Fig. 4.

(a) Schematic of the measurement setup for metalens characterization. (b) Direct image of the beam without fiber scanning at the illumination distance $z=5\mathrm{mm}$. A Gaussian blur (radius = $12\mu \mathrm{m}$) is applied to the image to reduce the noise. After that, the intensity distribution along $x$-axis (the red dashed line) is subtracted and fitted with a Gaussian distribution function. The measured and fitted intensity distribution is plotted on the image. (c) Longitudinal beam intensity distribution, constructed from a series of images similar to (b) taken at $z$ ranging from 0 to 15 mm with an increment of 0.1 mm. The beam power from the fiber and the exposure time are fixed. (d) Beam intensity distribution along $x$-axis for various illumination distance $z$, corresponding to the three red dashed line in panel (c). (e) FWHM of the beam intensity along $x$-axis versus $z$ at the center of the field. Blue circles correspond to a series of images with a fixed exposure condition. Red stars correspond to images with dynamically optimized exposure conditions to maximize the signal-to-noise ratio. The magenta dashed line is a fit to the red stars, assuming the beam is a perfect Gaussian beam, according to Eq. (3). Fitted Gaussian beam waist width is $w_0=59.5\mu \mathrm{m}$ (corresponding $\mathrm{FWHM}=70.1\mu \mathrm{m}$), and the waist position is at $z=1.58\mathrm{mm}$. The black dashed line is the PSF calculated in Zemax according to Huygens method.