2019 Fiscal Year Annual Research Report
Near-field polarization analysis for an optical nanofiber
| Project/Area Number |
18F18367
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| Research Institution | Okinawa Institute of Science and Technology Graduate University |
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Principal Investigator |
NICCHORMAIC SILE 沖縄科学技術大学院大学, 量子技術のための光・物質相互作用ユニット, 教授 (10715288)
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| Co-Investigator(Kenkyū-buntansha) |
TKACHENKO GEORGIY 沖縄科学技術大学院大学, 量子技術のための光・物質相互作用ユニット, 外国人特別研究員
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| Project Period (FY) |
2018-11-09 – 2021-03-31
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| Keywords | optical nanofiber / polarization control / directional coupling / whispering gallery mode / optomechanical probe / two-photon atomic spectroscopy |
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| Outline of Annual Research Achievements |
This year, we have published (Physical Review Applied) the key paper that established our method for the complete control of polarization at the waist of a single-mode optical nanofiber. In this paper, we study near-field directional coupling between crossed nanofibers. The sum and difference of optical power in the counterpropagating modes of the output fiber depend on the polarization state of light at the waist region of the input fiber. This dependence allowed us to reverse the unknown polarization change produced by the tapered fiber before the waist, thus achieving the complete control over the polarization at the waist by means of a free-space compensator.
In the following project, we have combined the crossed nanofibers with a whispering-gallery-mode microcavity to achieve ten-fold enhancement of the directional coupling between the nanofibers (ACS Photonics).
Further, we have realized the polarization compensation via far-field imaging of the light escaping from the nanofiber waist due to scattering of the guided mode from natural nanoscale imperfections of the fiber shape or material. This light, imaged by a lens with a high enough numerical aperture, allows us to adjust the polarization compensator, without the need for a near-field probe (Journal of Optics).
This contactless method has been applied for achieving accurate polarization control in two experimental studies: light-induced orbiting of dielectric microparticles around a single-mode nanofiber (Optica), and nanofiber-based two-photon spectroscopy for cold rubidium atoms (in process).
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
Originally, we planned to use a nanofabriacted plasmonic probe to perform in-situ polarimetry of the evanescent field around an optical nanofiber. By exploring directional coupling between two crossed nanofibers, we found that a nanofiber itself can serve as a simple and efficient near-field probe for the polarization state. This finding has advanced our work significantly, since a nanofiber can be quickly and precisely fabricated without the need for clean room facilities. The further achievement of cavity-enhanced directional coupling was an unexpected result as well.
Our plan was to apply the near-field polarization control in optomechanical experiments with colloidal particles near a single-mode nanofiber, and perhaps in atomic experiments, if a nanoprobe could be placed and manipulated inside a high-vacuum chamber where the laser-cooled atomic cloud is prepared and interrogated. The success of the contactless polarization compensation based on scattering imaging allowed us to conveniently and accurately control polarization for nanofibers in both above experimental environments.
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| Strategy for Future Research Activity |
In FY2020, we plan to publish the finished study on nanofiber-based two-photon spectroscopy for cold rubidium atoms. We will also use the imaging-based method for controlling polarization in an ongoing experimental study aimed at optomechanical detection of transverse spin angular momentum of light. In this study, the transverse spin is a feature of the evanescent field around an optical nanofiber. We are probing the spin by observing its direct mechanical action on a probe dielectric particle held and spun by optical tweezers next to a nanofiber waist.
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