Local tuning of photonic crystal cavities using chalcogenide glasses

Abstract

We developed a method to locally tune refractive index in photonic crystals. The technique, based on photodarkening of chalcogenide glasses, enables 3nm resonance tuning of GaAs photonic crystal cavities operating at 940nm.Photonic crystals represent one of the most promising platforms for on chip integration [1] of optical components. However, they are very sensitive to fabrication imperfection, so a practical method to post-tune their optical properties is needed. Here we present such a method based on chalcogenide glasses. Chalcogenide glasses quasi-permanently change their optical properties when illuminated with light above their band gap, and have been used to tune optical devices as quantum cascade lasers [2]. The tuning of PCs devices directly fabricated in chalcogenide glasses has already been shown in Ref. [3], but many other applications rely on PC fabricated in other materials such as group IV and III-V semiconductors.In our approach, a photosensitive chalcogenide glass layer is deposited on prefabricated GaAs/InAs devices. Linear three-hole defect [4] PC cavities were first fabricated in a 150 nm thick GaAs membrane containing a central layer of InAs quantum dots (QDs) as described in Ref. [5]. Arsenic trisulphide films with thickness between 30 nm and 100 nm were deposited onto the photonic crystals using thermal evaporation.The experiment was performed at cryogenic temperature (less than ∼ 60K) to obtain luminescence from the embedded InAs quantum dots, as needed for quantum information processing applications. This illustrates that the method works at low temperatures, though we stress that it is applicable to room temperature nanophotonic circuits. The sample was placed inside a continuous-flow liquid helium cryostat at 10K and the QD photolumi- * Electronic address: faraon@stanford.edu nescence was used to measure the cavity resonance. A confocal microscope setup and a laser tuned at 780 nm excited quantum dot luminescence while a spectrometer monitored the signal. A 543 nm HeNe laser (1μW ) focused to ∼ 1μm 2 through the same confocal setup was used for photodarkening of the As 2 S 3 layer (Fig.1). This wavelength was chosen because it is close to the 527 nm bandgap of As 2 S 3 .The thickness of the As 2 S 3 influences both the quality factor of the cavity and the maximum tuning range. For this reason we experimented with three different thicknesses: 30, 60 and 100 nm (samples S30, S60 and S100). For each sample, the spectrum of the cavities was recorded before and after the deposition of the chalcogenide layer. For samples S60(S30), the deposition caused the quality factor to degrade by ∼ 5%(30%) from an average value of ∼ 8500(10000) while the resonant wavelength shifted by ∼ 40 nm(28 nm). For sample S100 the Q degradation was more severe, from ∼ 6500 to ∼ 1000 and for this reason we mainly concentrate on samples S30 and S60.With the chips mounted in the cryostat, we focused the 543 nm laser on the PC cavities for a fixed time and recorded the cavity spectrum. For sample S60, the cavity reso...

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