WASHINGTON UNIVERSITY IN ST. LOUIS
Research Report
Laboratory of Micro/Nano Photonics Research Group
Samuel Wood
7/15/2013
Table of Contents
Executive Summary .......................................................................................... 3
Current Nano-Scale Detection Techniques ....................................................... 4
Background Information ...................................................................................6
Whispering Gallery Modes and Total Internal Reflection ..................................... 6
WGA Microresonators ............................................................................................ 6
Microresonator Types and Properties .................................................................... 7
Basic Experimental Layout ..................................................................................... 7
Microtoroid Fabrication ......................................................................................... 8
Laser Coupling and Tapered Optical Fiber ........................................................... 8
Past Research ................................................................................................... 10
Current and Future Research ...........................................................................14
Conclusion .......................................................................................................16
References .......................................................................................................17
1
List of Figures
1.
2.
3.
4.
5.
6.
7.
8.
9.
SEM .........................................................................................................4
TEM .........................................................................................................4
Microresonators ....................................................................................... 6
Microtoroid Fabrication Process ............................................................. 8
Basic Coupling Process ...........................................................................9
Coupling with Mode Splitting ............................................................... 10
Mode Transmission Results ..................................................................11
KCL Size Standard Deviation ............................................................... 11
Field Pattern for Two Scatterers ............................................................ 12
2
Executive Summary
Electron microscopy is currently the only accurate method for nanoparticle detection and
sizing. Unfortunately, it is not a valid option for field use because of its great size, sample
preparation and cost. There is a strong desire in many scientific fields for a more practical
method for nanoparticle detection. Potential applications for a new form of nanoparticle
detection would involve bio-defense, medicine and nanotechnology as well as many more.
The Washington University in St. Louis Laboratory of Micro/Nano Photonics Research
Group, run by Professor Lan Yang, is currently working on using WGM microresonators called
microtoroids to sense nanoparticles. The lab has already accomplished much in the way of using
these microtoroids and continues to conduct research to reach its goals.
Yang’s lab has already published work on single and multiple particle detection using
spherical nanoparticles and virions. All of the techniques that gave size estimations held up well
to both computer simulations and experimental testing.
Research being conducted now is focused on perfecting nanoparticle sensing by
increasing the detectable size range, expanding sensing to non-spherical nanoparticles, using
doped microtoroids for low or zero energy Raman lasing, and exploring bio-molecule detection.
3
Current Nano-Scale Detection Techniques
Currently, the only method for nanoparticle detection is
through the use of an electron microscope. An electron
microscope is a type of microscope that uses an electron beam to
attain extremely high resolution. The first electron microscope
was built by Ernst Ruska and Max Knoll in 1931 and was
improved upon in 1933 so it could attain a resolution higher than
any optical microscope. The most basic electron microscope has
Figure 2: Image of a SEM
the electron beam in a vacuum so that the electrons can easily travel the
distance needed. The body of the electron microscope evacuates the
air through pumps and the specimens are introduced using an air lock.
The magnification of an electron microscope is determined not by
fixed focus lenses but by the value of the current through the
intermediate and projector lens coils. The image is a result of the
scattering of electrons by atoms in the specimen.
Figure 1: Image of TEM
There are two basic types of electron microscopes, each with their own advantages and
disadvantages. The original electron microscope, the transmission electron microscope (TEM),
gets its image from a high voltage electron beam. The scanning electron microscope (SEM) uses
an electron beam to probe the specimen by scanning across a rectangular area and recording the
energy that is lost. The difference between SEMs and TEMs lies in their resolution and sample
capacity. SEMs produce lower resolution images but have the ability image bulk samples.
TEMs have high resolution but cannot image many samples at a time.
4
Unfortunately, electron microscopes are considered impractical for field use. They are so
large that they usually take up a full room and must be stored underground in a stable building
under special fields such as a magnetic field canceling system. The samples are processed
slowly and must undergo pretreatments like labeling with florescent dyes in order to be viewed.
There is a definite need for a nanoparticle detection technique that is quick, cheap, and portable
to allow for field measurements.
