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Abstract
With the novel capabilities of engineering the optical wavefront at the nanoscale, the dielectric metalens has been utilized for fluorescence microscopy imaging system. However, the main technical difficulty is how to realize the achromatic focusing and light modulation simultaneously by a single-layer metalens in the two-photon excitation STED (TPE-STED) endomicroscopy imaging system. Herein, by combining the spatial multiplexing technology and vortex phase modulation, a single-layer multitasking vortex-metalens as a miniature microscopy objective on the end of fiber was proposed. The multitasking vortex-metalens with 36-sectors interleaving (diameter of 100 μm) could focus the excitation beam (1050 nm) and depletion beam (599 nm) to the same focal distance, modulate a doughnut-shaped depletion spot with vortex phase and reshape the focal spots to further make improvement in the quality and symmetry. According to the TPE-STED theory, a symmetrical effective fluorescent spot with the lateral resolution of 30 nm was obtained by the proposed metalens. Thus, with the advantage of ultra-compact and lightweight, we prospect that the subminiature multitasking metalens will help guide future developments in high-performance metalenses toward high-resolution and real-time images for deep biological tissue in vivo and enable scientific high-end miniature endomicroscopy imaging system.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Stimulated emission depletion (STED) microscopy is a powerful imaging tool beyond the diffraction limit, which can create super-resolution images by inhibiting the spontaneous emission at the periphery of the diffraction-limited fluorescence spot through stimulated emission [1,2]. To realize a deeper penetration and less photodamage in scattering biological tissue, the two-photon excitation STED (TPE-STED) microscopy system has been demonstrated by utilizing near-infrared (NIR) excitation beam [3,4]. However, a typical two-photon excitation microscope can image only within 1 mm depth in vivo owing to technical limitations [5]. So the flexible miniature fiber-optic endomicroscopy system is a better alternative [5–7]. Furthermore, the technologies for generating a perfect doughnut profile and shrinking the zero-intensity central region of the depletion focal spot are essential to improve the resolution of the STED microscopy system [8]. Hence, the major challenges of the fiber-optic TPE-STED endomicroscopy system are how to realize the achromatic focusing of the excitation and depletion beams and the modulation of the doughnut-shaped depletion spot by an ultra-compact focusing unit [9,10]. To the best of our knowledge, the subminiature focusing unit which can integrate multiple functions to satisfy the requirements of the light modulation and achromatic focusing has not been employed for the fiber-optic TPE-STED endomicroscopy system.
Dielectric metalenses, single flat devices with nanoscale thickness, consist of the periodic dielectric arrays of subwavelength optical meta-atoms [11,12]. Moreover, metalenses can flexibly manipulate the amplitude, phase and polarization of incident waves by introducing the generalized laws of refraction and reflection [13,14]. Recently, metalenses have various applications in the optical imaging, such as achromatic visible imaging [15], two-photon microscopy imaging [16,17], and wide-angle visible imaging [18]. In the TPE-STED imaging, the most significant function to be fulfilled is the achromatic focusing. To realize the achromatic focusing, many designs of achromatic metalenses have been demonstrated to achieve double-wavelength confocal capacity, which are based on spatial multiplexing technology [11,17], polarization-controlled broadband achromatic [19], birefringence [16], doublet [20], and multilayer [21]. Besides, for the TPE-STED endomicroscopy, another crucial function is the modulation of the depletion beam. To obtain a doughnut-shaped focal spot, focusing optical vortex generators [13], holographic plasmonic metasurfaces [22], and meta-vortex-lenses [23] have been reported to engineer helically structured wavefront. Owing to these types of metasurfaces are usually designed to achieve only a single function, they cannot be utilized as an independent focusing unit for the TPE-STED imaging system, which is the main limiting factor to be used in an ultra-compact TPE-STED endomicroscopy. Ultimately, based on the significant progress having been made to date, it is necessary to obtain a single-layer metalens which can achieve not only the achromatic focusing with the excitation beam (NIR light) and depletion beam (visible light), but also the modulation of two focal spots with different modes (a doughnut-shaped depletion spot and a solid excitation spot). This multitasking metalens as a miniature endomicroscopy objective lens satisfies the essential requirements of the TPE-STED imaging, which is of great interest and practical value in super-resolution nanoscopy.
