The first article by Turunen and Friberg deals with a class of fields with propagation-invariant properties such as the optical intensity distribution. Coherent and partially coherent stationary and pulsed solutions are treated in view of scalar and electromagnetic approaches. Approximations of ideal propagation-invariant fields and methods for their generation are discussed. Finally, some application areas are covered.

Propagation-Invariant Optical Fields

“Non-diffracting” beams do not spread as they propagate. This property is useful in many areas. Here, the theory, generation, properties, and applications of various “non-diffracting” beams, including the Bessel beam, Mathieu beam, and Airy beam is reviewed. Applications include imaging, micromanipulation, nonlinear optics, and optical transfection.

Light beats the spread: ``non-diffracting'' beams

A particular class of Montgomery's self-imaging objects that we call continuously self-imaging gratings (CSIG's) is introduced. When they are illuminated by a plane wave, these objects produce a field whose intensity profile is a propagation- and wavelength-invariant biperiodic array of bright spots. The mathematical construction of these objects and their intrinsic properties are described. On a practical level, CSIG's are compact and achromatic nondiffracting array generators. We show that a good CSIG approximation can be realized by a two-level phase grating that is experimentally tested.

Generation of achromatic and propagation-invariant spot arrays by use of continuously self-imaging gratings.

Propagation invariant light fields such as Bessel light beams are of interest in a variety of current areas such as micromanipulation of atoms and mesoscopic particles, laser plasmas, and the study of optical angular momentum. Considering the optical fields as a superposition of conical waves, we discuss how the coherence properties of light play a key role in their formation. As an example, we show that Bessel beams can be created from temporally incoherent broadband light sources including a halogen bulb. By using a supercontinuum source we elucidate how the beam behaves as a function of bandwidth of the incident light field.

White light propagation invariant beams.

The propagation and transmission of Bessel beams through nano-layered structures has been discussed recently. Within this framework we recognize the formation of unguided diffraction-free waves with the spot size approaching and occasionally surpassing the limit of a wavelength when a Bessel beam of any order n is launched onto a thin material slab with grazing incidence. On the basis of the plane-wave representation of cylindrical waves, a simple model is introduced providing an exact description of the transverse pattern of this type of diffraction-suppressed localized wave. Potential applications in surface science are put forward for consideration.

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Diffraction-free beams in thin films.

Multiple quasi-Bessel beams are generated by transmission of sub-30-fs pulses from a Ti:sapphire laser through refractive thin-film microaxicon arrays. Time-integrated intensity distributions at several axial positions and for pulse durations of 26 and 12.5 fs reveal significant changes of contrast, envelope function, and spatial frequency spectrum in comparison with continuous wave data. Evidence is presented that strong space-time coupling results in a time-dependent interference zone.

Generation of femtosecond Bessel beams with microaxicon arrays.

Summary form only Thin-film microoptical arrays are promising tools for ultrashort-pulse laser beam shaping and characterization Short optical path lengths, minimum spectral dispersion, low absorption and broadband transmission are advantageous features for the handling of high-power few-cycle pulses The beam shaping in space, time and spectrum is of great practical interest for many applications (multiphoton excitation, material processing, acceleration, atom guiding, measurement techniques) and also relevant for an evaluation of damage risks In our experiments, we investigated three different kinds of interference phenomena: (a) free-space interference of converging beams (quasi-Bessel beams with extended focal zones and transversal fringes), (b) self-imaging of array structures (Talbot effect with regular intensity patterns), (c) interference caused by internal reflectance of spatially variable Fabry-Perot etalons (Newton rings)

Femtosecond interference effects generated by refractive thin-film micro-optics

Femtosecond quasi-Bessel beams have been generated by thin-film micro-axicon arrays. Time-integrated, spatially resolved interference patterns indicate space-time coupling effects for pulse durations of 12.5...30 fs by significant changes of contrast and spatial frequency.

F. Tschirschwitz

Papers

Generation of femtosecond Bessel beams with microaxicon arrays.

Femtosecond interference effects generated by refractive thin-film micro-optics

Thin-film microoptics for the generation of femtosecond Bessel beam arrays