A precision laser pattern generator for writing arbitrary diffractive elements was developed as an alternative to Cartesian coordinate laser/electron-beam writers. This system allows for the fabrication of concentric continuous-relief and arbitrary binary patterns with minimum feature sizes of less than 0.6 µm and position accuracy of 0.1 µm over 300-mm substrates. Two resistless technologies of writing on chromium and on amorphous silicon films were developed and implemented. We investigated limit characteristics by writing special test structures. A 58-mm f/1.1 zone plate written directly is demonstrated at a λ/50 rms wave-front error corresponding to a 0.06-µm pattern accuracy. Several examples of fabricated diffractive elements are presented.

In this paper, we present the results of thermochemical LIPSS formation on a chromium film with a thickness in the range of 28-350 nm induced by femtosecond laser radiation (λ = 1026 nm, ν = 200 kHz, τ = 232 fs). The period, height, morphology and chemical composition of TLIPSS as a function of the metal film thickness and focusing configuration are investigated. The growth of TLIPSS period from 678 nm to 950 nm with increasing thickness of the film has been explained by a formation of oxides with different stoichiometry composition. So, the CrO₂ oxide prevails in the composition for the case of TLIPSS formed on thin films which have the minimal period, whereas Cr₂O₃ oxide is dominant in the case of TLIPSS formed on thick chromium films which have the maximal period value. The results obtained are in agreement with numerical modeling of a period defined by the interference between an incident radiation and a scattered one from a single oxide ridge with a different chemical composition.

Enhancing the diffraction efficiency of continuous-relief diffractive optical elements fabricated by direct laser writing is discussed. A new method of zone-boundary optimization is proposed to correct exposure data only in narrow areas along the boundaries of diffractive zones. The optimization decreases the loss of diffraction efficiency related to convolution of a desired phase profile with a writing-beam intensity distribution. A simplified stepped transition function that describes optimized exposure data near zone boundaries can be made universal for a wide range of zone periods. The approach permits a similar increase in the diffraction efficiency as an individual-pixel optimization but with fewer computation efforts. Computer simulations demonstrated that the zone-boundary optimization for a $6\mathrm{\mu m}$ period grating increases the efficiency by 7% and 14.5% for $0.6\mathrm{\mu m}$ and $1.65\mathrm{\mu m}$ writing-spot diameters, respectively. The diffraction efficiency of as much as $65\%–90\%$ for $4–10\mathrm{\mu m}$ zone periods was obtained experimentally with this method.