掃描式近場光學顯微鏡(SNOM)為現今主要量測低於波長二分之ㄧ解析度之儀器，它是利用尖端極細的光纖探針(孔徑一般約50 nm)，外圍包覆著金屬膜將光限制在探針開孔附近，利用精密控制技術使探針在量測表面數十奈米處震盪，由於在一個波長的距離以內，因此解析度並不受繞射極限限制。但是此種近場光學顯微鏡有些先天的缺點，其解析度受限於探針孔徑尺寸，過小之孔徑無法傳遞電磁波，因此目前最佳解析度約為20 nm。此外，探針利用光纖傳播，對於量測電磁波之波長有範圍限制。針對以上缺點開發出新的無孔徑型掃描式近場光學顯微鏡，成為近幾年來研究之重點。此項技術的特點是將原子力顯微鏡改裝，利用探針與試件表面因為外加電磁場產生交互作用散射光，量測此種散射光，即可得到奈米級表面量測。由於解析度由探針尖端所決定，現今的技術可達10 nm 以下。此種技術最大的困難點在於雜訊光的去除，因此許多的研究團隊都在做相關之研究，但大多屬於實驗定性方面之研究，無法全面得到雜訊與訊號之資訊與量測方法之最佳化。

本文針對散射型掃描式近場光學顯微鏡首先說明其基本近場作用原理，然後建立近場分析模型，這是第一次對於複雜調變訊號進行定量分析，不管是Homodyne 或者Heterodyne 的各階調變頻率均有詳細之推導與相關參數之物理含意解釋，文中說明訊號雜訊與Bessel方程式及調變深度息息相關，並提出相關文獻驗證，對於實驗或儀器設計者提供有根據之參數以提高量測結果之訊噪比，包括：波長、入射光角度與探針振幅，改變以往認為波長與入射光角度與量測結果無關之認知。文中並說明Heterodyne的訊噪比較佳之原因，並不只是藉由參考光放大近場量測訊號而已，而是藉由高強度之量測訊號可改變相關參數如階數或探針振幅所達成，。此外，針對探針掃描方式或平台掃描的方式對近場影像影響也提供定量說明，原則上平台式掃描對於量測訊號之相位差不會影響，但探針式掃描量測則是具備成本低與快速量測的特性，文中分析得知，在採用高階探針頻率與量測範圍小的情形下，探針掃描量測是可行的方式。

近場影像由相當之複雜訊號所組成，因此有許多因素會影響影像之失真，本文提出文獻中常見造成影像失真的原因，包含：(1)試件的表面形貌改變與(2)不連續邊界造成之失真，藉由本文推導之式子很容易且清楚地得知其原因與解決之道。

本文根據推導結果提出新的兩種概念(1)高調變深度(>1)情形下Bessel 方程式之零點與(2)改變探針振盪方式為鋸齒式，在滿足文中所述條件下，純粹的近場訊號是有可能實現，打破以往不可能得到純近場訊號的概念。

本文利用DMD技術開發出數位控制光源頻譜合成系統，可任意將光源頻譜分解合成，功能相當強大。此技術可用於探討無孔徑近場光學顯微鏡與入射光頻譜的關係，因為探針強化效應與光頻譜有很強之相關性，可作為將來奈米光學頻譜分析之研究。

For hundreds of years, the resolution of optical microscopy was limited to the order of approximately as a result of the far-field diffraction effect. Aperture scanning near-field optical microscopes (SNOM) are one of the most commonly-employed instruments for obtaining optical resolutions below the diffractive limit. In such systems, the tapered metal-coated optical fiber aperture confines the illumination and detection electric field range to the near-field regime, and thus effectively eliminates background noise. However, SNOM techniques have a number of fundamental drawbacks. For example, the maximum attainable resolution is limited to just approximately 50 nm in the visible light range due to the finite skin depth of the metal used to define the aperture. Furthermore, the very small apertures typical of SNOM devices severely restrict the light throughput and this cannot be compensating simply by increasing the incident power level due to the risk of thermal damage to the probe. Accordingly, an alternative SNOM configuration was proposed in which the optical fiber was replaced with small scatter, yielding an enhanced resolution of approximately 10 nm depending on the tip diameter. In this configuration, the incident light illuminates the small scatter and induces an enhanced electric field between the tip and the sample whose magnitude depends on the dipole effect. Measuring the near-field interaction electric field is the operating principle. This device is conventionally referred to as the apertureless scanning near-field optical microscope (A-SNOM). However, in A-SNOM, the near-field electric field is seriously affected by a background interference electric field and therefore it is necessary to develop techniques for eliminating the background-scattering noise from the detected signal in order to improve the imaging resolution.