In this paper, based on the spatial multiplexing and vortex phase modulation technology, we proposed a single-layer multitasking vortex-metalens for the TPE-STED endomicroscopy imaging system as a miniature microscopic objective (diameter of 100 μm) on the end of fiber. With the illuminations of the same mode, a symmetrical doughnut-shaped STED spot (599 nm) and a solid pump spot (1050 nm) could focus on the same position (f = 10 μm), respectively. Besides, to improve the quality and symmetry of the focal spots, the focal spots were reshaped by the optimized structure of 36-sectors interleaving in this proposed metalens. According to the TPE-STED theory, a symmetrical effective fluorescent spot was obtained with the corresponding lateral resolution of 30 nm. Therefore, an ultra-compact TPE-STED endomicroscopy imaging system is achieved with the proposed subminiature metalens, which can open a new window of high resolution and minimally invasive imaging in deep regions of biological tissues.
2. Designs and theory
2.1. Design of two-photon excitation STED endomicroscopy imaging system
The scheme of the proposed fiber-optic TPE-STED endomicroscopy imaging system is illustrated in Fig. 1(a). This imaging system consists of two laser sources, a beam delivery system, a plug-and-play focusing unit and a detector. The pump laser source is an ultra-short pulse laser with the wavelength of 1050 nm to realize two-photon excitation, while the STED source is a continuous-wave laser with the wavelength of 599 nm to achieve stimulated emission depletion. The wavelength-division multiplexer (WDM) can couple the excitation beam and depletion beam to the transmission fiber simultaneously. The optical fiber can transmit illuminating beams and fluorescent signals stably, whereas the optical fiber coupler is used to split the fluorescence and illuminating beams. In the plug-and-play focusing unit, the fiber tip is connected to the beam delivery system with a mounting collar. As shown in Fig. 1(b), a miniaturized and polarization-insensitive multitasking vortex-metalens is fabricated on the end of fiber, which is utilized to focus double illuminating beams, modulate a doughnut-shaped STED spot and reshape focal spots simultaneously. The holder is placed on the surface of the sample to support the focusing unit during imaging. Figure 1(c) illustrates the generation process of the effective fluorescent spot by the pump spot and STED spot based on the TPE-STED theory. With the achromatic focusing and depletion spot modulation via the multitasking vortex-metalens, the size of the effective fluorescent spot is largely reduced by the stimulated emission depletion, which will further analyze in the next section.
Fig. 1. (a) Schematic illustration of the fiber-optic two-photon excitation STED endomicroscopy imaging system based on the multitasking vortex-metalens. (b) A plug-and-play endomicroscopy probe with a single metalens on the end of fiber. (c) The schematic of the effective spot generation via the pump spot and STED spot by the TPE-STED theory.
