Posted at 03.10.2018
Various aspects of nonlinear optical phenomena in the infrared range
Nonlinear optics is a branch of optics, which details the patterns of light in nonlinear mass media, where the dielectric polarization P reactions nonlinearly to the electric field of the light E. That is a very extensive concept. On this thesis, we concentrate our analysis on three areas of nonlinear optical phenomena in the infrared wavelength range: the characterization of a mid-infrared ultrashort laser beam by autocorrelation predicated on Second Harmonic Technology (SHG), the influence of the beam method on the connections between laser and mass media during nonlinear propagation of femtosecond near-infrared pulses in water, and the dynamics of the ablation of solid examples submerged in liquid utilizing a long nanosecond near-infrared laser beam.
Many energy of molecules and lattice vibrations are in mid-infrared wavelength range of 2. 5-25 m. Because of this, this wavelength range is called chemical fingerprint zone. Infrared absorption spectroscopy using source of light in this wavelength range has been trusted identify different covalent bonds in many types of samples. Besides, by irradiation associated with an intense and short laser beam pulse whose wavelength is tuned to the resonance, a particular molecular band absorbs the pulse energy, and specific chemical substance reaction is thrilled. Because of this, tunable mid-infrared ultrafast lasers have a whole lot of potential applications in energy and materials research, i. e. , the production of alcohol or hydrogen from H2O and CO2, and the introduction of next-generation solar panels.
Kyoto School Free-electron Laser beam (KU-FEL) can be an oscillator-type free-electron laser beam, which works in the mid-infrared wavelength selection of 5-13 m. In temporal domain name, the pulses from KU-FEL have a dual-pulse structure. Within a macropulse with the length of a few microseconds, thousands of micropulses sit with the interval of 350 ps between one another. Due to its special lasing dynamics, the wavelength instability of this kind of Free-Electron Laser (FEL) is relatively worse compared with optical lasers, i. e. , at the working wavelength of 12 m, this instability is just about a huge selection of Gigahertzes, which is related to the bandwidth of the vibrational settings. For those potential applications in which resonances are participating, stabilization of the wavelength of KU-FEL is necessary. And before that, we have to first know the amount of wavelength instability. Besides, similar to all other ultrashort pulse lasers, micropulse length of time of KU-FEL is vital information for applications such as nonlinear optics. For these purposes, in this thesis, we report the measurements of both length and wavelength instability of KU-FEL micropulses using the approach of Fringe-Resolved AutoCorrelation (FRAC).
For temporal characterization of ultrashort pulses, standard techniques such as Frequency-Resolved Optical Gating (FROG) and Spectral Stage Interferometry for Direct Electric-field Reconstruction (SPIDER) are invented more than a decade ago, which can give a single-shot solution for both amplitude and the period of the electric field, even for the pulses with the durations down to few circuit. Both FROG and SPIDER are spectrum-resolved dimension, that the 2D array detector (CCD) must gauge the single-shot spectrum. However, such kind of detectors for the mid-infrared wavelength range is very expensive, and not available inside our institute. Under this condition, we perform an autocorrelation way of measuring of KU-FEL, and look for the information about pulse length of time and wavelength instability for the results.
Autocorrelation is a kind of well-known strategy, which is created more than thirty years back. It really is usually used for a harsh estimation of the pulse duration of ultrashort laser pulses. Within this thesis, by a systematic research of the effect of the wavelength instability on the sign of FRAC measurement, we first propose a way of calculating the wavelength instability of micropulses of any oscillator-type FEL by FRAC. Besides, we find that, by integrating the FRAC in the hold off time, we can gauge the period of an ultrafast pulse, without knowing the chirps in advance. To the best of our knowledge, this finding has not been reported somewhere else, and it can save us from yet another Power AutoCorrelation (IAC) way of measuring.
Both of all these methods work very well when applied to an FRAC dimension of KU-FEL at the wavelength of 12 m. The durations and the wavelength instability of the microoulses are assessed to be ~0. 6 ps and 1. 3%. This technique can be also requested characterization of ultrashort pulses at other wavelengths, where 2D array detectors are not easily available, i. e. , for the extreme-ultraviolet circumstance.
Since our autocorrelation way of measuring is based on SHG, which really is a second order nonlinear process, good focusablity of the laser beam must reach the high depth at the emphasis position. To test the focusibility of the KU-FEL, a way of measuring of M2 factor of KU-FEL is carried out by the 2D knife-edge method before the autocorrelation dimension. The easiest way to measure the M2 factor of the laser beam is to gauge the beam profile at different distances from the focus by the beam profiler, and review the results. The reason why we choose the old-fashioned knife-edge method is still having less 2D array detector in this wavelength range. The beam profiles at different distances from the emphasis are reconstructed from the results of knife-edge scanning in both horizontal and vertical directions. Through the data evaluation, the beam of KU-FEL is available to really have the non-Gaussian beam profile. As a result, the analytical methods developed for Gaussian beams under the knife-edge dimension do not be employed by our case. Taken the non-Gaussian property of the beam into consideration, some special and original treatments are used through the data research.
