Jenny Yu, Dr. Keith Roper, Department of Bioengineering, 2007 Abstract Introduction Experimental Results Summary References Acknowledgements Varying cone angle and tip diameter in chemical etch of single mode tapered optical fibers Single-mode tapered optical fiber is used in near-field scanning optical microscopy (NSOM) or to obtain molecular spectroscopy. We varied exposure time of optical fiber to a silicone-oil/hydrofluoric (HF) acid etchant and composition of the etchant. Cone angles are 16° to 45° and tip diameters are 70 nm to 1.13 µm. Argon laser transmission through tapered tips increased as cone angle increased. Two mathematical models, phase-matching and axisymmetric Young-Laplace equation, are used to compare and identify the critical angle for surface plasmon. Single-mode tapered optical fibers (TOF) as NSOM tips provide resolution that is determined by the tip diameter and the electromagnetic wavelength (Rasmussen & Deckert, 2005; Burgos et al., 2003). Two common ways to fabricate TOFs: CO 2 laser-heated pulling (Burgos et al., 2003; Maruyama et al., 2006) and HF-silicone oil mixture etching (Mononobe et al., 1997; Wong et al., 2002). Four chemical etching variations (at room temperature). Variation I: Fiber submerged in silicone oil/49% HF. Variation II: Second etch with 49% HF solution diluted four-fold with deionized H 2 O to 12.25% (Mononobe et al.,1997). Variation III: The third variation was silicone oil/49% HF etch followed by second etch with buffered HF (BHF) with volumetric ratio of 4:1:3 (40 wt% NH 4 F: 49% HF: H 2 O). Variation IV: The first etch was silicone/49% HF, and second etch was BHF solution with ratio of 10:1:1, followed by a third etch with four-fold diluted HF (Mononobe et al.,1997). Fig.1. Optical fiber in silicone oil/49%HF etchant (not drawn to scale). Fig.2. SEM images of TOFs a) Variation I: silicone oil/HF etch for 120 minutes. (b) Variation II: 50-minute first etch, 40-minute second etch. (c) Variation III: 50-minute first etch, 40-minute second etch. (d) Variation IV: 50-minute first etch, 120 minute second etch, 90-second third etch. Fig.5. Measurement of power transmission of uncoated TOFs with different cone angles. The dotted line is Excel exponential fit to demonstrate the general trend of increase. Fig.4. Schematic of contact angle (  )in tapered region presented by theoretical shape of Young-Laplace equation 90-  /2 = ,  is the cone angle of the TOF. (Chang et al., 2006),  = 180-  -  (not drawn to scale). Fig.3. Plot of actual and theoretical tip profiles for TOFs of Fig.2. Cone angle etch rates: etch rate of cladding is 3.54 ± 0.17  m/min silicone/49% HF; 150 nm/min 4x- diluted HF. Cut-off radius: cut-off radius (R c ) (Keilmann, 1999), λ is operating wavelength, η c is RI of fiber core : Critical angle from phase matching: Surface Plasmon Polariton (SPP) and phase matching condition expressed by equation below (Chang et al., 2006). η c is RI of fiber core,  is the critical angle,  m and  c are the dielectric constants of Au and water. Table 1. Calculated values for  s based on phase-matching.  c is 1.769,  c is N/A indicates that at the given wavelength, it is not possible to match the phase for surface plasmon. Critical angle from asymptotic expansion: The fiber radius and meniscus height by derivation of Young-Laplace equation (Wong et al., 2002),  is angle between the meniscus and r axis, r 0 is fiber radius, z’ = z/r 0 and r’ = r/r 0 are normalized parameters, l 0 is (  /  g) 0.5,  = r/l 0 is perturbation parameter,  is Euler’s constant (0.5772). From phase-matching relation, best cone angle is 30  - 35  to generate SPR on Au film, and experiment should be carried out in water. Actual profiles of the TOFs deviate considerably from the axisymmetric Young-Laplace model. Ratio of BHF effects the tip diameter, lower concentration of NH4F (ratio 4:1:3) was more beneficial in producing smaller and smoother tip than higher concentration of NH4F (in ratio 10:1:1). Data suggests it is harder to fabricate small tip diameter with big cone angle. Trade-off between cone angle and desired tip diameter. Burgos, P., Lu, Z., Ianoul, A., Hnatovsky, C., Viriot, M., Johnston, L.J., Taylor, R.S. (2003) Near-field scanning optical microscopy probes: a comparison of pulled and double-etched bent NSOM probes for fluorescence imaging of biological samples. J. Microsc. 211, Chang, Y., Chen, Y., Kuo, H., Wei, P. (2006) Nanofiber optic sensor based on the excitation of surface plasmon wave near fiber tip. Journal of Biomedical Optics 11(1), Drezet, A., Hohenau, A., Krenn, J.R., Brun, M., Huant, S. (2007) Surface plasmon mediated near-field imaging and optical addressing in nanoscience. Micron, 38, Frey, H., Keilmann, F., Kriele, A., Guckenberger, R. (2002) Enhancing the resolution of scanning near- field optical microscopy by a metal tip grown on an aperture probe. Appl. Phys. Lett. 81, Maruyama, K., Ohkawa, H., Ogawa, S., Ueda, A., Niwa, O., Suzuki, K. (2006) Fabrication and characterization of a nanometer-sized optical fiber electrode based on selective chemical etching for scanning electrochemical/optical microscopy. Anal. Chem. 78, Mononobe, S., Uma Maheswari, R., Ohtsu, M. (1997) Fabrication of a pencil-shaped fiber probe with a nanometric protrusion from a metal film for near-field optical microscopy. Optics Express, 1, , Rasmussen, A. & Deckert, V. (2005) New dimension in nano-imaging: breaking through the diffraction limit with scanning near-field optical microscopy. Anal. Bioanal. Chem. 381, Wong, P.K., Wang, T.H., Ho, C.M. (2002) Optical Fiber Tip Fabricated by Surface Tension Controlled Etching. Hilton Head 2002 Solid State Sensor, Actuator and Microsystems Workshop, pp , South Carolina, USA. Thanks to Dr.Yanil Dall'Asén, Post Doctorate for assistance and guidance.