Previously we studied the spectral Gouy rotation as a specific rotational phenomenon of conical polychromatic light fields shaped by spiral gratings. The rotation of spectral anomalies around singularities results from accumulated spectrally dependent Gouy phase shift. We proposed to apply radially chirped spiral structures to obtain an axial modulation of the rotational characteristics. Here we present related experimental results with non-uniform spiral gratings which were programmed into a 10-Megapixel, phase-only, liquid-crystal-on-silicon (LCoS) spatial light modulator (SLM). A propagation-dependent variation of the Gouy rotation was indicated. More complex non-uniform geometries are considered.
The large bandwidth and high intensity of ultrafast vortex pulses, i.e. pulses with orbital angular momentum (OAM), open new prospects for applications in communication, imaging or nonlinear photonics. In previous experiments, we demonstrated the peculiar spatio-spectral behavior of pulsed polychromatic vortex beams in the vicinity of phase singularities. It was shown that the rotation of characteristic, so-called “spectral eyes” and the spectral dependent Gouy phase are closely connected. For practical applications, a controlled variation of spatio-spectral distributions is required. Here we report on our most recent studies concerning the dependence of time-integrated spectral maps on key optical parameters. It is shown that the speed of rotation of spectral eyes during the propagation is essentially determined by the angular and spectral profiles. This enables to modify the spectral rotation characteristics by applying low-dispersion, adaptive optical components. The performance of reflective liquid-crystal-on silicon spatial light modulators (LCoSSLMs) is compared to diffractive spiral gratings with variable illumination. Moreover, the generation of wavepackets with a time-dependent orbital angular momentum (self-torque) by superimposing multiple tailored vortex pulses is proposed. This allows for extending the capabilities vortex pulses by defined non-stationary spatio-spectral and topological characteristics.
Recently it was reported that free-space propagating, ultrashort-pulsed polychromatic beams with orbital angular momentum (OAM) show a spectral Gouy rotation (SGR) of red- and blue-shifted areas around singularities. In femtosecond laser experiments with different types of spiral phase gratings, pulse propagation in spectral domain was studied with high resolution and sensitivity. By analyzing maps of spectral moments it was found that the interference of multiple OAM beams leads to a periodical revival of SGR by diffractive Talbot self-imaging. If the wavefront twist of the sub-beams is synchronized (co-rotating vortices), an optimum performance is found. In contrast, SGR echoes of counter-rotating beams are periodically distorted by destructive interference. Thus, the fine structure of self-imaged spectral maps enables to sort partial beams from interference patterns by even extremely weak imprinted vorticity information. It may further have implications for highly nonlinear processes and opens new prospects for applications in metrology, optical computing, or interferometry.
Spatially resolved spectroscopy of vortex beams is able to test the state of optical systems, to decode specific information or to sensitively indicate light-matter interactions. Spectral maps of ultrashort vortex pulses generated by hybrid diffractive-reflective spiral phase plates were studied experimentally and theoretically. Local spectral maps were detected by high-resolution scanning with a fiber-coupled spectrometer. Distributions of spectral centers of gravity and second moments were analyzed for femtosecond pulses. Gouy rotation of characteristic spectral features in the proximity of a phase singularity as a function of propagation distance was indicated in the spectral domain. Angular rotation was found to be modulated by weak oscillations. Analysis of spectral meta-moments indicates a fast switching and twisting behavior of spatial chirp.
Array-specific propagation effects are relevant for evaluating cross-talk or coherent coupling in multichannel processing and designing complex interference maps. At pulse durations in few-cycle range, an important goal is to combine the flexibility of shaping structured beams with a high-quality temporal transfer. Therefore, low-dispersion actuator arrays have to be applied in a diffraction-free approach. Flexible structuring of sub-3-cycle Ti:sapphire laser pulse arrays was studied with collectively or individually tunable liquid crystal devices and thermally actuated mirrors. It is shown that the classical diffractive Talbot effect can be complemented by the spatio-temporal self-imaging of pulsed nondiffracting needle beam arrays.
