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.
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.
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.
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.
By using virtual imaging through dielectric microspheres as well as through coverslips with embedded high-index microspheres, point-like nanoscale objects are imaged in two different configurations - without and with nanoplasmonic arrays. The nanoplasmonic arrays with various periods are used to provide structured illumination of fluorescent nanospheres and/or F-actin protein filaments. To study the nanoplasmonic contributions to the resolution of nanoscale structures, we investigated several combinations of spectral emission bands (blue, green fluorescent nanospheres) with various spectral positions of localized surface plasmonic resonances (LSPRs) in the Au and Al arrays located in close proximity to nanospheres. It is demonstrated that the resolution about λ/7, where λ is the emission wavelength, can be obtained under conditions of spectral overlap between the emission and LSPRs. We show that the microsphere-based imaging has a solid immersion lens-limited resolution, however use of short-period nanoplasmonic arrays allows reaching ~λ/7 resolution. The mechanism of the observed superresolution effects is related to efficient coupling of the object’s emission to diffraction orders of the nanoplasmonic array, resembling the basic physics of localized plasmonic structured illumination microscopy and far-field superlens. However, we show that neither postimaging processing nor all-optical hardware after superlens such as additional layers of metal with surface gratings are required in our approach.
KEYWORDS: Super resolution microscopy, Biomedical optics, Super resolution, Proteins, Image resolution, Near field optics, Near field, Plasmonics, Nanoplasmonics, Lenses
The mechanisms of super-resolution imaging by contact microspherical or microcylindrical nanoscopy remain an enigmatic question since these lenses neither have an ability to amplify the near-fields like in the case of far-field superlens, nor they have a hyperbolic dispersion similar to hyperlenses. In this work, we present results along two lines. First, we performed numerical modeling of super-resolution properties of two-dimensional (2-D) circular lens in the limit of wavelength-scale diameters, λ ≤ D ≤ 2λ, and relatively high indices of refraction, n=2. Our preliminary results on imaging point dipoles indicate that the resolution is generally close to λ/4; however on resonance with whispering gallery modes it may be slightly higher. Second, experimentally, we used actin protein filaments for the resolution quantification in microspherical nanoscopy. The critical feature of our approach is based on using arrayed cladding layer with strong localized surface plasmon resonances. This layer is used for enhancing plasmonic near-field illumination of our objects. In combination with the magnification of virtual image, this technique resulted in the lateral resolution of actin protein filaments on the order of λ/7.
Recently, it was experimentally demonstrated (K.W. Allen et al., APL 108, 241108 (2016)) that microspheres can be used as contact microlenses to enhance the efficiency of collection of light by individual pixels in mid wave infrared (MWIR) focal plane arrays (FPAs). In this work, using finite difference time domain (FDTD) modeling, we optimized the designs of such FPAs integrated with microspheres for achieving maximal angle of view (AOV) as a function of the index of refraction and diameter of the spheres. We also designed structures where the spheres are partly immersed in a layer of photoresist. Our designs are developed for both front-side and back-side illuminated structures. Compared to standard microlens arrays, our designs provide much larger angle of view reaching ~15 degrees for front-illuminated and ~4 degrees for back-illuminated structures. Our designs allow decreasing the sizes of photosensitive mesas down to wavelength-scale dimensions determined by the minimal waists of the focused beams produced by the dielectric microspheres, so-called photonic jets. This opens a principle possibility to reduce the dark current and increase the operating temperature of MWIR FPAs. We also discuss the techniques of fabrication of such FPAs integrated with a large number of microspheres and show that suction assembly of microspheres is a promising method of obtaining massive-scale integration of microspheres onto the individual pixels with very small concentration of defects.
The transmission properties of side-coupled circular cavity systems are studied based on numerical two-dimensional finite-difference time domain modeling. The spatial asymmetry is introduced due to different separations between the circular resonators and side-coupled stripe waveguides. These structures can be viewed as 4-port routers where different ports are connected due resonant coupling between the guided modes in stripe-waveguides and whispering gallery modes in circle resonators. It is found that due to strongly asymmetric geometry, significant optical losses, and mode conversion processes, such structures display strongly asymmetric optical transmission properties for the waves propagating in forward and backward directions between the ports. In non-optimized single microcavity structures, it results in isolation ratios on the order of 10 dB for wavelengths resonant with WGMs. In structures formed by two closely spaced circular resonators, WGMs are strongly coupled leading to formation of bonding and antibonding photonic molecular modes. It is shown that at the wavelengths resonant with hybridized molecular modes the isolation ratios can be increased beyond 20 dB. At the same time, different wavelengths can be preferentially coupled to different ports resulting in wavelength demultiplexing functionality.
