Phase-sensitive interferometry with ultrashort optical pulses

Artificial dielectric (AD) materials, where the optical properties of a material are altered through the introduction of subwavelength periodic structuring, have extensive applications in optoelectronic and photonic components to implement properties such as antireflectivity, birefringence, or tuning of the refractive index. However, as the sophistication of AD structures grows, a detailed understanding of the near-field effects in the structure becomes critical. For dense device integration or multifunctional device designs, near-field effects may result in performance degradation. A tool capable of accurately modeling the near fields in an AD is crucial for the study of these effects and for advanced device design. In order to achieve these goals, we have developed a new method which extends the capabilities of rigorous coupled-wave analysis (RCWA) to compute the internal (in addition to the external) fields of an AD with quasimonochromatic or ultrashort pulse illumination. We then apply this technique to investigate one important application of near-field effects: field concentration within an AD. For example, in the study of nonlinear optical interactions with matter, ultrashort pulse lasers are commonplace, as the nonlinear effects are greatly enhanced by the extremely high peak power resulting from the temporal concentration of the field. By using an appropriate AD structure, field localization in the transverse direction can also be achieved. To demonstrate this idea, we design two AD structures that rely on transverse field concentration: (i) a device which exhibits field confinement in the transverse direction when illuminated by an ultrashort optical pulse, thereby allowing enhancement of nonlinear effects; and (ii) a multifunctional device combining an effective l-D photonic crystal (PC) color filter with an AD for field localization, allowing for enhanced detection efficiency.

Theoretical analysis of noncollinear phase-matched optical parametric generation in BBO crystal

A variety of phenomena involves atomic motion on the femtosecond time-scale. These phenomena have been studied using ultrashort optical pulses, which indirectly probe atomic positions through changes in optical properties. Because x-rays can more directly probe atomic positions, ultrashort x-ray pulses are better suited for the study of ultrafast structural dynamics. One approach towards generating ultrashort x-ray pulses is by 90° Thomson scattering between terawatt laser pulses and relativistic electrons. Using this technique, the author generated ~ 300 fs, 30 keV (0.4 Å) x-ray pulses. These x-ray pulses are absolutely synchronized with ultrashort laser pulses, allowing femtosecond optical pump/x-ray probe experiments to be performed. Using the right-angle Thomson scattering x-ray source, the author performed time-resolved x-ray diffraction studies of laser-perturbated InSb. These experiments revealed a delayed onset of lattice expansion. This delay is due to the energy relaxation from a dense electron-hole plasma to the lattice. The dense electron-hole plasma first undergoes Auger recombination, which reduces the carrier concentration while maintaining energy content. Longitudinal-optic (LO) phonon emission then couples energy to the lattice. LO phonon decay into acoustic phonons, and acoustic phonon propagation then causes the growth of a thermally expanded layer. Source characterization is instrumental in utilizing ultrashort x-ray pulses in time-resolved x-ray spectroscopies. By measurement of the electron beam diameter at the generation point, the pulse duration of the Thomson scattered x-rays is determined. Analysis of the Thomson scattered x-ray beam properties also provides a novel means of electron bunch characterization. Although the pulse duration is inferred for the Thomson scattering x-ray source, direct measurement is required for other x-ray pulse sources. A method based on the laser-assisted photoelectric effect (LAPE) has been demonstrated as a means of measuring ultrashort x-ray pulse durations. LAPE may also serve as the basis for a gated x-ray detector.

The interface and bulk magnetization dynamics of single-crystalline, wedge-shaped Fe(001) thin films with Cr cap layers have been studied by time-resolved magneto-optical Kerr effect (MOKE) and time-resolved magnetization-induced second harmonic generation (MSHG) using an all-optical pump-probe technique. We observed long-lived (≈1 ns) MOKE and MSHG oscillations excited by ultrashort (≈150 fs) optical pulses. They exhibit the same main resonance frequency f and damping constant. However, a 90° phase shift was observed between linear and nonlinear responses proving that MOKE and MSHG oscillations are related to the temporal variations of different magnetization components Mz and My. Additionally, we found weak oscillations at the double frequency 2f. Comparing the results of static and dynamic MSHG measurements we evaluate the in-plane amplitude of the optically excited interfacial magnetization oscillations.

