Nanolithography Using Bow-tie Nanoantennas Rouin Farshchi EE235 4/18/07 Sundaramurthy et. al., Nano Letters, (2006)

Outline Near-field optics and Nanoantennas Nanolithography Bow-tie nanoantennas - lithography - FDTD modeling Summary 2

Near-field Optics 3 Sanchez, PRL 82, 4014 (1999) Near-field: immediate vicinity of “light source” with dimensions <. Near-field Probes: Sharp tips (ANSOM), coated tapered optical fibers (NSOM) Nanoantennas: plasmon resonance coupling Nanopartice arrays, Pairs of nanoparticles Rechberger, Opt. Comm. 220 (2003) 137–141 Produce greatly enhanced fields upon laser excitation (up to 10 3 ), confined to regions ~20nm, significantly defeating diffraction limits:  microscopy, SERS, lithography ~ 300 nm Hecht, JPC 112, 7761 (2000)

Near-field optical lithography 4 Yin et. al., Appl. Phys. Lett (2002) Achieve ~ / 10 resolution by focusing femtosecond laser beam onto Au coated AFM tip in close proximity to SU-8. Two-photon polymerization occurs in SU-8 over confined regions due to local enhancement of electric field by surface plasmons on AFM tip.

Bow-tie Nanoantennas [3] Sundaramurthy et al., Physical Rev. B, (2005) [1] Schuck et al., Phys. Rev. Lett. 94, (2005) 5 [2] Fromm et al., Nano Lett. 4, 957 (2004) 10 3 field enhancement to <30 nm regions 10% efficiency (define) vs ~10 -5 for NSOM - ~10 3 enhancement of incident intensity - confined to 650 nm 2 region Au triangles on ITO (fabrication in [1]) Effects: -Plasmon resonance in each triangle -Coupling across gap Finite difference time domain (FDTD) for computation of [3]: - intensity enhancement - scattering efficiency - resonant wavelengths

Bow-tie Fabrication 6 ITO substrate Ti sticking layer ~4 nm ~20 nm ~75 nm Au SU nm ~80 nm Au Define with e-beam lithography Sundaramurthy et al., Nano Letters, (2006) Schuck et al., Phys. Rev. Lett. 94, (2005) Measured with TPPL [Schuck]

Exposure of SU-8 on bowties 7 SU-8 Excitation source: Ti:sapphire laser 120 fs, f = 75 MHz = 800 nm Focus beam to diffraction-limited spot With 1.3 NA 100x obvective lens Exposure powers: 27  W – 10 mW Sundaramurthy et al., Nano Letters, (2006) Sundaramurthy et al., Physical Rev. B, (2005) Measured with TIR microscopy polarizer, beam-splitter

AFM / SEM of exposed SU-8 8 AFM: At high exposure powers, SU-8 ablation at bow-ties SU-8 TPP away from bow-ties Blanket TPP TPP only at bow-tie gap No TPP Nano-lithography: - Exposure + develop, bow-tie nanoantennas covered with SU-8 Sundaramurthy et al., Nano Letters, (2006) TPP in vicinity of bow-tie

AFM of exposed SU-8 Nano-lithography: - Exposure + develop, bow-tie nanoantennas covered with SU-8 9 Au bow-ties “capture” energy of diffraction limited spot and concentrate it at two small areas near the gap, exceeding exposure threshold. record 30 nm features with near-field lithography using record low power of 27  W Sundaramurthy et al., Nano Letters, (2006)

Theory- FDTD displacement current in gap current in metal region frequency dependant (RIT) far-field radiation power scattering cross-section incident power scattering efficiency 16nm gap 500nm gap 0.13  A peak 0.05  A peak 10 Sundaramurthy et al., Physical Rev. B, (2005)

Theory- FDTD 11 The FDTD simulations predict an intensity enhancement of 107 at 4 nm above each of the triangle tips exposed at 27  W, in good agreement with experimental value of 150 from experiment. FDTD Calculated enhancement peaks occur within 4 nm of SU-8 peak locations from AFM measurement. Sundaramurthy et al., Nano Letters, (2006)

Conclusion - large electric-field enhancement in highly confined regions at tips of gold bow-tie nanoantennas 12 - Allows for local exposure of SU-8 resist to record low dimensions (<30nm) using record low power (~27  W) - Intensity enhancement thought to be due to coupling of plasmon resonance at tips of triangles, as suggested by theoretical modeling. Thank you!