2.2. Design of multitasking vortex-metalens with 36-sectors interleaving
The focal length is inversely proportional to the incident wavelength λ for the traditional metalens designed at a single operation wavelength. In the TPE-STED endomicroscopy imaging, the excitation (λ = 1050 nm) and depletion (λ = 599 nm) beams with a large gap in wavelength will result in the focal positions being far apart due to the strong chromatic dispersion. In addition, considering the requirement of the transverse mode of excitation and depletion focal spots, the incident illuminations should be modulated into a solid pump spot and a doughnut-shaped STED spot simultaneously by the same objective. To achieve these goals without other additional optical devices, a single-layer multitasking vortex-metalens is designed based on the achromatic focusing technology of spatial multiplexing and the vortex phase modulation. So the blue sectors can modulate the excitation beam to a tight pump spot, while the orange sectors can modulate the depletion beam to a focusing optical vortex spot, as shown in Fig. 2(a). The structure of 36-sectors interleaving in this metalens provides the effect of spots shaping to further improve the quality of focal spots by minimizing the diffractive artifacts. The radial lengths of different sectors in the proposed metalens are 20 μm, 15 μm, and 15 μm, respectively. Since the diameter of metalens is 100 μm (focal length f = 10 μm and numerical apertures NA = 0.98), the size of the endomicroscopy probe is largely reduced. With the advantage of high refractive index and the much lower absorption than amorphous silicon (a-Si) at visible wavelengths, the high contrast dielectric crystalline silicon (c-Si) was utilized to implement the metalens [24,25], which can increase the light confinement to each meta-atom and reduce the near-field coupling between adjacent nanoposts [26]. The corresponding refractive index and absorption coefficient of c-Si are 3.933 + 0.01867i at 599 nm and 3.559 + 0.00013i at 1050 nm [27]. As a result, the high contrast c-Si permits a small lattice constant and increase the sampling density in the design of high numerical apertures (NA) metalens.
Fig. 2. (a) Schematic of the multitasking vortex-metalens with 36-sectors interleaving based on the spatial multiplexing technology (blue sectors: λpump = 1050 nm, orange sectors: λSTED = 599 nm). (b) and (c) The side view and top view of the meta-atom which consist of the nanopost (c-Si) and the hexagonal substrate (SiO2). (d) The inset illustrates the details of the discontinuous spiral arrangement (topological charge of +1) of nanoposts in orange sectors. The dark background color is just utilized for showing the details of orange sectors clearly.
Figure 2(b) and (c) show that the meta-atom is composed of a cylindrical c-Si nanopost (with radius R and height H) and periodic hexagonal silica substrate (with lattice constant U). For the design of normal incidence, the lattice constant U must to satisfy $U \le 2\lambda /\left( {\sqrt 3 {n_{\textrm{silica}}}} \right)$ to prevent the higher order diffractions from propagating in the substrate [11]. In addition, U should also satisfy U < λ/(2NA) to avoid the spherical aberrations based on the Nyquist sampling criterion [12]. Therefore, the optimized lattice constant is 300 nm in orange sectors (λSTED = 599 nm) and 500 nm in blue sectors (λpump = 1050 nm). Then, based on the 3D finite-difference-time-domain (FDTD) method, the phase shifts and transmission of the meta-atoms were simulated, which is shown in Fig. 3(a) and (b). In order to realize the full 2π phase delay and obtain a high transmission, the diameter of nanoposts and height H are swept in the simulation of the 599 nm and 1050 nm beams, respectively. A low height will achieve less than 2π phase coverage and extremely low transmission at the excitation wavelength (λ = 1050 nm). Thus, the height H of 690 nm is an optimized choice by trading off the calculation results of dual-wavelength. The third figures of Fig. 3(a) and (b) are the polynomial fitting cure of the diameter as a function of the phase shift and transmission at the two different wavelengths, respectively. With the fitting function, the required spatial phase can be realized by adjusting the diameter of nanoposts. It is also shown that the meta-atoms possess high transmission (> 70% at 599 nm and > 95% at 1050 nm). The normalized magnetic field intensity profiles of meta-atoms are illustrated in Fig. 3(c) and (d), respectively. The light is concentrated inside the nanoposts and implied the weak near-field coupling between the adjacent nanoposts.
Fig. 3. (a) and (b) Simulated phase shifts and transmission of meta-atoms as a function of the nanopost height and the nanopost diameter at λSTED = 599 nm (USTED = 300 nm) and λpump = 1050 nm (Upump = 500 nm), respectively. The third figures profile the horizontal dashed lines (the nanoposts height of 690 nm) in the phase and transmission figures. The shaded part is excluded. (c) and (d) The top and side views of the normalized magnetic field intensity at λSTED = 599 nm (diameter of 100 nm) and λpump = 1050 nm (diameter of 180 nm), respectively. The dashed white frames depict the boundaries of the c-Si nanoposts.