Biological hard tissues, such as those found in bone and teeth, are complex tissues that build a strong mineral structure over an organic matrix framework. The laser-matter interaction for bone hard tissues holds great interest to laser surgery and laser dentistry; the use of short/ultrashort pulses, in particular, shows interesting behaviors not seen in continuous wave lasers. High laser energy densities in ultrashort pulses can be focused on a small irradiated surface (spot diameter is 10-50 μm) leading to rapid temperature rise and thermal ablation of the bone tissue. Ultrashort pulses, specifically those in the picosecond and femtosecond ranges, impose several challenges in modeling bone tissue response. In the present paper we perform time-dependent thermal simulations of short and ultrashort pulse laser-bone interactions in singlepulse and multipulse (set of ultrashort pulses) modes of laser heating. A comparative analysis for both radiation modes is discussed for laser heating of different types of the solid bone on the nanosecond, picosecond and femtosecond time scales. It is shown that ultrashort laser pulses with high energy densities can ablate bone tissue without heating tissues bordering the ablation creator. This reaction is particularly desirable as heat accumulation and thermal damage are the main factors affecting tissue regrowth rates, and thus patient recovery times.

erated optical pulses are 207 femtoscconds and 1 58 watts, respectively, making these pulses both the shortest andmost intense ever generated from an all semiconductor diode laser system. The nonlinear gain dynamics of theamplification process in these devices are also investigated using standard pump-probe techniques. The resultsyield information as to how the gain dynamics effects the pulse shaping mechanisms during modelocking and thepotential difficulties which arise in compressing and amplifying femtosecond optical pulses with semiconductortraveling wave optical amplifiers.Applications utilizing these optical pulses will also be discussed, with an emphasis on optical clock distribu-tion. In this type of application, we take advantage of the large output power and low timing jitter provided bythe modelocked semiconductor laser system.2. Femtosecond Pulse Generation with Traveling Wave Amplifiers.Compact and efficient sources of high repetition rate, high peak power ultrashort optical pulses are needed toreplace the standard ultrashort optical pulse generation schemes due to their relatively large size and poor wallplug efficiency. It has been previously demonstrated that an external cavity hybrid mode locked semiconductorlaser system has been able to produce optical pulses as short as 460 fsec in duration with over 70 watts of peakpower I 11. In this section, a modification of this system which incorporates a four prism sequence to compensate

\n Direct laser writing (DLW) techniques with ultrashort laser pulses have been extensively used for materials processing. The nonlinear nature of the interaction between ultrashort pulses and matter confers high spatial localization to these techniques and allows the fabrication of compact devices in a single step with resolution in the micro/nanoscale. In this context, we can highlight the fabrication of microstructures in transparent materials, like polymers and glasses, since they allow for the realization of three-dimensional devices. For instance, high-quality direct laser-written waveguide circuits have been fabricated and applied to different fields, such as telecommunications, quantum optics, sensors, etc. As photonic circuits often guide high light intensity, the characterization of the nonlinear optical properties of these structures becomes essential to direct their technological application. In this work, we realized the third-order nonlinear optical characterization of fs-laser written waveguides, which were fabricated in special glasses: Corning® Gorilla® and Eagle XG® Glass. The nonlinear characterization was performed through the Dispersive-scan (D-scan) technique, the temporal analog of Z-scan that can be applied to waveguides. The results obtained for the Gorilla® Glass waveguides indicate that the fs-laser writing process is responsible for reducing the magnitude of the nonlinear refractive index, when compared to the bulk material. This effect depends on the writing parameters, and the reduction is stronger when higher energy pulses are employed. Raman spectroscopy measurements revealed that this reduction could be related to the generation of non-bridging oxygen hole centers (NBOHCs) upon fs-laser irradiation. The characterization of Eagle XG® Glass waveguides, fabricated in different writing regimes (repetitive and cumulative), confirms the reduction of the nonlinear refractive index in the repetitive regime. However, it also revealed that little to no modification occurs when the cumulative regime is used, meaning that heat accumulation can be responsible for erasing NBOHCs and restoring the optical nonlinearities. Our results bring new light to what is currently known about the light-matter interaction when ultrashort pulses are involved. Furthermore, they can help tailor the application of fs-laser written glass waveguides, according to the desire for high or low optical nonlinearities.\n

