Diffraction of circularly polarized light from near-®eld optical probes D. J. SHIN*², A. CHAVEZ-PIRSON* & Y. H. LEE³ *NTT Basic Research Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan ³Department of Physics, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-ku, Taejon 305-701, Korea Key words. Circularly polarized light, near-®eld optical probe, near-®eld optics, polarization, vector diffraction. Summary Diffracted ®elds from 100-nm aperture near-®eld scanning optical microscopy (NSOM) probes and uncoated tapered ®bres are measured and analysed. Using a solid angle scanner, the two-dimensional intensity distribution and polarization state of the diffracted light are resolved experimentally. Polarization analyses show that circularly polarized input light does not maintain its polarization state for all diffraction angles, and is completely ®ltered into linearly polarized light at large polar diffraction angles. This drastic decomposition originates from the vector nature of light diffracted by the sub-wavelength aperture. There is a fundamental dif®culty in generating circularly polarized light near the aperture of NSOM probes owing to polarization-dependent diffraction in the near-®eld regime. This is illustrated by the Bethe-Bouwkamp model using circularly polarized input light. 1. Introduction The most important breakthrough which has launched the rapid progress in near-®eld scanning optical microscopy (NSOM) is the use of metal-coated tapered optical ®bre probes. The diffraction problem of a small hole in an in®nite conducting plane, which can be regarded as the limiting case of an NSOM probe, was solved rigorously 50 years ago (Bethe, 1944; Bouwkamp, 1950). The far-®eld angular intensity distribution of NSOM probes and its relation to the aperture size were reported (Obermuller & Karrai, 1995). The near-®eld distribution was also investigated experimen- tally and numerically (Novotny et al., 1995; Decca et al., 1997). However, for research into NSOM and for image interpretation, the known optical properties of NSOM probes are still not suf®cient, especially when polarization is involved. It is interesting to know whether the diffraction of circularly polarized light from a sub-wavelength aperture, such as a near-®eld optical probe, poses any intrinsic limit for experiments using circularly polarized light. In a recent experiment using a near-®eld optical probe to study excitonic spins in a magnetic semiconductor heterostruc- ture, it was observed that the contrast between left-handed and right-handed circularly polarized luminescence degraded as the probe approached the surface (Levi et al., 1996). A possible explanation for the loss of polarization contrast is that evanescent ®elds do not propagate angular momentum and cannot couple to circularly polarized light in the far-®eld. To explore this issue further requires a full understanding of the diffraction from sub-wavelength apertures, since the vector diffraction process itself strongly in¯uences the polarization state. Here, we present our experimental and theoretical studies of the polarization- dependent diffraction from near-®eld optical probes in the near- and far-®eld regimes. 2. Experiment We constructed a solid angle scanner to measure the far- ®eld intensity distribution and polarization state from optical ®bre probes used for NSOM. Polarization-resolved diffraction patterns are studied with this instrument. We note that there are technical dif®culties associated with measuring the diffracted intensity from NSOM probes. First, the emission covers a wide range of angles, even extending into the backward direction. Secondly, the diffracting object, the ®bre probe, should be ®xed as ®rmly as possible to avoid polarization ¯uctuation due to ®bre movement. Finally, Journal of Microscopy, Vol. 194, Pt 2/3, May/June 1999, pp. 353±359. Received 6 December 1998; accepted 4 February 1999 q 1999 The Royal Microscopical Society 353 Correspondence to: Dr A. Chavez-Pirson. Tel: (81) 462 40 3683; fax: (81) 462 70 2342; e-mail: chavez@wave.brl.ntt.co.jp. ² Current address: Department of Physics, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-ku, Taejon 305-701, Korea.