In terms of performance, we have
measured Q-factors of the order of
2. 2 × 103 in scattering mode for a polystyrene microsphere inserted inside
a three-hole, collapsed core MOF. The
WGM signature spanned from the visible to near infrared. We also achieved
reverse coupling operation with this
Our other experiments include
stacking several identical microspheres
into the same MOF capillary to improve
Q-factor and investigating slow light
applications. We continue to look into
using high refractive index glass microspheres to reach greater Q-factor values,
while exploiting photorefractivity
effects of the microsphere for spectrally
tuning WGM combs.
Researchers have long been using micro- sphere optical resonators to localize
light by means of whispering gallery modes
(WGMs). 1 WGMs are a powerful photonic tool
for realizing ultrasensitive biological probes or
for slow light applications. 2, 3 However, efficient
light localization in spherical microresonators
requires a high-yield excitation scheme. This
is often achieved with light coupling through
evanescent waves emerging from optical fiber
tapers. WGM photonic devices based on such
excitation methods depend on strict alignment
conditions. It is also important to note that
the microsphere’s exposure to ambient atmosphere results in particle contamination, thus,
We have demonstrated a photonic platform
which offers improved excitation and interrogation capabilities for WGMs localized inside
microsphere resonators. 4 Our platform consists
of an optical microsphere integrated inside a
microstructured optical fiber (MOF) capillary,
utilizing dispersion into a liquid medium. By
making the diameter of the microsphere to be
only fractionally smaller than the size of the
MOF capillary, we can ensure that the optical
resonator is efficiently excited by the light
guided into the MOF core and that the microsphere remains trapped in the capillary.
In our design, the microsphere’s encapsulation inside the MOF increases resonator
integration and compactness, addresses
contamination effects, and readily allows
liquid infiltration. The MOF-encapsulated
microsphere resonator can be applied to a
number of liquid and gas sensors and light
switching devices. Notably, our resonator
design makes it possible for researchers to
integrate/stack microspheres of different
materials, surface affinities and emission
properties into a single MOF that can boost
photonic device functionality.
(a) Microsphere-MOF resonator system with light excitation and collection modes.
(b) SEM photos of an MOF infiltrated with 10 μm-diameter polystyrene microspheres. (c) Scattered spectrum from a 10 μm-diameter polystyrene microsphere
for excitation through the fiber endface. (d) Typical spectrum of a WGM located at
the visible wavelength band.
Gianluigi Zito and
Institute of Electronic
Structure and Laser,
Institute of Photonic Technology, Jena, Germany
1. K.J. Vahala. Nature
424, 839 (2003).
2. F. Vollmer et al. Proc.
Nat. Acad. Sci. 105,
3. K. Totsuka and M.
Tomita. J. Opt. Soc.
Am. B 23, 2194 (2006).
4. K. Kosma et al. Opt.
Lett. 38, 1301 (2013).
Integrating Microsphere Resonators
Inside Microstructured Optical Fibers
576 577 578 560 580 600 620 640
Wavelength (nm) Wavelength (nm)
Q = 2. 2 × 103
n = 2, l = 76
n = 1,
l = 82