Chapter 29 - Ultrafast time reversal mirror at 800 nm based on photosensitive naphthalocyanine doped solids

The fundamental building block of quantum information processing technologies is the quantum-bit a ‘qubit.’ These technologies require the ability to prepare, control, and read out a qubit state. Spins confined in self-assembled quantum dots are promising candidates for a quantum bit, because semiconductors are compatible with mature modern opto- and micro-electronics. These quantum dot systems offer two more advantages: they are excellent interfaces between the spin state—an anchored qubit and a photon—a ‘flying qubit’ and they provide means to coherently control the spin qubit by ultrashort optical pulses. In this review, we thoroughly discuss the qubit provided by an optically-excited electron in a quantum dot–the exciton qubit. We demonstrate its spin state initialization, coherent control and read-out using ultrashort optical pulses.

The present work is devoted to the study of the dynamics of multi-photon processes in semiconductor heterostructures. A time-dependent description is important for understanding in detail the transient response of semiconductors excited by ultrashort optical pulses. In the first part of this thesis, we set up a phenomenological model based on rate equations, in order to investigate the possibility of measuring degenerate two-photon gain in a semiconductor microcavity. The amplification predicted by the model is fairly low ( 2%) and mainly limited by the intra-band relaxation of the carriers, which leads to rapid saturation. In the second part, we develop a general theory for the dynamics of multi-photon processes in semiconductors. It will give insight into complex effects related to the coherence between the bands, which are not included in usual absorption coefficients or susceptibilities. For this purpose, we derive effective multi-band Bloch equations that include resonant multi-photon processes induced by two linearly polarized electromagnetic pulses of frequency close to the band gap and close to the half of the band gap respectively. The benefit of the proposed approach is two-fold. First, the description of the dynamics is restricted to a reduced number of bands. However, the discarded bands are not neglected, but consistently taken into account in the higher order processes. Second, all quantities appearing in the effective multi-band Bloch equations vary on the same time scale, which makes the numerical integration much more efficient. The time-dependent polarization current, as well as some susceptibilities, are derived on a consistent level of approximation, and are discussed in detail. The propagation of the electromagnetic fields is neglected. Such a model is appropriate for the description of low-dimensional quantum confined systems (e.g. quantum wells or quantum wires) excited by two colinearly propagating pulses. It accounts for various linear and nonlinear optical processes, such as one- and two-photon absorption, second-harmonic generation, difference-frequency mixing, or coherent control of photocurrent. In this thesis, the general theory is applied to the study of three specific physical situations. First, we investigate the charge and spin current in a symmetric AlGaAs/GaAs quantum well, injected by interference between one- and two-photon inter-band transitions. We identify new coherent terms in the expression of the current, which contribute significantly to the terahertz emission. The effects of the Stark shifts and the inter-valence band two-photon transitions are also calculated and discussed. Second, we calculate the anisotropic two-photon absorption spectra of an AlGaAs/GaAs V-shaped quantum wire with realistic band structure. The Coulomb interaction is taken into account within the Hartree-Fock approximation. The various excitonic peaks are identified with respect to the involved subbands and to the symmetry properties. We also show that excitons that are dark for one-photon excitation may become bright for two-photon spectroscopy, when the light is polarized perpendicularly to the growth direction, but not along a symmetry axis of the wire. Finally, the last application focuses on the optical injection of current in the presence of excitonic effects. Concentrating on the same AlGaAs/GaAs V-shaped quantum wire, we show that the Coulomb interaction within the Hartree-Fock approximation induces terahertz oscillations in the injected charge current. The oscillation frequency corresponds to the energy spacing between the two lowest excitonic resonances, slightly below the band gap, excited respectively by the laser pulse with frequency close to the band gap, and the one with frequency close to the half of the band gap.

