Control of semiconductor quantum dot emission intensity and polarization by metal nanoantennas

We have studied the amplified emission properties of nanoislands with CdSe quantum dots in ZnSe/CdSe/ZnSe heterostructures surrounded by metallic antennas. It has been found that variations of the optical antenna length give rise to periodic amplification of the integral emission intensity. The period of the discovered oscillations corresponds to the wavelength of the surface plasmon-polariton mode propagating in the metallic antenna. The nature of observed periodicity was confirmed by results of numerical simulations for linear antennas. It has been established that the velocity of surface polaritons depends not only on the parameters of the dielectric constants of the metal and of the semiconductor substrate but also on the width of the metallic antenna. The influence of antenna antisymmetry (its helicity) on selective amplification of the degree of circular polarization of photoexcitation has been investigated. We found that plasmon-polariton standing waves induced in $S$-type (curved) antennas by circularly polarized light, which was used for quantum dot photoexcitation, result in enhanced polarization selectivity of the quantum dot emission. The selectivity of the polarization of photoexcitation is a periodic function of the helical antenna length.

Figure 1

(top) View of part of the structure used for the investigations. The structure was divided into substructures, each of which comprised 16 mesas ($4\times 4$ structure seen in the figure) with QDs. The distance between the mesas was 10 $\mu \mathrm{m}$, so that the substructure size was $50\times 50$ $\mu \mathrm{m}$. In each substructure two of the 16 mesas had no nanoantennas and were used for comparison. The remaining 14 mesas were embedded into nanoantennas with different geometric parameters. (bottom) The schematic representation of the geometry parameters for both linear and $S$-type antennas. For $S$-type antennas $L$ is the average length between the outer and inner edges of one antenna arm.

Figure 2

Emission spectra of a single mesa with CdSe/ZnSe QDs under photoexcitation by a laser ($\lambda =488$ nm) with left and right circular polarization.

Figure 3

Dependence of the integral emission intensity of single mesas on mesa size. The insets show photos of single mesas with $d\sim 80$ nm obtained with an electron-beam microscope.

Figure 4

Typical photos of structures with linear metallic nanoantennas obtained using an electron-beam microscope.

Figure 5

Enhancement factor of the emission of QDs embedded in silver linear antennas ($w=200$ nm and gap $D=150$ nm) as a function of antenna length.

Figure 6

Enhancement factor of the emission of quantum dots embedded in silver linear antennas (with $w=150$ nm and gap $D=150$ nm) as a function of antenna length.

Figure 7

Dependence of the oscillation period of the antenna enhancement factor on antenna width.

Figure 8

Dependence of the enhancement factor on gap width, measured for an antenna width of 200 nm.

Figure 9

Numerically simulated dependence of the LFIE factor at a frequency of 615 THz as a function of the antenna length $L$ for $w=140$ nm, ${\epsilon }_D=2$, and different gap sizes, $D=60$ nm (line with squares) and $D=70$ nm (line with circles). The inset shows the instantaneous distribution of the $E$-field $x$ component for $L=520$ nm.

Figure 10

Typical photos of the studied structures with curved antennas with clockwise and counterclockwise helicity.

Figure 11

Emission spectra measured for mesas with CdSe/ZnSe QDs embedded in 400-nm-length curved antennas with opposite helicity measured under (a) left- and (b) right-circularly polarized laser light (wavelength of 488 nm).

Figure 12

(a) Enhancement factor of the emission of QDs embedded in curved antennas (with $w=200$ nm, gap $D=150$ nm) as a function of antenna length. (b) Strength of the selective polarization dichroism effect induced by photoexcitation with circular polarization as a function of the length of curved antennas.