Fig. 1. (a) Schematics of the 940 nm HCG-VCSEL. The grating period is Λ, a is the width of the grating bar, the duty cycle (DC) is defined as a/Λ, and tg is the thickness of the grating. (b) Field distribution of the resonance mode of our HCG-VCSELs.

Fig. 2. (a) Reflectivity contour of the HCG as a function of normalized thickness (tg/Λ) and normalized wavelength (λ/Λ) under normal incidence. (b) Reflectivity spectra of the HCGs for different bar widths for a grating period of 648 nm and a thickness of about a half wavelength.

Fig. 3. Field distribution of the fundamental mode of the designed HCG-VCSEL with an oxide aperture of 4 μm in diameter. The resonant wavelength is 941.6 nm.

Fig. 4. Fabrication process flow of the HCG-VCSEL.

Fig. 5. (a) Infrared microscope image of the mesa after oxidation. The dashed ellipse indicates the profile of the oxidation edge. The size of the oxide aperture is about 4  μm×8  μm. (b) SEM image of a typical air-suspended HCG with two posts of the HCG-VCSEL.

Fig. 6. (a) L-I-V curves of an HCG-VCSEL. (b) Lasing spot image from the CCD of the HCG-VCSEL at 2 mA. (c) L-I-V curves of the device without an HCG. (d) Image from the CCD of the devices without HCGs at 6 mA.

Fig. 7. (a) Spectra of the HCG-VCSEL under CW operation. (b) Spectra of the device without an HCG at different currents.

Fig. 8. Effective mode lengths of the HCG-VCSELs with different pair numbers of the p-DBR. The TM HCG has a grating period of 380 nm and a bar width of 230 nm. The thickness of the HCG is about a half wavelength.

Fig. 9. Calculated small-signal modulation responses of the TM HCG-VCSEL with a λ/2-cavity and an air thickness of one-quarter wavelength beneath the HCG at different currents.