The dissertation first time develops a comprehensive interference-based model with which to analyze the amplitude and phase of the heterodyne detection signal at different harmonics of the tip vibration frequency. The analysis considers a reflective-type A-SNOM since this device represents one of the most commonly used forms of A-SNOM system for measuring the surface properties of materials at the nanoscale. The study indicates that the high-order harmonic tip scattering noise decays more rapidly with a high-order Bessel function for small phase modulation depths than the near-field interaction signal. It is also shown that the signal-to-noise (S/N) ratio improves as the wavelength of the illuminating light source is increased or the incident angle is reduced. Those concepts did not be mentioned before. It is also demonstrated that sample stage scanning yields an improved imaging result, but tip scanning provides a reasonable low-cost solution that scanning is performed using a lock-in detection technique with an order higher than two in the modulation frequency, and a small scan area.

Finally, it discusses the modulation depth in Bessel function zero points and offers new thoughts to deal with the signals, and moreover it is shown that the pure near-field signal can be theoretically obtained using tip ramp function vibration instead of sinusoidal function. The two new thoughts tell us that the pure near-field interaction can be obtained in some specific conditions.

Like aberrations in traditional optical image system, there are many factors which affect the image quality in A-SNOM. (1)Image artifacts are induced by variations in the topography of the scanned sample surface and are essentially fake near-field signals. It is clear that the problem of image artifacts in heterodyne A-SNOM detection can be easily resolved by performing a sample stage scanning operation rather than a tip scanning operation. (2)The image error is caused by the variety amplitude of the tip vibration. The amplitude is affected by changes in the discontinuous geometry and/or material. In order to correct these image errors, the tip scanning rate should be carefully controlled in such a way as to allow the AFM feedback control scheme sufficient time to trace the variations in the amplitude of the tip vibration and to take appropriate remedial actions to maintain a constant amplitude, A.

The results presented in this study provide an improved understanding of the complex signal detected in the heterodyne A-SNOM technique and suggest potential means of improving its S/N ratio.

A DMD pattern scanning calibration method is developed and applied to the synthesis of various infrared C-band (1530-1565 nm) spectral profiles, including a fast programmable tunable light source with bandwidth approximately 3.8 nm, a square profile, a saw tooth waveform and a triangular spectrum profile. The spectrum synthesis system is suitable for the study in A-SNOM like surface-enhanced Raman spectroscopy (SERS), because the tip enhancement depends on incident light spectrum.

