Department of Physics, Dalian University of Technology, Dalian

Transcription

1 Solid State Phenomena Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Fabrication of a brush-shaped bent fiber probe for Near-Field optics by heated pulling combined with chemical etching Pengfei Li, Shi Pan, Shifa Wu, Yinli Li, Wei Sun Department of Physics, Dalian University of Technology, Dalian Lpfplane2004@yahoo.com.cn Keywords: Optical fiber probe, heated pulling, chemical etching,probe tip Abstract. We developed a method to fabricate bent fiber probe: heated pulling combined with chemical etching. Based on sampling theory, the resolving power of SNOM relates to the size of the probe tip. If the apex of the probe is smaller, the resolution ability of SNOM is higher. At the same time it can collect more near-field information through bigger cone angle of probe. Through improving the operation and optimizing the parameters of the heated-pulling instrument, we can obtain the probe with bent angle fiber varying from degree, the cone angle varying from degree and the apex diameter less than 100nm. With this new brush shape of probe, the collecting efficiency for near-field light is improved. That is to say that more near-field information is obtained. The successful ratio of the produce is more than 70%. 1 Introduction Because of the diffraction limit of traditional optics imaging, ordinary optic instrument s ability of resolving power is probably λ/2. SNOM (Scanning Near-field optical Microscope) is a special kind of near-field microscope, it is able to break through the diffractive limit of ordinary optical microscope and achieve the nanometer level [1, 2]. PSTM (Photon Scanning Tunneling Microscope) is a kind of near-field microscope, too. It has been applied to physics field, chemistry field and so on. But the application of PSTM is limited because it can only get the images of special samples, such as the samples with wave of surface, or the samples with the variation of refractive index. If the samples have the characters of surface and the variation of refractive index (such as the biologic samples), we get the mixed images. AF/PSTM can get better images without artifacts [3, 4]. Now how to improve the resolving power of PSTM and collect the near-field optics is the main question. Then how to fabricate good fiber probe is important for development of AF/PSTM. It is required that the probe has high-transmission efficiency and highly sensitivity. The parameters, including the shape of probe, the size of tip and the cone angle, directly affect the transmission efficiency and sensitivity of system. A good probe should have a bigger cone angle and smaller diameter of tip. In this paper we introduce the method, heated pulling combined with chemical etching, to fabricate bent fiber probe. The shape of probe and the cone angle can be controlled through heated pulling. With chemical etching the probe with small diameter of apex can be produced. Then, the brush-shaped probe with big cone angle and small diameter of tip, can be produced with the developed method. 2 The Implement and the Method Different with normal straight-tip probe, we adopt the method to fabricate bent fiber probe: heated pulling combined with chemical etching in different temperature [5]. (1) Heated pulling *This research was sponsored by national science natural science foundation of China (No ) and by All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans China Ministry of Education (No ) Tech Publications, (ID: , Pennsylvania State University, University Park, USA-04/03/16,02:34:56)

2 690 Nanoscience and Technology We use instrument of GQR-3 to heat and pull fiber. It is available in the experiment. Fig.1 the instrument of GQR-3 and its controller (GQR-3 is used to produce probe in this experiment. The left image is the instrument of GQR-3, used in the experiment. The right image is its controller, regulating the electrode current and the heating time) The instrument of GQR-3 is simple and convenient to operate. We get the bent fiber by heated pulling the fiber by using single mode fiber. Its diameter is 125μm and its core is 9μm. Firstly, before pulling the fiber, we should peel off the coating of fiber, revealing the length of naked fiber about mm then wiping it with alcohol. Secondly, heating the core directly, the current is controlled about 12 ma and the heating time is probably 3-5 seconds, repeating it 12 times. Finding that the fiber formed a big cone angle, it is time to adjust the stage toward the middle and make the heated fiber shrink. Then, through time after time heating, we can get bent optical probe with shape of brush. When the heating area is too thin to see, we stop heating. Thirdly, uplift the fiber and it will break at the heated position because of gravitation. Now the probe is ready. Using microscope we may see a bent optical probe with shape of brush. The ratio of repetition is about 90% and the diameter of apex is less than 5μm. Fig.2 the process of producing advanced fiber probe: (First: heating the fiber with electrode current. Second: adjusting the stage toward the middle, what makes the heated fiber shrinking.then the brush-shaped probe is formed) Fig.3 shows that Left image is the product of ordinary fiber probe and the right image is advanced fiber probe.

