent years. Two additional sections describe mobile instruments, due to the increasing interest in this area, and commercial instruments, which, after a premature commercialization in the 1960s and 1970s, have only recently become available again.…”

“…The excitation by photons is made using lamps, called flashlamps (such as a xenon glow discharge lamp), and, more recently, another laser, such as a diode laser (a laser whose cavity is placed within a light emitting diode). 5,[51][52][53][54] As the LIBS technique involves the application of a laser pulse on a sample surface, the laser source must employ some mechanism to extract a pulse of radiation with a reproducible duration time, also called pulse duration, from the laser cavity. This can be made simply by pulsing the flashlamp, where the laser pulse will be produced as long as the energy amplification in the cavity is above its losses until the end of the flashlamp pulse, or, in a more complex way, by techniques such as modelocking, much employed in ultra-short pulse lasers (discussed later), and Q-switching.…”

“…Q-switching is based on preventing radiation amplification from successive traveling in the active medium by increasing the energy losses and, suddenly, enabling emission by fast loss reduction. [51][52][53][54][55] In active Q-switching, the variation of the energy losses is made by a combination of a polarizer and some device for polarization rotation inside the laser cavity, between the active medium and the rear mirror. 51 Therefore, after the initially generated radiation passes through the polarizer, becoming polarized, the switching between no polarization rotation and 90 o rotation enables fast switching between activation and deactivation of the laser beam, allowing the production of laser pulses with a few nanoseconds of pulse duration, up to tens of hertz of pulse repetition rates, and up to hundreds of millijoules of pulse energy.…”

“…Initially, active and passive mode-locking techniques were used, which are similar to those of active and passive Q-switching, but employing a periodic variation of the energy losses (loss modulation) above and below the constant cavity gain, with a shorter time below the gain (which defines the pulse duration) and a period depending on the cavity length and the propagation velocity of the laser pulse (cavity round-trip time). [52][53][54] The first modelocked lasers were developed in the mid-1960s using ruby or a Nd:Glass (a Nd 3+ -doped silicon oxide crystal), enabling the production of laser pulses with durations on the order of picoseconds for the first time. 85,86 The first femtosecond lasers were colliding-pulse mode-locked dye lasers, which also employ a dye-chain amplifier for energy amplification, developed in 1980s, and allowing pulse durations down to 30 fs.…”

“…Chirped pulse amplification is based on stretching the laser pulse, which leads to a reduction in its maximum intensity, followed by its amplification and a subsequent recompression back to the femtosecond scale. 52,90,91 In recent years, most papers dealing with ultrashort pulse lasers applied to LIBS use the self mode-locked Ti:saphire lasers employing chirped pulse amplification. 75,[92][93][94][95][96] In LIBS, when a laser pulse with a duration on the order of femtoseconds is applied to the sample, as this time is shorter than the thermal coupling time constant in matter (about 1 picosecond), the mechanisms leading to plasma formation are dominated by multiphoton ionization over thermal decomposition, with a subsequent plasma decrease without further interaction with the laser pulse.…”