This convenient nanoparticle detection is possible through the use of micro scale sensors
called microresonators. They use the frequency of modes excited from a laser to sense attached
particles. The Washington University in St. Louis Laboratory of Micro/Nano Photonics
Research Group run by Professor Lan Yang is currently using silica microtoroids to develop an
effective technique using sensing that allows for the necessary detection and characterization of
nanoparticles and structures.
5
Background Information
Whispering Gallery Modes and Total Internal Reflection
Whispering Gallery Modes (WGMs) were discovered in the whispering gallery at St.
Paul’s Cathedral and were first explained by Lord Rayleigh in 1878. In the gallery, whispering
against the circular wall could be clearly heard at any other point on the wall, if a listener held
their ear to it. Rayleigh explained that because of the curvature of the wall the waves were
allowed to propagate along it and the sound’s intensity decayed directly proportionally to the
distance from the source. This is unlike waves in free space where the intensity decays
proportionally to the square of the distance from the source. WGMs can be summarized as
specific resonances of a wave field inside a smoothed edged resonator. They do not just apply to
sound waves; light can also act as a WGM. Light that forms a WGM in a resonator does so by
traveling along the resonator wall through total internal reflection. Total internal reflection is an
occurrence that happens when a wave hits a medium, with a lower refractive index, at a larger
angle than the critical angle of the medium.
WGA Microresonators
Optical WGM microresonators are of interest for a
variety of applications including sensing and lasing. When
light is coupled to a microresonator, the photons are trapped
Figure 3: a. microring, b. microdisk, c. microtoroid,
d. microsphere, e microchanel, f. microbottle
inside through total internal reflection and cycle around the
resonator until imperfections allow them to escape. This allows for the confinement of light in a
very small volume and leads to high light intensity inside the resonator, enabling enhanced
interactions between light and structures placed inside the cavity.
6
Microresonator Types and Properties
There are many different types of WGA microresonators shapes. Some resonator
geometries include microspheres, microdisks, microrings, microbottles, and microtoroids.
Microresonators can also be made out of a variety of different materials. Some materials that
can be used to build the structures are liquid droplets, silica, semiconductor and polymer. We
have chosen to go with microtoroids constructed with silica because of their ultra-high-quality
factor and their easy on-chip production. The quality factor (Q) is the ratio the energy stored in
the resonator over the energy dissipated per cycle around the resonator. The mode volume V is
the volume that the resonant mode occupies when the energy is equally distributed throughout
the mode volume at the peak value. Mode volume affects the intensity of the light in the
resonator. When a cavity is smaller the mode volume usually is smaller and thus there is a
higher light intensity.
Basic Experimental Layout
The basic layout of any testing that is done in our lab involves a tunable external cavity
continuous-wave laser, basic optical fiber cable, microtoroids, a photo detector, a function
generator and an oscilloscope. The optical cable is tapered so that the light from the laser will
couple with the toroid, inducing WGMs in the cavity of resonator. The function generator sends
a wave function to the laser for it to actuate through the optical fiber. The toroid is placed near
the tapered portion of the optical fiber and light is coupled with the toroid. The light returns to
the fiber and travels to a photo detector and the resonance frequency information to the
oscilloscope. The oscilloscope results can then be interpreted into usable data. Most of the
experiments that are done in the lab are accomplished with a variation of this basic setup.
7
Microtoroid Fabrication
Microtoroids are developed on silica
coated silicon chips in lab. The fabrication
process for these toroids is a fairly simple
Figure 4: Six step production of microtoroids.
chemical process. We start by coating the
silica chips with a material called Photo Resist (PR) using a spin-coater at 115 degrees Celsius
for 3 minutes. PR is a material that, after being exposed to UV light, will break down when
placed in a photo developer. After being coated, a mask is placed upon the chip that blocks UV
light in certain places. The chip with the mask is placed under a UV lamp for 35-40 seconds and
then it is run through the photo developer. The photo developer gets rid of the PR except for
where the mask blocked the UV rays. The places where PR remains will act like cookie cutters
for the microtoroids made on the chip. Next comes the hydrogen fluoride (HF) etching also
known as wet etching. The chips are placed in a highly concentrated HF solution and PR and the
silica that was not covered are etched away by the HF. The disks are then cleaned using acetone.