3 Solid State Phenomena Vols In the experiment the space of electrodes decides the size of cone and the distance of stage moved decides the length and angle of the bent part of fiber. The professional machine is expensive and the fiber probes are mostly direct tip. The instrument of QDR-3, which we use, is cheaper and easily operated and can make a bent fiber probe with shape of brush. With the method of heated pulling it can produce good fiber probe. Fig.4 fiber probe before etched: (the scale is 50μm.The apex is less than 5μm.) (2) Etching the probe In the experiment we use 40% HF acid solution etching the semi-manufactured fiber probe product [6, 7]. Meniscus etching [8] is used for taping the fiber. The parameter of temperature and etch time are important for the probe in the experiment. At different temperature we erode the semi-manufactured goods, so we can get different probes, whose size of apexes are different. The time is different and the tip is different too. We get perfect probe by controlling time and temperature. Etching temperature is at 15 in the experiment. After fixing the semi-manufactured goods on the etch shelves, we deep the naked probes 5 mm in 40% HF acid solution. Etching time is 15 min. When time is over, we dip them in special water for 24 hours, which has been taken out HF ions. The probes are glazed because they are heated pulling firstly. Fig.5 shows the finished product: (The left image is 400 by optics microscope, scale is 50μm. The right image is by scanning electron microscope, scale is 500 nm. From the images it can be known that the apex of tip is less than 100nm.) 3 Resolute and Discussion

4 692 Nanoscience and Technology From Fig.5 it can be known that the apex of probe produced with the method of heated pulling combined with chemical etching in different temperature is less than 100 nm. In Fig.3 we can see that the probe is bigger than normal probe, which would affect the frequency of mechanical resonance. We have found the discussion about straight-tip [10] but there are not any data about bent probe. So we adopt the method: controlling the distance of probe and sample. Sticking the probe on one side of a tuning fork and shaking with the tuning fork. When fiber probe approaches a sample, the oscillation of tuning fork will change for the influence of shear-force [11] damp. Then we can get the near-field curve. Contrast with a standard curve, we can validate that the probe is good. Conclusion: The method we mend is simple, easily operated. The successful ratio is approximately 90%. The cone angle of fiber probe is between 40-90, which has a better ratio of light transmission. On the other hand, it is credible for near-field imaging. References [1] Ferrel T L, Warmack R J and Reddick: Photon scanning tunneling microscopy. United states Patent. (1991) [2] R.C.Reddick, R.J.Warmack and T.L.Ferrel: New form of scanning optical microscopy, Phys (1989), p.767 [3] Shifa Wu,June Yao,Guoshu Jian,Ning Guo:The development of PSTM.ACTA OPTICA SINICA(1998).p [4] Shifa Wu,Shi Pan,Jiang Zhanf,Wei Liu,JingZhi Wang: The Method for Separating Image of the Bent Optical Fiber Tip Resonated AF/PSTM.Engineering Science VOL 3(2001).NO.8 [5] Liyuan Zhang,Yonggui Li,Qiang Wang:Preparation of Optical Fibers For Scanning Near-Field Infrared Microscopy By Etching Methode.ACTA PHYSICA SINICA.VOL.50(2001) [6] Jialin Sun,Guangyan Tian,Qin Li,Jun Zhao,Jihua Guo: FABRICATION of large cone angle Optical Fiber Probe by Dynamic Chemical Etching Method.ACTA PHYSICA SINICA.VOL 50(2001). [7] Shuji Mononobe and Motoichi Ohtsu : Fabrication of a Pencil-Shaped Fiber Probe for Near-Field Optics by Selective Chemical Etching. Journal of LightWave Technology VOl.14 (1996).NO.10 [8] K,M, Takahashi: Meniscus shapes on small diameter fibers. J.Colloid Interface SCI VOL.134 (1990), p [9] S.Mononobe, R.Uma Maheswari, T.Saiki, M.Naya, and M.Ohtsu, in Proc.Conf. Near Field Optics-3. VOL.8 of EOS Topical Meeting Digests (1995), p [10] Xiumei Liu,Jia Wang,Dacheng Li: Fabrication and Characterization of Fiber-Optic Nano-Probes with High Transmission Efficiency and High Resolution.ACTA OPTICA SINICA.VOL.20(2000). NO.5 [11] E.Bezig, P.L.Finn, and J.S.Weiner: Combined shear force and near-field scanning optical microscope. Alppl.Phys.Lett.VOL.60 (1992),p.2484

5 Nanoscience and Technology / Fabrication of a Brush-Shaped Bent Fiber Probe for Near-Field Optics by Heated Pulling Combined with Chemical Etching /