In recent years, optical super-resolution by microspheres and microfibers emerged as a new paradigm in nanoscale label-free and fluorescence imaging. However, the mechanisms of such imaging are still not completely understood and the resolution values are debated. In this work, the fundamental limits of super-resolution imaging by high-index barium-titanate microspheres and silica microfibers are studied using nanoplasmonic arrays made from Au and Al. A rigorous resolution analysis is developed based on the object’s convolution with the point-spread function that has width well below the conventional (~λ/2) diffraction limit, where λ is the illumination wavelength. A resolution of ~λ/6-λ/7 is demonstrated for imaging nanoplasmonic arrays by microspheres. Similar resolution was demonstrated for microfibers in the direction perpendicular to the fiber axis with hundreds of times larger field-of-view in comparison to microspheres. Using numerical solution of Maxwell’s equations, it is shown that extraordinary close point objects can be resolved in the far field, if they oscillate out of phase. Possible super-resolution using resonant excitation of whispering gallery modes is also studied.
Resonant light pressure effects provide new degrees of freedom for optical manipulation of microparticles. In particular, they can be used for optical sorting of photonic atoms with extraordinary uniform resonant properties. These atoms can be used as building blocks of structures and devices with engineered photonic dispersions. To study the spectral shape of the force peaks, we developed a method to precisely control the wavelength detuning between the tunable laser emission line and central position of the whispering gallery mode (WGM) peaks in tapered fiber-to-microsphere water-immersed couplers. Our method is achieved by integrating optical tweezers to individually manipulate microspheres and based on preliminary spectral characterization of WGM peak positions followed by setting a precise amount of laser wavelength detuning for optical propulsion experiments. We demonstrated dramatic enhancement of the optical forces exerted on 20 μm polystyrene spheres under resonant conditions. Spectral properties of the resonant force enhancement were studies with controlled laser line detuning. In addition, we observed the dynamics of radial trapping and longitudinal propelling process and analyzed their temporal properties. Our studies also demonstrated a stable radial trapping of microspheres near the surface of tapered fiber for high speed resonant optical propulsion along the fiber.
Vitreoretinal surgery is performed using mechanical dissection that sometimes results in iatrogenic complications, including vitreous hemorrhage, retinal breaks, incomplete membrane delamination, retinal distortion, microscopic damage, etc. An ultraprecise laser probe would be an ideal tool for cutting away pathologic membranes; however, the depth of surgery should be precisely controlled to protect the sensitive underlying retina. The ultraprecise surgical microprobe formed by chains of dielectric spheres for use with the erbium:YAG laser source (λ=2940 nm ), with extremely short optical penetration depth in tissue, was optimized. Numerical modeling demonstrated a potential advantage of five-sphere focusing chains of sapphire spheres with index n=1.71 for ablating the tissue with self-limited depth around 10 to 20 μm. Novel detachable microsphere scalpel tips formed by chains of 300 μm sapphire (or ruby) spheres were tested on ophthalmic tissues, ex vivo. Detachable scalpel tips could allow for reusability of expensive mid-infrared trunk fibers between procedures, and offer more surgical customization by interchanging various scalpel tip configurations. An innovative method for aiming beam integration into the microsphere scalpel to improve the illumination of the surgical site was also shown. Single Er:YAG pulses of 0.2 mJ and 75-μs duration produced ablation craters in cornea epithelium for one, three, and five sphere structures with the latter generating the smallest crater depth (10 μm) with the least amount of thermal damage depth (30 μm). Detachable microsphere laser scalpel tips may allow surgeons better precision and safety compared to mechanical scalpels when operating on delicate or sensitive areas like the retina.