Spatially resolved spectroscopy of vortex beams is able to test the orbital angular momentum state of optical systems, to
decode specific information or to sensitively indicate light-matter interactions. Spectral maps of ultrashort vortex pulses
were studied experimentally and theoretically. Local spectra were detected by scanning with a spatially highly resolving
fiber-coupled spectrometer. Characteristic distributions of spectral statistical moments were analyzed for ultra-broadband
near-infrared pulses with pulse durations in few-cycle range. It is shown that the spectral moments can be used for
improving the contrast of vortex recognition and localization as well as for the data transfer via orbital angular
momentum maps. In combination with time-resolved wavefront data, a more complete characterization of dynamic
vortices is feasible. Gouy phase effect and radial oscillatory behavior of spectral maps of vortex pulses are demonstrated.
Further implications of the spatio-spectral information content for singular optics and related applications will be
The characterization of laser pulses with pulse durations in few-cycle range is highly challenging because the transfer of spatial and temporal information is sensitive against even slight amplitude and phase distortions. It can be improved by exploiting the propagation features of distortion-tolerant nondiffracting beams. The availability of novel types of MEMS components enables to realize smart and robust autocorrelators which combine low-dispersion and adaptive functionality of MEMS mirrors with the self-reconstructing properties of non-diffracting beams shaped by axicons. Two basic concepts of adaptive non-collinear, nonlinear autocorrelation are presented here: (a) autocorrelation with adaptive selfreconstruction, and (b) discrete phase shifting methods. By tuning the superposition angle in non-collinear autocorrelation it is possible to bypass the corruption of temporal information by distortions. This is demonstrated by performing autocorrelation experiments with a MEMS-type Fresnel mirror with hysteresis compensation. It is show that a spatially located distortion can be sampled in temporal domain. Phase-shifting approaches promise improvements with respect to the time resolution.
The formation of periodical nanostructures with femtosecond laser pulses was used to create highly efficient substrates for surface-enhanced Raman spectroscopy (SERS). We report about the structuring of silver and copper substrates and their application to the SERS of DNA (herring sperm) and protein molecules (egg albumen). The maximum enhancement factors were found on Ag substrates processed with the second harmonic generation (SHG) of a 1-kHz Ti:sapphire laser and structure periods near the SHG wavelength. In the case of copper, however, the highest enhancement was obtained with long-period ripples induced with at fundamental wavelength. This is explained by an additional significant influence of nanoparticles on the surface. Nanostructured areas in the range of 1.25 mm2 were obtained in 10 s. The surfaces were characterized by scanning electron microscopy, Fast Fourier Transform and Raman spectroscopy. Moreover, the role of the chemical modification of the metal structures is addressed. Thin oxide layers resulting from working in atmosphere which improve the biocompatibility were indicated by vibration spectra. It is expected that the detailed study of the mechanisms of laser-induced nanostructure formation will stimulate further applications of functionalized surfaces like photocatalysis, selective chemistry and nano-biology.
Ultrashort-pulsed Bessel and Airy beams in free space are often interpreted as "linear light bullets". Usually, interconnected intensity profiles are considered a "propagation" along arbitrary pathways which can even follow curved trajectories. A more detailed analysis, however, shows that this picture gives an adequate description only in situations which do not require to consider the transport of optical signals or causality. To also cover these special cases, a generalization of the terms "beam" and "propagation" is necessary. The problem becomes clearer by representing the angular spectra of the propagating wave fields by rays or Poynting vectors. It is known that quasi-nondiffracting beams can be described as caustics of ray bundles. Their decomposition into Poynting vectors by Shack-Hartmann sensors indicates that, in the frame of their classical definition, the corresponding local wavefronts are ambiguous and concepts based on energy density are not appropriate to describe the propagation completely. For this reason, quantitative parameters like the beam propagation factor have to be treated with caution as well. For applications like communication or optical computing, alternative descriptions are required. A heuristic approach based on vector field based information transport and Fourier analysis is proposed here. Continuity and discontinuity of far field distributions in space and time are discussed. Quantum aspects of propagation are briefly addressed.