Now, xenon diflouride (XeF2) etching or wet etching is performed to create microdisks. The
XeF2 reacts with the silicon etching it away but not affecting the silica disks, leaving pillars of
silicon under the disks for support. Last, a high powered CO2 laser is used to reflow the silica
disks which cause them to collapse into microtoroids.
Laser Coupling and Tapered Optical Fiber
Coupling occurs when the laser is allowed to wrap into and out of the microresonator.
The light traveling into the resonator is trapped and forced to travel around many times until
imperfections allow it to escape. This coupling is accomplished by using a tapered optical fiber.
Optical fibers consist of three layers: a core, a cladding layer, and a buffer or protective layer.
8
The core, in this case, consists of silica and the cladding layer is a
material that has a higher refractive index than the core. This allows
light to travel through the optical fiber by total internal reflection. A
tapered optical fiber is one that has been stripped of its outer two
Figure 5: Basic coupling process with
microtoroid and tapered fiber.
layers. Tapered fibers are made in the lab through careful heating and
pulling of the original optical fiber. Light from a taper is allowed to travel into a microtoroid
because they are both made of silica and therefore have the same refractive index. This light is
then trapped by total internal reflection until imperfections let it escape and it travels back into
the taper by the same principle as before.
9
Past Research
Yang’s lab has been working with microtoroids for a long
time now and has done a lot of interesting research with them. One
recent project went into Rayleigh particle detection and sizing using
single and multiple scatter induced mode splitting. A Rayleigh particle
Figure 6: Coupling with mode splitting
caused by scattering.
has a radius that is much smaller than the wave length of the light that is used. Mode splitting is
a phenomenon where a single resonance mode splits into doublet as a result of coupling between
clockwise (CW) and counter-clockwise (CCW) modes. Mode splitting seen in the transmission
spectrum can be used to determine the size of a light scattering particle through the estimation of
the particles polarizability. Polarizability is the ability for a particle or molecule to become
polarized and is represented by the following equation.
Polarizability can be calculated by the ratio between the spectral distance and linewidth
of the two split modes or 2g and 2 Γ. Both can be estimated using the mode splitting observed
in the transmission spectrum using an oscilloscope.
With the polarizability equation is then used to calculate the particle radius:
Scattering occurs when a nanoparticle is in the evanescent field of WGMs. Some light
that scatters is lost to the environment but the rest couples into the oppositely propagating WGM
10
causing two standing- wave-modes (SWM) that are split in frequency from the superposition of
the CW and CCW modes. With a single particle, the symmetric mode (SM) locates the particle
at its anti-node and the asymmetric mode (ASM) locates it at its node.
The lab first ran some simulations using Comsol
Multiphysics that used a two dimensional resonator model
to obtain ideal results and to confirm that there predictions
would theoretically be possible. The simulations results
came out as expected with pairs Eigen modes representing
Figure 7: This graph shows simulated and actual mode
transmission results from single particle mode splitting
the standing wave modes from the mode splitting. The set up for sizing a particle involves the
regular experimental set up discussed in the background information but also a series of devices
that form and apply the nanoparticles to the toroid. The potassium chloride (KCL) and
polystyrene (PS) spheres 30-175 nm were used as the particles for this experiment. Before the
particles were applied, the transmission showed a single
Lorentzian resonance. When the first particle is shot onto
the toroid, the standing wave modes form and the
degeneracy lifts. More particles are applied and the
splitting distance and line width are both increased. With
enough data the statistics of splitting change could be
used to estimate the average size of the particles. The lab
used KCl and PS spheres of different sizes to approximate
the sizes. The results indicated that the methods were a good
Figure 8: Shows standard deviation in predicted size of KCL
particles compared to actual size.
approximation of the size. The KCl sizes were taken by
measuring over 100 particles using a scanning electron microscope and the PS particles were
11
sized according to the information given by the
manufacturer of the equipment. There are some limitations
to these sizing processes. For instance, if the nanoparticle is
very small the amount of splitting is less than the sum of the
frequency linewidth and the additional dampening rate
Figure 7: field pattern for two particle scattering
which makes it undetectable.