[1] E. H. Synge, “A suggested method for extending the microscopic resolution into the ultramicroscopic region,” Phil. Mag., Vol. 6, p.p. 356-362, 1928.
[2] G. Binnig and H. Rohrer, “Scanning tunneling microscopy,” Helv. Phys. Acta., Vol. 55, p.p. 726-735, 1982.
[3] G. Binnig, C. F. Quate, and C. Gerber, “Atomic force miscopy,” Phys. Rev. Lett., Vol. 56, p.p. 930-933, 1986.
[4] D. W. Pohl, S. Denk, and M. Lanz, “Optical stethoscopy: image recording with resolution ,” J. Appl. Phys., Vol. 44, p.p. 651-653, 1984.
[5] J. D. Jackson, Classical electrodynamics, Wiley, 1999.
[6] J. Wessel, “Surface-enhanced optical microscopy,” J. Opt. Soc. Am., Vol. 2, p.p.1538-1540, 1985.
[7] H. K. Wickramasinghe and C. C. Williams, “Apertureless near field optical microscope,” US Patent 4 947 034, 1990.
[8] Y. Inouye and S. Kawata, “Near-field scanning optical microscope with a metallic probe tip,” Opt. Lett., Vol. 19, p.p. 159-161, 1994.
[9] Kirstein S., “Scanning near-field optical microscopy,” Current Opinion in Colloid & Interface Science, Vol. 4, p.p. 256-264, 1999.
[10] E. Betzig and R. J. Chinchester, “Single Molecules Observed By Near-Field Scanning Optical Microscopy,” Science, Vol. 262, pp.1422, 1993.
[11] L. A. Nagahara and H. Tokumoto, “Scanning near-field optical microscopy / spectroscopy of thin organic films,” J. of Vacc. Sci. & Tech. B, Vol. 14, pp. 800, 1996.
[12] S. K. Buratto and J. W. P. Hsu, “Near-Field Photoconductivity-Application to Carrier Transport In Ingaasp Quantum-Well Lasers,” Appl. Phy. Lett., Vol. 65, pp. 2654-265 ,1994.
[13] W. P. Ambrose et al, “Single-Molecule Detection and Photochemistry on a Surface Using Near-Field Optical-Excitation,” Phys. Rev. Lett., Vol. 72, pp. 160-163, 1994.
[14] S. Patane, G. G. Gucciardi, M. Labardi and M. Allegrini, “Apertureless near-field optical microscopy,” Rivita Del Nuovo Cimento, Vol. 27, pp. 1-46, 2004.
[15] B. Knoll and F. Keilmann, “Near-field probing of vibrational absorption for chemical microscopy,” Nature, Vol. 399, p.p. 134-137, 1999.
[16] L. Novotny, E. Z. Sanchez and X. S. Xie, “Near-field optical imaging using metal tips illuminated by higher-order Hermite-Gaussian beams,” Ultramicrosc. Vol. 71, p.p. 21-29, 1998.
[17] R. Hillenbrand, and F. Keilmann, “Complex optical constants on a subwavelength scale,” Phys. Rev. Lett., Vol. 85, pp.3029-3032, 2000.
[18] R. Hillenbrand, B. Knoll, and F. Keilmann, “Pure optical contrast in scattering-type scanning near-field microscopy,” J. Microsc., Vol. 202, Pt 1, pp.77-83, 2000.
[19] B. Knoll, and F. Keilmann, “Enhanced dielectric contrast in scattering-type scanning near-field optical microscopy,” Opt. Commun., Vol. 182, pp.321-328, 2000.
[20] J. A. Porto, P. Johansson, S. P. Apell, and T. Lopez-Rios, “Resonance shift effects in apertureless scanning near-field optical microscopy,” Phys. Rev. B, Vol. 67, 085409, 2003.
[21] R. Fikri, D. Barchiesi, F. H’Dhili, R. Bachelot, A. Vial, P. Royer, “Modeling recent experiments of apertureless near-field optical microscopy using 2D finite element method,” Opt. Comm. Vol. 221, p.p. 12-22, 2003.
[22] Y. C. Martin, F. Hamann, and H. K. Wickramasinghe, “Strength of the electric field in apertureless near-field optical microscopy,” J. Appl. Phys., Vol. 89, 5774, 2001.
[23] S. Hudlet, S. Aubert, A. Bruyant, R. Bachelot, P. M. Adam, J. L. Bijeon, G. Lerondel, P. Royer, and A. A. Stashkevich, “Apertureless near field optical microscopy: a contribution to the understanding of the signal detected in the presence of background field,” Opt. Commun., Vol. 230, pp.245-251, 2004.
