Fig. 1. Resonating and scattering behavior of a single GaAs nanodisk. (a), (b) Schematic of the dielectric nanodisk in free space for scattering (a) and Q (b) simulation, respectively; (c) scattering cross section following the variation of the nanodisk radius and wavelength. The nanodisk’s height is fixed at h=560  nm. The size of each hollow circle is proportional to the Q factor of the corresponding resonant mode, and the black arrow indicates the appearance of a quasi-BIC at r/h=0.732. (d) Relation between the resonant wavelength corresponding to the two labeled bright stripes in (c) and the nanodisk radius. The three pairs of points, labeled as H1 and L1,H2 and L2, and H3 and L3, correspond to r=345  nm, 410 nm, and 440 nm, respectively. The two black dashed lines illustrate the developing trend of the strong coupling between the low-Q and high-Q modes. (e) Patterns of the six resonant modes are labeled by blue and red circles in (d). The YZ plane through the nanodisk axis is the observing plane and the electric amplitude is normalized. White lines show the nanodisk boundary.

Fig. 2. Fabricating GaAs nanodisks with embedded InAs QDs. (a) Growing InAs QDs. An active multilayer is grown on a GaAs (100) substrate by MBE, between which there is a 500 nm GaAs buffer layer and a 500 nm AlAs sacrificial layer. The active multilayer consists of a 275 nm GaAs bottom cladding layer, 10 nm InAs dot-in-well gain material (2 nm In0.35Ga0.65As layer for buffering strain, 2.2 mL InAs QDs, and 6 nm In0.35Ga0.65As layer for relaxing strain), and a 275 nm GaAs top cladding layer. (b) Lifting off the active multilayer by wet etching off the AlAs sacrificial layer; (c) bonding the active multilayer onto a silicon substrate with a 3 μm top oxide layer; (d) spunning HSQ on the transferred active multilayer and patterning it by EBL; (e) etching out nanodisks of different radii by ICP-DRIE; (f) removing the HSQ hard mask; (g) AFM images of the top surface before and after the transferring of the active multilayer; (h) SEM images of the fabricated nanodisks. The insets show the magnified top and side views of one nanodisk.

Fig. 3. Resonating behavior of a single GaAs nanodisk on a silicon substrate with a 3 μm oxide layer. Calculated Q and resonant wavelength versus nanodisk radius. The Q factor reaches its maximum value (229) for the optimal parameters (r=400  nm, h=560  nm) at wavelength 1270 nm.

Fig. 4. Schematic diagram of the optical setup for characterizing the μ-PL spectrum. (a) The nanodisk can be positioned by the home-built microscopy and captured by the camera (CCD). A continuous laser (532 nm) focused onto the sample by an objective (20×, NA=0.5) is used to excite the quasi-BIC by a single nanoresonator. A long-pass filter was inserted to reject the incident laser and only the fluorescence signal arrives in the spectrometer. (b) Normalized PL spectrum measured from a 560 nm active multilayer.

Fig. 5. PL measurement for fabricated GaAs nanodisks. (a) Schematic of a GaAs nanodisk situated on a silicon substrate pumped by a 532 nm continuous laser. The inset is the exemplary SEM image of one fabricated nanodisk. (b) Measured PL spectra for various GaAs nanodisks of different radii; (c) Q factors are extracted from the measured PL spectra (black dots) and fitted by the Lorentzian curve for different GaAs nanodisks. The blue dotted line corresponds to the parameters at which the quasi-BIC appears.

Fig. 6. Influence of material loss and fabrication error on GaAs nanodisks. (a) The two angles representing imperfect fabrication may degrade the Q factor of the supported quasi-BIC. The insets are illustrating the side angle (θ) and the inclination angle (α) for the nanodisk, respectively. (b) Measured refractive index of the active multilayer. The black solid line represents the real part of the refractive index, while the red solid line is the imaginary part of the refractive index. The active multilayer has a refractive index of about n=3.45 with an unignorable material loss k=0.055 in the wavelength range from 1100 to 1400 nm. (c) Q factors for different nanodisks are calculated according to the measured complex refractive index in (b).