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.
A mid-infrared optical parametric chirped pulse amplification (OPCPA) system generating few-cycle pulses with multi-GW peak power at a 1 kHz repetition rate is presented. The system is pumped by a high-energy 2-μm picosecond source to exploit the high nonlinearity of ZnGeP2 (ZGP) crystals for parametric amplification. Employing a dispersion management scheme based on bulk materials and a spatial light modulator pulses as short as 75 fs are obtained in the idler at a center wavelength of 5 μm. The maximum generated pulse energy of 1.2 mJ translates into a peak power of 14 GW. Moreover, damage considerations of ZGP crystals at high 2 μm pump pulse intensities in the few-ten picosecond range are explored.
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.
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.
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.
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.
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.