Frequency Selective Surfaces for Filtering Radiation ...

3rd International Congress on Advanced Electromagnetic Materials in Microwaves and Optics

Frequency Selective Surfaces for Filtering Radiation Harmonics of Novosibirsk Terahertz Free Electron Laser S. A. Kuznetsov1,2, V. V. Kubarev1, A. V. Gelfand3, N. I. Fedorinina3, P. V. Kalinin1, B. G. Goldenberg1, N. A. Vinokurov1 1

Budker Institute of Nuclear Physics SB RAS, Lavrentiev av. 11, 630090 Novosibirsk, Russia Research-and-Education Centre “Nanosystems and Modern Materials”, Novosibirsk State University, Pirogova str. 2, 630090 Novosibirsk, Russia Fax: +383-330-71-63; email: [email protected] 3 Institute of Semiconductor Physics SB RAS, Novosibirsk Branch “TDIAM”, Nikolaeva street 8, 630090 Novosibirsk, Russia. Fax: +383-339-17-26; email: [email protected] 2

Abstract In this presentation we describe the results on development of frequency selective surfaces designed for selecting radiation harmonics of Novosibirsk terahertz free electron laser. Single layer FSS with resonant and nonresonant elements of different topologies are considered: thin-film inductive crosses and capacitive loops, thick self-bearing inductive meshes with round openings etc. Technological aspects of FSS fabrication by techniques of conventional photolithography, electroforming and LIGA are discussed. The obtained experience is used in further development of more sophisticated FSS both for filter and metamaterials applications at subterahertz and terahertz frequencies.

1. Introduction The Novosibirsk terahertz free electron laser (NovoFEL) operating at the Siberian Centre of Photochemical Research, Novosibirsk, Russia, is considered as a unique source of high-power monochromatic and coherent THz-radiation [1]. Based on 12 MeV energy recovery linac, the first stage of the NovoFEL facility enables to generate radiation which fundamental frequency harmonics can be smoothly varied within a spectral range 1.3−2.5 THz with monochromaticity 3·10-3 (FWHM). The radiation is generated as a continuous train of 40–100 ps pulses at the pulse repetition rate of 2.8–11.2 MHz. Radiation peak and average power reaches ~1 MW and 500 W respectively that opens up possibilities for unique experiments in fundamental and applied physics, chemistry, biology, medicine etc. Measurements show that along with the fundamental harmonics the NovoFEL radiation spectrum contains the higher spectral components. In spite of small relative power of the higher harmonics (~≤1%) their absolute values are considered to be high enough for some user experiments where short-wave radiation is required. Necessity of creation of adequate frequency filters, providing selection of desired radiation harmonics (in the first place the 2nd and 3rd ones) at strong suppression of the powerful fundamental one, stimulated our technological works on development of FSS-based THz-filters for NovoFEL applications [2-5]. In this report we present recent results of development of single layer planar THz-FSSs using different techniques: thin-film photolithography, electroforming and LIGA.

2. FSS on polymeric film substrates We have adapted a conventional semiconductor industry technique employing Si-wafer-based photolithography for creating FSSs on the surface of low absorbing supporting polymeric films metallized by 0.5µm-thick-Al via a thermal vacuum deposition method. Two polymers were chosen for THzapplication due to their satisfactory physical and optical characteristics: polyimide (PI) and polypropylene (PP). In comparison with PI, PP exhibits very low dispersion and dielectric losses within a very wide frequency range (see [6]), whereas PI has higher mechanical fastness and thermostability and is well-suitable for high-quality thin film growing from a liquid phase by a spin coating and thermal annealing processes inappropriate for PP.

ISBN 978-0-9551179-6-1

608

©2009 Metamorphose-VI


The examples of fabricated single layer thin-film FSSs of different topologies are illustrated in Fig.1. The cases a,b,e,f correspond to 3.5µ-PI-film based FSSs with inductive cross elements optimized for band-pass selection of the 2nd and 3rd NovoFEL radiation harmonics (νII ≈ 4 THz, νIII ≈ 6 THz) against a background of the fundamental generation frequency νI ≈ 2 THz. Fig.1 c,d,g correspond to the 20µPP-film based FSSs with capacitive loop elements designed for resonant rejection of νI at 2.25 THz. The results of FT-characterization performed at the “Bruker IFS 66v/s” spectrometer are in satisfactory agreement with Ansoft HFSS v.11 simulations (nPI=1.8, tgδPI≈0.04; nPP=1.53, tgδPP≈0.001).