According to the generalized laws of refraction and reflection, the abrupt phases for the full 2π coverage are modulated by the meta-atoms with the different diameters of the nanoposts. The strong lagging phase can be obtained by the larger diameters of nanoposts. Thus, to introduce the achromatic focusing effect at dual-wavelength and vortex phase modulation at STED wavelength of 599 nm, the spatial phase distribution profiles of the blue sectors and orange sectors can be expressed as follows:
We proposed the expected sample fabrication methods in the future experimental project of the multitasking vortex-metalenses with 36-sectors interleaving. Firstly, the c-silicon layer from a silicon on insulator (SOI) wafer can be transferred to the end of fiber by adhesive wafer bonding and deep reactive ion etching technology [24]. Then, the metalens patterns are defined with electron-beam lithography in the e-beam resist. Finally, the pattern transfer is etched using the inductively coupled plasma etcher technology.
3. Multitasking results and discussion
3.1. Performance of dual-wavelength achromatic focusing
To demonstrate the multitasking performances of the proposed metalens, we theoretically simulated the focusing electric field of this multitasking vortex-metalens. With the advantages of non-diffractive and tight focusing to the Bessel beam, the illumination was right-handed circular polarized zeros-order Bessel-Gauss beam at 599 and 1050 nm, respectively. Figure 4(a) and (b) show the normalized electric energy density distribution of the axial plane (left) and the focal plane (right) at 599 and 1050 nm, respectively. The focal lengths are 10 μm at both wavelengths (NA = 0.98), which demonstrate the performance of achromatic focusing in the proposed metalens. In addition, a weak secondary focal spot (f ≈ 23 μm) can be observed on the axial plane (y-z plane) at 599 nm in Fig. 4(a). The secondary depletion spot with low intensity is caused by the error of the phase shifts (over 2π phase coverage) for 599 nm wavelength, which resulted from the nanoposts designed to operate at 1050 nm wavelength [11]. In contrast, there is no higher-order pump spot on the axial plane (y-z plane) at 1050 nm in Fig. 4(b), since the nanoposts designed to operate at 599 nm wavelength results in less than 2π phase shifts for 1050 nm wavelength. In general, the cross-talk is mainly caused by the meta-atoms which are designed for only single desired wavelength. In practice, however, the meta-atoms work under dual-wavelengths which have some phase errors at the undesired wavelength. However, the weak secondary depletion spot is far away from the main focal spots, which cannot influence the overlap of the depletion spot and pump spot. Thus, the weak cross-talk between 599 nm and 1050 nm has little effect on the depletion process during TPE-STED imaging. Although the polarization-dependent configuration for the dual-wavelength can suppress the cross-talk [16], the depletion and pump beams with the different polarization modes will obtain a poor depletion ratio in the STED microscopy [29]. Eventually, the FWHM of the effective fluorescence spot will increase. Meanwhile, the collected fluorescence is half of the total emitted fluorescence due to the polarization-dependent configuration. Therefore, the design of polarization-insensitive in the proposed metalens is beneficial for the TPE-STED imaging and improve the ability of fluorescence collection. Overall, without other traditional optical devices, the performance of achromatic focusing is achieved by this single-layer multitasking metalens. These focal spots satisfy the requirement of the efficient overlap to produce a minimal effective fluorescent spot in the TPE-STED endomicroscopy.
Fig. 4. (a) and (b) Simulated normalized electric energy density distribution in the y-z (left) and x-y (right) planes for the multitasking vortex-metalens with 36-sectors interleaving at 599 nm and 1050 nm, respectively. Scale bar, 1 μm. (c) The electric field intensity Ex phase distribution of x-y focal plane with the topological charge of +1. (d) and (f) The intensity profile along with the horizontal and vertical line cutting through the center of the focal spots at 599 and 1050 nm, respectively. The blue dashed curve is the intensity profile of an ideal Airy spot at 1050 nm. (e) The schematic of the multitasking vortex-metalens with 36-sectors interleaving.
