High-angle deflection of metagrating-integrated laser emission for high-contrast microscopy (1).pdf
Juodėnas et al. Light: Science & Applications  (2023) 12:251  Ofﬁcial journal of the CIOMP 2047-7538 https://doi.org/10.1038/s41377-023-01286-0 www.nature.com/lsa A R T I C L E O p e n A c c e s s High-angle deﬂection of metagrating-integrated laser emission for high-contrast microscopy Mindaugas Juodėnas 1✉, Erik Strandberg2, Alexander Grabowski 2, Johan Gustavsson2, Hana Šípová-Jungová1, Anders Larsson2 and Mikael Käll 1✉ Abstract Flat metaoptics components are looking to replace classical optics elements and could lead to extremely compact biophotonics devices if integrated with on-chip light sources and detectors. However, using metasurfaces to shape light into wide angular range wavefronts with high efﬁciency, as is typically required in high-contrast microscopy applications, remains a challenge. Here we demonstrate curved GaAs metagratings integrated on vertical-cavity surface-emitting lasers (VCSELs) that enable on-chip illumination in total internal reﬂection and dark ﬁeld microscopy. Based on an unconventional design that circumvents the aspect ratio dependent etching problems in monolithic integration, we demonstrate off-axis emission centred at 60° in air and 63° in glass with > 90% and > 70% relative deﬂection efﬁciency, respectively. The resulting laser beam is collimated out-of-plane but maintains Gaussian divergence in-plane, resulting in a long and narrow illumination area. We show that metagrating-integrated VCSELs of different kinds can be combined to enable rapid switching between dark-ﬁeld and total internal reﬂection illumination. Our approach provides a versatile illumination solution for high-contrast imaging that is compatible with conventional microscopy setups and can be integrated with biophotonics devices, such as portable microscopy, NIR-II range bioimaging, and lab-on-a-chip devices. Introduction Wavefront engineering using collections of sub- wavelength nanostructures—or metasurfaces—started with simple demonstrations of lensing and anomalous refraction1,2 but has now evolved toward a variety of complex applications, including ranging3, polarization imaging4, holography5, optical manipulation6, and non- linear photonics7. However, metaoptics are so far mostly used as stand-alone components, even though one of the most enticing advantages over traditional optics—fabri- cation process compatibility—intuitively suggests direct integration with optoelectronic devices. Indeed, recent reports demonstrated such integration on LED8, OLED displays9, and semiconductor lasers10. One application area that could signiﬁcantly beneﬁt from further progress in this direction is optical biosensing and microscopy, which still heavily rely on bulky external components such as lasers and compound microscopes. Flat metaop- tics integrated with on-chip light sources, detectors, and microﬂuidics, could lead to extremely compact and cost- effective biophotonics devices able to drastically facilitate life-science research and applications. In recent years, the ﬁeld of biophotonics has placed signiﬁcant emphasis on label-free techniques for analyz- ing individual nanoscopic objects like biological nano- particles, biomacromolecules, and drug carriers, primarily utilizing light scattering methods11–15. The scattering cross-section of sub-wavelength particles decreases rapidly with decreasing particle size, which necessitates efﬁcient reduction of background caused by unscattered light. The traditional means of achieving this are various types of dark-ﬁeld (DF) or total internal reﬂection (TIR) © The Author(s) 2023 OpenAccessThisarticleislicensedunderaCreativeCommonsAttribution4.0InternationalLicense,whichpermitsuse,sharing,adaptation,distributionandreproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Correspondence: Mindaugas Juodėnas (mindaugas.juodenas@chalmers.se) or Mikael Käll (mikael.kall@chalmers.se) 1Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden 2Department of Microtechnology and Nanoscience, Chalmers University of Technology, 412 96 Gothenburg, Sweden 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; illumination setups based on ultrahigh numerical aperture (NA) immersion optics or waveguides coupled to bulky external light sources16–18. Full integration of a miniature light source into a planar chip for such high-contrast illumination has not yet been demonstrated, though a recent report on metasurface-coupled TIR microscopy based on an external light source indicates its feasibility19. In this work, we present curved metagratings mono- lithically integrated on vertical cavity surface-emitting lasers (VCSELs) with the aim to produce efﬁcient emis- sion at high deﬂection angles for high-contrast DF and TIR microscopy (Fig. 1). The VCSEL is a semiconductor emitter that, because of its high efﬁciency and suitability for wafer-scale produc- tion, is widely used in face recognition, proximity sensors, augmented reality, among other