Vitreoretinal surgery is performed using mechanical dissection that sometimes results in iatrogenic complications, including vitreous hemorrhage, retinal breaks, incomplete membrane delamination, retinal distortion, microscopic damage, etc. The laser probe would be an ideal tool for cutting away pathologic membranes, however the depth of surgery should be precisely controlled to protect the retina. In this study, we optimized the design of such ultraprecise surgical microprobe formed by chains of dielectric spheres assembled directly inside the cores of the mid-infrared flexible delivery systems used in such surgeries. Specifically, our design is optimized for use of Erbium:YAG laser sources with extremely short optical penetration depth in tissue. By using numerical modeling, we demonstrate a potential advantage of five-sphere focusing chains of sapphire or ruby spheres with index n=1.71 for ablating the tissue with self-limited depth around 10-20 μm. We fabricated and tested such optimized structures formed by 300 μm ruby spheres with ophthalmic tissues, ex vivo. Single Er:YAG pulses of 0.2 mJ and 75 μs duration produced ablation craters in cornea epithelium for one, three, and five sphere structures with the latter generating the smallest crater depth (10 μm) with the least amount of thermal damage depth (30 μm). We show that integration of the ultraprecise laser ablation capability with illumination and suction tools would produce a single headpiece with versatile functionality in ultraprecise intraocular surgery.
It is shown that microsphere-chain waveguides have strong polarization-dependent attenuation properties that can be used for developing passive filters of radially polarized beams. By using numerical modeling, it is demonstrated that the principle of operation of such devices is based on filtering periodically focused modes in chains of dielectric spheres occurring for a narrow range of indices of refraction 1.7<n<1.8. It is shown that for 10- and 20-sphere long chains a degree of radial polarization in excess of 0.8 and 0.9, respectively, can be obtained that allows developing novel polarization components with the light focusing capability.
We study super-resolution capability of liquid-immersed high refractive index (n~1.9–2.1) barium titanate glass
microspheres with diameters from several microns up to hundreds of microns. Imaging is provided in a conventional
upright microscope with the spheres placed in a contact position with various semiconductor and metallic
nanostructures. Using a commercial Blu-ray disk, we demonstrate an ability to discern 100 nm feature sizes which
cannot be resolved by conventional microscopy. Using silver nanowires with diameter about 100 nm, we
demonstrate ~1.7 times improvement in spatial resolution compared to conventional diffraction-limited far field
microscopy. Using two-dimensional nanoplasmonic arrays, we demonstrate high resolution imaging by using
objectives with surprisingly small numerical apertures. The last property is attractive for high-resolution imaging at
long working distances. This imaging technique can be used in biomedical microscopy, microfluidics, and
nanophotonics applications.
Ophthalmic surgery may benefit from use of more precise fiber delivery systems during laser surgery. Some current ophthalmic surgical techniques rely on tedious mechanical dissection of tissue layers. In this study, chains of sapphire microspheres integrated into a hollow waveguide distal tip are used for erbium:YAG laser ablation studies in contact mode with ophthalmic tissues, ex vivo. The laser’s short optical penetration depth combined with the small spot diameters achieved with this fiber probe may provide more precise tissue removal. One-, three-, and five-microsphere chain structures were characterized, resulting in FWHM diameters of 67, 32, and 30 μm in air, respectively, with beam profiles comparable to simulations. Single Er:YAG pulses of 0.1 mJ and 75-μs duration produced ablation craters with average diameters of 44, 30, and 17 μm and depths of 26, 10, and 8 μm, for one-, three-, and five-sphere structures, respectively. Microsphere chains produced spatial filtering of the multimode Er:YAG laser beam and fiber, providing spot diameters not otherwise available with conventional fiber systems. Because of the extremely shallow treatment depth, compact focused beam, and contact mode operation, this probe may have potential for use in dissecting epiretinal membranes and other ophthalmic tissues without damaging adjacent retinal tissue.