Fig. 1: Frequency response of thin-film THz-FSSs (a-d) and microphotographs of FSS’s patterns (e-g).

3. Thick self-bearing FSS In conditions of high incident radiation power the application of thin-film FSSs can be seriously restricted. Thick self-supporting substrate-free FSSs of the inductive type serve as the best alternative. We successfully used combination of photolithography and electroforming techniques for fabricating a set of nonresonant substrate-free FSSs (Ni-meshes) with round hexagonally packed openings a few ten microns thick (Fig.2). Such FSSs exhibit high-pass (or “band-pass”-like) properties and much more effective for suppressing the NovoFEL’s low-frequency harmonics than single layer thin-film FSSs due to a wave-guide cut-off effect. For the most small scale and high aspect FSSs (N7, 8) we revealed noticeable discrepancy of simulations and measurements (Fig.2 b) that is explained by conicity of the FSSs openings. Conicity, resulted from parasitic Fresnel photo-diffraction, can be eliminated by switching to a deep X-ray lithography technique (LIGA) [3-5]. Fig.3 illustrates our recent polymeric LIGA-structure created as a mask with deep vertical walls for the further electroforming of a thick high-aspect FSS.

ISBN 978-0-9551179-6-1

609



Fig. 2: Frequency response of thick substrate-free THz-FSS.

Fig. 3: Example of polymeric LIGA-structure (SU-8, thickness – 440 µm, aspect ratio – 11).

4. Conclusion We have successfully mastered and tested three key techniques for fabricating different single layer THz-FSSs with various metallization thickness t and aspect ratios a: thin-film photolithography (t ≤1 µ, a < 0.1); photolithography + electroforming (t ≤ 45 µ, a ≤ 2); LIGA (t > 45 µ, 2 ≤ a ≤ 11). Though our researches were focused mainly on the problems of filtering powerful THz-radiation of the NovoFEL, the obtained experience served as a basis for further development of more sophisticated FSSs and metastructures operating at subTHz- and THz-wave bands. At present, our main efforts are concentrated on upgrading the photolithographic technique for creating low-loss PP-film-backed multilayered subterahertz FSS-filters and metasurfaces for non-high-power applications (see [6]).

Acknowledgement This work is partially supported by the Russian Foundation for Basic Research (project 07-02-01459-а).

References [1] [2] [3] [4] [5] [6]

V. P. Bolotin, N. A. Vinokurov, D. A. Kayran et al., Status of the Novosibirsk terahertz FEL, Nuclear Instr. Meth., vol. A543, pp. 81-84, 2005. S.A. Kuznetsov, V.V. Kubarev, P.V. Kalinin, and N.A. Vinokurov, 1-D and 2-D metal mesh based quasioptical selective components for high-power applications at Novosibirsk terahertz FEL, Conf. Digest of Joint “IRMMW-THz 2007” Conf., vol. 1, pp. 303-304, Cardiff, UK, 3rd-7th September, 2007. S. A. Kuznetsov, V. V. Kubarev, P. V. Kalinin et al., Development of metal mesh based quasi-optical selective components and their application in high-power experiments at Novosibirsk terahertz FEL, Proc. 29th Int. FEL Conf. “FEL-2007”, pp. 89-92, Budker INP, Novosibirsk, Russia, Aug 26-31, 2007. S. A. Kuznetsov, B. G. Goldenberg, V. V. Kubarev et al., Development of thick metal mesh THz-filters by LIGA-technology for high-power applications at Novosibirsk terahertz FEL, Conf. Digest of Joint “IRMMW-THz 2007” Conf., vol. 2, pp. 978-979, Cardiff, UK, 3rd-7th September, 2007. S. A. Kuznetsov, B. G. Goldenberg, P. V. Kalinin et al., Development of copper mesh structures for frequency and spatial selection of THz-radiation from Novosibirsk free electron laser, J. Surface Investigation. X-Ray, Synchrotron and Neutron Tech., Issue 9, 2009 (in press). S. A. Kuznetsov, A. V. Arzhannikov, V. V. Kubarev et al., FSS-Filters and Metasurfaces for Subterahertz and Terahertz Frequencies, These Proceedings.

ISBN 978-0-9551179-6-1

610