applications. VCSELs are well-suited for monolithic integration with metasurfaces because they can be designed for single-mode emission normal to the epitaxial structure of the chip, which means that the beam shaping structure can be easily integrated in the process ﬂow for fabrication of bottom-emitting VCSELs on transparent substrates. There have been sev- eral reports on structures etched into the VCSEL facet that modify its emission, for example into beam arrays20, orbital angular momentum beams21, and deﬂected cir- cularly polarized beams22. Nanostructures have also been integrated with the mirrors of the VCSEL cavity to pro- vide both optical feedback and to generate orbital angular momentum23 or circularly polarized output24. The most widely used and the most general form of a beam shaping metastructure is a phase-gradient meta- surface. It consists of discrete building blocks placed on the nodes of a uniform, subwavelength lattice by matching their calculated phase and amplitude response to the phase map of the intended functionality. Pioneering results on monolithic integration of such phase-gradient metaoptics and VCSELs was provided by Genevet and co- workers25–27. They showed on-chip beam shaping, including collimation, deﬂection, Bessel and OAM beam generation, etc. The phase mapping approach works well in the paraxial regime, where angles of deﬂection are small, but it fails at high deﬂection angles. In such high NA conditions, the spatial extent that encompasses a 2π linear phase gradient becomes comparable to the sub- wavelength unit cell size of the metasurface, leading to either insufﬁcient or varying spatial sampling rate28. It is in principle possible to circumvent this problem by optimizing a full (2π phase space) super-unit cell28,29, but this approach generally leads to open areas of varying width between structures which, in the case of mono- lithically integrated metasurfaces, is problematic because of the effect known as aspect-ratio dependent etching (ARDE). Metasurfaces are typically fabricated by a top-down process—etching the high-index material (e.g., silicon on a fused silica substrate) through a lithographically deﬁned mask. The process of etching, especially at the nanoscale, is often limited by ARDE, which manifests as a reduction of etch rate as the ratio of the etched depth to the opening in the mask becomes larger. An etch-stop layer is the common solution, because it allows to prolong the etch time and let smaller openings to catch up. In this case it means depositing another material on the backside of the VCSEL chip with reasonable etching selectivity against GaAs. However, this would inevitably lead to a less efﬁ- cient device and negate the advantages of monolithic fabrication on a pristine GaAs crystal because of defect induced absorption, refractive index and thermal expan- sion discontinuities. Here we circumvent this problem by instead using metagratings, or subwavelength binary high-contrast gratings, which can be considered a proto-concept of metasurface ﬂat optics30–35. The metagratings are opti- mized such that all etched trenches have equal width, which completely eliminates ARDE and results in >90% deﬂection efﬁciency. Furthermore, based on a simple axicon concept, we introduced grating curvature able to collimate the inherently divergent VCSEL emission out- of-plane while in-plane divergence is maintained. The fabricated structures are interfaced to microﬂuidic chips for proof-of-concept DF and TIR microscopy of nano- particles. We also demonstrate switchable DF/TIR over a wide ﬁeld-of-view using on-chip illumination. Results Concept of an offset axicon metagrating The metasurface concept that we use in this paper is based on a uniform metagrating with constant period and unit cell size across the sample. Such a metagrating can be transformed into an axicon by curving the ridges into Top mirror VCSEL Dark Field/TIR Imaging Oxide aperture Bottom mirror GaAs substrate Gaussian beam Integrated curved metagrating 5 m 50 m Fig. 1 Overview of the metagrating-integrated laser for microscopy concept. Schematic illustration of an oxide-conﬁned GaAs-VCSEL and its integration with a metagrating that produces off- axis emission at high angles for high-contrast DF and TIR microscopy Juodėnas et al. Light: Science & Applications  (2023) 12:251  Page 2 of 9 curvature in the metagratings to change the illuminated area size, the location of the area of interest can be facilitated before switching to a more concentrated illu- mination with full collimation. Such integrated multi- functional light source can become a versatile platform for rapid screening of samples without the need of full microscope reconﬁguration, e.g., switching between TIR and DF condensers. Ideally, our proposed illumination module would be combined with ﬂuorescence imaging, but there is so-far a lack of efﬁcient ﬂuorophores operating at ~1000 nm wavelength. However, this is an actively researched area because excitation within the NIR-II range (1000–2000 nm) features dramatically reduced auto- ﬂuorescence and light toxicity compared to conventional visible-range excitation. We therefore expect our demonstration to prove useful also for ﬂuorescence ima- ging, once suitable functional ﬂuorophores are readily available39–41. Finally, the metagrating concept demonstrated here can easily be reoptimized and integrated with both, InP-based VCSELs42 that emit at longer wavelengths, as well as with VCSELs emitting in the visible wavelength spectrum, though these are still at an early development stage43,44. Materials and methods Grating simulations Metagratings were simulated using the Wave Optics module of COMSOL Multiphysics software in 2D. Peri- odic boundary conditions were deﬁned on the sides of the computation domain, while the bottom and top bound- aries were assigned as exciting and receiving ports with in-plane polarization. To ﬁnd the optimal structure, the optimization module was used with the Nelder-Mead algorithm and the transmittance of +1 diffraction order as the ﬁgure of merit. Depending on the unit cell size, either three or four trenches were deﬁned in the calculation domain and were allowed to change their position within the unit cell freely, as well as their width and height. The trenches were constrained to be equal width. The opti- mization was allowed to run until converged. GaAs refractive index data was taken from Papatryfonos et al45. VCSEL fabrication The VCSEL epitaxial structure is grown by MOCVD and features an active region with three strained InGaAs quantum wells sandwiched between two distributed Bragg reﬂectors (DBR), with 28 and 20 AlAs/GaAs and/or AlGaAs/GaAs pairs on the topside and substrate side, respectively, for bottom emission through the GaAs substrate. The top DBR has one AlGaAs layer with a 98% aluminium content which is selectively oxidized to form an oxide aperture for lateral current and optical mode conﬁnement. The VCSELs were fabricated using standard processing steps of oxide conﬁned GaAs-based VCSELs. First, Ti/Pt/Au top contact rings were evaporated on a highly doped p-type layer on the top DBR. After which alignment marks are transferred to the backside of the chip to align the metagrating with the centre of the VCSEL. A VCSEL mesa with 21 µm diameter is then dry Switchable TIR/DF Mode (Water) 20× 1 m 50 m 50 m 2 3 1 2 3 1 10 m 10 m 10 m 10 m VCSEL a b c d e f g h i 625 1000 460 Thickness, m TIR TIR DF-TIR BF BF-TIR DF-TIR TIR DF Glass Slide Cover Slip Cover Slip Au Colloid Fig. 5 VCSEL with an integrated metagrating in a mirror-symmetric conﬁguration provides clean TIR illumination. a 3D render of the integrated mirrored metagrating concept; b SEM micrograph of the fabricated metagrating; c Schematic of the on-chip microscopy setup, where Au nanoparticles in a colloid solution are illuminated by only TIR laser light; d, e Optical micrographs of light scattered by colloidal Au nanorods (180 × 88 nm2) by combined DF/TIR illumination (d) and TIR illumination only (e), insets show corresponding intensity cuts; f–i Optical micrographs (60× objective) of a sample with nanorod-incubated cells: bright ﬁeld (f), bright ﬁeld with TIR (g), DF-TIR (h), and TIR only (i) modes Juodėnas et al. Light: Science & Applications  (2023) 12:251  Page 7 of 9 etched by an inductively coupled plasma (ICP) reactive ion etching (RIE) using Ar/SiCl4 gas mixture. The oxide aperture is created by selective wet oxidization at 420 °C. The targeted aperture size is 2 µm to achieve single-mode lasing. After oxidation, a Ge/Ni/Au bottom contact is evaporated and annealed to create an ohmic contact to the n-doped bottom mirror. The VCSEL chip is passi- vated by a 350 nm thick layer of SiNx which is dry etched to open connections to large, sputtered Ti/Au pads for electrical injection. Finally, 250 nm of SiNx is sputtered to protect the VCSELs during the following metasurface fabrication. More details can be found in the supple- mentary information. Metasurface fabrication Either semi-insulating GaAs substrates or VCSEL chips in the bottom-emitting conﬁguration were used to fabri- cate monolithic metagratings. SiO2 layer was sputtered on the substrate to serve as an etching mask. Optimized patterns were exposed in positive ArP 6200.13 e-beam resist. After development, Ni was evaporated and lifted- off. Patterns were then etched into SiO2 in CF4/O2 plasma. Finally, GaAs was etched using a switching pro- cess, alternating between SiCl4 and O2 plasma processing steps. Since there is no etch-stop layer, the etching depth was controlled by timing and stopping the process manually. More details can be found in the supplementary information. Optical setup The fabricated metasurfaces on GaAs substrates and integrated on VCSEL chips were characterized using a custom-built optical microscope equipped with 40× NA = 0.95 dry objective. GaAs substrates were illumi- nated using a loosely focused, linearly polarized 976 nm laser beam. A lens focused on the back-focal plane of the objective could be