By using polystyrene microspheres with index n=1.59 as a model system we study light focusing and transport
properties of chains formed by spheres with diameters varying from 2 to 30 μm. We used techniques of imaging based
on light scattering perpendicular to the axis of the chain to visualize and study periodically focused beams in such
structures. The results demonstrate good agreement with geometrical optics modeling for sufficiently large spheres,
D>>10λ, where D is the sphere diameter and λ is the wavelength of light. For mesoscale structures with 4<D/>><10 we
observed two effects which cannot be explained by geometrical optics. One is a "beam tapering" effect which is stronger
in normalized units than it is theoretically possible in the limit of geometrical optics in short chains. Another effect is
reduced power attenuation in sufficiently long chains which is found to be smaller than it is possible in the limit of
geometrical optics. Both effects are ascribed to the increased role of physical optics properties. We can also suggest
some role of the microjoints developing between polystyrene microspheres in self-assembled structures. The results are
important for developing applications requiring focusing of multimodal beams such as mid-infrared laser surgery.
We study the propulsion of polystyrene microspheres along water immersed silica tapered fibers. We observed a nearly
linear increase of the propulsion velocity with the sphere diameter increasing from 3 to 20 μm. By measuring the fiber
transmission spectra we demonstrate efficient evanescent coupling of light to whispering gallery modes (WGMs) in large
(>10 μm) polystyrene spheres. For 20 μm spheres we observed the depth of resonant dips ~ 3.5 dB in combination with
the Q-factors ~ 103. Due to small losses in the fiber ~1-2 dB we are able to determine the power in the tapered region and
to characterize quantitatively the optical propelling forces. The maximum value of the propelling velocity was 260 μm/s
and was observed for 15 μm spheres with guided power of only 43 mW. Such velocities are nearly an order of magnitude
higher than those observed for similar powers on waveguide structures. Using simple physical arguments we show that
for spheres with diameters larger than 10 μm the experimentally observed velocities of propelling are too high to be
explained by the conventional nonresonant scattering forces. We propose that these high velocities indicate that the
optical forces are enhanced in such cases due to resonant coupling effects.
Photonic nanojet-induced modes (NIMs) have emerged as a new paradigm for understanding the optical properties of
chains of dielectric microspheres with wavelength-scale diameters (D≤10λ). Here we show that light focusing and
transport properties of chains of submillimeter spheres (D>100λ) are dominated by periodically focused modes (PFMs)
which can be considered a geometrical optics analog of NIMs. We review recent geometrical optics results on this
subject showing that Brewster angle conditions for TM polarized rays can be periodically reproduced in chains of
spheres with index n= √3 =1.73205... giving rise to lossless PFMs with the 2D period. In this work we studied the
phase properties of PFMs using a novel method based on Fourier analysis of the high resolution transmission spectra of
chains of submillimeter ruby spheres with n~1.75 at λ=1.2 μm. We demonstrated that PFMs are the best surviving modes
in long chains of spheres. Finally, we considered applications of PFMs for focusing multimodal beams in ultra-precise
laser surgery.
Ophthalmic surgery may benefit from the use of more precise fiber delivery systems for laser surgery. In this study,
chains of sapphire microspheres integrated into the distal tip of a hollow waveguide are used for preliminary mid-infrared,
Erbium:YAG laser ablation studies in contact mode with ophthalmic tissues, ex vivo. The combination of the
Er:YAG laser's short optical penetration depth and small spot diameters achieved with this novel fiber probe may
provide more precise tissue removal. One, three, and five microsphere chain structures were assembled and compared,
resulting in spot diameters of 67, 32, and 30 μm, respectively. Single laser pulses of 0.1 mJ energy and 75 μs duration
produced craters with average widths of 44, 30, and 17 μm and depths of 26, 10, and 8 μm, for one, three, and five
sphere structures, respectively. Chains of microspheres produced spatial filtering of the multimode Er:YAG laser
beam and fiber, thus providing spot diameters not otherwise available for precise tissue ablation using conventional
fiber delivery systems. With further probe development, this novel approach to mid-IR laser ablation may provide an
alternative to mechanical tools for ultra-precise surgical dissection and removal of ophthalmic tissues.
We investigate high-Q microsphere resonators with whispering gallery modes using a tapered optical microfiber
immersed in a liquid inside a microfluidic platform. The strength of the coupling between the cavity and the
microfiber taper is shown to depend on the contact position of the microsphere along the taper and on the
refractive index contrast between the microsphere and the liquid environment. We demonstrate that barium
titanate glass beads with index around 1.9 are promising candidates for developing sensor and optomechanical
applications of such resonator systems.
