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Photonics

Nanophotonics

Nanojets

We study the combined system of coupled micro-cavities and colloidal quantum dots.

Fig. 1

Figure 1. Con-focal luminescence microscopy of 5 micron spheres with surface layer of quantum dots. The different colours are due to dots of different sizes.

Micro-resonators are important components in optoelectronic devices. come in a variety of shapes and sizes, with the common property of confining light in long lived modes. On its own a photonic micro-resonator, in particular one with spherical symmetry, can be considered as a "photonic atom". Photons are confined by the radial discontinuity in refractive index in an analogous way that electrons are confined by the electronic potential in an atom[1]. The modes in these resonators are however leaky and have a finite Quality Factor (Q-factor, Q). Light is not completely confined within the resonator and the mode intensity decays quasi exponentially into the surrounding medium. Using this evanescent field we can couple light in and out of the micro-cavity. These systems are very well described analytically due to their spherical and circular symmetry.



When individual resonators are brought in close proximity such that their evanescent fields overlap, the modes of the individual resonators split in an analogous way to modes in simple molecules [2]. There is also a corresponding redistribution in intensity. The level of splitting can be controlled by the inter-resonator distance, and type of modes are dictated by the individual resonator shape[3, 4], and the symmetry of the coupled molecule. This offers a great deal of flexibility, and by tuning these parameters one can create a desired spectral response from the photonic molecule. We aim to couple this flexibility with that of self assembled semiconductor Quantum Dots (QDs) . Quantum dots are small (5-30nm) accumulations of semiconductor material, whose optical response can be tuned simply by varying the size of the structure[5, 6]. The spontaneous emission from the QDs is effected by the change in the density of states of the surroundings, and light is emitted preferentially at the modes of the cavity. Using the Photo luminescence (PL) from the QDs, we can probe the spectral distribution of modes in the coupled cavity.

Current work

We coat dielectric microspheres with diameters from 2-10um with colloidal Quantum Dots, CdTe or CdSe. When the QDs are brought within close proximity of the spheres surface the change in photonic density of states causes a change in the spontaneous emission rate, thus the PL of the dots is modified by the presence of the cavity. The cavity induced enhancement allows us to make very sensitive measurements on the quantum dots. For example we can see Raman scattering and anti-stokes emission from a single CdTe monolayer [7-9].

Fig. 2

>Figure 2. Emission spectra of a single sphere (a), a triangular structure (b), a 5-sphere ring (c) and a 7-sphere cyclic photonic molecule (d). The microspheres have a nominal diameter of 5.374 μm. The red arrows in (b) indicate additional peaks in the coupled spectrum compared to the single sphere spectrum.

We also note a focusing effect in these spherical micro-cavities. When light with wavelength of order of the diameter of the sphere is focused on these molecules we see high intensity spots at points of symmetry, with low divergence beams [10]. We attribute this effect to the formation of "Photonic Nanojets" in the molecule [11]. This is an intensity pattern on the shadow side of the sphere with a narrow low divergence peak and higher order lobes caused by interference of the incident field and scattered field from the spherical resonator.

We also note a focusing effect in these spherical micro-cavities. When light with wavelength of order of the diameter of the sphere is focused on these molecules we see high intensity spots at points of symmetry, with low divergence beams [10]. We attribute this effect to the formation of "Photonic Nanojets" in the molecule [11]. This is an intensity pattern on the shadow side of the sphere with a narrow low divergence peak and higher order lobes caused by interference of the incident field and scattered field from the spherical resonator.

Fig. 3

>Figure 3.left: Triangular photonic molecule with attached single sphere. Light is coupled into the attached sphere by nanojet coupling. Inset top right: Image of the triangular photonic molecule with attached sphere in white light. The crosshair indicates the focus position of the laser. Inset bottom right: A merged image of the triangular structure with attached sphere under laser illumination and under white light, showing the spatial distribution of the emission within the structure. Right: Image shown in inverse colours. Dark colours indicate high intensities.

Fig. 4

> Figure 4. (a) Photonic jet formed by focusing of 410nm light by a 3um diameter sphere, refractive index 1.68, simulated via Finite element method. Light is linearly polarised in plane of the page. (b) Close up of jet distribution in (a).The white line plots the FWHM of the intensity distribution. The FWHM at the surface of the sphere is 160nm. (c) Light of the same wavelength focused using a high NA Microscope objective (NA=1). The white line indicates the FWHM of the intensity distribution. The beam waist is 130nm(FWHM~260nm), and the divergence angle is 76 deg (d) Comparison of the axial intensity profile of a tightly focused Gaussian beam and a Photonic Jet.

Individual cavities are widely used as sensors, as the position and Q-factor of the modes depends strongly on the surrounding refractive index[12]. The modes of these coupled cavities have been shown to be more sensitive to changes in the refractive index of the surrounding medium than the individual modes [13, 14], also the localised intensity can produce hotspots which are more sensitive. We have shown the ability of coupled microspheres to act as optical delay lines with wavelength tuneable delay[15].Microspheres can also act efficient light sources with directional emission[10].

References:

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