inserted for Fourier plane imaging. The relative efﬁciency of the metagratings was determined by capturing Fourier images, subtracting the background, integrating the pixel values in the corresponding areas of the Fourier plane, and referencing to the total sum of pixel values on the image. DF and TIR images of nanoparticles and cells were taken in the same setup using a 20×, NA = 0.5 and 60×, NA = 0.7 dry objectives respectively. Free space measurements The power in each diffraction order was captured using large-area silicon photodiode, Hamamatsu S2281-01. Three IPV measurements were performed for each device, one for each diffraction order of the VCSEL with the metagrating, by placing the photodiode at the appropriate angle for each diffraction order. The beam proﬁles were measured by illuminating a diffusive plate, Ophir WB-I SWIR. The image on the diffusive plate was then focused and captured by a CCD camera, Ophir SP932U. Acknowledgements This work was funded by the Knut and Alice Wallenberg Foundation and performed in part at Myfab Chalmers. The authors acknowledge Emelie Tornéus for assistance preparing the cell samples and Ruggero Verre for assistance with the nanofabrication. Cell samples were a kind gift from Caroline Adiels. Nanorods were synthesized by Lei Shao. Author contributions M.J. proposed, optimized, fabricated, and characterized the metagratings, designed and carried out the dark ﬁeld and total internal reﬂectance experiments, analyzed the metagrating and microscopy data; E.S. fabricated and characterized the VCSELs without the integrated metagratings and analyzed the VCSEL data; M.J. and E.S. characterized the VCSELs with the integrated metagratings and co-wrote the initial draft; A.G. supervised the VCSEL fabrication; A.G., J.G., and A.L. co-designed the VCSEL architecture; H.Š.-J. assisted with cell microscopy experiments; M.K. supervised the project, proposed the microscopy applications and edited the paper. All authors commented on and revised the ﬁnal version of the draft. Funding Open access funding provided by Chalmers University of Technology. Data availability The data supporting the ﬁndings of this study are available from the corresponding author on reasonable request. Conﬂict of interest The authors declare no competing interests. Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41377-023-01286-0. Received: 26 May 2023 Revised: 14 September 2023 Accepted: 14 September 2023 References 1. Yu, N. et al. Light Propagation with Phase Discontinuities: Generalized Laws of Reﬂection and Refraction. Science 334, 333–337 (2011). 2. Bomzon, Z. et al. E. Space-variant Pancharatnam–Berry phase optical elements with computer-generated subwavelength gratings. Opt. Lett. 27, 1141–1143 (2002). 3. Martins, R. J. et al. Metasurface-enhanced light detection and ranging tech- nology. Nat. Commun. 13, 5724 (2022). 4. Rubin, N. A. et al. Matrix Fourier optics enables a compact full-Stokes polar- ization camera. Science 365, eaax1839 (2019). 5. Zheng, Y. Q. et al. Enriching Metasurface Functionalities by Fully Employing the Inter-Meta-Atom Degrees of Freedom for Double-Key-Secured Encryption. Adv. Mater. 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Light: Science & Applications  (2023) 12:251  Page 9 of 9
Title: High-angle deflection of metagrating-integrated laser emission for high-contrast microscopy ​
Authors: Mindaugas Juodėnas, Erik Strandberg, Alexander Grabowski, Johan Gustavsson, Hana Šípová-Jungová, Anders Larsson, Mikael Käll ​
Publisher:  Science & Applications, Official journal of the CIOMP 
Date: 2023-10-13 00:00:00
Abstract: Flat metaoptics components are looking to replace classical optics elements and could lead to extremely compact biophotonics devices if integrated with on-chip light sources and detectors. ​ However, using metasurfaces to shape light into wide angular range wavefronts with high efficiency, as is typically required in high-contrast microscopy applications, remains a challenge. ​ Here we demonstrate curved GaAs metagratings integrated on vertical-cavity surface-emitting lasers (VCSELs) that enable on-chip illumination in total internal reflection and dark field microscopy. ​ Based on an unconventional design that circumvents the aspect ratio dependent etching problems in monolithic integration, we demonstrate off-axis emission centred at 60° in air and 63° in glass with > 90% and > 70% relative deflection efficiency, respectively. ​ The resulting laser beam is collimated out-of-plane but maintains Gaussian divergence in-plane, resulting in a long and narrow illumination area. ​ We show that metagrating-integrated VCSELs of different kinds can be combined to enable rapid switching between dark-field and total internal reflection illumination. ​ Our approach provides a versatile illumination solution for high-contrast imaging that is compatible with conventional microscopy setups and can be integrated with biophotonics devices, such as portable microscopy, NIR-II range bioimaging, and lab-on-a-chip devices. ​