The generation method of spatial-dispersion-free optical vortices using polarization singularity due to an axially-symmetric polarizer was proposed and demonstrated with an ultrabroad-band light (~350 to ~1000 nm) and an ultrashort optical pulse (~50 fs).

In recent years, the wide variety of digital micromirror device (DMD) applications has extended significantly across various fields of optics, including ultrafast optics. Despite these advances, the interaction between DMDs and ultrashort pulses remains poorly understood. To address this gap, this study presents a comprehensive characterization of the behaviour of a DMD system when interacting with ultrashort laser pulses. In this work, the fluence threshold for multi-shot damage was first determined to be 0.12 J/cm-2. Regarding the temporal effects, the group delay dispersion (GDD) of intrinsic materials was experimentally measured for the zeroth order and was determined to be 190 fs2. The temporal dispersion introduced by the DMD was then theoretically quantified for higher diffraction orders, showing that it generates a broadening and a spatiotemporal shift that depend on the diffraction order. Concerning spatial effects, the lateral chromatic aberration for a broad wavelength range was analysed, revealing the spatial separation of different wavelength components due to the wavelength dependence of the order of diffraction. Finally, the capability of the DMD to analyse the intensity spatial distribution of the light beam was demonstrated using a single-pixel imaging technique. These findings contribute to the understanding of the effects resulting from the interaction of ultrashort pulses with the DMD, thereby facilitating applications.

Ultrashort pulse (USP) laser processing has set a new standard in the manufacturing industry due to its high precision and its capacity to engrave nearly all materials. This technology has been introduced in sectors such as automotive, aerospace and electronics for tasks such as micro-drilling or surface structuring. However, some challenges to the widespread use of USP laser processing include improving fabrication speed and exibility to meet industrial demands. Beam shaping techniques, which involves altering the amplitude, phase, or polarization of laser beams, have emerged as a convenient solution to address these issues, as they allow for more ecient, versatile, and high-resolution material processing. In this thesis, various beam shaping strategies are explored to boost the throughput and applicability of USP laser manufacturing processes. In particular, the experimental work is divided into three main parts, each focusing on a dierent type of beam shaping. The rst part investigates the optimization and application of Bessel beams for generating volume diraction gratings in dierent glasses. It includes the study of nonlinear laser-matter interactions during irradiation of each material and examines its eect on the morphological and optical properties of the resulting volume modications. By ne-tuning the laser writing parameters, ecient phase elements suitable for multiple applications are produced in all types of glass in a single laser pass. The second part examines the use of triangular beams to streamline asymmetrical surface machining. This novel manufacturing method was employed to engrave grooves with a wide range of geometries in a single step. Additionally, it enabled the straightforward fabrication of blazed gratings on stainless steel, with features similar to those manufactured using conventional methods. The third part focuses on the generation of multilevel at-top beams and its application in USP laser polishing. To optimize light structuring, several diractive optical elements (DOEs) were manufactured using a femtosecond (fs) laser-assisted lithographic process. The results of this line of work demonstrate the importance of the design and fabrication of the DOEs to ensure homogeneous beam shaping and uniform processing using an USP laser source. Overall, this thesis presents new strategies to enhance the precision and eciency of ultrashort pulse (USP) laser material processing. The ndings contribute to the broader eld of laser microfabrication and have clear implications in industrial applications.

The dynamics of an ultrashort optical pulse in order—disorder ferroelectrics in the presence of defects without limitations on the pulse power is considered in the framework of the microscopic pseudospin formalism within the unidirectional propagation approximation. The effective equations for the electric field amplitude and the ferroelectric polarization are derived and numerically solved. Quasi-soliton modes of propagation of ultrashort optical pulses are revealed, and the dependences on the microscopic Hamiltonian parameters are analyzed. The influence of defects of different types on the propagation of ultrashort pulses is established.

We demonstrate with realistic numerical simulations that fiber optical parametric chirped pulse amplification is able to amplify ultra-short optical pulses. Such amplifiers driven by two-pump waves can amplify pulse bandwidth twice as large as the one of a single pump configuration. We show that pulses as short as 50 fs can be directly amplified. In addition, we take benefit from the saturation regime to achieve spectral broadening which makes possible to reduce pulse duration down to 15 fs.