The control of the orbital angular momentum (OAM) of ultrashort laser pulses with highly compact, low-dispersion and flexible devices opens new prospects for momentum-sensitive applications in plasmonics, materials processing, biochemistry, microscopy or optical data transfer. We report on the generation of few-cycle vortex pulses of variable topological charge from a Ti:sapphire laser oscillator with novel types of thermally tunable reflective, spiral-phase micro-electro-mechanical systems (MEMS). The spatial and temporal properties of the pulses were characterized by a reconfigurable, nondiffracting Shack-Hartmann wavefront autocorrelator. The intensity propagation can be described by a Laguerre-Gaussian beam with slight distortions caused by the line of maximum phase step. The different topological charges were indicated by quantitatively comparing the lengths of measured transversal Poynting-vector components to corresponding numerical simulations.
TiO2 is well known as a low-cost, highly active photocatalyst showing good environmental compatibility. Recently it was found that TiO2 nanotubes promise to enable for high photocatalytic activity (PCA). In our experiments, we studied the PCA and spectroscopic properties of TiO2 nanotube arrays formed by the anodization of Ti. The PCA efficiency related to the decomposition of methylene-blue was measured. To obtain reliable data, the results were calibrated by comparing with standard materials like Pilkington Activ™ which is a commercially available self cleaning glass. The studies included a search strategy for finding optimum conditions for the nanotube formation and the investigation of the relationship between PCA and annealing temperature. TiO2 nanotubes of different shapes and sizes were prepared by an anodization of Ti foil in different electrolytes, at variable applied voltages and concentrations. The photo-dissociation of methylene-blue was detected spectroscopically. For the optimized material, an enhancement factor of 2 in comparison to the standard reference material was found. Furthermore, femtosecond-laser induced photoluminescence and nonlinear absorption of the material were investigated. Possibilities for further enhancements of the PCA are discussed.
The temporal self-reconstruction of pulsed Bessel-like needle beams was studied. Arrays of nondiffracting sub-7-fs
needle beams were shaped from Ti:sapphire oscillator pulses by programming multiple axicons in a phase-only spatial
light modulator. Defined distortions in the time domain were induced by local spectral filtering. By differently shading
parts of selected sub-beams, the self-reconstruction was analyzed under variable conditions. Pulse duration maps were measured with two-dimensional second order autocorrelation based on the Shack-Hartmann sensor principle of
wavefront division. Completely distorted pulses were found to have a pulse duration of > 13 fs whereas partially
distorted sub-beams returned to pulse durations close to the initial ones. Specific applications are proposed.
For a growing number of applications in nonlinear spectroscopy, micro- and nano-machining, optical data processing, metrology or medicine, an adaptive shaping of ultrashort pulsed, ultrabroadband laser beams into propagation-invariant linear focal zones (light blades) is required. One example is the femtosecond laser high-speed large area nanostructuring with moving substrates and cylindrical optics we reported about recently. Classical microoptical systems, however, distort the temporal pulse structure of few cycle pulses by diffraction and dispersion. The temporal pulse transfer can be improved with innovative types of reflective MEMS axicons based on two integrated rectangular mirrors, tilted by a piezoelectric bending actuator. In contrast to pixelated liquid-crystal-on-silicon (LCoS) based devices, cutoff frequencies in multi-kilohertz range, a purely reflective setup and continuous profiles with larger phase shift are realized which enable for shaping extended propagation-invariant zones at a faster and more robust operation. Additionally, a fixed phase offset can be part of the structure. Here, the performance of a prototype of linear mechanically tunable MEMS axicon is demonstrated by generating a pseudo-nondiffracting line focus of variable diameter and depth extension from a femtosecond laser pulse. The temporal transfer of 6-fs pulses of a Ti:sapphire laser oscillator is characterized with spectral phase interferometry for direct electric-field reconstruction (SPIDER) and spatially resolved nonlinear autocorrelation. Spatial and temporal self-reconstruction properties were studied. The application of the flexible focus to the excitation of plasmon-polaritons and the self-organized formation of coherently linked deep sub-wavelength laser-induced periodic surface structures (LIPSS) in semiconductors and dielectrics is reported.
