Super-resolution imaging with patchy microspheres

Qingqing Shang,1 Fen Tang,2 Lingya Yu,3 Hamid Oubaha,4 Darwin Caina,4,5 Sorin
Melinte,4 Chao Zuo,6 Zengbo Wang,3,7 and Ran Ye2,6,8

1Key Laboratory for Opto-Electronic Technology of Jiangsu Province, Nanjing
Normal University, Nanjing, 21033, China

2School of Artificial Intelligence, Nanjing Normal University, Nanjing, 21033,
China

3School of Computer Science and Electronic Engineering, Bangor University,
Bangor, LL57 1UT, UK

4Institute of Information and Communication Technologies, Electronics and
Applied Mathematics, Universiteé catholique de Louvain, Louvain-la-Neuve,
Belgium, 1348

5Faculty of Engineering, Physical Sciences and Mathematics, Universidad
Central del Ecuador, Quito, 170402 Ecuador

6Smart Computational Imaging Laboratory (SCILab), School of Electronic and
Optical Engineering, Nanjing University of Science and Technology, Nanjing,
210094, China

<EMAIL_ADDRESS>
<EMAIL_ADDRESS>
Abstract: The diffraction limit is a fundamental barrier in optical
microscopy, which restricts the smallest resolvable feature size of a
microscopic system. Microsphere-based microscopy has proven to be a promosing
tool for challenging the diffraction limit. Nevertheless, the microspheres
have a low imaging contrast in the air, which hinders the application of this
technique. In this Letter, we demonstrate that this challenge can be
effectively overcome by using partially Ag-plated microspheres. The deposited
Ag film acts as an aperture stop that blocks a portion of the incident beam,
forming a photonic hook with oblique near-field illumination. Such a photonic
hook significantly enhanced imaging contrast, as experimentally verified by
imaging Blu-ray disc surface and silica particle arrays.

## Introduction

Optical microscopes (OMs) are one of the most important tools for scientific
research. Due to the Abbe diffraction limit, conventional OMs cannot resolve
two objects closer than 0.5$\lambda$/NA, where $\lambda$ is the incident
wavelength and NA the numerical aperture of the microscope. Therefore, an OM
equipped with a near-unity NA objective and a white light source ($\lambda$
$\sim$ 550-600 nm) has a resolution limit of 300 nm. Within this context, many
different methods have been proposed to overcome this limitation. In 2011,
Wang et al. demonstrated the 50 nm-resolution of dielectric microspheres with
white light illumination [1]. Since then, super-resolution imaging using
dielectric microspheres has attracted considerable scientific interest.
Microsphere-assisted imaging has the advantages of simple operation, no
fluorescent labelling and good compatibility with commercial OMs. To obtain
high-quality images, various parameters affecting the imaging performance of
microspheres have been studied, such as illumination conditions [2],
microsphere diameters [3], immersion mode [4, 5, 6] and immersion materials
[7, 8, 9].

Currently, most microsphere-assisted imaging methods use high-refractive-index
microspheres in a liquid environment [10, 11, 12, 13, 14, 15]. Nevertheless,
the imaging process is affected by the shape of the air-liquid interface as
well as the refractive index distribution inside the liquid. Moreover, samples
may be contaminated or damage in liquid. Only a few research has been done to
improve the imaging performance of microspheres in air, such as improving
illumination conditions [2], optimizing the diameter and the refractive index
of microspheres [16], using plano-convex-microsphere (PCM) lens design [17,
18] and using a microsphere lens group [19].

This Letter presents the performance of super-resolution imaging in the air
using patchy microspheres. To the best of our knowledge, this is the first
study showing that patchy microspheres are suitable for super-resolution
imaging. The patchy particles can generate a curved photonic jet, i.e.
photonic hook, due to the structural asymmetry [20, 21, 22], which is shown to
be useful in boosting imaging contrast and quality in this work. The results
will contribute to the further advancement of the microsphere-based optical
nanoscopy/microscopy techniques and facilitate their applications in
nanotechnology, life sciences, etc.

## Results and Discussion

Fig.1 (a) illustrates the schematic drawing of patchy microsphere fabrication
by glancing angle deposition (GAD) method [23]. BaTiO3 (BTG, 27 - 35 $\mu$m
diameter, Microspheres-Nanospheres, USA) were self-assembled into monolayers
by drop-casting of a small amount of BTG powders on a glass slide followed by
using water to compact them together. The microsphere arrays were then coated
with 100 nm thick Ag film by physical vapor deposition (PVD) (1 Å/s) at a
constant angle ($\alpha$ = 60∘). After deposition, a stream of deionized water
was used to transfer the microspheres from the glass slide to the observation
sample. We call the fabricated patchy BTG microsphere as p-BTG particle.

