Novel types of thin-film microoptical components have been found very advantageous for beam shaping of high-power and ultrashort-pulse lasers. Measuring techniques, nonlinear optics, materials processing, and further advanced photonic applications, will benefit from specific advantages. Compared to conventional microoptics, low dispersion and absorption as well as added degrees of freedom in structure and functionality are accessible. Single or multilayer designs, spherical and non-spherical profiles, extremely small angles, and flexible substrates enable key components for the tailoring of lasers in spatial, temporal, and spectral domain at extreme parameters. By vacuum deposition and selective etching transfer, a cost-effective fabrication of single or array-shaped refractive, reflective, or hybrid components is possible. During the last decade significant progress in this field could be achieved. Including very recent applications for spatio-temporal shaping and characterization of complex and non-stationary laser fields, the state of the art is presented here. Particular emphasis is put on the generation of localized few-cycle wavepackets from Ti:sapphire laser beams by the aid of broadband microaxicons. Special microoptics are capable of transforming vacuum ultraviolet radiation. Wavefronts of strongly divergent sources can be analyzed by advanced Shack-Hartmann sensors based on microaxicon-arrays. Single-maximum nondiffractive beams are generated by different approaches for self-apodizing systems. Prospects for future developments, like robust multichannel information processing with arrays of self-reconstructing X-pulses, are discussed.
Ultrashort-pulse single-maximum nondiffracting beams of microscopic radius and large axial depths are interesting for applications in nonlinear optics and spectroscopy, for acceleration and manipulation of particles, measuring techniques, materials treatment or information processing. Here we report on the experimental generation of such beams by self-apodized truncation of Bessel and pseudo-Bessel beams from a Ti:sapphire oscillator. Small angle operation was enabled by thin-film structures. To obtain self-apodization, the diameter of the truncating diaphragm was adapted to the first minima of Bessel distribution. The propagation of (a) Bessel beams of meter-range axial extension shaped by axicon mirrors, and (b) microscopic pseudo-Bessel beams of millimeter-range extension shaped by Gaussian-shaped microaxicon lenses was studied. In case (a), single-maximum beams of > 20 cm depth were produced. To generate comparable focal zones from Gaussian beams, a much larger distance (10x) is necessary, and axial stretching of spectrum destructs the temporal structure. In case (b), the focal zone length was increased by a factor of >5 compared to a Gaussian beam. Arrays of truncated Bessel beams were generated as well. The experimental results indicate that truncated Bessel beams enable more compact setups than corresponding Gaussian beams and are in particular advantageous for ultrashort pulses. Further improvements are possible by combining coherent addition in resonators with truncation outcoupling.
For spatiotemporal transformation and processing of ultrashort-pulse laser beams, serious design constraints arise from dispersion and diffraction. At pulse durations in 10-fs range, temporal and spatial parameters of propagating wave packets are coupled and significant inhomogeneities appear. To enable a controlled shaping or encoding and a reliable detection or decoding with 2-D spatial resolution, specific advantages of thin-film micro-optical arrays can be exploited. Transmitting and reflecting components of extremely small conical angles are used to generate multiple nondiffracting beams and self-imaging phase patterns. With novel-type metal-dielectric microaxicons, low-dispersion reflective devices are realized. Beam propagation is simulated numerically with Rayleigh-Sommerfeld diffraction theory. For ultrafast time-space conversion, matrix processors consisting of dielectric thin-film microaxicons are tested. Transversally resolving linear and nonlinear autocorrelation techniques are applied to characterize the space-time structure of localized few-cycle wave packets shaped from Ti:sapphire laser beams at pulse durations down to 8 fs. Bessel-like X waves are generated and their propagation is studied. In combination with autocorrelation, wavefront analysis of ultrashort-pulse lasers with Bessel-Shack-Hartmann sensors operated in reflection setup is demonstrated.