For an extended wavefront analysis, structured materials processing, optical information technologies, or
superresolving microscopy with ultrashort pulses, more flexible and robust techniques of beam shaping are required.
Non-Gaussian fringe-free Bessel beams ("needle beams") can be generated with programmable phase maps of
phase-only displays. Such beams behave propagation invariant over relatively extended regions with respect to their
characteristic spatio-temporal signatures. Here, we extend the concept of needle pulses towards other types of
nondiffracting fields including significantly more complex ones. It is shown that also nondiffracting light slices,
tubular beams or pixellated images can be composed from simple nondiffracting constituents of higher degree of
symmetry. With arrangements of multiple small phase axicons programmed into liquid-crystal-on-silicon spatial
light modulators, a large variety of non-conventional nondiffracting beams of even highly asymmetrically partitions
can be achieved with widely propagation invariant spectral and temporal properties. Modified Shack-Hartmann
sensors with integrated temporal sensitivity, advanced types of multichannel autocorrelators and adaptive materials
processing with variable focal spots are proposed.
The formation of laser induced periodic surface structures (LIPSS) is to a large extent of self-organizing nature and
in its early stages essentially influenced by optical scattering. The evolution of related mechanisms, however, has
still to be studied in detail and strongly depends on materials and laser parameters. Excitation with highly intense
ultrashort pulses leads to the creation of nanoripple structures with periods far below the fundamental wavelength
because of opening multiphoton excitation channels. Because of the drastically reduced spatial scale of such laser
induced periodic nanostructures (LIPNS), a particular influence of scattering is expected in this special case. Here
we report on first investigations of femtosecond-laser induced nanostructuring of sputtered titanium dioxide (TiO2)
layers in comparison to bulk material. The crucial role of the optical film quality for the morphology of the resulting
LIPNS was worked out. Typical periods of nanoripples were found to be within the range of 80-180 nm for an
excitation wavelength of 800 nm. Unlike our previously reported results on bulk TiO2, LIPNS in thin films appeared
preferentially at low pulse numbers (N=5-20). This observation was explained by a higher number of scattering
centers caused by the thin film structure and interfaces. The basic assumptions are further supported by
supplementary experiments with polished and unpolished surfaces of bulk TiO2 single crystals.
Programmable liquid-crystal devices for high-resolution spatial shaping of ultrashort-pulsed laser beams promise to be
an alternative approach to passive microoptical structures. In former experiments we demonstrated that depositionfabricated
nanolayer lenses and axicons can serve as low-dispersion, damage resistant, ultrabroadband microoptical
components. With small-angle layer microaxicons, robust wavefront sensors and 2D autocorrelators were built up with
them which took advantage of stable and tilt-independent nondiffracting propagation. The flexibility of the thin-film
design, however, was limited with respect to the dynamic range. For adaptive applications, information encoding, image
transfer and data storage, addressable and phase variant components are required. Recently, phase-only reflective liquidcrystal-
on-silicon spatial light modulators (LCoS-SLMs) became available. By analyzing the pulse transfer behavior in
spectral and temporal domain it was shown that selected versions of LCoS-SLMs are capable to shape 10-fs pulses with
marginal distortion. Variable arrays of pulsed Bessel-like beams and nondiffracting complex patterns were shaped
experimentally and related applications are discussed. The adaptive correction of aberrations in nondiffracting tubular
beams on microscale is demonstrated. The unique properties of programmable beam patterns of well controlled
propagation promise the coverage of fields of entirely new photonic applications.
The combination of sample translation and line focusing by cylindrical optics is shown to be a convenient and highly
effective way of generating laser induced coherent periodic surface structures (LIPSS) in TiO2 over significantly
extended areas. Compared to known techniques based on a sample translation relative to a circular symmetric focus, the
approach is much less time consuming and requires only a single translation stage. The capability of the method to form
both high and low spatial frequency LIPSS (HSFL, LSFL) at the second harmonic wavelengths of a Ti:sapphire-laser
(around 400 nm) at properly chosen scanning velocity and laser pulse energies is demonstrated. Structured multi-mm2
areas with periods of 80 nm and 325 nm were obtained corresponding to distinct sets of optimized parameters.
