It is shown that the resolution of portable and lightweight smartphone-based microscopy can be increased up to the diffraction limit by using imaging through ball lenses with refractive index sufficiently close to n=2 under contact conditions with nanoplasmonic and biomedical samples. It is demonstrated that the millimeter-scale ball lenses made from LASFN35 glass with n=2.02 at λ = 600 nm allow achieving extraordinarily high image magnification up to 50u. Under these conditions, it is possible to move away from the conventional resolution limitations of cellphone microscopy determined by the pixilation of the images and to reach the diffraction-limited resolution ~600 nm. It is shown that the dispersion of n allows tuning of magnification in a very broad range. The magnification values are explained by the exact numerical solution of the Maxwell equations and a good agreement with the experiment is demonstrated. We performed smartphone imaging of different biomedical samples such as melanoma and human aorta and demonstrated that the quality of imaging is comparable to that in conventional microscopy with the 10× objectives. The in-principle possibility is suggested of melanoma diagnostics based on observation of distribution of lymphocyte cells by application of cellphone microscope to the patient’s skin without a need to make histological samples.
Silicon has been commonly used in the microelectromechanical systems (MEMS) community for the past sixty years, and anisotropic wet etching with tetramethylammonium hydroxide (TMAH) has been central to these fabrication efforts. It has been shown anisotropic wet etching of silicon is an ideal method to rapidly fabricate components with huge production volume, and low cost. Although TMAH has been commonly used to create integrated circuits, the technology hasn’t been fully explored for optical applications. Four types of light concentrating arrays were fabricated with this technology in this work: (i) micropyramids with 54.7˚ sidewall angle, (ii) microcones with 45˚ sidewall angle, (iii) inverted square pyramids with 54.7˚ sidewall angle, and (iv) inverted triangular pyramids with different sidewall angles. The 54.7˚ sidewall angle arrays have smooth sidewalls due to etching with TMAH, which reduces scattering loss. The 3-D microphotonic structures were created through a comparable fabrication protocol, but require different photoresists, mask alignments, and if the surfactant Triton X-100 was utilized during the etch. The fabricated structures can be either heterogeneously integrated with front-illuminated focal plane arrays (FPAs) created in material systems with high quantum efficiency, or be monolithically integrated with metal/silicide Schottky barrier photodetectors for operation in short-wave infrared (SWIR) or mid-wave infrared (MWIR) regimes. The light concentrating capability of the proposed structures was tested by finite-difference time-domain modeling. Experimentally, the formation of photonic nanojets at the tips of micropyramids was demonstrated using illumination by a 2.96 μm wavelength Er:YAG laser.
One of the challenges of mid-wave infrared (MWIR) silicon (Si) photonics is related to the low absorption of Si-based photodetector focal plane arrays (FPAs), and therefore the reduced quantum yield. Another challenge is related to the significant thermal noise in uncooled FPAs which spoils the quality of imaging. It is proposed that the technology of Si anisotropic wet etching, capable of fabricating light concentrating arrays, can be used for solving these problems. The proposed designs are based on monolithic integration of Si micropyramids with metal/silicide Schottky barrier photodiodes (SBD). By using finite-difference time-domain (FDTD) modeling, it is shown that the photons can be spatially concentrated and resonantly trapped near the tips of the pyramids, allowing for multiple passes in the silicide layer and thus increasing the likeliness of photon absorption. This potentially leads to multispectral imaging functionality at the resonant frequencies. In addition, these resonances can be excited in a broad range of angles leading to MWIR FPAs with a wide angle-of-view. To demonstrate the proposed concept, micropyramidal arrays with three different geometrical parameters were fabricated and integrated with nickel/silicide (NiSi) SBDs. The choice of Ni was determined by the simplicity of short-wave IR (SWIR) testing at room temperature, but in the future, similar studies can be performed in the MWIR range by using Au or Pt. Preliminary testing results revealed a stronger photoresponse from micropyramids with smaller tops, but further studies are required to compare the performance of such novel photodetector arrays with an extensive range of geometrical parameters.
Collection of light in photodetector focal plane arrays (FPAs) can be enhanced by microlenses or metasurfaces. We propose an alternative approach based on using microconical waveguide arrays integrated with mid-wave infrared (MWIR) FPAs which allows increasing photon collection efficiency with large angle-of-view (AOV). The light incident on FPA is collected by the wider base of microconical waveguides with diameter (Dt) and delivered to their narrow base with diameter (Db) which is coupled to the photodetector mesa of FPA. A parameter to determine the light-concentrating ability is a power enhancement factor (PEF) defined as the ratio of the powers delivered to the same photodetector with and without the microconical waveguide. By using finite-difference time-domain modeling, the PEF and AOV parameters of the proposed structures are studied as a function of geometrical parameters of microcones. It is demonstrated that the maximal PEFs in excess of 100 require use of sufficiently elongated small-angle microcones with a wavelength-scale diameter of the narrow base. To demonstrate the light concentrating capability, slightly suboptimal microconical arrays with Dt/Db = 60 μm/8 μm and with 150 μm length of microcones were fabricated in photoresist by using a nanoscribe tool directly on top of the front-illuminated Ni/Si Schottky-barrier short-wave infrared photodetectors with 22 μm mesas, and three-time enhancement in the photocurrent response was observed. Due to expected reduction of the thermal noise for compact photodetector mesas, the proposed approach permits increase of the SNR and the operation temperature of the MWIR imaging devices.
Developing megapixel large-area CCD and CMOS sensor arrays in the 2000-s stimulated ideas about developing microscope systems operating without heavy and bulky microscope stands and objectives by using microoptics approach in combination with imaging by cellphone cameras. Due to limited magnification, however, the best resolution of such systems is currently limited by the finite size of the pixels at ~1.5 μm level. We propose a novel approach to designing such microscope systems based on using contact ball lenses with index of refraction close to 2, which are capable of imaging biomedical and nanoplasmonic objects with extraordinarily high magnification and resolution. By using ball lenses made from glass with index n = 2.02 at λ = 600 nm we build a cellphone camera-based microscope system with up to x50 magnification and resolution fundamentally limited at ~600 nm level due to diffraction of light. It is demonstrated that the operation of such system is a subject for strong dispersive effects in glass leading to a complicated tradeoff of magnification, resolution, and field-of-view (FOV) in the proximity to critical index of 2. Using this system, we performed imaging of melanoma samples which shows a potential of developing biopsy-free in vivo histology of skin using ball lensassisted smartphone microscopy.
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