Femtosecond laser fabrication of metamaterials for high frequency devices Marian Zamfirescu *a), Razvan Dabu *, Marius Dumitru *, George Sajin **, Florea Craciunoiu ** * NILPRP – Bucharest, Atomistilor 409, Magurele, Romania ** IMT Bucharest, Str.Erou Iancu Nicolae 126A, Bucharest, Romania a) 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 150fs pulse duration, 775nm wavelength, and energy of tens of nJ. 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. Film of 100 nm Au on Si substrate (Threshold : about 50 nJ at 150 fsec) 1 µm/sec1000 µm/sec 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. Pulse energy = 110 nJ Focused z = 0 Unfocused z – 1 µm Unfocused z + 1 µm 10 µm lc = 50 µm ls = 100 µm width of digits = 5 µm N = 5 pairs of fingers Composite Right/Left-Handed Metamaterials (CRLH - MTM) Threshold Fluency The lateral size of structures are obtained by adjusting the laser intensity above the threshold of ablation. Femtosecond ablation of metallic films bellow diffraction limit DLW Setup Determination of the best parameters for DLW The CRLH-TL structure is constituted of series interdigital capacitors and inductors fabricated on metallic film. Equivalent circuit model in a LC network p Y’Y’ Z’Z’ L’ R p C’ L / p C’RpC’Rp L’ L / p Conclusions Using the actual setup, structures with resolutions below 1 µm were obtined. Metamaterial structures can be easily fabricated on metal films by femtosecond lasers. 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. Depending on the its lateral size, the fabricated structures can behave as metamaterials in the microwave spectral region (tens to hundreds GHz). Negative refractive index (NRI) 2D CRLH structures 30µm lc = 30 µm ls = 30 µm width of digits = 3 µm Transmission Line (TL) References LASER 775nm 200 fsec 2KHz Translations XYZ Microscope Objective (NA>0.5) Attenuation Dichroic Mirror /2 Glan Polarizer PC Spatial Filter HeNe laser 100X PMT 5X AD converter Self focusing system Laser control unit Gas Eject CCD AFM measurements The AFM measurements show the minimum lateral size of the created structure of about 650 nm, and the ablation depth of about 260 nm. Equivalent circuit model of 2D - CRLH - laser energy: 50 nJ - writing speed 0.1 mm/s. 1D CRLH structures 50µm 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. Series and shunt resonance frequencies : Permittivity and permeability of CRLH material: [1]V.G. Veselago, Soviet Phys. Usp. 10, 509 (1968). [2]J. B. Pendry et al, J. Phys. Condens. Matter 10, 4785 (1998). [3]C. Caloz, T. Itoh. Electromagnetic metamaterials: transmission line theory and microwave applications. John Wiley & Sons, L R = 0.25 nH C R = 2 pF L L = 0.5 nH C L = 1 pF n<0 up to 5 GHz p = 12 mm From scaling proprieties: for p=70 µm  n<0 at tens of GHz ( p << )