Sizing and detection of multiple particles is much trickier than for a single particle
because the resonance spectra cannot easily be interpreted. The simplest case of multiple
scatterers would be two Rayleigh particles. When one scatterer is on the resonator, the SM
places its anti-node at the location of the particle and the ASM places its node. However, when
two particles are on a resonator, it is much more complicated because the particles are usually no
longer at a node or anti-node. Back-scattering occurs and interferes with the SWMs which
makes it impossible to use the same coefficients to denote coupling in single scatterer sizing.
This problem is solved by altering rate equations so that the SWMs are distributed with one
having the maximum light path and the other having the minimum. This in turn maximizes the
coupling rate between the two counter-propagating modes which leads to a new equation for
polarizability:
The radius of the Nth particle can then be solved using the following equation:
12
Verification of this conclusion was completed through simulation. The simulation had
two fixed particle sizes and only changed the position of the second particle. The results of the
simulation matched closely to the predictions. After this actual control, tests took place with the
microtoroids. The experimental set up is normal except for the introduction of two nanoprobes
that produce higher electromagnetic fields for higher signal output. The results of this
experiment were supportive of the proposed outcome and the simulations. After SWM coupling
was accomplished with two Rayleigh particles, the lab jumped to using multiple particles.
According to Fermat’s principle, with each new scatterer that enters the toroid mode volume, the
WGMs redistribute themselves to maximize the mode splitting. The lab used this information to
come up with new equations for change in frequency and linewidth experienced and the
polarizability of the Nth scatterer. The size of a spherical nanoparticle then can be solved for
using the relation between the polarizability, refractive index and size of the particle. The lab
tests this by using purified and inactivated Influenza virions X-31 A/AICHI/68 purchased from
Charles River Laboratories, as well as gold and polystyrene spheres. Influenza A virions were
delivered to the toroid through the same nozzle used to deliver the inorganic particles. The
virions refractive index was assumed to be 1.48 as deduced from information from a previous
study. The calculated size of the virions ranged from 46-55nm within the normal range of an
Influenza A virion. Gold nanospheres of 50nm and 100nm and polystyrene nanospheres of
100nm and 135nm were used in testing. All the inorganic estimated particle ranges were close to
their actual size standard deviations.
13
Current and Future Research
There are a couple of promising routes that the lab is currently working on or looking to
work on in the near future with the microtoroids. One of them is using the microresonator to
generate high quality lasers using a gain medium that can be generated using very little power.
Gain is the measure of the ability of a system to increase the power of a signal from input to
output. Gain mediums are a source of optical gain within a laser. A general method for
incorporating gain medium into silica is by doping two rare-earth ions with either Erbium or
Ytterbium. Doping is just the introduction of impurities into an intrinsic object, in this case a
microresonator, to modify its properties. Raman scattering would allow for, if enough power is
pumped into a microtoroids, a Raman laser to be generated. Raman scattering is the inelastic
scattering of a photon. This just means a photon scattered by an excitation. These photons have
a lower frequency than those of normally scattered photons and can create a Raman laser through
stimulating Raman scattering which amplifies light. Detection using a microcavity laser is done
in a similar fashion to a passive microresonator, except that the lasers beatnote frequency is
monitored for indications of mode splitting. These lasers could be used to detect objects too
small to be detected by just passive resonator sensors because their detection limit is set by the
laser linewidth which is much smaller than the resonance linewidth of a passive resonator.
Another future endeavor for the lab will be developing techniques in bio-molecule
detection. The techniques discussed earlier provide an effective procedure for detecting
nanoparticles from 10nm to 175nm. This covers many important biological particles but further
improvements in technique are needed to accomplish detection and sizing of non-spherical
particles. Another problem with bio-molecule detection is that they are usually in an aqueous
environment. This requires either the microtoroids to be placed in water or the molecules to be
14
brought to a dry environment. Putting a microresonator in water requires a change in the size of
the toroid because of the higher refractive index of the medium which degrades the quality of the
resonator. Also, ultra pure water must be used along with purified and diluted samples. Also
due to different forces that are present in an aqueous environment the surface of the resonators
must be modified because of dramatically lower rates of particle binding. Drying methods can
be used but they will in turn often modify the particles. Unfortunately, measurements must also
be conducted in a vacuum. There is a long way to go before we have effective bio-molecule
detection using microresonators.
