Particle deposition procedure

In this section, we introduce in detail how we deposit single gold nanoparticle on the optical nanofiber experimentally. Since the optical nanofiber is extremely fragile and the transmission is highly influenced by the unexpected dusts on the surface. The deposition of single nanoparticles is realized with special technique using nanoparticle suspension.

The experimental procedure is as following:

First, we prepare a highly diluted gold nanoparticle suspension in water. The concen-tration of the nanoparticle suspension needs to be low enough to ensure that the number of possible nanoparticle deposited on the optical fiber for each contact is less than 1. In practice, this technique effectively avoids depositing clusters.

Then, we take 5 µL of gold nanoparticle suspension in water from the evenly dis-tributed suspension with a pipette tip (10 µL volume). Leaving the sample in ultrasonic bath for ten minutes before deposition will be helpful to distribute the nanoparticles ho-mogeneously in the suspension. We carefully squeeze the nanoparticle suspension out of the pipette tip and create a droplet of about 3 mm in diameter. The droplet needs to be big enough so that we can slightly tilt the pipette to move it to the side of the pipette

36 CHAPTER 1. SUBWAVELENGTH OPTICAL NANOFIBER tip. And at the same time, it can not be too big so that due to the surface tension of the water, the small droplet can maintain its shape without dripping.

To precisely control the approaching of the droplet, we mount the pipette on a three-dimensional displacement stage. We adjust the focus plane of the camera filming the nanofiber from the side to be on the fiber. In this way, we can move the droplet to the same focus plane and see clearly the distance between curved droplet surface and the nanofiber. As shown in Fig. 1.17-a, the arc on the top of the image is the reflection from the edge of the droplet, and the thin line at the bottom is the scattering from the defects on the nanofiber.

By controlling the space between the droplet and the nanofiber, we move the droplet vertically towards the nanofiber. The instant when the droplet touches the nanofiber is easily detectable, as light is strongly scattered at the contact. Then, we quickly lift the droplet to disengage it from the nanofiber.

After several “attaching and disengaging" cycles, a single nanoparticle is deposited on the nanofiber. On the camera, it appears to be a spot with strong scattering, as shown in Fig.1.17-b. At the same time, the transmission drops about 20 %.

Conclusion

Optical nanofibers with subwavelength diameter can be used as a platform for light matter interaction thanks to the transverse confinement of the evanescent field.

In this section, we first introduced the light guiding by optical nanofibers and the fabrication of optical nanofiber with adiabatic profile.

Then, we introduced the coupling between gold nanoparticles and optical nanofiber with surface evanescent field both in detail. The scattering from gold nanoparticles ex-hibits dipolar like emission. When gold nanoparticles coupled to an optical nanofiber as dipole, the guided light shows chiral property.

Chapter 2

Polarization control of linear dipole radiation using an optical nanofiber

Publication: Joos, M., Ding, C., Loo, V., Blanquer, G., Giacobino, E., Bramati, A., Krachmalnicoff, V. and Glorieux, Q. (2018). Polarization control of linear dipole radiation using an optical nanofiber. Physical Review Applied, 9(6), 064035.

In free space, a linear dipole will emit linearly polarized light. However, when we combine a linearly polarized dipole with a subwavelength waveguide, this can change dramatically. A subwavelength waveguide has a strong longitudinal component along the light propagation direction, therefore, the guiding of light shows totally property than in the free space. This phenomena has been reported in our previous work [21] by coupling a gold nanorod to a nanofiber waveguide, and also by Martin Neugebauer and all [31] by coupling to crossing waveguides. In this chapter, we explain this mechanism and show that a linear dipole is not restricted to emit linearly polarized light, and our experiment provides the evidence that the design of nanophotonic environment will strongly modify the emission diagram and polarization.

2.1 Linear dipole

A nanoparticle has, by definition, much smaller dimensions than the wavelength of visible light with which it interacts which could naively suggest that its shape does not play an important role in phenomena such as absorption or diffusion. It is in fact otherwise;

the optical response of metallic nanoparticles largely depends on their shapes and is significantly different from the macroscopic properties of the metal that constitutes them.

A rigorous treatment of the diffraction of a monochromatic plane wave by a metallic nanoparticle involves solving Maxwell’s equations as well as the boundary conditions for the system studied. This tedious work presents analytical solutions only for a few simple geometries such as the cylinder, the sphere and the ellipsoid. In addition, this technique does not allow us to develop intuition for cases where there are no analytical solutions but which however interest us. Thus, we introduce a classical microscopic model, the Lorentz model [32, 33], to describe qualitatively the interaction of light with a metallic nanoparticle. In addition to fueling intuition, this model qualitatively predicts the optical properties of gold nanoparticles as we will present in this chapter.

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38 POLARIZATION CONTROL OF LINEAR DIPOLE