Planar photonic crystal cavities with far-field optimization for high coupling efficiency and quality factor

Abstract

Different types of planar photonic crystal cavities aimed at optimizing the far-field emission pattern are designed and experimentally assessed by resonant scattering measurements. We systematically investigate the interplay between achieving the highest possible quality (Q) factor and maximizing the in- and out-coupling efficiency into a narrow emission cone. Cavities operate at telecommunications wavelengths, i.e. around approximately 1.55 microm, and are realized in silicon membranes. A strong modification of the far-field emission pattern, and therefore a substantial increase of the coupling efficiency in the vertical direction, is obtained by properly modifying the holes around L3, L5 and L7 type PhC cavities, as we predict theoretically and show experimentally. An optimal compromise yielding simultaneously a high Q-factor and a large coupling to the fundamental cavity mode is found for a L7-type cavity with a measured Q congruent with 62000, whose resonant scattering efficiency is improved by about two orders of magnitude with respect to the unmodified structure. These results are especially useful for prospective applications in light emitting devices, such as nano-lasers or single-photon sources, in which vertical in- and out-coupling of the electromagnetic field is necessarily required.

Efficient continuous-wave nonlinear frequency conversion in high-Q gallium nitride photonic crystal cavities on silicon

“…Contrary to the near-Gaussian distribution of the far-field pattern of the L3 cavity, two lobes appear around the center of the reciprocal space for the H0 cavity, arising from the odd parity of the fundamental mode, given the even parity of the injector design. Cavity Q-factors gradually dropped with increasing injector-hole radius, 13 down to a value of 2.7-4.4 × 10 4 for the largest considered injector sizes (∆r i = 8 nm and 10 nm for the L3 and H0 cavities, respectively). We have selected three injector sizes to be fabricated for each cavity (∆r i = [3,5,8] nm for the L3 cavity and ∆r i = [5,7,10] nm for the H0 cavity) to cover a coupling efficiency (η) range from approximately 2% to 20%.…”

Electrical conduction and optical properties of doped silicon-on-insulator photonic crystals

“…For each background doping level, we realized L3 nanocavities, with a lattice parameter of 420 nm and f Ϸ 28% that operate at wavelengths around 1.55 m. The cavities were realized as air-bridges ͑while the electrical characterization was done on-substrate͒ and the far-field pattern of the cavities was optimized as discussed in Ref. 12, using hole enlargements of 0, +3 and +9 nm to improve the out-of-plane coupling efficiency. This hole enlargement ͑⌬rЉ = rЉ − r͒ is clearly visible in the SEM image shown in Fig.…”

Room-temperature emission at telecom wavelengths from silicon photonic crystal nanocavities

“…This clearly demonstrates the effectiveness of far-field optimization for enhancing the vertical out-coupling efficiency. 15 Besides the fundamental mode, seven additional resonances are clearly visible in the spectra. A comparison with theoretical photonic band structure calculations by means of guided-mode expansion 17 allowed us to identify the whole spectrum of L3 cavity modes in each case, yielding excellent agreement for resonance wavelengths and number of modes ͑see Fig.…”

“…The fabrication procedure is the same as that reported in Ref. 15. Several L3 cavities with a lattice constant of a = 420 nm and a hole radius of r / a = 0.29 were fabricated.…”

“…On the other hand, an upper limit for the collection efficiencies from our PhC nanocavities is estimated from finitedifference time-domain calculations as C = 0.9, 0.2, 0.8 for ⌬r = +18 nm, 0 nm, Ϫ18 nm, respectively. 15 Since the maximum increase in extraction efficiency ͑i.e., without any Purcell effect͒ is C / M , the experimental Purcell factor is then given by F P Ӎ ␣ M / C , yielding very similar values of F P Ӎ 12 in each case. This is consistent with the experimental observation that all the three different cavities display almost the same Q Ӎ 3 ϫ 10 3 when measured in PL while independent resonant scattering spectra 21 yield Q-factors of 7.3ϫ 10 3 , 7.6ϫ 10 4 , 9.6ϫ 10 3 for ⌬r = +18 nm, 0 nm, Ϫ18 nm, respectively, in agreement with theoretical values.…”