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2nd Conference on Centro Fermi’s Projects, Rome 19-20 April 2012
OPTICAL MICRORESONATORS &
BIOPHOTONIC SENSORS PROJECT
Simone Berneschi
Centro Fermi Grants
CNR, Institute of Applied Physics “Nello Carrara”
Project Coordinator:
Stefano Pelli
CNR, Institute of Applied Physics “Nello Carrara”
OUTLINE
•
•
•
•
Motivations;
Objectives;
WGM microresonators: a brief overview;
Applications & Results;
NL effects;
biosensors;
• Conclusions
MOTIVATIONS
« …smaller objects in nature are not just
scaled replicas of similar big objects and
in fact they have improved properties…»
Galileo «Dialogue Concerning Two New Sciences» (1638)
• Light – matter interaction
increases in the presence
of small objects;
• High Q microcavities, with strong spatial
localization of the field, well respond to this
principle and receive an even greater interest
in many fundamental processes in photonics
(e.g.: QED & NL processes; biosensing….)
OBJECTIVES
Investigating Whispering Gallery Modes (WGMs)
microcavities for:
• Developing highly sensitive, label free
biosensors (early diagnosis); (microsphere/microbubble)
• Developing all-optical switch by means of NL
polymeric coating; (microsphere)
• Studing possible integration solutions with
optical planar devices. (millidisk)
WGMs RESONATORS
• The Whispering Gallery phenomenon was initially described by
Lord Rayleigh based on observations in St. Paul’s Cathedral in
London;
• a whisper spoken close to the wall can be heard all the way
along the gallery, 42 m to the other side, thus the term
“whispering gallery”
Lord Rayleigh (1842 – 1919)
Whispering Gallery under the cupola of the
St. Paul’s Cathedral in London
L. Rayleigh, “The Problem of the Whispering Gallery,” Philosophical Magazine 20, 1001–1004 (1910).
WGMs RESONATORS
•
•
•
Microspheres
Microbubble
Microdisks
• light can be resonantly guided by total
internal reflection, along an equatorial plane,
with long cavity lifetime and strong spatial
confinement;
Maxwell + boundary conditions:
Field radial component
for the fundamental
mode
Evanescent
field tail
Field polar component for the
fundamental mode (spherical
Legendre function)
Field azimuthal
component
(periodical function)
WGMs RESONATORS
Efficient and robust coupling of the light to the cavity requires:
phase matching and mode overlap!
Approaches for efficient evanescent coupling of light into the
microspheres:
Prism
Tapered fiber
Surface waveguide
Hybrid fiber-prism
WGMs RESONATORS
Q factor measurement: experimental setup
• From WGM spectral linewidth dν
• Q=ν/dν
camera
Vis. LD
Tunable
LD
dn=300 KHz
Dn=1.5 GHz
camera
Monitor
Mux
Detector
piezo
Modulator
2 (Energy stored into the cavity)
Q
Energy loss/cycle
Scope
dn
WGMs RESONATORS
SiO2 microspheres by fusion splicing
electrodes
Fiber Tip
• A cleaved tip of the fiber is inserted
between two metal electrodes;
• Arc discharges partially melts the
fiber tip;
D = 2R = 150 – 350 μm
depending on the
number of shots
• Surface tension forces produce the
spherical shape.
WGMs RESONATORS
Crystalline microdisk by polishing
• Partial melting + surface tension effect cannot be applied to
crystals.
• Polishing procedure by using a home-made lapping station. The
almost spherical profile of the edges is obtained through a
rotational stage whose pivot point can be finely adjusted.
Polishing protocol:
• Grinding phase steps
(abrasive disk);
• Fine polishing phase
(diamond suspensions);
WGMs RESONATORS
CRYSTALLINE MICRODISK INTEGRATION
Transmission (a.u.)
3.4
3.2
3.0
n=1.5 MHz,
8
Q=1.3 x 10
Q = 1.3  108
2.8
320
360
400
Detuning (MHz)
The system is all in guided
integrated optics architecture
(LiNbO3) !