To further demonstrate the performance of achromatic focusing of the proposed metalens, we obtain the transmission efficiencies are 50% at 599 nm and 80% at 1050 nm, respectively. The different transmission efficiencies and energy loss are mainly caused by the absorption of c-Si at 599 nm and the interface reflectivity at both wavelengths. The focusing efficiency was defined by the ratio of the power passing through a diameter of 10 μm circular aperture at the focal plane to the transmission power of illumination [24,30]. Then, the focusing efficiencies are 30% at 599 nm and 35% at 1050 nm, which are suffered from the division of spatial multiplexing. Moreover, there is a trade-off between the focusing efficiency and NA of the metalens, since it is well-known that the focusing efficiency of metalens decreases with increasing NA [31]. So the NA of the proposed metalens is up to 0.98, which results in decreasing the focusing efficiency of metalens. In conclusion, due to the slightly poor focusing efficiency, the higher incident power of depletion and pump beam should be improved to ensure the high enough collected fluorescence intensity and image sharpness during imaging. However, the excessive incident power will increase the photodamage to the biological tissues. Thus, the incident power of illuminations should be optimized to realize the two-photon excitation and depletion process in order to obtain the proper functionality of the imaging system.
3.2. Performance of vortex phase modulation for depletion spot
Figure 4(c) shows the electric field intensity Ex phase distribution of x-y focal plane at 599 nm. The 0-2π vortex phase distribution can be observed in the center of the focal plane corresponding to the same position of the STED spot. A single period of vortex phase demonstrates the topological charge number of +1 in the design of vortex-metalens. The PPD of the STED spot is 488 nm in Fig. 4(d). To illustrate the performance of vortex phase modulation of the proposed metalens in detail, a single-wavelength (λSTED = 599 nm) vortex-metalens with the same parameters was designed for comparison, which is shown in Fig. 5. The normalized electric energy density distribution in the y-z (left) and x-y (right) planes for the single wavelength vortex-metalens (NA = 0.98, f = 10 μm) with the same illumination are plotted in Fig. 5(a). Figure 5(b) is the structure of the single wavelength vortex-metalens. Figure 5 (c) shows the 0-2π vortex phase in the center of the Ex phase distribution in the x-y focal plane. It is clear that a single period of 0-2π vortex phase in the center of Fig. 4(c) is the same as Fig. 5(c). Furthermore, the PPD of the line profile in Fig. 4(d) is equal to the PPD in Fig. 5(d) of 488 nm, which reveals that the design of discontinuous 36-sectors has little influence on the modulation of vortex phase. Thus, with the discontinuous spiral arrangement, the proposed multitasking vortex-metalens successfully implements vortex phase modulation for the depletion spot.
Fig. 5. The vortex phase modulation results of the single-wavelength vortex-metalens. (a) The simulated normalized electric energy density distribution in the y-z (left) and x-y (right) planes for a single wavelength vortex-metalens at λSTED = 599 nm. Scale bar, 1 μm. (b) The top view of the single wavelength (λ = 599 nm) vortex-metalens. (c) The electric field intensity Ex phase distributions of the x-y focal plane with the topological charge of +1. (d) The intensity profile along with the horizontal and vertical line cutting through the center of the x-y focal planes.
