Coherent Raman scattering imaging with a near-infrared achromatic metalens

Abstract

Miniature handheld imaging devices and endoscopes based on coherent Raman scattering are promising for label-free in vivo optical diagnosis. Toward the development of these small-scale systems, a challenge arises from the design and fabrication of achromatic and high-end miniature optical components for both pump and Stokes laser wavelengths. Here, we report a metasurface converting a low-cost plano-convex lens into a water-immersion, nearly diffraction-limited and achromatic lens. The metasurface comprising amorphous silicon nanopillars is designed in a way that all incident rays arrive at the focus with the same phase and group delay, leading to corrections of monochromatic and chromatic aberrations of the refractive lens, respectively. Compared to the case without the metasurface, the hybrid metasurface-refractive lens has higher Strehl ratios than the plano-convex lens and a tighter depth of focus. The hybrid metasurface-refractive lens is utilized in spectroscopic stimulated Raman scattering and coherent anti-Stokes Raman scattering imaging for the differentiation of two different polymer microbeads. Subsequently, the hybrid metalens is harnessed for volumetric coherent Raman scattering imaging of bead and tissue samples. Finally, we discuss possible approaches to integrate such hybrid metalens in a miniature scanning system for label-free coherent Raman scattering endoscopes.

FIG. 1.

Schematic diagram of a hybrid water-immersion achromatic metalens for CRS imaging and theoretical analysis on the effect of chromatic aberration. (a) Illustration of a hybrid metalens to achromatically focus the pump and Stokes beams. The hybrid metalens consists of a plano–convex glass lens attached to a metasurface comprising α-silicon nanopillars. ω_p and ω_s label the frequencies of the pump and Stokes beams. Ω_vib is the targeted Raman transition, which is equal to the energy difference between the pump and Stokes beam (i.e., Ω_vib = ω_p − ω_s). Ω_as is the frequency of newly generated coherent anti-Stokes light (i.e., Ω_as = 2ω_p − ω_s). ΔI_p and ΔI_S represent the energy loss (i.e., stimulated Raman loss) at the pump beam and the energy gain (i.e., stimulated Raman gain) at Stokes beam before (solid curve) and after (dashed curve) interaction with the molecules, respectively. (b) Simulated SRS intensity when there is a focal length difference (Δf) between the pump beam of λ_p = 800 nm and the Stokes beam of λ_S = 1040 nm. The numerical aperture (NA) of the focusing lens is assumed to be 0.4. (c) Effects of Δf on SRS intensity (red curve) and axial resolution (blue curve). The axial resolution is defined as the full width at half maximum (FWHM) of the longitudinal SRS intensity profile in the focal region.

FIG. 2.

Principle and design of a hybrid water-immersion achromatic metalens. (a) Calculated phase profile of the metasurface along the x-axis passing through the lens center. (b) Simulated focal shift and (c) root-mean-square of wavefront aberration function (WAF_RMS) with and without the metasurface. The red and brown curves show the WAF_RMS at 0°, 1°, and 2° angle of incidence. (d) Designed nanostructures and their phase shifts for λ_p = 800 nm and λ_s = 1040 nm and their transmission at λ = 800 nm (shown by colors). The inset illustrates the shape of eight α-silicon nanopillars. (e) Scanning electron microscopy (SEM) images from a region of the metasurface. Scale bar: 1 µm. The inset shows an oblique view (scale bar: 200 nm). (f) An image taken while aligning the metasurface to the refractive lens. Scale bar: 2 mm.

FIG. 3.

Characterization of the hybrid metalens. (a) Point spread function (PSF) measurement setup, (b) PSFs of the refractive lens with and without the metasurface, (c) FWHM of focus, (d) Strehl ratio of focus, and (e) focusing efficiency of the hybrid metalens, measured from λ = 750 to 1100 nm with 10 nm interval. The red and blue curves represent the measurements of the hybrid metalens with and without the metasurface, respectively. For each wavelength in (c) and (d), intensity profiles of the focal spot along horizontal, vertical, and diagonal directions were measured. An Airy disk function was then used to fit the measured intensity profiles to determine FWHMs and Strehl ratios. The average value of these quantities at each wavelength is shown in (c) and (d) as data points, with the standard deviation plotted as error bars. The black line in (c) is the theoretical FWHM of the focal spot calculated when NA = 0.4.

FIG. 4.

A metalens SRS microscope and its imaging performance. (a) Schematic. AOM: acousto-optic modulator; L: lens; OBJ: objective lens; PD: photodiode; DM: dichroic mirror; M: mirror. (b)–(e) Performance comparison between the hybrid metalens and refractive lens (i.e., without the metasurface) by imaging a 1-µm polystyrene bead at 3060 cm⁻¹ Raman shift. (b) and (c) are the intensity profiles along the x-direction, and their inset images are the x–y cross section view of the bead. (d) and (e) are the intensity profiles along the z-direction, and their inset images are the x–z cross section view of the bead. All solid curves are fitted with a Gaussian function. δx and δz are the FWHM of the fitted curves and defined as the lateral and axial resolutions, respectively. All scale bars in the inset images are 5 µm.

FIG. 5.

Metalens SRS and CARS imaging of mixed 3-µm polymethyl methacrylate (PMMA) and 2-µm polystyrene (PS) beads. (a) SRS images and (b) spectra of PMMA (green) and PS beads (red) at the forward detection mode. (c) CARS images and (d) spectra of PMMA (green) and PS beads (red) in the backward (epi-)detection mode. Scale bars are 5 µm.

FIG. 6.

Metalens-based CRS imaging of lipid content in an ex vivo mouse ear. (a) Forward SRS image and (b) epi-detected CARS image at 2884 cm⁻¹. Scale bar: 50 µm.

FIG. 7.

Performance of the hybrid metalens in volumetric SRS imaging PMMA bead and ovarian cancer tissue samples. (a) and (b) are images from the sample of 10 µm PMMA beads imaged (2955 cm⁻¹ Raman shift) by the hybrid metalens and refractive lens, respectively. Different depths are labeled. (c) and (d) Ovarian cancer tissue imaged at 2900 cm⁻¹ Raman shift by the hybrid metalens and refractive lens, respectively. The bright spots are lipid droplets. Scale bars are 40 µm.