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Nonlinear Optical Phenomena in the Infrared Range

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Various aspects of nonlinear optical phenomena in the infrared range

  • Yu Qin

Nonlinear optics is a branch of optics, which describes the behavior of light in nonlinear media, where the dielectric polarization P responses nonlinearly to the electric field of the light E. This is a very broad concept. In this thesis, we focus our study on three aspects of nonlinear optical phenomena in the infrared wavelength range: the characterization of a mid-infrared ultrashort laser by autocorrelation based on Second Harmonic Generation (SHG), the influence of the beam mode on the interaction between laser and media during nonlinear propagation of femtosecond near-infrared pulses in liquid, and the dynamics of the ablation of solid samples submerged in liquid using a long nanosecond near-infrared laser.

Many energy levels of molecules and lattice vibrations are in mid-infrared wavelength range of 2.5-25 µm. For this reason, this wavelength range is called chemical fingerprint zone. Infrared absorption spectroscopy using light source in this wavelength range has been widely used identify different covalent bonds in many kinds of samples. Besides, by irradiation of an intense and short laser pulse whose wavelength is tuned to the resonance, a specific molecular band absorbs the pulse energy, and specific chemical reaction is excited. For this reason, tunable mid-infrared ultrafast lasers have a lot of potential applications in energy and material science, i.e., the production of alcohol or hydrogen from H2O and CO2, and the development of next-generation solar cells.

Kyoto University Free-electron Laser (KU-FEL) is an oscillator-type free-electron laser, which works in the mid-infrared wavelength range of 5-13 µm. In temporal domain, the pulses from KU-FEL have a dual-pulse structure. In a macropulse with the duration of a few microseconds, thousands of micropulses sit with the interval of 350 ps between each other. 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 µm, this instability is around hundreds of Gigahertzes, which is comparable to the bandwidth of the vibrational modes. For those potential applications in which resonances are involved, stabilization of the wavelength of KU-FEL is necessary. And before that, we should first know the amount of wavelength instability. Besides, similar to all other ultrashort pulse lasers, micropulse duration of KU-FEL is very important information for applications such as nonlinear optics. For these purposes, in this thesis, we report the measurements of both the duration and wavelength instability of KU-FEL micropulses using the technique of Fringe-Resolved AutoCorrelation (FRAC).

For temporal characterization of ultrashort pulses, standard techniques such as Frequency-Resolved Optical Gating (FROG) and Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER) are invented more than ten years ago, which can give a single-shot measure for both the amplitude and the phase of the electric field, even for the pulses with the durations down to few cycle. Both FROG and SPIDER are spectrum-resolved measurement, for which the 2D array detector (CCD) is required to measure the single-shot spectrum. However, such kind of detectors for the mid-infrared wavelength range is very expensive, and not available in our institute. Under this condition, we perform an autocorrelation measurement of KU-FEL, and try to find the information about pulse duration and wavelength instability for the results.

Autocorrelation is a kind of well-known technique, which is invented more than thirty years ago. It is usually used for a rough estimation of the pulse duration of ultrashort laser pulses. In this thesis, by a systematic study of the influence of the wavelength instability on the signal of FRAC measurement, we first propose a method of measuring the wavelength instability of micropulses of an oscillator-type FEL by FRAC. Besides, we find that, by integrating the FRAC over the delay time, we can measure the duration of an ultrafast pulse, without knowing the chirps in advance. To the best of our knowledge, this finding has not been reported anywhere else, and it can save us from an additional Intensity AutoCorrelation (IAC) measurement.

Both of the above mentioned methods work well when applied to an FRAC measurement of KU-FEL at the wavelength of 12 µm. The durations and the wavelength instability of the microoulses are measured to be ~0.6 ps and 1.3%. This technique can be also applied for characterization of ultrashort pulses at other wavelengths, where 2D array detectors are not easily available, i.e., for the extreme-ultraviolet case.

Since our autocorrelation measurement is based on SHG, which is a second order nonlinear process, good focusablity of the laser beam is required to reach the high intensity at the focus position. To test the focusibility of the KU-FEL, a measurement of M2 factor of KU-FEL is carried out by the 2D knife-edge method before the autocorrelation measurement. The most convenient way to measure the M2 factor of a laser is to measure the beam profile at different distances from the focus by a beam profiler, and analyze the results. The reason why we choose the old-fashioned knife-edge method is still the lack of 2D array detector in this wavelength range. The beam profiles at different distances from the focus are reconstructed from the results of knife-edge scanning in both horizontal and vertical directions. During the data analysis, the beam of KU-FEL is found to have the non-Gaussian beam profile. As a result, the analytical methods developed for Gaussian beams under the knife-edge measurement do not work for our case. Taken the non-Gaussian property of the beam into consideration, some special and original treatments are taken during the data analysis.

