Femtosecond laser fabrication of metamaterials for high frequency devices

1. Self focusing system PMT AD converter 5X Spatial Filter Gas Eject INFLPR LASER 775nm 200 fsec 2KHz Attenuation CCD Dichroic Mirror PC Glan Polarizer Laser control unit l/2 Microscope Objective (NA>0.5) 100X HeNe laser Translations XYZ L’R p C’L/ p Z’ C’Rp L’L/ p Y’ p 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 126A, 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 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. Determination of the best parameters for DLW Femtosecond ablation of metallic films bellow diffraction limit DLW Setup 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. Threshold Fluency Focused z = 0 Pulse energy = 110 nJ Unfocused z – 1 µm Unfocused z + 1 µm 10 µm 1 µm/sec 1000 µm/sec The lateral size of structures are obtained by adjusting the laser intensity above the threshold of ablation. Film of 100 nm Au on Si substrate (Threshold : about 50 nJ at 150 fsec) Composite Right/Left-Handed Metamaterials(CRLH - MTM) Negative refractive index (NRI) Transmission Line (TL) Series and shunt resonance frequencies : The CRLH-TL structure is constituted of series interdigital capacitors and inductors fabricated on metallic film. LR = 0.25 nH CR = 2 pF Equivalent circuit model in a LC network LL = 0.5 nH CL = 1 pF Permittivity and permeability of CRLH material: p = 12 mm (p << l ) n<0 up to 5 GHz AFM measurements 1D 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 nJ - 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 2D 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] 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, 2006. Equivalent circuit model of 2D - CRLH