Raman spectroscopy is widely used for biomedical applications such as skin analysis and early cancer detection [1–9]. Recently there has been a lot of development in miniaturizing Raman spectroscopy-based devices for in vivo studies [10–12]. We have earlier reported a handheld portable Raman spectroscopy device for skin analysis [13]. The confocal configuration allowed data collection at different dermal layers in a less invasive manner compared to the conventional ex vivo tape stripping method. The dual-wavelength excitation configuration allowed an ultrawide band characterization of the stratum corneum thickness, natural moisturizing factor, and water content. The scanning time was about 2 min for 10 depths reducing 50% of time compared to a commercial Raman spectroscopy system. However, there are several limitations. The first is that the dual-wavelength excitation was sequential by using a fiber array multiplexing module. A simultaneous excitation would be ideal so that different biological features can be measured at the same time. The second is that the excitation and collection beam spots were not focused at the same spot on the sample because the excitation and collection fibers were put side by side and the wavelengths were different. This limitation is a common issue faced by many fiber-based endoscopic probes [12, 14–17]. A coincidence of the beam spots would be ideal to achieve the best spatial resolution. The third is that the overall handheld probe size was on the order of hundreds of millimeters. Although it has already been miniaturized from a benchtop device to a portable handheld device, a further size reduction would enable the device to perform measurements for hard-to-reach areas and endoscopic applications. It is worth mentioning that the limitations mentioned above may also be experienced by non-Raman spectroscopy-based fiber sensors.

The metasurface technology is a rising star in the nanophotonics field and is explored here to address these limitations. It is based on specially designed nanostructures that allow manipulation of the light properties, such as phase and amplitude of the transmitted, reflected, or scattered light, at a sub-wavelength scale. It provides the potential to redesign conventional refractive optics-based optical components in an ultrathin footprint (approximately 10³ times thinner) and achieve new functionalities that never existed before. Over the last decade, research groups worldwide have demonstrated metasurface-based optical components with superior performance to conventional optical components, such as combining multiple functions in a single flat optics device and correcting aberrations to achieve diffraction-limited focusing [18–26]. The concept has started attracting attention in biomedical imaging and sensing too [27, 28]. Pahlevaninezhad et al. implemented the concept of bijective illumination collection imaging for optical coherence tomography of tissues [29]. It was realized through the use of two metasurface lenses (metalens), each bent the incident light at an angle and focused it into a line for the excitation path and the collection path, respectively. The excitation and collection spots coincided and a lateral resolution of 3.2 μm over a depth of 1.25 mm was achieved at a wavelength of 1.3 μm. Beyond single-wavelength metalenses, recent research has shown the merits of broadband achromatic metalenses. Chen et al. has designed broadband achromatic metalens for focusing and imaging in the visible range between 470 and 670 nm [30]. This was achieved by controlling the phase, group delay, and group delay dispersion of the metalens using TiO₂ nanofins of differing widths and lengths and height 600 nm. Wang et al. has demonstrated a broadband achromatic metalens for imaging in the near-infrared (NIR) range 650–1000 nm [31]. The authors achieved high efficiency with an impressive TiO₂ nanopillar height of 1500 nm and 90^o vertical side walls.

Inspired by the previous work, we designed and simulated five different metasurface components that can be integrated on the fiber facets to replace the optical filters and lenses in the current handheld confocal dual-wavelength Raman spectroscopy device. Specifically, this all-metasurface design includes two narrow band pass filters and two off-axis focusing metalenses allowing for simultaneous excitation of two beams at 671 nm and 785 nm and one broadband achromatic on-axis focusing metalens from 810 nm to 910 nm for the collection beam. The focal spots of the excitation beams and collection beam completely overlap so the optimum lateral and axial resolutions were achieved. The new design reduced the overall probe size by approximately 100 times, from 200 mm to 2 mm in diameter into a needle size. We made sure that the design could be fabricated and put into practical use. The parameters of the metasurface nanostructures were based on literature-reported fabricated devices and all metasurfaces have achieved high efficiency of 70% across the entire wavelength range of interest. The methodology and results presented here not only help resolve the limitations of our current probe design, but also extend the confocal Raman spectroscopy applications to oral cavity examinations [32], small animal experiments [14], medical endoscopic experiments [33], and neonatal or infant skin analysis for hard-to-reach areas [34], as shown in Figure 1.

