JTh2A.59.pdf
CLEO:2014 © 2014 OSA
Enhanced spontaneous emission by embedding light emitters inside hyperbolic metamaterials Lorenzo Ferrari,1 Dylan Lu,2 Dominic Lepage,2 and Zhaowei Liu,1,2,* 1
2
Materials Science and Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA Department of Electrical and Computer Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA * E-mail:
[email protected], Tel: 1-858-822-3470, Fax: 1-858-534-1225
Abstract: The inclusion of an emitter inside a Ag/Si multilayer yields a 3-fold enhancement of the Purcell factor over its outer value. The radiation is outcoupled to the far-field via a triangular and a rectangular grating. OCIS codes: (160.3918) Metamaterials; (250.5403) Plasmonics.
Enhancing the recombination rates of point-source emitters is crucial not only for light generation (single photon sources, light emitting diodes and optical amplifiers), but also for other nanotechnology applications ranging from biosensing, fluorescence imaging and DNA targeting to the high speed modulation and detection of optical signals. Two figures of merit quantify the performance of the emitter, with respect to its behavior in a homogeneous environment: the Purcell factor (PF) [1], which describes the decay speed enhancement, and the radiative enhancement (RE), which represents the far-field increase of emitted power. Hyperbolic metamaterials (HMMs) have been employed in the recent past in spontaneous emission (SE) engineering: in contrast to resonant structures like cavities or single metallic/dielectric interfaces, these media exhibit a PF that is broadband and tunable in frequency [2–4]. However, in the current configurations the emitters are dispersed on top of the HMM, which does not maximize the radiation-matter coupling.
Fig. 1. (a) Uniform Ag/Si ML, (b) triangular and (c) rectangular grating.
The present study optimizes the SE enhancement at visible frequencies of a point source, modeled as a dipole, by including it inside a multilayered silver (Ag)/ silicon (Si) HMM. The structure consists in a stack of 10 Ag/Si periods, each 20 nm thick (Fig. 1(a)). First, we demonstrate the tunability of the PF for an outer emitter by varying the percentage of Ag, or filling ratio (ρ), in a single period of the multilayer (ML). After fixing ρ=0.6, which corresponds to the highest PF (110-fold enhancement at λ0=554 nm), we move the dipole inside each individual Si layer and compute the respective Purcell enhancements (Fig. 2). We observe a significant enlargement in bandwidth, with spectral features dependent on the dipole polarization. A parallel or perpendicular orientation of the emitter excites indeed ML modes with different symmetry. The PF inside the structure reaches a value of almost 300, 3 times larger than that achieved outside. To scatter the radiation trapped in the HMM into the far-field, we implement a 1D triangular grating with a pitch of 300 nm, and a 1D rectangular grating with a pitch of 200 nm and a slit width of 40 nm (Figs. 1(b),(c)). The emitter is embedded in a layer of polymethyl methacrylate (PMMA), a host material commonly used in fabrication. The RE is defined as:
RE(θ ) =
P↑r (θ ) P↑0 (θ )
.
(1)
where P↑r (θ ) and P↑0 (θ ) are the powers emitted by a dipole respectively inside the ML and in free space into a cone with vertex in the emitter, axis oriented as the positive z direction and half-angle θ .
JTh2A.59.pdf
CLEO:2014 © 2014 OSA
A 10-fold and 6-fold RE is obtained in the triangular and rectangular grating respectively (Fig. 3). This work provides a new route to design high speed and high efficient opto-electronic devices, which may be applied to the field of light emitting diodes and photo detectors.
Fig. 2. (a) Positioning of a dipole (red dot) outside and inside selected Si layers (ochre stripes) of the uniform ML. (b) PF of a parallel or (c) orthogonal dipole for the outer (dashed line) and inner positions (solid lines). The insets show the coupling of the dipole field, directed from the tail to the tip of the triangles and proportional in magnitude to their size, to the Ag (grey)/Si (yellow) interfaces. (d) PF of an isotropic emitter for the positions considered in (b) and (c).
Fig. 3. (a) Isotropic PF for the triangular grating (blue curve) the rectangular grating (green curve) and an unpatterned structure with an emitting layer of 20 nm (red curve, circular markers). (b) RE at different emission angles for an isotropic dipole, provided by the triangular grating, and (c) by the rectangular grating, sketched in the insets.
4. References [1] E. M. Purcell, "Spontaneous Emission Probabilities at Radio Frequencies," Phys. Rev. 69, 681 (1946). [2] Z. Jacob, I. I. Smolyaninov, and E. E. Narimanov, "Broadband Purcell effect: Radiative decay engineering with metamaterials," Appl. Phys. Lett. 100, 181105 (2012). [3] C.L. Cortes, W. Newman, S. Molesky, and Z. Jacob, “Quantum nanophotonics using hyperbolic metamaterials,” J. Optics-UK 14, 063001 (2012). [4] D. Lu, J.J. Kan, E.E. Fullerton, and Z. Liu, "Enhancing spontaneous emission rates of molecules using nanopatterned multilayer hyperbolic metamaterials," Nat. Nano. 9, 48–53 (2014).