With the development of the Ti:sapphire laser beam and the chirped pulse amplification (CPA) system, high power at the order of Terawatt becomes available at the wavelength of around 800 nm. It has attracted a great deal of hobbies on the studies of nonlinear optics, like the generations of attosecond pulses, Terahertz radiations, high order harmonics, and supercontinuum spectra. From the beginning of this century, the filamentation induced by femtosecond pulses during propagation in nonlinear press is a hot topic. During the nonlinear propagation of femtosecond pulses, due to the balance between self-focusing, plasma defocusing, and nonlinear reduction, the intense area of the laser collapses to an area with very small diameter, which can propagate for a distance much longer than the Rayleigh period. This phenomenon is named filamentation. Due to the long focal depth of the filamentation, it offers many applications such as laser machining, Laser Imaging, Recognition and Ranging (LADAR), and long distance Laser-Induced Break down Spectroscopy. Besides, strong spectral broadening occurs during filamentation, and the coherent white light is made at the central area of the beam. This impact is widely used for pulse compression. And then for the reason why of high time image resolution, this coherent white light also acts as a good light source in spectroscopy.
Most of the studies about filamentation have used Gaussian beams as the event beams. Recently, the axicon lens has made the era of Bessel beam much easier. Many teams have concentrated their studies on the filamentation induced by Bessel beams. Compared with Gaussian beams, Bessel beams keep carefully the high on-axis power for even longer propagation distance, thus can produce longer filamentation. We execute a comparison research of filamentations produced by Gaussian and Bessel beams. Since the pulses we can use are splitted from a CPA system, which contain the vitality of 200 J, we choose the water as the nonlinear mass media. Weighed against gaseous media, liquid has much larger nonlinear coefficient, so the nonlinear effect can be observed at much lower incident electric power, and in a much shorter propagation range. Besides, unlike solid media, we may use the liquid test for very long time during experiment, without fretting about the laser-induced destruction. During this experiment, we have affirmed the resistance of Self Phase Modulation during the propagation of Bessel beam, which is also reported in some papers by other groups. The experimental results and qualitative explanations are reported in this thesis.
When an strong laser pulse is targeted on the material, plasma is generated. In this process, small part of the material to be analyzed gets atomized and thrilled, and produces light. By collecting and studying the spectra of the emitted light, we can find the constituents of the material, or even the comparative abundance of every constituent element. This technique is called Laser-Induced Malfunction Spectroscopy (LIBS).
Compared with other similar techniques, LIBS has many advantages, i. e. , in principle, it can find all elements, and can review any matter no matter its physical status, be it solid, liquid or gas. Since during a one shot in the LIBS way of measuring, the mass of the ablated material is in the number of picogram to nanogram, the LIBS is known as to be non-destructive. Another important good thing about LIBS is the easiness of the sample preparation. For most of the instances, the sample will not require any treatment before LIBS dimension. For this reason, LIBS can be applied for in-situ multi-elemental research. And due to its fast analysis time, LIBS can be utilized for a realtime composition measurement.
Nd:YAG laser at fundamental wavelength (1064 nm) is frequently used during LIBS tests. It includes several advantages, i. e. , the scattered laser light does not influence the measurement of the visible spectra, and compared with shorter wavelength, laser as of this wavelength has better heating up influence on the laser-induced plasma.
Compared with LIBS of sound sample in gaseous media, LIBS of stable sample under liquid is more difficult. In such condition, if the solitary nanosecond pulse is employed for ablation, the measured spectra are always deformed and broadened, which is due to the strong confinement of plasma plume in liquid environment. One solution of this problem is to use the two times pulses LIBS, during which the first pulse can generate a bubble nearby the surface of the sample, where the plasma produced by the second pulse can develop. Another solution is by using the long nanosecond pulses, that have the durations of more than 100 ns. During long pulse LIBS, the diameter of the laser-induced bubble can reach hundreds of micrometers at the trailing part of the pulse, which gives a space with low density for the plasma plume to develop. Weighed against the two times pulses LIBS, the benefit of the long pulse LIBS is that, it could be applied for the measurement under very high pressure. However, if the dual pulses LIBS is applied under such condition, the bubble made by the first pulse can not develop to a size large enough for the plasma plume made by the next pulse to grow inside. And as a result, the dual pulses LIBS manages to lose its benefits.
In this thesis, we survey our experimental review of long pulse LIBS of sound samples under water. Two tests are included. The first one is to enhance the laser concentrate position, and the next one is to review the effect of solvent temperature on the ablation dynamics. The results of these tests can help us better understand the dynamics of ablation during long pulse LIBS of solid sample submerged into liquid.