Furthermore, the appearance of nano-bumps on 30 nm scale on the surface of the LSFL is reported. Basic technical
issues are discussed and potential applications of LIPSS in rutile-type TiO2 like superwetting, friction control, catalysis
and photovoltaic are proposed.
Spatial light modulators based on liquid-crystal-on-silicon micro-displays were investigated with respect to their
capability to flexibly shape complex wavefields from femtosecond pulses. Experiments were performed with a
Ti:sapphire laser oscillator emitting linearly polarized radiation at pulse durations in 10 fs range. It is shown that the
transfer characteristics well enable for an undistorted adaptive shaping of microoptical phase profiles which are linearly
dependent on the gray values at such ultrashort pulses. In particular, beam arrays consisting of individually
programmable nondiffracting Bessel-like beams, needle beams and beam slices of high aspect ratios were generated. By
composing complex patterns of nondiffracting subbeams, image information was propagated nearly undistorted over
certain distances ("flying images"). Cross-talk was minimized by diffractive background management. Further
applications like adaptive wavefront sensing, advanced autocorrelation as well as statistical encoding are discussed.
Light distributions of Bessel-Gauss and Laguerre-Gauss type carry an orbital angular momentum and thus can be
regarded as particular types of optical vortex beams. Optical vortices in highly intense femtosecond laser pulses are
expected to lead to a variety of specific applications like momentum selective spectroscopy, nonlinear laser-material
interaction or quantum information processing. Here we report on experiments with a Ti:sapphire laser oscillator at
wavelengths around 800 nm. To compare the pulsed and cw case, the system was driven with and without mode-locking.
At the minimum pulse duration of about 10 fs, a FWHM spectral bandwidth of 120 nm was available. By applying
diffractive spiral phase elements, beams with topological charges of m = 1 and m = 2 were formed. The specific
propagation behavior was studied by detecting spatially resolved intensity and spectral maps. In addition to the helical
beam generation with fixed phase patterns, adaptive approaches based on liquid-crystal microdisplays are considered.
Recently, we proposed a new approach of a noncollinear correlation technique for ultrashort-pulsed coherent optical
signals which was referred to as Bessel-autocorrelator (BAC). The BAC-principle combines the advantages of Bessellike
nondiffracting beams like stable propagation, angular robustness and self-reconstruction with the principle of
temporal autocorrelation. In comparison to other phase-sensitive measuring techniques, autocorrelation is most straightforward
and time-effective because of non-iterative data processing. The analysis of nonlinearly converted fringe
patterns of pulsed Bessel-like beams reveals their temporal signature from details of fringe envelopes. By splitting the
beams with axicon arrays into multiple sub-beams, transversal resolution is approximated. Here we report on adaptive
implementations of BACs with improved phase resolution realized by phase-only liquid-crystal-on-silicon spatial light
modulators (LCoS-SLMs). Programming microaxicon phase functions in gray value maps enables for a flexible variation
of phase and geometry. Experiments on the diagnostics of few-cycle pulses emitted by a mode-locked Ti:sapphire laser
oscillator at wavelengths around 800 nm with 2D-BAC and angular tuned BAC were performed. All-optical phase shift
BAC and fringe free BAC approaches are discussed.
We present a synthesized sub-ps dual-wavelength laser source for digital holographic interferometry with a wide
reconstruction range. The developed laser source generates two spectrally separated parts within one pulse. The sub-ps
pulse duration desensitizes the holographic setup to environmental impacts. A center wavelength distance of only 12 nm
with a high contrast was demonstrated by spectral shaping of the 50 nm broad seed spectrum of a CPA Ti:sapphire laser
system centered at 800 nm.
Time-resolved two-wavelength contouring requires the simultaneous and separable recording of two holograms. In
general, a single CCD-camera is applied, and the spectral separation is realized by different reference wave tilts, which
requires ambitious interferometric setups. Contrary to this, we introduce two CCD-cameras for digital holographic
recording, thus essentially simplifying the interferometric setup by the need of only one propagation direction of the
reference wave. To separate the holograms for the simultaneous recording process, a Mach-Zehnder interferometer was
extended by a polarization encoding sequence.
