Emission Enhancement in a Nanowire Coupled to a Nanoantenna

Masaaki Ono1,3, Eiichi Kuramochi1,3, Guoqiang Zhang1, Hisashi Sumikura1,3, Yuichi Harada2,
David Cox4, and Masaya Notomi1,3
1Optical Science Laboratory, 2Physical Science Laboratory, 3NTT Nanophotonics Center,
4National Physical Laboratory, U.K.

Nanoemitters such as quantum dots and nanowires have small active regions, and are good candidates for realizing optical devices with low power consumption, a small footprint, and high-speed responsivity. On the other hand, it is difficult to couple them efficiently to light because they are much smaller than the wavelength of light. Therefore, we have focused on plasmonics, which overcomes the diffraction limit of light, and studied a bowtie nanoantenna to enhance the light-matter interaction. The modes of bowtie nanoantennas are localized in a region of the order of 10 nm in size, which is close to the size of the nanoemitters. However, it is very difficult to place nanoemitters in the subwavelength region precisely, and a dispersion technique has been mainly used for nanoemitter placement [1]. In this study, we fabricated a nanowire-nanoantenna coupled system, where we placed a single InP nanowire within the gap of a gold bowtie nanoantenna by using a nanomanipulator installed in a focused ion beam system [Fig. 1(a)]. Moreover, we observed significant emission enhancement in the fabricated coupled system [2].

We positioned the InP nanowire in the gap during in-situ scanning electron microscope observation [Fig. 1(b)]. We investigated the emission characteristics at a temperature of 80 K by photoluminescence (PL). The excitation and emission wavelengths were 636 and 875 nm, respectively. Figure 2 shows a map of the PL intensity. Here, E (E//) is the light polarized perpendicular (parallel) to the nanoantenna. The black dashed line (red solid line) was obtained under E (E//) excitation and E (E//) detection. The PL intensity (I) was normalized by the intensity at a point R (IR), which is far from the nanoantenna. Although I/IR was close to unity at the antenna position for E polarization, that for E// polarization was 6.1, which indicates that the emission was strongly enhanced by the nanoantenna. When we consider that the diameter of excitation laser was much larger than the antenna mode, the intrinsic enhancement factor was as large as 110. The obtained enhancement is due to the double enhancement of the excitation and emission, and our numerical analysis agrees very well with the experimental results. Moreover, a numerical analysis indicated that nanowire-nanoantenna interplay leads to additional enhancement in our coupled system. Here, we established a nanomanipulation technique for subwavelength nanowires, and achieved emission enhancement by efficiently utilizing the plasmonic structure.

A. Kinkhabwala et al., Nature Photon. 3, 654 (2009).
M. Ono et al., 2014 IEEE Photonics Conference, WH2.1 (2014).

Fig. 1. (a) Schematic structure of the coupled system. (b) SEM image of the fabricated sample. We inserted a nanowire with a diameter of 60 nm and a length of 7 μm.

Fig. 2. Normalized PL intensity map on the nanowire. Inset: SEM image that shows the antenna and reference positions.