Lasing up to T = 339 K in Subwavelength Nanowire-Induced Photonic Crystal Nanocavities
Authors: Sergent Sylvain , Takiguchi Masato , Tsuchizawa Tai , Taniyama Hideaki , Notomi Masaya 
Published: September 2020

We report on lasing operation up to 339K in nanocavities constituted of subwavelength ZnO nanowires integrated in SiN photonic crystals. With thresholds as low as 4MW.cm-2, the investigated nanolasers outpeported subwavelength ZnO nanowire lasers operating at high-temperature. 1. Introduction Because of their unique properties, semiconductor nanowires have been key building blocks for a range of advanced devices such as nanolasers or single photon detectors and emitters. There has been a great deal of interest in integrating such unique objects in photonic platforms for information processing and sensing applications. Our group has especially focused on realizing photonic devices by integrating subwavelength nanowires in photonic crystal waveguides [1-4] and photonic crystal disks [5]. This has led to the realization of femtoJoule optical switches [1], photodetectors [2] and 10-Gb/s-modulated nanolasers operating at telecommunication wavelengths [3]. So far, such nanolasers have only been demonstrated at cryogenic temperatures but we report here that lasing action in nanowire-induced photonic crystal nanocavities can actually be obtained up to 339 K. 2. Results The nanowire-induced nanocavities investigated in this work are constituted of subwavelength ZnO nanowires embedded in two-dimensional photonic crystal line-defect waveguides. The ZnO nanowires present lengths ranging between 1.6 qm 4.2 qm whereas diameters range between 40 and 80 nm. The photonic crystals are obtained by processing a 108-nm thick SiN slab on silicon according to the methods described in reference [4]. They are constituted of a hexagonal array of circular air holes in the SiN slab with a central line-defect. An air groove is also processed at the center of the line-defect. The ZnO nanowires are transferred onto the SiN photonic crystal surface, manipulated with the tip of an atomic force microscope and moved into the central groove. One of the 15 fabricated nanocavities is shown in Fig. 1 a. The nanocavities are investigated by microphotoluminescence using a 1kHz pulsed laser as the excitation source (pulse width 0.35 ps, emitting wavelength 266 nm). If subwavelength nanowires do not present any cavity mode when lying on the SiN surface, the microphotoluminescence spectra of nanowire-induced nanocavities show the appearance of resonances in the near-bandedge range (Figure 1c) corresponding to the fundamental and higher order-modes, as confirmed by three-dimensional finite-difference time-domain calculations (Figure 1b). Power- dependent experiments show clear indications of lasing of the fundamental mode at room-temperature with the S- shape of the light-light curve, the blue shift of the mode wavelength and the reduction of the mode linewidth from 0.4nm down to 0.2 nm (see Figure 1d to 1f). Thanks to the low optical losses of its fully dielectric design, the threshold of the nanolaser at room-temperature is as low as 4 MW.cm-2, which constitutes a significant improvement over previously reported subwavelength ZnO nanowire lasers [6]. Within our set of 15 cavities, threshold actually varies between 4 MW.cm-2 and 15 MW.cm-2 at room temperature. When increasing the nanocavity temperature from 298 K to 339 K, lasing can still be observed owing to the intrinsic temperature-resistant optoelectronic properties of ZnO and the robustness of our design. The lasing mode redshifts with temperature from 383.5 nm to 385.3 nm (Figure 1g) in good agreement with the thermo-optic coefficients of ZnO of SiN, whereas the threshold rises from 4 MW.cm-2 to 12 MW.cm-2 (Figure 1g) owing to increasing non-radiative recombinations. 2 5 10 20 Power [MW. cm .* 370 380 390 Wavelength [nm] Fig. 1. (a) Scanning electron microscope image of a ZnO subwavelength nanowire embedded in a grooved SiN photonic crystal waveguide. (b) Fundamental nanocavity mode as calculated by the three-dimensional finite-difference time-domain method. (c) Room-temperature photoluminescence spectra at various excitation power densities: 0.5 Pth, Pth = 4 MW.cm-2 and 2.5 Pth. (d) Mode intensity, (e) wavelength and (f) linewidth as a function of the excitation power. The insets of panel (d) represent the lasing wavelength and threshold as a function of temperature. 3. Conclusion As a conclusion, we have demonstrated high-temperature lasing in a nanowire-induced nanocavity integrated into a photonic crystal waveguide. Relying on a fully dielectric platform with low optical losses, such nanolasers can outperform their plasmonic counterparts in terms of lasing thresholds with values as low as 4 MW.cm-2. We should note that despite such striking results, there is still room for improvement with alternative designs based on photonic crystal disks that present higher confinement factors and smaller mode volumes [5].