Figure 2 gives an overview of the design methodology. Figure 2A shows a schematic of the probe head made of three optical fibers including two excitation fibers on the side, one collection fiber in the middle, and five metasurface devices at the distal end of the fiber facets, labeled as (1)–(5). The excitation fibers are large-mode area single-mode photonic crystal fibers with a core diameter of 20 μm and a wavelength range of 600–1700 nm (LMA-20, Thorlabs). The collection fiber has a core diameter of 25 μm and a wavelength range of 800–1700 nm (LMA-25, Thorlabs). Single-mode fibers were used because metasurface works only with single-mode incident beams. However, traditional single-mode fibers have a small core diameter of 5 μm and a narrower wavelength range. To maximize the interaction of the light and metasurfaces at the fiber facets, photonic crystal fibers with large mode areas are needed. Our new metasurface-based designs were based on commercially available large mode area single-mode photonic crystal fibers, but it has been shown in the literature that the mode area can be as large as 100 μm [35]. The collection fiber has a low cut-off wavelength at 800 nm so that any excitation light at shorter wavelengths will be blocked. The five metasurfaces in our design include 1) 671 nm narrow band pass filter (NBPF), 2) 671 nm off-axis focusing metalens, 3) 785 nm narrow band pass filter, 4) 785 nm off-axis focusing metalens, and 5) 810–910 nm broadband achromatic on-axis focusing metalens. Metasurface 1)–4) were designed to replace all the optical components in the excitation beam path in the earlier design [13], i.e., metasurface 1) and 3) for the dual-passband laser cleaning filters and metasurface 2) and 4) for the fiber array multiplexing module, the collimator, and the objective. Metasurface 5) was designed to replace the optical components in the collection beam path including the objective, the notch filter, and the collimator.

Figure 2B summarizes the design parameters for the three metalenses, i.e., 2), 4), and 5). The diameters of the metalenses were calculated based on the core diameters of the photonic crystal fibers and their divergence due to the metalens thickness and NBPF thickness. The diameters were 45.9 μm, 51.1 μm, and 51.8 μm for metalens 2), 4), and 5), respectively. The focal lengths were calculated along the beam path. The focal lengths were 356.9 μm, 356.9 μm, and 260.4 μm. The focusing angles were calculated based on the equation, θ = c o s − 1 f o n − a x i s f o f f − a x i s , where f is the focal length. The focusing angles were 43.1^o, −43.1^o, and 0. The numerical apertures (NA) were calculated based on the equation, NA = sin t a n − 1 D 2 f , where D is the diameter. The NAs were 0.06, 0.07, and 0.10. It is noted that although the NAs are small, the focusing angles for the off-axis metalenses are large which adds challenges to the metalens design.

Figure 2C is the schematic of the NBPF metasurface and off-axis metalens combined structure for the excitation fiber. The substrate side will be attached to the fiber facet. The output light from the fiber will first pass through the substrate (SiO₂), then the NBPF layers (Ag/Poly (methyl methacrylate) or PMMA/Ag), and finally the metalens (TiO₂). The optical properties of TiO₂ could be found in the supplementary information. The purpose of the NBPF was to only pass through the excitation wavelengths and filter out the autofluorescence signals generated from the optical fibers. The NBPF was a Fabry-Pérot cavity consisting of the PMMA material sandwiched by two Ag mirrors, which allowed only optical waves in resonance to pass through. The bandwidth of the transmission peaks (i.e., linewidth) and the free spectral range between each transmission peak (i.e., finesse) can be adjusted by the cavity length, the material of the cavity, and the thickness of the cavity. The metalens on top of the NBPF focuses incoming light at an angle. The metalens was made of TiO₂ circular pillars. The parameters of the unit cell design were height = 1500 nm, diameter = 50–130 nm (for λ = 671 nm) and 50 to 170 (for λ = 785 nm), and period = 160 nm (for λ = 671 nm) and 200 nm (for λ = 785 nm). The phase maps and transmission maps are presented in the supplementary information, see Supplementary Figure S1. The phase coverage was 2π and the transmission was close to 1. Note that all the design parameters were proved to be fabricatable by literature [30, 36]. The design equation for the off-axis focusing function can be found in [37], where each metalens element at position x , y needs to provide a phase shift φ that satisfies: φ = − 2 π λ x − x f 2 + y − y f 2 + z f 2 − f (1) where f = x f 2 + y f 2 + z f 2 is the focal length from the center of the metalens to the focal point at the position x f , y f .

