Bernd Schnabel

High-efficiency dielectric reflection gratings: design, fabrication, and analysis

We report on reflection gratings produced entirely of dielectric materials. This gives the opportunity to enhance the laser damage threshold over that occurring in conventional metal gratings used for chirped-pulse-amplification, high-power lasers. The design of the system combines a dielectric mirror and a well-defined corrugated top layer to obtain optimum results. The rules that have to be considered for the design optimization are described. We optimized the parameters of a dielectric grating with a binary structure and theoretically obtained 100% reflectivity for the -1 order in the Littrow mounting for a 45 degrees angle of incidence. Subsequently we fabricated gratings by structuring a low-refractive-index top layer of a multilayer stack with electron-beam lithography. The multilayer system was fabricated by conventional sputtering techniques onto a flat fused-silica substrate. The parameters of the device were measured and controlled by light scatterometer equipment. We measured 97% diffraction efficiency in the -1 order and damage thresholds of 4.4 and 0.18 J/cm(2) with 5-ns and 1-ps laser pulses, respectively, at a wavelength of 532 nm in working conditions.

Efficient Coupling into Polymer Waveguides by Gratings

Investigations of highly efficient grating couplers for polymer slab and strip waveguides fabricated by electron-beam lithography are reported. A maximum input efficiency of 67% is achieved. The electron-beam direct-writing technique allows one to replicate the original gratings into polymer substrates by embossing. An all-polymeric optical chip with efficient grating couplers is demonstrated. Waveguide grating couplers with blazed profile and variable grating depth are investigated. Thus, the intensity distribution of the outcoupled light is matched to a Gaussian-like profile. A focusing blazed grating that couples the light with an efficiency of 42% into a polymer strip waveguide is reported. A curvature correction of the grating lines allows one to improve the focusing properties.

Study on polarizing visible light by subwavelength-period metal-stripe gratings

One method for influencing the polarization of light is the use of wire-grid polarizers. For the visible region, this type of element can be realized as a metal-stripe grating with periods less than the wavelength. We fabricate metal-stripe gratings with periods down to 190 nm in thin chromium layers of 35-nm thickness using electron-beam lithography and ion-beam etching. A detailed investigation of the influence of grating period and duty cycle on the polarization effect is carried out to verify the conformity of rigorous diffraction theory and experimental results. The comparison between the two indicates good performance. Polarization ratios of the order of 5 with transmission efficiencies of about 60% in TM polarization are obtained. The connection of the polarization effect real- izable and the fabrication technology used is discussed.

POLARIZATION MULTIPLEXING OF DIFFRACTIVE ELEMENTS WITH METAL-STRIPE GRATING PIXELS

Diffractive elements with polarization multiplexing for the visible spectral region are demonstrated. The polarization-multiplexing property of the element is based on the polarization-dependent transmission characteristics of metal-stripe subwavelength period gratings. The proper dimensions of these gratings are estimated by rigorous calculations. The principle of polarization multiplexing by use of metal-stripe subwavelength period gratings is described for a diffractive element that has a binary amplitude transmission per polarization channel and is demonstrated by experimental results.

Evidence for bandedge lasing in a two-dimensional photonic bandgap polymer laser

Optically pumped organic semiconductor lasers are fabricated by spincoating a thin film of a methyl-substituted ladder-type poly-(para-phenylene) onto a nanopatterned circular grating distributed Bragg reflector. Varying the period of the grating allows for tunability of the laser wavelength. Comparing the emission spectra below and above the threshold shows that the lasing is spectrally located at the bandedge of the Bragg dip. This is in excellent agreement with the calculated dispersion relation of the laser system.

Apodized annular-aperture diffractive axicons fabricated by continuous-path-control electron beam lithography

Binary diffractive axicons, designed to generate a uniform axial line focus over a specified interval, are fabricated by electron beam lithography. Continuous-path-control mode and dynamic beam expansion of a low-voltage electron beam pattern generator are employed to avoid quantization errors in the patterning of the circular zones. Apodization by local control of diffraction efficiency is shown to be an effective method to reduce axial intensity oscillations along the focal line. A novel split-groove apodization method is introduced to suppress second-order interference.

Low energy lithography; energy control and variable energy exposure

The paper describes an electron beam lithography system for structures as small as 10nm and pattern samples produced on it. The system works with electron energies from 20keV down to 1keV. A special objective lens allows to realize probes of 5nm diameter for the lower energy range. Depending on how much energy is applied, the electrons will penetrate more or less deeply into the material. This may be used to create three-dimensional profiles in the resist. Provided in the system is a quasi-continuous path control to generate protracted curved structures with minimum edge roughness. Automatic marker detection algorithms and metric coordinate-control facilities enable the use of mix-and-match technologies.

E-beam lithography: a suitable technology for fabrication of high-accuracy 2D and 3D surface profiles

The fabrication of surface profiles may become an interesting technology in the field of micromachining. Recently, surface profiles are known and widely used in optics, especially in diffractive optics. E-beam lithography is a suitable technology for the fabrication of such profiles. This paper gives an overview about the experiences and results achieved at the Friedrich-Schiller-University Jena, Germany, on this field for some years. At first we describe the challenges and obstacles of typical optical profiles with regard to e-beam writing technology. After introducing our two different e-beam writers ZBA 23H (variable shaped beam) and LION LV1 (high resolution gaussian beam) we demonstrate different writing strategies for the fabrication of binary, multilevel and continuous surface profiles. Variable energy writing is a new technology extending the abilities of the well-known variable dose writing. Some selected examples of interesting patterns and profiles, as holograms, gear wheels, lenses, gratings and encoder discs, demonstrate different aspects to be considered and the possible solutions for some problems.

E-beam lithography: an efficient tool for the fabrication of diffractive and micro-optical elements

The fabrication of surface profiles for microoptical and micromechanical devices becomes more and more important. Especially the high accuracy necessary for the realization of the functional components benefits from the use of e-beam lithography, either for the fabrication of tools for optical lithography and replication techniques or for the direct writing of the pattern. After a classification of the pattern and an introduction into the processes for the realization of surface structures we show examples for the basic structure classes as well as for the integration of several properties within one profile resulting in elements with higher complexity.

Enhanced e-beam pattern writing for nano-optics based on character projection

The pattern generation for nano-optics raises high demands on resolution, writing speed and flexibility: nearly arbitrary complex structures with feature sizes below 100 nm should be realized on large areas up to 9 inches in square within reasonable time. With e-beam lithography the requirements on resolution and flexibility can be fulfilled but the writing time becomes the bottle neck. Acceleration by Variable Shaped Beam (VSB) writing principle (geometrical primitives with flexible size can be exposed with a single shot) is sometimes not sufficient. Character Projection (CP) is able to speed up the writing drastically because complex pattern of a limited area can be exposed by one shot [1]. We tested CP in the Vistec SB350 OS for optical applications and found a shot count reduction up to 1/1000, especially for geometries which are hard to approximate by geometrical primitives. Additionally, the resolution and the pattern quality were influenced in a positive way. Another benefit is the possibility to spend a part of the gain in writing speed to the use of a high resolution but low sensitive resist like HSQ. The tradeoff between speed and flexibility should be compensable by a large number of characters available.