G.Nunzi Conti et al., Opt. Express, 19, 3651 (2011)
APPLICATIONS
NL EFFECTS IN COATED
MICROSPHERES
pump
PUMP-PROBE Configuration:
All-optical switching for a
probe signal Iprobe by a
resonant pump beam Ipump
which change the coating
refractive index and hence
the resonance position.
probe
Motivation: optical switch based on electronic Kerr effect
(n = n0 + n2 I) on spherical WGMR coated by a nonlinear medium;
Large resonance shift obtained on low time scales (10-12 s) using
intensities well below the damage thresholds of the polymers.
NL EFFECTS IN COATED
MICROSPHERES
Coated microspheres
Dipping
Wet layer
formation
Polymer: liquid crystal polyfluorene
Solvent
evaporation
(λpeak = 379 nm; n2  Re ((3)) = 2  10-10 cm2/W; β  Im ((3)) = = 2  10-7 cm/W)
Solution: 0.1 mg/ml of polymer in toluene
NL EFFECTS IN COATED
MICROSPHERES
Q factor from spectral linewidth
Uncoated microsphere
Coated microsphere
Transmittance [a.u.]
5.0
4.5
4.0
3.5
3.0
2500
2750
3000
3250
Detuning [MHz]
3500
3750
Q = 1.5  108
Q = 5  106
(@ 1550 nm)
Coating thickness < 100 nm
NL EFFECTS IN COATED
MICROSPHERES
An optically induced shift of WGM of up to 250 MHz is obtained in the CW
pump regime, which is nearly an order of magnitude smaller as compared to
the pulsed probe regime.
Such a difference of the values of the shift induced optically by the power of
the pump radiation is an indicator of the nonlinear-optical mechanism of the
shift.
S. Soria et al. Opt. Express (2011)
SiO2 MICROSPHERES
AS OPTICAL BIOSENSORS
WGMs are morphological dependent:
any change in its surrounding environment (i.e. refractive index) or on its
surface (due to some chemical and/or biochemical bonding) causes a
shift of the resonances and reduces the Q factor value.
By measuring this shift, it is possible to obtain the refractive index
change and/or the concentration of the analyte.
R’
R
Wavelength
Sweep
Generator
DFB
Laser
•
Power
Detector
res
from the resonance condition:
res

 r n

r
n
SiO2 MICROSPHERES FOR
PROTEIN APTASENSORS
Aptamers: are RNA or DNA molecules (ca. 30 to 100 nucleotides)
that recognize specific ligands and that are selected in vitro from
vast populations of random sequences [so named in 1990 by
Ellington and Szostak].
They exhibit:
- comparabile affinity and specificity
- more reproducibility and higher stability
- reversible denaturation and ease of modification
SiO2 MICROSPHERES FOR
PROTEIN APTASENSORS
Functionalization procedure
a)
b)
c)
d)
Activation
Silanization
Thrombin Binding Aptamer (TBA) immobilization
Passivation (mercapto-ethanol 1mM 1h)
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
100 m
Piranha treatment:
H2SO4: H2O2
4:1
for 3 minutes
a)
Mercaptopropyltrimethoxy silane
1% v/v toluene
for 10 minutes at 60°C
b)
Dithiol-TBA
5'-GGTTGGTGTGGTTGG- 3'
10M in carbonate
buffer 0.5M pH9
for 2 hours at 60 rpm
c)
SiO2 MICROSPHERES FOR
PROTEIN APTASENSORS
Q factor measurement
(@ 773 nm)
bare microsphere
Q = 4.0  107
silanized microsphere
Q = 4.0  106
(in aqueous environment)
Thrombin binding microsphere
Q = 8.0 
105
(in buffer solution)
Uniform distribution of the film
Coating thickness < 100 nm
L. Pasquardini et al. , J. of Biophotonics (2012)
SiO2 MICROSPHERES FOR
PROTEIN APTASENSORS
Set – Up measurement
Detected Proteins:
Thrombin: coagulation factor
It involves many pathological
diseases like:
Aatherosclerosis;
marker for some cancer;
VEGF (Vascular Endothelial
Growth Factor): regolator
for angiogenesis;
SiO2 MICROSPHERES FOR
PROTEIN APTASENSORS
Binding measurements showed that derivatized glass
microspheres can act as efficienta) aptasensors in complex
matrices: buffer and no filtered human serum.