3.3. Performance of excitation and depletion spots shaping
As shown in Fig. 4(a) and (b), the intensity distributions in the focal plane (x-y plane) show the weak sidelobes around the main focal ring (599 nm) or spot (1050 nm. Furthermore, Fig. 4(d) and (f) show the intensity profile along with the horizontal and vertical line cuttings through the center of the focal spots at 599 and 1050 nm, respectively. The central FWHM of the doughnut in STED spot is 264 nm in Fig. 4(d). As shown in Fig. 4(f), the diffraction-limited Airy profile (FWHM = 0.514λ/NA) is plotted in a blue dashed curve. The full width at half-maximum (FWHM) of pump spot is 580 nm (∼ 0.55λ) at 1050 nm, which is close to the FWHM of ideal Airy spot.
As illustrated in Fig. 6(c), to further demonstrate the quality of focal spots shaping, an achromatic metalens formed by the simple division with the same parameters was designed for comparison. In Fig. 6(a) and (b), the normalized electric energy density distribution shows the visible sidelobes around the main focus ring in both the y-z plane and x-y plane at 599 nm, which is stronger than the depletion spot of the proposed metalens in Fig. 4(a). In addition, Fig. 6(d) and (e) show the intensity profiles in the horizontal and vertical line cuttings at 599 and 1050 nm, respectively. The PPDs of the 599 nm STED focal ring in x and y line cuttings are 500 nm and 479 nm, respectively. The intensity of sidelobes of the main peaks in x and y line cuttings are 50% and 25%, respectively. Hence, it is clear that the transverse intensity profile of the STED focal ring is severe asymmetrical by the simple division in the STED spot shaping. However, Fig. 4(a) and (d) show that the symmetrical focal ring with weak sidelobes was achieved by the multitasking metalens with 36-sectors interleaving, which reveals that the more excellent performance of the proposed metalens in reshaping depletion spots. Moreover, the FWHMs of the 1050 nm pump spot (Fig. 4(f)) of the proposed metalens are equal to the FWHMs in Fig. 6(e). Although the structure of 36-sectors interleaving in this proposed metalens has no effect on the size of the pump spot, the visible circular sidelobes can be seen in the x-y focal plane (Fig. 6(a)) at 1050 nm. Whereas, there are no sidelobes in Fig. 4(b), which demonstrates that this proposed multitasking metalens also can shape the pump spot to improve the quality of the focal spot at 1050 nm. Therefore, with the discontinuous arrangement in the structure of 36-sectors interleaving, the proposed metalens realize the performance of the focal spots shaping. By the optimization of the focal spot shaping, the lateral resolution will further improve by the high-quality focal spots in the TPE-STED endomicroscopy.
Fig. 6. The focusing and light modulation results of the achromatic metalens with the simple spatial multiplexing division. (a) and (b) The simulated normalized electric energy density distribution in the y-z (left) and x-y (right) planes at 599 and 1050 nm, respectively. Scale bar, 1 μm. (c) The schematic of the achromatic metalens designed with the simple spatial multiplexing division. (d) and (e) The intensity profile along with the horizontal and vertical line cutting through the center of the x-y focal planes at 599 and 1050 nm, respectively. The blue dashed curve is the intensity profile of an ideal Airy spot at 1050 nm.
