Narrow-linewidth carbon nanotube emission in silicon hollow-core photonic crystal cavity
Résumé
Polymer-sorted semiconducting single-walled carbon nano-tubes (SWNTs) provide room-temperature emission at near-infrared wavelengths, with potential for large volume production of high-quality solutions and wafer-scale depo-sition. These features make SWNTs a very attractive material for the realization of on-chip light sources. Coupling SWNT into optical microcavities could enhance and guide their emission, while enabling spectral selection by cavity resonance engineering. This could allow the realization of bright, narrowband sources. Here, we report the first demonstration of coupling SWNTs into the resonant modes of Si hollow-core photonic crystal cavities. We exploit the strong evanescent field in these resonators to interact with SWNT emission, coupling it into an integrated access wave-guide. Based on this concept, we demonstrate narrowband SWNT emission resonantly coupled into a Si bus wave-guide with a full width at half-maximum of 0.34 nm and an off-resonance rejection exceeding 5 dB. Single-walled carbon nanotubes (SWNTs) are a versatile material with outstanding electrical [1,2] and optical properties [3,4]. Indeed, the same SWNT-based device can be reconfigured to perform as a light emitter, detector, or transistor, just by changing the applied voltage [5]. Hence, they have an immense potential to serve as an efficient connection bridge between the electrical and optical planes in future silicon optoelectronic circuits. In the optical domain, semiconducting SWNTs are a direct bandgap material exhibiting room-temperature electro-and photolumi-nescence in the visible and near-infrared [6,7]. Recent advances in solution processing [8] and wafer-scale selective deposition [9] have opened a new path toward the realization of on-chip light sources for the Si photonic platform, based on the use of SWNTs. These could become a cost-effective alternative to current solutions relying on III-V materials. However, the emission from SWNTs has a low quantum efficiency [10] and a wide spectrum that hinder their use as narrowband sources [11]. These two drawbacks could be circumvented by integrating the SWNTs into Si photonic microcavities. The resonant light confinement in small volumes in those cavities could be used to enhance light emission through the Purcell effect [12], while their high quality factors could be exploited to spectrally narrow the wideband SWNT emission. Spectral narrowing has been demonstrated for the emission of individual SWNTs coupled to suspended 1D photonic crystal microcavities [11,13]. This configuration has shown very promising results for quantum photonic applications relying on single-photon sources [14]. However, the intensity of a single emitter could not suffice for datacom or sensing applications. One possible solution to increase the emission intensity is to use a network of SWNTs [7,15–18]. Networks of high-purity polymer-sorted semiconducting SWNTs have shown room-temperature photoluminescence [7] and intrinsic gain [15]. It is worth noting that this sorting technique results in polymer wrapped semiconducting SWNTs. This precludes photo-luminescence quenching when the SWNTs are in contact with a silicon substrate, obviating the need to suspend them in air. Hence, polymer-sorted SWNTs allow cost-effective and large volume on-chip deposition based on drop casting or spin coating techniques. In this context, remarkably narrowband emission with quality factors of Q 3800 has been demonstrated for networks of polymer-sorted SWNTs in 2D silicon photonic crystal microcavities [16]. Still, no coupling between cavity-enhanced SWNT emission and on-chip waveguides has been reported for this kind of structure. Resonant enhancement of the emission from SWNT networks and coupling to Si pho-tonic waveguides has been shown for Si micro-ring resonators [19]. Nevertheless, these traveling wave resonators have comparatively large mode volumes that limit their potential for Purcell emission enhancement.