VEGF165 concentration of 0,3mg/ml in buffer
6
stability
VEGF165
Frequency shift [GHz]
5
b)
4
3
2
1
0
0
200
400
600
800
1000
1200
Time [s]
Measure conditions:
Thrombin concentration of 0,3mg/ml in non
filtered 10% diluted human serum
L. Pasquardini et al. , J. of Biophotonics (2012)
1400
FROM MICROSPHERES TO
MICROBUBBLES (MB)
The optical microcavity (microsphere) &
coupling
system (taper
fiber)
are
immersed in a liquid medium (fluidic cell)
Problems: possible instability on the
resonance position due to the induced
perturbations by the liquid environment on
the coupling system.
No integrated solution.
The fluidics is integrated inside the device
(microbubble)
& coupling system
(tapered fiber) is external to the fluidics
Advantage: Possibility to test liquid or
gas flows inside the microbubble without
disturb the microfiber alignment.
Integrated solution.
Systems based on
Bulk Microresonators
Modulator
Laser
Systems based on
Hollow Microresonators
Modulator
Laser
WHAT IS AN OPTICAL MB:
THE BASIC IDEA
Antoine De Saint-Exupéry
Le Petit Prince (“The Little Prince”) - 1945
M. Sumetsky et al., Opt. Lett. 35, p. 898 (2010)
Similarly to the snake which has swallowed an elephant, an optical microbubble is a
resonant microcavity structure, obtained starting from a microcapillary preform (the
snake in the corresponding picture) by means of a particular fabrication process
which locally increases the radial dimension of the hollow microtube (the elephant)
along the axial direction.
OPTICAL MB FABRICATION:
A NEW PROCEDURE
Modified Fusion Splicer
The electrodes were moved outside the splicer and
placed in a U shaped holder able to rotate by 360°
by means of a step by step motor.
A pair of electrical wires connects the electrodes to
the splicer
Uniform heating of the pressurized
capillary is obtained by rotation of the U
shaped holder around the capillary.
OPTICAL MICROBUBBLE
RESONATORS
Q factor measurement
Contact - Critical coupling condition
Postnova
Z-DI
160481
UFE
capillary
Outer Capillary
Diameter (µm)
280
122
Capillary Wall
Thickness (µm)
20
21
MBR Outer
Diameter (µm)
380
240
MBR Wall
Thickness (µm)
4
6
Parameters
S. Berneschi et al., Opt. Lett. (2011)
Postnova
Microbubble
No Contact – undercoupling condition
OPTICAL MICROBUBBLE:
REFRACTOMETRIC TEST
A peristaltic pump is connected to the microbubble
Postnova
Microbubble
Router = 190 μm
w = 4 μm
Sensibility:
0.5
nm/RIU
Different water – ethanol
solutions:
Detection Limit:
10 -6 RIU
(4:1, 4:2, 4:3) in volume
S. Berneschi et al., Opt. Lett. (2011)
CONCLUSIONS & PERSPECTIVES
• Possibility to obtain high Q WGM resonators in different
materials and with different fabrication process;
• Possibility to integrate optical WGMRs in planar structures
(LiNbO3 millidisk)
add-drop filters & optoelectronics
oscillators in RF systems;
• Demonstration
microspheres
of
all – optical switch
add-drop configuration;
by
NL
coated
• Demonstration of optical microsphere aptasensors for protein
detection (in human serum)
take the detection to the limit;
• Demonstration of optical microbubble resonators
possibility to use this structures for biosensing;
RELATED PROJECTS
&
COLLABORATIONS
Aramos Project
EDA
Optoelectronics Oscillators
CNRS, LAAS & Univ. de
Toulouse, France
Naomi
National Project
Biosensors (protein essays)
FBK
(Fondazione Bruno Kessler),
Trento; Ospedale di Careggi
(Firenze).
Short term mobility
program CNR
Collaboration with different
european Research Institutes
&
Universities
(Moscow,
Budapest, Trento,..)