With the development of the Ti:sapphire laser and the chirped pulse amplification (CPA) system, high power at the order of Terawatt becomes available at the wavelength of around 800 nm. This has attracted a lot of interests on the studies of nonlinear optics, such as 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 media has been a hot topic. During the nonlinear propagation of femtosecond pulses, due to the balance between self-focusing, plasma defocusing, and nonlinear loss, the intense part of the laser beam collapses to a spot with very small diameter, which can propagate for a distance much longer than the Rayleigh length. This phenomenon is called filamentation. Because of the long focal depth of the filamentation, it has many applications such as laser machining, Laser Imaging, Detection and Ranging (LADAR), and long distance Laser-Induced Breakdown Spectroscopy. Besides, strong spectral broadening occurs during filamentation, and the coherent white light is generated at the central part of the beam. This effect is widely used for pulse compression. And for the reason of high time resolution, this coherent white light also serves as a good light source in spectroscopy.

Most of the studies about filamentation have used Gaussian beams as the incident beams. Recently, the axicon lens has made the generation of Bessel beam much easier. Many groups have focused their studies on the filamentation induced by Bessel beams. Compared with Gaussian beams, Bessel beams keep the high on-axis intensity for even longer propagation distance, thus can produce longer filamentation. We perform a comparison study of filamentations generated by Gaussian and Bessel beams. Since the pulses we can use are splitted from a CPA system, which contain the energy of 200 µJ, we choose the liquid as the nonlinear media. Compared with gaseous media, liquid has much larger nonlinear coefficient, so that the nonlinear effect can be observed at much lower incident power, and in a much shorter propagation range. Besides, unlike solid media, we can use the liquid sample for long time during experiment, without worrying about the laser-induced damage. During this experiment, we have confirmed 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 intense laser pulse is focused on the material, plasma is generated. During this process, small portion of the material to be analyzed gets atomized and excited, and emits light. By collecting and analyzing the spectra of the emitted light, we can detect the constituents of the material, or even the relative abundance of each constituent element. This technique is called Laser-Induced Breakdown Spectroscopy (LIBS).

Compared with other similar techniques, LIBS has many advantages, i.e., in principle, it can detect all elements, and can analyze any matter regardless of its physical state, be it solid, liquid or gas. Since during a single shot in the LIBS measurement, the mass of the ablated material is in the range of picogram to nanogram, the LIBS is considered to be non-destructive. Another important advantage of LIBS is the easiness of the sample preparation. For most of the cases, the sample does not require any treatment before LIBS measurement. For this reason, LIBS can be applied for in-situ multi-elemental analysis. And due to its fast analysis time, LIBS can be used for a realtime composition measurement.

Nd:YAG laser at fundamental wavelength (1064 nm) is most often used during LIBS experiments. It has several advantages, i.e., the scattered laser light does not influence the measurement of the visible spectra, and compared with shorter wavelength, laser at this wavelength has better heating effect on the laser-induced plasma.

Compared with LIBS of solid sample in gaseous media, LIBS of solid sample under liquid is more complicated. In such condition, if the single nanosecond pulse is used 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 double pulses LIBS, during which the first pulse can generate a bubble near the surface of the sample, in which the plasma produced by the second pulse can expand. Another solution is to use the long nanosecond pulses, which 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 provides a space with low density for the plasma plume to grow. Compared with the double pulses LIBS, the advantage of the long pulse LIBS is that, it can be applied for the measurement under very high pressure. However, if the double pulses LIBS is applied under such condition, the bubble generated by the first pulse can not grow to a size large enough for the plasma plume generated by the second pulse to expand inside. And as a result, the double pulses LIBS loses its advantage.

In this thesis, we report our experimental study of long pulse LIBS of solid samples under liquid. Two experiments are included. The first one is to optimize the laser focus position, and the second one is to study the influence of solvent temperature on the ablation dynamics. The results of these experiments can help us better understand the dynamics of ablation during long pulse LIBS of solid sample submerged into liquid.


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