To study our approach of time-resolved digital holographic two-wavelength contouring, an adaptive fluidic PDMS-lens
with integrated piezoelectric actuator served as test object. The PDMS-lens consists of an oil-filled lens chamber and a
pump actuator. If a voltage is applied to the piezoelectric bending actuator the fluid is pumped into the lens chamber
which causes a curvature change of the 60-μm thick lens membrane and thus a shift of the focal length. The dynamic
behavior of the PDMS-lens, driven at a frequency of 1 Hz, was investigated at a frame rate of 410 frames per second.
The measured temporal change of the lens focal length between 98 and 44 mm followed the modulation of the
piezoelectric voltage with a 30 V peak-to-peak amplitude. Due to the performed time-resolved two wavelength
contouring, we are able to extract the optical path length differences between center and perimeter of the lens. From the
calculated phase difference maps we estimated large optical path differences of larger than 10 μm, corresponding to
more than 15 times of the source wavelength.
The considerable potential of advanced thin-film microoptics for tailoring light fields of pulsed high-power lasers even at
extreme parameters like ultrashort pulse durations, broad spectral bandwidths or vacuum ultraviolet wavelengths is
demonstrated. A comprehensive review of the state of the art and the most relevant aspects of this branch of modern
optics is given. In particular, applications of structured dielectric, metallic and compound layers and programmable
liquid-crystal devices for control and diagnostics of ultrashort pulses in space and time are discussed. Recent theoretical
and experimental results of wavefront sensing, pulse diagnostics, multichannel materials processing and information
encoding into the phase maps of arrayed pulsed beams of nondiffracting propagation characteristics are presented here.
Fringe-resolved noncollinear autocorrelation extracts information about the pulse duration of ultrashort optical signals
from analyzing the intensity envelope of fringes. By detecting nonlinear autocorrelation functions after frequency
conversion, even an evaluation of temporal asymmetry and frequency chirp are enabled. Here we report on a modified
approach based on replacing crossed plane waves by Bessel-like beams. In comparison to the conventional method,
appropriate mathematical transforms have to be applied. The method is simple and single-shot capable and takes
advantage of specific advantages of pseudo-nondiffracting beams. First proof-of-principle experiments with few-femtosecond
pulse durations were performed and compared to simulations. In multishot operation regime, the
implementation of phase-shifting procedures by spatial light modulators promises considerable improvements of the time
resolution analogous to the known principle of phase-shift interferometry.
Recently developed Shack-Hartmann sensors with axicon beam shapers show an enhanced robustness compared to
setups with spherical microlenses. With ultraflat axicon arrays, further improvements were obtained. Very extended,
fringeless nondiffracting beams or "needle beams" with self-reconstructing properties can be produced. Specific
advantages of thin-film structures like low dispersion and reflective operation can be implemented. Here we report on
first systematic studies of angular tolerance and displacement sensitivity of different types of refractive, reflective and
diffractive Shack-Hartmann devices. A quantitative description of the functionality is given on the basis of higher order
spatial statistical moments. This method enables for identifying optimum parameter ranges to determine wavefront
curvatures under extreme conditions.
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.
Pseudo near-field intensity distributions of 1-cm diode-laser bars in slow-axis plane were recorded without parasitic feedback by automated scanning with a slanted highly-reflecting thin metal wire. Irregularities in field patterns are correlated with local curvature and twisting of facets. Information on mode guiding within the semiconductor laser can be derived.
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.
We report on specular reflectivity measurements at the position of the waveguide at front facets of commercial diode laser arrays. Since the waveguide thickness is such semiconductor structures amounts about 1 micrometers an even better spatial resolution of the probe light spot is required. For this purpose, a micro-reflectance setup was designed and implemented. For re-locating the optically active region, e.g. after stepped-up operation time, we employ the photosensitivity of the active region by using the photocurrent induced by the probe beam for auto- alignment of the setup. We show for coated InGaAlAs/GaAs- single chip devices that during long-term operation the diode laser front facet reflectivity at the position of an emitter is almost constant with a slight tendency (about 0.002 at 633 nm) to increase. The results are explained in the framework of defect-induced refractive index changes within the semiconductor material close to the interface between waveguide and facet coating.