The p-BTG particles were observed with a commercial reflected light microscope
(Axio AX10, Carl Zeiss) for super-resolution imaging [Fig.1 (b)]. The
microspheres collected near-field nanoscale information from the Blu-ray Disk
(BD) surface and generated a magnified real image above the sample, which was
then captured by the objective lens. This is confirmed by the imaging plane
that is above the substrate, in contrast to virtual imaging whose image plane
is down into the substrate. The entire imaging process was performed in air. A
20 $\times$ objective (NA = 0.4, EC EPIPLAN, Carl Zeiss) were used for imaging
with the p-BTG particles. The system was illuminated by a white light source
(HAL 100, Carl Zeiss). All the experimental results were recorded using a
high-speed scientific complementary metal-oxide-semiconductor (CMOS) camera
(DFC295, Leica).

Fig.1 (c) shows the scanning electron microscopic (SEM) image of the top
surface of the BD sample in this study. It has a strip pattern with 300 nm
periodicity, including 200 nm track width and 100 nm gap between two adjacent
tracks. The pattern of the BD cannot be resolved by conventional OM method
with diffraction-limited resolution of $\lambda$/2NA = 550/0.8 = 687.5 nm
[Fig.1 (d)]. The p-BTG microspheres after depositing 100 nm Ag by GAD method
($\alpha$ = 60∘) are shown in Fig.1 (e). There are some elliptical shadows on
the left side of the microspheres [white arrow, Fig.1 (e)], because the
microspheres blocked the transportation of Ag vapor from the source to the
substrate during deposition. The corresponding SEM image of p-BTG arrays also
confirms the presence of Ag patches on the microspheres [Fig.1 (f)]. Fig.1 (g)
shows a p-BTG on a BD sample, in which both the Ag patch and the strip pattern
can be observed.

In this study, the imaging performance of BTG and p-BTG microlenses in air
were compared with each other. As shown in Fig.2 (a), the BTG in air formed a
magnified, low-contrast, real image of the strip pattern above the BD. The gap
between two neighboring tracks was 2.5 $\mu$m at the clearest image position,
corresponding to a magnification factor (M) $\sim$ 8.3 $\times$. Here, the
super-resolution image formed by BTG has a poor quality, with very low imaging
contrast, which is not sufficient for most of the practical applications. On
the contrast, as shown in Fig.2 (b), the new p-BTG microspheres generate
significantly improved super-resolution images, both in quality and contrast.
The imaging contrast has been boosted by a factor of $\sim$ 6.5 $\times$, as
shown in Fig.2 (c) by retrieved intensity profile along the white dash lines
in Fig. 2 (a) and (b). The measured magnification factor for p-BTG lens is
about 3.9 $\times$, which is smaller than that of unpatched particles (M = 8.3
$\times$). This is caused by different focusing characteristics of BTG and
p-BTG particles which will be discussed below.

Interestingly, as shown in Fig.2 (d), we observed two patchy textured pattern
in each p-BTG particle, but only one side of the microspheres was coated with
Ag films. The two paired patterns have a rotation angle of 180∘ between them
around the center of the microsphere. This phenomenon could be attributed to
the internal reflection occurring inside the high-index microspheres, in which
case the Ag film prevents part of the incident light from entering the
microsphere and causes a shadow with the same shape on the other side of the
microsphere after multiple internal reflections.

In another imaging test, silica particles with 230 nm mean diameter
(Nanorainbow Biotechnology, China) were self- assembled into arrays [Fig.2
(e)] [24] and observed through BTG and p-BTG particles in air. The silica
particle arrays were coated with 20 nm Ag before observation to enhance their
reflectivity. As shown in Figure 2 (f), (g), the p-BTG particle again
demonstrates a better imaging performance over the BTG particle when imaging a
sub-diffraction-limited nanoparticle array.

In our experiments, we also found that the p-BTG particles deposited on the
sample surface may have different appearance: bright, dark, or textured, and
different imaging performance, depending on the positions of Ag coatings. As
shown in Fig.3 (a), when the Ag film is at the bottom of the microsphere, it
is like a concave mirror that reflects the incident light backwards in a
convergent way, so that more light can be collected by the objective, leading
to a bright appearance. On the contrary, the p-BTG looks dark when the Ag film
is on top of the microsphere. As shown in Fig.3 (b), the Ag film acts like a
convex mirror that reflects the incident beam divergently at a large angle, so
that most of the reflected light cannot be captured by the objective. The
p-BTG lens shows a textured appearance and form a magnified real image when
the Ag film is on the side of the microsphere [Fig.3 (c)], in which case the
Ag film acts as an aperture stop, enhancing the contrast of the image and
forming a photonic hook inside the microsphere.

