Micromachined solid immersion lenses and optical antennas for scanning near-field optical microscopy.

摘要

The optical microscope is a powerful and ubiquitous measurement and observation tool in science, medicine and industry. In spite of this, however, the resolving power of the optical microscope is fundamentally limited by diffraction. In this work we demonstrate two methods to overcome this limitation based on micromachined Solid Immersion Lenses (SILs) and optical antennas.; In the first method for improving optical resolution, the Solid Immersion Lens (SIL), light is focused in a high refractive index lens held close to the sample. Silicon nitride SILs with diameters of 7 micron integrated with atomic force microscope cantilevers are fabricated by surface micromachining. A scanning optical microscope based on the micromachined SIB and operating in reflection and transmission modes at a wavelength of 400nm is presented. The full width at half maximum spot size of the SIL-based microscope is measured to be ∼130nm, which is ∼1.9 times better than the spot size without the SIL (256nm). Furthermore, the optical transmission efficiency of the SIL is ∼64% (with losses due to reflection and absorption), which is significantly better than that of the tapered fiber nearfield scanning optical microscope (typically ∼0.001–0.01%).; The second method for improving resolution uses antennas operating at optical wavelengths to enhance the optical fields in regions whose dimensions are much smaller than the wavelength. We present a numerical study based on the finite difference time domain (FDTD) technique, showing that the optical intensity is enhanced by three orders of magnitude in a region whose dimensions are less than ∼lambda/40. A study on the factors influencing intensity enhancement is presented. Optical antennas operating at infrared wavelengths (∼2–10 micron) are fabricated by electron-beam lithography. Far-field measurements on the optical antennas are carried out and found to be in good agreement with FDTD calculations.; Lastly, we present a technique in which the contact stiffness between the sample and a vibrating atomic force microscope tip is used to study the elastic modulus and thickness of thin films. We report experimental results for thin metal and polymer films deposited on silicon substrates and compare them with the predictions of a theoretical model.