[24] F. Formanek, Y. D. Wilde, and L. Aigouy, “Analysis of the measured signals in apertureless near-field optical microscopy,” Ultramicrosc., Vol. 103, pp.133-139, 2005.
[25] P. G. Gucciardi, G. Bachelier, and M. Allegrini, “Far-field background suppression in tip-modulated apertureless near-field optical microscopy,” J. Appl. Phys., Vol. 99, Art. No. 124309, 2006.
[26] P. G. Gucciardi, G. Bachelier, M. Allegrini, J. Ahn, M. Hong, S. Chang, W. Jhe, S. C. Hong, and S. H. Baek, “Artifacts identification in apertureless near-field optical microscopy,” J. Appl. Phys., Art. No. 064303, 2007.
[27] L. Billot, M. Lamy de la Chapelle, D. Barchiesi, S. H. Chang, S. K. Gray, J. A. Rogers, A. Bouhelier, P. M. Adam, J. L Bijeon, G.. P. Wiederrecht, R. Bachelot, and P. Royer, “Error signal artifact in apertureless scanning near-field optical microscopy,” Appl. Phys. Lett., Vol. 89, Art. No. 023105, 2006.
[28] L. Gomez, R. Bachelot, A. Bouhelier, G. P. Wiederrecht, S. Chang, S. K. Gary, F. Hua, S. Jeon, J. A. Rogers, M. E. Castro, S. Blaize, I. Stefanon, G. Lerondel and P. Royer, “Apertureless scanning near-field optical microscopy: a comparison between homodyne and heterodyne approaches,” J. Opt. Soc. Am. B, Vol. 23, pp. 823-833, 2006.
[29] A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Analytical model for quantitative prediction of material contrasts in scattering-type near-field optical microscopy,” Opt. Express, Vol. 15, pp. 8550-8565, 2007.
[30] I. Stefanon, S. Blaize, A. Bruyant, S Aubert, G. Lerondel, R. Bachelot, and P. Royer, “Heterodyne detection of guided waves using a scattering-type scanning near-field optical microscope,” Opt. Express, Vol. 13, p.p. 5553-5564, 2005.
[31] R. Hillenbrand B. Knoll, and F. Keilmann, “Pure optical contrast in scattering-type scanning near-field microscopy,” J. Microsc. Vol. 202, p.p. 77-83, 2000.
[32] J. N. Walford, J. A. Porto, R. Carminati, J. J. Greffet, P. M. Adam, S. Hudlet, J. L. Bijeon, A. Stashkevich, and P. Royer, “Influence of tip modulation on image formation in scanning near-field optical microscopy,” J. Appl. Phys. Vol. 89, p.p. 5159-5169, 2001.
[33] A. Bek, Apertureless SNOM: a new tool for nano-optics, (Ph.D. Thesis, Max Planck Institute for Solid State Research, Germany, 2004).
[34] F. Keilmann, and R. Hillenbrand, “Near-field microscopy by elastic light scattering from a tip,” Phil. Trans. R. Soc. Lond. A., Vol. 362, pp.787-805, 2004.
[35] N. Ocelic, A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett., Vol. 89, Art. No. 101124, 2006.
[36] M. Micic, N. Klymyshyn, Y. D. Sun, and H. P. Lu, “Finite element method simulation of the field distribution for AFM tip-enhanced surface Raman Scanning Microscopy,” J. Phys. Chem. B., Vol. 107, pp.1574-1584, 2003.
[37] P. G. Gucciardi, G. Bachelier, and M. Allegrini, “Far-field background suppression in tip-modulated apertureless near-field optical microscopy,” J. Appl. Phys.Vol. 99, Art. No. 124309, 2006.
[38] Y. Song, and B. Bhushan, “Atomic force microscopy dynamic modes: modeling and applications,” J. Phys.: Condens. Matter, Vol. 20, Art. No. 225012, 2008.
[39] L. J. Hornbeck, “Current status and future applications for DMD-based projection displays” http://www.dlp.com./tech/research.aspx,1998.
[40] L. J. Hornbeck, “Digital Light ProcessingTM for high-Brightness, high-resolution applications,” http://www.dlp.com./tech/research.aspx,1997.
[41] A. N. Riza and M. J. Mughal, “Broadband optical equalizer using fault-tolerant digital micromirrors,” Opt. Express, Vol. 11, pp. 1559-1565, 2003.