An automated, compact system for high-accuracy measurements of specular reflectivity at different laser wavelengths (633 nm, 822 nm) on the basis of lock-in technique and LabView has been developed. With a spatial resolution of less than 2 micrometers , a signal resolution of less than 10-4 was obtained. Micro-optical components like microlenses with angular-compensating antireflection coatings, nonuniform micro-mirror arrays, broad-stripe diode laser facets and low-numerical-aperture graded-index microlenses have been characterized by two-dimensional reflectivity mapping. Errors caused by the angular spectrum of the focused polarized probe lasers were analysed.
Multilayer micro-optics combines refractive beam shaping with wavelength selective multiple interference. Layer composition and thickness distribution were optimized by advanced simulation software. Components were fabricated by mask-shaded vapor deposition with planetary rotation. Surface profiles were characterized interferometrically. For two-dimensional reflectance mapping, a high-accuracy automated system was developed. Micro-mirror arrays for self-imaging resonators, mode selective mirrors for miniaturized solid-state lasers and angular-adapted graded AR-coatings for microlenses are presented as applications.
Shading masks consisting of regular grids of thin metallic wires have been used for the vacuum deposition of micro- optical thin film components. The fabrication of cylindrical microlenses with single- and two-step procedures has been demonstrated. Refractive as well as partially reflective arrays with pitches greater than or equal to 50 micrometer have been realized with SiO2 and SiO2:HfO2 layers on glass, quartz and polymer substrates. The thickness profiles have been characterized interferometrically.
To improve the beam properties of a short-pulse KrF excimer laser (FWHM < 3.8 ns) with a low number of roundtrips (2...3) and limited magnification (< 5), unstable resonator designs have been developed. Resonators containing mirrors with super-Gaussian reflectivity distributions of different super-Gaussian orders have been investigated theoretically by beam propagation method as well as experimentally by measuring the near- and far-field beam profiles. Mirrors with radially varying multilayer systems have been fabricated by mask-shaded vacuum deposition of dielectric layers with planetary rotation of the substrate. It has been demonstrated that the (delta) -factor of the laser can be significantly improved with compact resonator configurations. For a resonator length of 400 mm, a beam spot size on the output coupler of 2 w equals 1.6 mm and a magnification of M equals 5, the optimum operation characteristics has been found for a super-Gaussian order of about k equals 10. Compared with a plane-parallel system, the (delta) -factor was enhanced by a factor of up to 7. The characteristic beam parameter M2 (times-diffraction- limited-factor) could be reduced in x- and y-directions by factors of 4 and 2, respectively. The experimental results are in good agreement with our theoretical predictions.
Refractive and refractive-reflective arrays of circular as well as cylindrical thin-film microlenses in linear, hexagonal and rectangular arrangements on solid glass/quartz plates and flexible polymer foils have been produced with an improved vapor-deposition technique using shading hole masks and a planetary rotation system. Up to 1000 solitary elements with pitches of 170...350 micrometers and diameters of 50...350 micrometers have been deposited on solid quartz plates and flexible polymer substrates. Focal lengths of typically 1.5...20 mm have been realized. Additional global envelope functions of phase and/or reflectance have been realized. Different types of segmented lasers including imaging and self-imaging arrays as coupling, outcoupling and focussing components have been tested. Graded reflectance micro-mirror arrays (GRMMA) have been used as refractive-reflective elements for solid-state lasers with unstable Talbot resonators which deliver arrays of phase-coupled, focused partial beams. Arrays of diode laser beamlets have been generated by optical transformers combining rod lenses (fibers) with microlens arrays. New schemes for side-on and end-on pumped solid-state lasers and fiber array lasers containing microlens arrays have been developed.
Apodized holographic gratings and dielectric mirrors have been designed, fabricated, and tested as outcoupling components of unstable CO2-laser resonators. The influence of the imaging properties of profiled dielectric layers on the laser field has been investigated.