Polarization Properties in Apertureless-Type Scanning Near-Field Optical Microscopy
© Ishibashi and Cai. 2015
Received: 25 August 2015
Accepted: 30 August 2015
Published: 29 September 2015
Polarization properties of apertureless-type scanning near-field optical microscopy (a-SNOM) were measured experimentally and were also analyzed using a finite-difference time-domain (FDTD) simulation. Our study reveals that the polarization properties in the a-SNOM are maintained and the a-SNOM works as a wave plate expressed by a Jones matrix. The measured signals obtained by the lock-in detection technique could be decomposed into signals scattered from near-field region and background signals reflected by tip and sample. Polarization images measured by a-SNOM with an angle resolution of 1° are shown. FDTD analysis also reveals the polarization properties of light in the area between a tip and a sample are p-polarization in most of cases.
Recently, nano-optics are attracting great attentions not only for measuring physical properties such as optical responses [1, 2], Raman scattering , and magnetic properties but also for reactions of molecules and/or controlling of spins. In those cases, polarization states in nano-optics are very important, because those phenomena are dependent on the polarization of light. For example, spin manipulation with light is attracting attentions because light may have abilities of controlling the direction of magnetization in a time scale of several tenth nanoseconds. Stanciu et al. reported that magnetization direction of GdFeCo is optically switched by a 40 fs circularly polarized light (CPL) pulse . Satoh et al. demonstrated the spin-wave emission and the directional control of propagation using the CPL or linearly polarized light (LPL) . These new phenomena may lead to controlling single spin with light. On the other hand, CPL has been also used to measure circular dichroism (CD) due to the geometric and electromagnetic chiral properties of single molecule , biomolecules , and singe-wall carbon nanotube . In those cases, obviously, each single molecule can be selectively investigated if nano-sized CPL is available.
For high resolved polarization imaging with a spatial resolution of ~10 nm, scanning near-field optical microscopies (SNOM) have been developed in the 1990s [9–12]. Aperture-type SNOMs using optical probe having an aperture on top of tapered optical fiber with metal were extensively studied, because it was considered that polarization properties were maintained and background signal was relatively small. Magneto-optical (MO) measurements that measure optical responses of magnetic materials for circular polarization are one of the most important purpose at that time [11–22], because size of magnetic recording marks had became smaller than the wavelength of the visible light. Several groups use aperture-type optical fiber probes collecting the evanescent wave, which is referred to as a transmission-mode MO-SNOM. However, there are disadvantages. When the evanescent wave propagates through the optical probes, the light intensity decreases extremely, the polarization of the wave is deteriorated, and the spatial resolution of the imaging was limited to the size of the aperture, which is typically larger than 50 nm. In addition, it was difficult to measure opaque materials in reflection-mode, because the thickness of metallic coating contributes to enlargement of probe end.
Another type of SNOM, apertureless-type SNOM (a-SNOM), has also been developed, in which cantilever tips for atomic force microscopy were used instead of the optical fibers probe with an aperture. In a-SNOM, the scattered light generated at the area where evanescent waves are formed between the tip’s extremity and the sample’s surface. Considering the principle of a-SNOM, we can expect higher spatial resolution depending on the radius of tips, which is smaller than 10 nm in the commercially available tip, with higher intensity and good polarization properties. In addition, it is considered that a-SNOM is suitable for measurements of opaque materials, since there is no obstacle around the extremity of tips. One of the most successful applications of a-SNOM is tip-enhance Raman scattering spectroscopy [23–27]. On the other hand, polarization property of a-SNOM has not been sufficiently understood yet. In this paper, we report the polarization properties of a-SNOM, including experimental and simulation results.
Results and Discussion
We measured a chromium film deposited on a quartz substrate with a checker pattern with a thickness of 20 nm and a period of 2 × 2 μm2. Figure 3 shows a topographic image and two SNOM images of the chromium pattern measured with the s-polarized illumination . Figure 3b, c was measured at demodulation frequencies of Ω and 2Ω, respectively. In those SNOM images, the checker pattern corresponding to the topography in Fig. 3a is clearly observed. We found that the intensity of area of chromium film (higher parts) is higher than that of quartz area (lower parts), and they showed opposite sign of phase ϕ for the chromium and the quartz. This result indicates that the signal depends on not only the reflectivity but also the phase. Therefore, we consider that the contrast obtained in those SNOM images is due to difference in the complex permittivity of the materials. In Fig. 3c, a dark area surrounding the chromium patterns is observed. We believe this is due to the edge darkening effect. Spatial resolution was determined to be 14 nm by measuring a cross section of SNOM images measured at 2Ω.
Polarization properties of apertureless-type scanning near-field optical microscope (a-SNOM) were studied. Polarization SNOM images were successfully measured with the spatial resolution of ~14 nm and the angle resolution better than 1°. We found that the polarization properties of optical signals could be decomposed into SNOM signal scattered from near-field region and background signal reflected by a tip and a sample. Signal to noise ratio was improved by choosing the angles of the incident light and the analyzer. We also found that a-SNOM worked as a wave plate described by Jones matrix and the polarization state of the electric field between a tip and a sample was p-polarization.
This research was supported by the National Institute of Information and Communications Technology (NICT) and KAKENHI, Grants-in-Aid for Scientific Research (B) (23310073).
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