To understand the main focusing properties of the p-BTG microsphere lens,
computational modeling was performed using the two-dimensional (2D) finite-
difference-time-domain (FDTD) method (3D sphere modelling is not possible due
to limited computing resource). As shown in Fig.4 (a), (b), cylinders (D = 35
$\mu$m, n1 = 1.9) were created for FDTD simulation. The background medium was
set to air (n2 = 1). A plane light ($\lambda$ = 550 nm) propagating in the Y
direction forms a photonic jet inside the cylinder [Fig.4 (a)]. Fig.4 (b) is
the FDTD simulation result of the intensity field distribution in the vicinity
of a cylinder partially covered with a 100 nm thick Ag film (90∘ central
angle, $\theta$ = 10∘). We can see the formation of a ‘photonic hook’, with
the light path off-centered and curved due to the asymmetry property of the
incident beam caused by the Ag coating. In terms of imaging, the oblique near-
field illumination can help the lens to capture higher orders of diffraction
from the sample [25], which turned out to be very beneficial in boosting the
imaging quality and the imaging contrast in microsphere-based super-resolution
imaging. This enhancement mechanism can play an important role in developing
more advanced and reliable microsphere super-resolution imaging systems.

From Fig. 4 (b) we can also see the angled ‘photonic hook’ beam leads to a
larger object-to-focus distance (O). Since magnification M = I/O, where I is
image plane position and O the object plane position, increasing O will lead
to decreased M which can explain why the p-BTG lens produces smaller
magnification factor as in experiments.

To illustrate the position effect of Ag film on BTG particle focusing, we
varied the $\theta$ angle in Fig. 4(b) from 0∘ to 80∘, while keeping the film
coating opening angle $\beta$ = 90∘. As shown in Fig.4 (c), the photonic hook
phenomenon is maximized at $\theta$ = 0, which gradually decreases as $\theta$
increases up to 60∘. After this angle, the focusing doesn’t show a curved hook
focusing effect. In our experiments, the p-BTG particle with $\theta$ = 10∘ \-
45∘ degree is recommended for overall best performance which is a balanced
choice between magnification factor and imaging contrast enhancement.

## Conclusion

In conclusion, BTG microspheres with patchy coating on their surface can
provide a new strategy for improving the quality of super-resolution images
obtained with high-index microspheres in air. Due to the formation of photonic
hook illumination condition, the super-resolution imaging contrast can be
improved by more than 6-fold which significantly boost the overall imaging
quality. This method enables achieving high-quality super-resolution imaging
without the use of immersion liquid, such as water or oil, opening a new path
to developing more advanced and reliable nano-imaging systems based on
engineered microsphere lenses.

## Funding

Postgraduate Research & Practice Innovation Program of Jiangsu Province
No.1812000024501

## Disclosures

The authors declare no conflicts of interest.

## acknowledge

The support of Fonds européen de développement régional (FEDER) and the
Walloon region under the Operational Program “Wallonia-2020.EU” (project
CLEARPOWER) is gratefully acknowledged.