The effects of periodical focusing of light were studied in chains of sapphire microspheres with 300 μm diameters
assembled either on a substrate or inside capillary tubing. Dye-doped fluorescent microspheres were used as multimodal
sources of light in experimental studies. Significant reduction of the focused spot sizes was observed for chains of
spheres compared to a single sphere case. Numerical ray tracing simulations were performed for similar chains
assembled inside hollow waveguides to be used as an optical delivery system with mid-infrared lasers for ultra-precise
surgery. The device designs were optimized for contact conditions during laser surgery involving short optical
penetration depths of light in tissue. It is shown that chains of spheres with n around 1.65-1.75 provide a two-fold
improvement of the spatial resolution over single spheres. Potential applications of these microprobes include ultraprecise
laser procedures in the eye and brain or piercing a cell, and coupling of multimodal beams into photonic
microstructures.
KEYWORDS: Optical spheres, Signal attenuation, Refraction, Spherical lenses, Optical fibers, Tissue optics, Ray tracing, Photonics, Hollow waveguides, Near field optics
Integration of microspheres inside micro-capillaries or hollow waveguides may allow development of compact focusing
tools for a variety of biomedical and photonics applications. However, problems associated with developing focusing
microprobes involve the multimodal structure of noncollimated beams delivered by fibers and waveguides. By using
numerical ray tracing, it is shown that serial spherical microlenses filter out spatially periodic modes which can be used
for obtaining tightly focused beams. Experimental studies are performed for spheres with sizes from 10 to 300 μm with
different indices of refraction ranging from 1.47 to 1.9. The chains were assembled inside plastic tubing with bore sizes
matching the size of the spheres. By using high index spheres, it is demonstrated that these structures are capable of
focusing light in contact with tissue. The beam attenuation properties of such chains are found to be in good agreement
with numerical modeling results. Potential applications of integrated microsphere arrays include ultra-precise intraocular
and neurosurgical laser procedures, photoporation of cells, and coupling of light into photonic microstructures.
Strong coupling between whispering gallery modes (WGMs) is studied in polystyrene bispheres by using spatially
resolved spectroscopy. The supermonodispersive pairs of spheres (size deviation <0.03%) were selected using their
uncoupled WGM peaks' positions. By using novel geometries of capturing light by the imaging spectrometer we
observed clear spectral signatures of strong coupling between WGMs for bispheres with mean sizes from 2.9 to 10 μm.
In a special geometry with the collection of light along the axis of bisphere and with the slit of the spectrometer oriented
perpendicular to the substrate we observed unusual and characteristic kites in the spectral images of such photonic
molecules. We showed that such kites allow unambiguous relating of the split components (antibonding and bonding
modes) in the spectral image to their WGM eigenstates in the uncoupled cavities. In many cases such kites simplify the
interpretation of the dense spectral images of bispheres. Using various geometries of the experiments we quantified the
dependence of coupling constant on the sizes of spheres for maximally coupled fundamental modes located in the
equatorial planes of spheres on the substrate. The results show the feasibility of achieving a coherent WGM-based
optical transport in microsphere resonator circuits.
We report on the light transport phenomena in linear chains composed of several tens of touching spherical
microcavities. A new optical mode type, namely nanojet-induced modes (NIMs) is observed. These modes result from
the optical coupling of microspheres acting as a series of micro-lenses, which periodically focus propagating wave into
photonic nanojets. Theoretically, formation of periodic nanojets has been predicted in Z. Chen et al., Opt. Lett. 31, 389
(2006). The chains were produced by means of the self-assembly directed by micro-flows of water suspension of
polystyrene microspheres. The mean size of spheres was varied in the 2-10 micron range. To couple light to NIMs we
used built-in emission sources formed by several locally excited dye-doped microcavities from the same chain.
Conversion of modes emitted by the light source into the NIMs results in losses of several dB per sphere in the vicinity
(first few tens of spheres) of such sources. At longer distances we found an attenuation rate as small as 0.5 dB per sphere
that reveals low intrinsic propagation loss for NIMs. The NIMs have potential applications for coupling and guiding of
light in compact arrays of spherical cavities with extremely high quality (Q) whispering gallery modes.
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