15
Conclusion
In conclusion, the research that has been and will be completed in the lab focuses on
improving nanoparticle sensing techniques using WGM microresonators called microtoroids.
These microtoroids can be easily mass produced on silica coated silicon chips and have ultrahigh quality factors, making them an ideal candidate for being used in future sensing techniques.
Our goals for the future include full nanoparticle sensing with a greater size range, bio-molecule
sensing technology practical for in field use, and low or solar power microcavity lasing using
doped microtoroids. The lab has already made quite a bit of progress in using microtoroids for
sensing, including detection and sizing of single and multiple spherical nanostructures.
Current
and future research include working on single and multiple particle detection of different particle
shapes other than spheres, improving the detectable size range of nanoparticles, developing ways
to detect particles in an aqueous environment, and using doping to create microcavity Raman
lasing for smaller particle detection. Current goals are a long way off from being realized but the
research being done is slowly advancing us towards these goals.
16
References
[1]
electron microscope (instrument). (n.d.). Encyclopedia Britannica. Retrieved July 15,
2013, from http://www.britannica.com/EBchecked/topic/183561/electron-microscope
[2]
environmental scanning electron microscope (ESEM) (instrument). (n.d.). Encyclopedia
Britannica. Retrieved July 15, 2013, from
http://www.britannica.com/EBchecked/topic/1434429/environmental-scanning-electronmicroscope-ESEM
[3]
Gordon, J. R., Teague, C., & Serway, R. A. (n.d.). College Physics Vol. 2. Cengage
Learning.
[4]
He, L. (2012). Whispering Gallery Mode Microresonators for Lasing and Single
Nanoparticle Detection (Ph.D.). Washington University in St. Louis, United States -Missouri. Retrieved from
http://search.proquest.com.libproxy.wustl.edu/pqdtlocal1005748/docview/1011649317/a
bstract/13F49C32891564FE6DB/3?accountid=15159
[5]
He, L., Özdemir, ?., Zhu, J., Kim, W., & Yang, L. (2011). Detecting single viruses and
nanoparticles using whispering gallery microlasers.Nature Nanotechnology, 6(7), 428432. doi:10.1038/nnano.2011.99
[6]
Hecht, J. (2000). Laser guidebook. New York: Mcgraw-Hill.
[7]
Jones, R., Liu, A., Cohen, O., Hak, D., Fang, A., & Paniccia, M. (2005). A continuouswave Raman silicon laser. Nature, 433(7027), 725–728. doi:10.1038/nature03346
[8]
Raman, C. V. (1928). A new radiation. Article. Retrieved July 15, 2013, from
http://dspace.rri.res.in:8080/jspui/handle/2289/2135
17
[9]
scanning electron microscope (SEM) (instrument). (n.d.). Encyclopedia Britannica.
Retrieved July 15, 2013, from
http://www.britannica.com/EBchecked/topic/526571/scanning-electron-microscope-SEM
[10]
transmission electron microscope (TEM) (instrument). (n.d.). Encyclopedia Britannica.
Retrieved July 15, 2013, from
http://www.britannica.com/EBchecked/topic/602949/transmission-electron-microscopeTEM
[11]
Wright, O. (2012). Gallery of whispers. Phys. World February, 31-36.
[12]
Zhu, J. (2011). Ultra-high-Q Microresonator with Applications towards Single
Nanoparticle Sensing (Ph.D.). Washington University in St. Louis, United States -Missouri. Retrieved from
http://search.proquest.com.libproxy.wustl.edu/pqdtlocal1005748/docview/916604245/abs
tract/13F49C32891564FE6DB/1?accountid=15159
[13]
Zhu, J., Özdemir, ?., & Yang, L. (2011). Sensors: Bypassing the diffusion limit. Nature
Photonics, 5(11), 653-654. doi:10.1038/nphoton.2011.263
18