## References

  * [1] Z. Wang et al.  “Optical virtual imaging at 50 nm lateral resolution with a white-light nanoscope”  In _Nat. Commun._ 2.1 Nature Publishing Group, 2011, pp. 218  DOI: 10.1038/ncomms1211
  * [2] S. Perrin et al.  “Illumination conditions in microsphere-assisted microscopy”  In _J. Microsc._ 274.1, 2019, pp. 69–75  DOI: 10.1111/jmi.12781
  * [3] M. Guo, Y.-H. Ye, J. Hou and B. Du  “Size-dependent optical imaging properties of high-index immersed microsphere lens”  In _Appl. Phys. B-Lasers Opt._ 122 Springer Berlin Heidelberg, 2016, pp. 15  DOI: 10.1007/s00340-016-6335-x
  * [4] A. Darafsheh, G.. Walsh, L.. Negro and V.. Astratov  “Optical super-resolution by high-index liquid-immersed microspheres”  In _Appl. Phys. Lett._ 101, 2012, pp. 141128
  * [5] F. Wang et al.  “Microsphere-assisted super-resolution imaging with enlarged numerical aperture by semi-immersion”  In _Appl. Phys. Lett._ 112.2, 2018, pp. 023101  DOI: 10.1063/1.5011067
  * [6] Y. Zhou et al.  “Effects of immersion depth on super-resolution properties of index-different microsphere-assisted nanoimaging”  In _Appl. Phys. Express_ 11.3, 2018, pp. 032501  DOI: 10.7567/APEX.11.032501
  * [7] S. Lee et al.  “Immersed transparent microsphere magnifying sub-diffraction-limited objects”  In _Appl. Opt._ 52.30, 2013, pp. 7265–7270  DOI: 10.1364/AO.52.007265
  * [8] X. Hao et al.  “Microsphere based microscope with optical super-resolution capability”  In _Appl. Phys. Lett._ 99.20, 2011, pp. 10–13  DOI: 10.1063/1.3662010
  * [9] A. Darafsheh, C. Guardiola, J.. Finlay and A. Cárabe  “Super-resolution imaging by high-index microspheres embedded in an elastomer layer”  In _Opt. Lett._ 40.1, 2014, pp. 5–8  DOI: 10.1364/ntm.2015.nm2c.5
  * [10] A. Darafsheh  “The effect of the background medium on imaging performance of microsphere-assisted microscopy”  In _Opt. Lett._ 42.4, 2017, pp. 735–738  DOI: 10.1364/NTM.2017.NM3C.3
  * [11] R. Ye et al.  “Experimental far-field imaging properties of a $\sim$5-$\mu$m diameter spherical lens”  In _Opt. Lett._ 38.11, 2013, pp. 1829–1831  DOI: 10.1364/ol.38.001829
  * [12] B. Hou et al.  “Microsphere assisted super-resolution optical imaging of plasmonic interaction between gold nanoparticles”  In _Sci. Rep._ 7.1 Springer US, 2017, pp. 13789  DOI: 10.1038/s41598-017-14193-3
  * [13] H. Yang, N. Moullan, J. Auwerx and M..M. Gijs  “Super-resolution biological microscopy using virtual imaging by a microsphere nanoscope”  In _Small_ 10.9, 2014, pp. 1712–1718  DOI: 10.1002/smll.201302942
  * [14] G. Huszka, H. Yang and M..M. Gijs  “Microsphere-based super-resolution scanning optical microscope”  In _Opt. Express_ 25.13, 2017, pp. 86–90
  * [15] L. Li et al.  “Label-free super-resolution imaging of adenoviruses by submerged microsphere optical nanoscopy”  In _Light: Sci. Appl._ 2, 2013, pp. e104  DOI: 10.1038/lsa.2013.60
  * [16] S. Lee et al.  “Overcoming the diffraction limit induced by microsphere optical nanoscopy”  In _J. Opt._ 15.12, 2013, pp. 125710  DOI: 10.1088/2040-8978/15/12/125710
  * [17] B. Yan et al.  “Superlensing plano-convex-microsphere (PCM) lens for direct laser nano-marking and beyond”  In _Opt. Lett._ , 2020, pp. 1168–1171
  * [18] B. Yan et al.  “Unibody microscope objective tipped with a microsphere: design, fabrication, and application in subwavelength imaging”  In _Appl. Opt._ 59, 2020, pp. 2641–2648
  * [19] H. Luo et al.  “Enhanced high-quality super-resolution imaging in air using microsphere lens groups”  In _Opt. Lett._ 45, 2020, pp. 2981–2984
  * [20] Liang Yue et al.  “Photonic hook: a new curved light beam”  In _Opt. Lett._ 43, 2018, pp. 771–774  DOI: 10.1364/OL.43.000771
  * [21] I.. Minin et al.  “Experimental observation of a photonic hook”  In _Appl. Phys. Lett._ 114, 2019, pp. 031105  DOI: 10.1063/1.5065899
  * [22] G. Gu et al.  “Photonic hooks from Janus microcylinders”  In _Opt. Express_ 27, 2019, pp. 37771–37780
  * [23] A.. Pawar and I. Kretzschmar  “Patchy particles by glancing angle deposition”  In _Langmuir_ 24, 2008, pp. 355–358
  * [24] C. Fang et al.  “Fabrication of two-dimensional silica colloidal crystals via a gravity-assisted confined self-assembly method”  In _Colloid Interface Sci. Commun._ 37, 2020, pp. 100286
  * [25] C. Sanchez et al.  “Oblique illumination in microscopy: A quantitative evaluation”  In _Micron_ 105, 2018, pp. 47–54

Figure 1: (a) Fabrication of patchy BTG (p-BTG) particle by glancing angle
deposition method; (b) Experimental setup of super-resolution imaging with
p-BTG lens; (c) SEM image and (d) OM image of BD substrate (lines not
resolved); (e), (f) SEM images of p-BTG particle after Ag deposition; (g) SEM
image of p-BTG particle transferred on BD substrate. Figure 2: (a), (b) OM
images of the pattern on BD surface observed through (a) pristine BTG and (b)
p-BTG with the 20 $\times$ objective; (c) Optical intensity profiles across
the white dash lines; (d) OM image of the corresponding p-BTG; (e) SEM image
of the 230 nm-diameter SiO2 particle arrays; (f), (g) The SiO2 particle arrays
observed through (f) pristine BTG and (g) p-BTG microspheres. Figure 3: OM
images of p-BTG particles when the Ag film is (a) at bottom of the
microsphere, (b) on top of the microsphere and (c) on the side of the
microsphere. Figure 4: (a), (b) FDTD-simulated light field of (a) the pristine
BTG and (b) the p-BTG; (c) The influence of the position of Ag films on the
focusing of the p-BTG.