Femtosecond laser fabrication of metamaterials for high frequency

Femtosecond laser fabrication of metamaterials for high frequency devices Marian Zamfirescu*a), Razvan Dabu*, Marius Dumitru*, George Sajin **, Florea Craciunoiu ** *NILPRP – Bucharest, Atomistilor 409, 077125 Magurele, Romania **IMT Bucharest, Str. Erou Iancu Nicolae 126 A, 077190 Bucharest, Romania a) marian. zamfirescu@inflpr. ro Direct Laser Writing (DLW) is used for configuration of micro structures. Films of few hundreds nm of gold deposited on glass or silicon are precisely processed by tightly focusing a femtosecond laser with 150 fs pulse duration, 775 nm wavelength, and energy of tens of n. J. The laser fluence is hold just above the ablation threshold. Thus, the evaporated film leaves behind structures with thin lateral size of about 1µm. Using this technique the fabrication of micro-structures is demonstrated. The periodic structures are designed to behave as a left handed metamaterial in the GHz range. The fabricated metamaterials have applications for antenna, filters, couplers, etc. in the high frequency range. Femtosecond ablation of metallic films bellow diffraction limit Determination of the best parameters for DLW Setup Self focusing system PMT AD converter Threshold Fluency 5 X LASER 775 nm 200 fsec 2 KHz Attenuation Spatial Filter Pulse energy = 110 n. J CCD Dichroic Mirror Laser control unit Glan l/2 Polarizer He. Ne laser The lateral size of structures are obtained by adjusting the laser intensity above threshold of ablation. For a given material the quality and the resolution of the written structure depends strongly on few parameters such as: - writing speed; - focusing; - laser energy. 100 X Focused Unfocused z = 0 Unfocused z – 1 µm z + 1 µm PC Microscope Objective (NA>0. 5) 10 µm 1 µm/sec Gas Eject 1000 µm/sec Film of 100 nm Au on Si substrate (Threshold : about 50 n. J at 150 fsec) Translations XYZ Composite Right/Left-Handed Metamaterials (CRLH - MTM) Negative refractive index (NRI) Transmission Line (TL) The CRLH-TL structure is constituted of series interdigital capacitors and inductors fabricated on metallic film. Series and shunt resonance frequencies : LR = 0. 25 n. H CR = 2 p. F Equivalent circuit model in a LC network LL = 0. 5 n. H L’R p C’L/ p Permittivity and permeability of CRLH material: Z’ C’R p CL = 1 p. F p = 12 mm ( p << l ) L’L/ p Y’ n<0 up to 5 GHz p AFM measurements 1 D CRLH structures The AFM measurements show the minimum lateral size of the created structure of about 650 nm, and the ablation depth of about 260 nm. Network of interdigital capacitors and stub inductances are created on Gold thin films by tightly focusing the femtosecond laser. The desired design is obtained by PC controlled displacement of the sample with submicron precision. - laser energy: 50 n. J - writing speed 0. 1 mm/s. lc = 50 µm ls = 100 µm width of digits = 5 µm N = 5 pairs of fingers 50µm From scaling proprieties: for p=70 µm n<0 at tens of GHz Conclusions 2 D CRLH structures lc = 30 µm ls = 30 µm width of digits = 3 µm Metamaterial structures can be easily fabricated on metal films by femtosecond lasers. Using the actual setup, structures with resolutions below 1 µm were obtined. Depending on the its lateral size, the fabricated structures can behave as metamaterials in the microwave spectral region (tens to hundreds GHz). The resolution as small as few hundreds nm can be reached by methods based on radially polarised laser, second harmonics, or/and shorter pulse duration. References 30µm [1] [2] [3] Equivalent circuit model of 2 D - CRLH V. G. Veselago, Soviet Phys. Usp. 10, 509 (1968). J. B. Pendry et al, J. Phys. Condens. Matter 10, 4785 (1998). C. Caloz, T. Itoh. Electromagnetic metamaterials: transmission line theory and microwave applications. John Wiley & Sons, 2006.