Figure 2D is the schematic of the broadband achromatic on-axis metalens for the collection fiber. The substrate material was SiO₂ and the metalens material was TiO₂. Based on the design principles described in [30] for a metalens to focus achromatically, not only the phase shift at each position needs to be satisfied, but also the group delay. Therefore, we simulated a pool of pillars with different shapes and sizes including a circular, cross, and square shape, as shown in Figure 3A. These shapes are symmetrical, so the design is independent of the polarization of the incident light. The height of the nanopillars was 1500 nm which was much higher than the off-axis design because the increased height will increase the group delay coverage of the unit cells [31]. The diameters and lengths were 50–200 nm for all three shapes. For on-axis focusing, the phase equation follows Eq. 1 by setting x f = y f = 0 , f = z f , and λ = λ c e n t e r = 860   n m . The group delay equation is the first derivative of the phase equation with respect to the wavelength at λ c e n t e r = 860   n m . Figure 3B shows the required group delay in femtosecond versus phase in radians for the metalens and the satisfied circular, cross, and square unit cells. The selection of unit cells was based on the principle of minimum sum of squared residuals. Figures 3C, D are the actual phase and group delay realized by the selected unit cells as a function of radial positions of the metalens, respectively. The phase was matched perfectly, and the group delay was matched well. The discrepancy was due to insufficient group delay coverage. This can be improved by having a larger pillar height and a smaller pillar size, but the fabrication of such high aspect ratio pillars will be challenging. Here, the design parameters have been demonstrated on fabricated devices in [31]. Non-etheless, the current parameters are satisfactory to achieve desirable metalens performance which will be discussed in Section 3.

All simulations were performed using commercial FDTD software (Lumerical, Ansys Inc.). For the metasurface designs of the excitation beams, the mesh setting was auto-nonuniform with accuracy 1 and the boundary conditions were PML (stretched coordinate PML) in x-axis, Symmetric in y-axis, and PML in z-axis. For metasurface designs of the collection beam, the mesh setting was auto-nonuniform with accuracy 3 and the boundary conditions were Anti-symmetric in x-axis, Symmetric in y-axis, and PML in z-axis.

This section discusses the simulated results of five metasurfaces mentioned in the last section. It was divided into three subsections, one for the 671 nm excitation fiber, one for the 785 nm excitation fiber, and the last one for the 810–910 nm collection fiber.

In this paper, we have proposed an all-metasurface-enabled fiber needle probe for dual-wavelength excitation Raman spectroscopy devices. The new design reduces the overall size by 100 times and improves the optical performance of the existing handheld probe [13]. It consists of five metasurfaces that replace five conventional optical components in the original design. The metasurfaces designed for the excitation fibers at λ = 671   n m and λ = 785   n m include a hybrid configuration of a Fabry-Pérot cavity and a metalens which functions as a narrow band pass filter and an off-axis focusing lens. The total thickness of the hybrid metalens is in the range of 200 nm, not counting the substrate thickness. The metasurface designed for the collection fiber at λ = 810 − 910   n m is an achromatic broadband metalens that functions as an apochromatic objective lens (one of the most expensive objectives) which focuses the incident light at a constant focal length without chromatic and spherical aberration in a broad wavelength range. The total thickness of this metasurface is 1500 nm, not counting the substrate thickness. Our simulation results show that the two incident excitation beams were focused at an angle of 43.4^o and −43.4^o. The two excitation focal spots and the collection focal spot coincide at the same location on the sample. The profiles of the combined beam were characterized along the x-, y-, and z-axis which correspond to the effective sensing area on the sample of 239.1 μm³ ( 4.6   μ m × 4.6   μ m × 11.3   μ m ). All metasurfaces presented here were developed based on state-of-the-art design methodologies and supported by literature for fabricated devices. The parameters of the fiber probes discussed were also based on commercially available fibers, which means the entire proposed design framework can be readily deployed and put into practical use. The new needle probe will enable many new applications using Raman spectroscopy, for example, oral cavity examinations, small animal in vivo experiments, endoscopic experiments, and neonatal skin analysis for hard-to-reach areas. Finally, the metasurface design framework can also be generalized to other optical instruments for miniaturization and improved optical performances.