Recent progress in laser beam shaping and characterization with novel-type thin-film microoptics is presented. These novel microoptical devices offer several distinctive advantages, such as a short optical path, small angles, low roughness or multilayer design. These features allow shaping of laser beams at extreme parameters with respect to spectrum, angular distribution, intensity, or pulse duration. Particular emphasis is laid on (i) hybrid components for high-power diode laser collimation, (ii) spatio-temporal shaping of localized few-cycle wavepackets, and (iii) microoptics for the vacuum ultraviolet. For the fabrication of thin-film structures, vapor deposition with shading masks was used. To improve the efficiency of diode laser collimation, spatially variable AR coatings and integrated arrays of cylindrical microlenses were developed. Arrays of Bessel-like beams were generated from sub-10-fs Ti:sapphire laser pulses by refractive and reflective microaxicons. We further demonstrated the use of microaxicon arrays for spatially resolved autocorrelation of ultrashort pulses. Deposition and etching transfer of flat VUV-structures was studied. Finally, the generation of single-maximum nondiffracting beams by self-apodizing system design is discussed.
Spatially resolved wavefront sensing and time-resolved autocorrelation measurement of ultrashort pulses are usually separated procedures. For few-cycle pulses with significant spatial inhomogeneities and poor beam quality, a fully spatio-temporal beam characterization is necessary. Here we report on a new concept for a joint two-dimensional mapping of local temporal coherence and local wavefront tilt based on the combination of collinear autocorrelation and Shack-Hartmann wavefront sensing. Essentially for this "wavefront autocorrelation" is a splitting of the beam into a matrix of Bessel-like sub-beams by an array of thin-film microaxicons. The sub-beams are further processed by a two-dimensional collinear autocorrelation setup. The second harmonic distribution of sub-beams at a defined distance is imaged onto a CCD camera. The nondiffractive sub-beams ensure an extended depth of focus and a low sensitivity towards angular misalignment or axial displacement. With low-dispersion small-angle refractive-reflective shapers, wavefront-sensing of Ti:sapphire laser wavepackets was demonstrated experimentally for the first time.
For spatio-temporal processing of ultrashort-pulse laser beams, design constraints arise from dispersion and diffraction. In sub-10-fs region, temporal and spatial coordinates of propagating wavepackets get non-separable. To enable controlled shaping and detection with spatial resolution, specific advantages of thin-film microoptical arrays are exploited. Transmitting and reflecting components of extremely small conical angles were used to generate multiple nondiffracting beams and self imaging patterns. With novel-type metal-dielectric microaxicons, low-dispersion reflective devices were realized. Beam propagation was simulated with Rayleigh-Sommerfeld diffraction theory. For time-space conversion, matrix processors consisting of thin-film microaxicons were tested. Transversally resolving linear and nonlinear autocorrelation techniques were applied to characterize the space-time-structure of localized few-cycle wavepackets shaped from Ti:sapphire laser beams at pulse durations down to 8 fs. Bessel-like X-waves were shaped and their propagation was studied. In combination with autocorrelation, wavefront analysis of ultrashort-pulse lasers with Bessel-Shack-Hartmann sensors operated in reflection setup was demonstrated.
ZnO nanocrystalline thin layers are of growing interest for ultrafast optical applications. Previous investigations delivered different values of second order susceptibilities. The quantitative contribution of grain structure - depending on fabrication procedure - is not well understood. For our investigations, pure and doped polycrystalline and amorphous ZnO thin filmes of 0.1 to 1.5 μm thickness have been prepared by spray pyrolysis and alternative techniques. Texture, thickness and further structural properties of the layer have been characterized by SEM, AFM, XRD, and optical spectroscopy. Using 20-fs Ti:Sa laser pulses centered at 800 nm, we measured the angular dependence of SHG intensity and determined second order susceptibilities. For a small range of crystallinity parameters, pronouced SHG efficiencies appear. From our experiments, design parameters for ZnO nanolayers can be derived which enable a tailoring of sandwich structures for advanced non-linear processing and femtosecond laser autocorrelation.
Spectral interference caused by structured thin-film components has been used for shaping and characterization of few-cycle femtosecond laser beams. Array structures enable spatially resolved measurements of coherence and wavefront. The generation of spatially and temporally localized optical wavepackets with reflective and refractive axicons was demonstrated in theory and experiment.