4. Resolution of TPE-STED endomicroscopy imaging system
To evaluate the performance of multitasking vortex-metalens in the TPE-STED endomicroscopy imaging system, the effective fluorescent spot (effective spot) distributions was calculated by the vectorial model based on the Lorentzian fitting function for the fluorescent depletion process [29,32,33]. According to the most favorable depletion conditions when the excitation and depletion polarizations and the molecular transition dipole are all parallel, the depletion ratio is defined as the ratio of the effective fluorescent intensity to the total emission fluorescent intensity via pump beam excitation [32], which is given as:
where $\vec{n}$ is the unit vector that along the molecular transition dipole axis, ${\vec{E}_{\textrm{STED}}}$ is the intensity of STED electric field and $C = 3 \cdot {\sigma _{\textrm{dip}}} \cdot \tau $ is a molecular calibration constant that depends on the transition cross sections ${\sigma _{\textrm{dip}}}$ and the fluorescence lifetime τ of Rhodamine-6G [29]. The factor of 3 is obtained from averaging over random orientations. The constant C is determined by fitting the measured data, in which ${\sigma _{\textrm{dip}}}$ is 1.1×10−16 cm2 [33] and τ is 3.75 ns [34]. The maximum depletion beam photon flux is 1026 photons/cm2s (∼33.2 MW/cm2). Thus, the effective fluorescent distribution ${I_{\textrm{eff}}}({\vec{r}} )$ can be generated by: where ${I_{\textrm{pump}}}({\vec{r}} )$ is the intensity distribution of pump spot, and the squared in this equation due to a nonlinear process of the two-photon excitation for Rhodamine-6G dye. The lateral resolution is usually improved by increasing the maximum intensity of depletion beam. But the photobleaching and photodamage in the biological tissue will occur if the intensity of the depletion beam is excessively large. This means that the intensity of the depletion beam should be well optimized in order to obtain the proper functionality of the imaging system. Hence, the parameter setting of the maximum depletion beam photon flux of 1026 photons/cm2s in this work is more favorable for living cell imaging.The effective spot is calculated by the vectorial model with the optimized focal spots of the proposed multitasking vortex-metalens with 36-sectors interleaving. Then, the intensity distributions of pump spot and effective spot in the x-y planes are showed in Fig. 7(a) and (b). Furthermore, the intensity profile in horizontal line cuttings of the pump spot and effective spot are plotted in Fig. 7(c). The effective spot shrinks significantly below the diffraction limit by the depletion process. According to the intensity distribution of horizontal and vertical line cuttings for the effective spot in Fig. 7(d), the lateral FWHM of 30 nm can be obtained in this super-resolution effective spot, which is much smaller than the pump spot of 580 nm. Therefore, the imaging system can generate a symmetrical fluorescent spot beyond the diffraction limit and realize a high lateral resolution by the excellent multitasking performance of this vortex-metalens. Besides, based on the same analytical method, the FWHM of the effective spot achieved by the achromatic metalens formed by simple division (Fig. 6) is up to 46 nm. In consequence, the lateral resolution has been improved by nearly 35% due to the focal spots shaping via the multitasking vortex-metalens with 36-sectors interleaving.
Fig. 7. (a) Normalized electric energy density distribution in the x-y planes of the pump spot (λ = 1050 nm) of the proposed metalens. (b) The normalized electric energy density distribution in the x-y planes of effective spot by the TPE-STED numerical calculation. (c) The intensity profile along with the horizontal line cutting through the center of the focal spots in (a) and (b), respectively. (d) The intensity profile along with the horizontal and vertical line cutting through the center of the effective spot.
5. Conclusion
In summary, the multitasking performances of the single-layer vortex-metalens with 36-sectors interleaving for the TPE-STED endomicroscopy had been demonstrated theoretically. Based on the spatial multiplexing technology, this multitasking metalens could realize the excitation beam (λ = 1050 nm) and depletion beam (λ = 599 nm) achromatic focusing (f = 10 μm). Moreover, the doughnut-shaped depletion spot could be obtained by the design of vortex phase modulation in this proposed metalens. Meanwhile, the structure of 36-sectors interleaving in this multitasking metalens could reshape focal spots for further improving lateral resolution. Thus, the single-layer multitasking vortex-metalens can combine the functionalities of multiple optical devices in an ultra-compact TPE-STED imaging system. According to the TPE-STED theory, an effective fluorescence spot with the lateral resolution of 30 nm was generated by the pump spot and STED spot. With the proposed subminiature metalens, we expect that the ultra-compact TPE-STED endomicroscopy imaging system opens up possibilities for cellular resolution imaging in deep tissue regions, which the existing imaging technologies have far been unreachable.
Funding
Tianjin Municipal Education Commission (2018KJ084); National Natural Science Foundation of China (61701346).
Disclosures
The authors declare no conflicts of interest.
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