Subwavelength Optical Probing of Fluidic Microparticles

Abstract

[evdate 2007-05-01] President's Cup 2007 Final Report Subwavelength Optical Probing of Fluidic Microparticles

Subwavelength Optical Probing of Fluidic Microparticles Chan Wing Shan Lo Chi Fung Siu Kin Tung Supervisor: Prof. Andrew Poon Department of Electrical and Computer Engineering, Hong Kong University of Science and Technology Photonic jets with waists on the order of wavelength were generated by focusing 1550nm illumination with tapered optical fibers with different diameter. They were then applied to scanning fluid motions inside fabricated microfluidic channels. Intensity of the backscattering from the photonic jet was observed to be varying with fluidic motion. The generated photonic jet waists were measured with Shadow Image Measurement technique and Knife-edge Scanning Measurement technique. The narrowest jet measured was one-wavelength in waist, i.e. 1.5um with +/- 0.25um error, which was generated with fiber of 40um in diameter. For photonic jet generated with 125um optical fiber, the measured jet waist was 7um and its working distance was around 50um. With ultra-high resolving power and its compactness, the photonic jets can potentially enhance the performance of optical microscopes employed in various sensing applications, including the microfluidic bio-chips.

Introduction Microfluidic bio-chips are getting attention in the medical and biochemical industries for its potential as a speedy and compact bio-fluid analyzer. Having the size as small as a stamp, the bio-chips have fluidic channels with channel width less than 1 mm built on it. By inputting tiny amount of fluidic sample, built-in electro and magneto sensors together with the crossings of channels provide means for rapid analysis for scientists.

Since the fluids are flowing in miniature channels, the flow will be largely governed by surface tension and fluidic resistance. Some interesting properties such as zero velocity at the channel walls will be observed. Optical sensing of micro and nano-scaled particles flowing in such fluids is promising because of its high sensitivity. Yet at present optical imaging of microfluidic channels still relies heavily on bulky microscopes, contrasting the compactness of the biochip itself. In view of this, we propose the application of the novel photonic nanojet in microfluidic biochips.

The photonic nanojet is essentially a very narrow beam of light with a waist less than half the wavelength of the light, thereby breaking the theoretical diffraction limit in traditional geometrical optics. This idea was first demonstrated by a research group at Northwestern University by computer simulation [1] [2]. The simulation shows that a photonic jet can be generated at the shadow-side of a circular micro-particle illuminated by plane waves of light. When the microsphere is less than 10um in diameter, the nanojet will be formed (Fig. 1).

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The extremely narrow spot size of the nanojet has aroused interests of researchers because of its potential as a better imaging tool. The resolving power in traditional optical microscopes has always been limited by the diffraction limit, making it difficult to produce sharp images of small particles. Now with the nanojet, researchers have the chance to observe these particles in a more detailed way.

Circular dielectric Plane waves Photonic nanojet

Less than 10um Less than half-wavelength

Fig. 1 Schematic of the generation of nanojet

In 2006, another research group from Northwestern University scaled up the nanojet proposal from light waves to microwave and successfully demonstrated experimentally the existence of such nanojet [3]. When this microwave nanojet was used to scan across some particles, enhanced backscattering was observed. This further provides evidence for the existence of the nanojet phenomenon; however, the nanojet has never been successfully demonstrated in an optical dimension due to the difficulties in measuring the waist of such a narrow jet.

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Our group has been proving this nanojet phenomenon in an optical level and developing its application model for analysis in microfluidic bio-chips. In this report we will detail our experimental measurements of photonic jets and the proof of theory for scanning of microfluidic channels.

Scanning of Microfluidic Channels The strength of photonic nanojet is its subdiffractional accuracy in detecting tiny objects of interest. By integrating the nanojet into the microfluidic bio-chip, the bio-particles that are flowing through the focusing point of the nanojet would interact with the jet and would be imaged with ultra-high resolution. Here we propose 3 different ways of integrating nanojet onto the microchips. 1. A line of nanojet is produced across the microfluidic channel as laser strikes from top. The nanojet interacts with the bio-particles that are flowing through the channel, giving various backscattering patterns which are collected by a beam-splitter and a photo-detector for analysis purpose. This implementation will allow efficient 1-D localized scanning of the fluid (Fig. 2). Plane waves of laser Beam-splitter Backscattering light to photo-detector Fiber Bio-chip Channel

Fig. 2 1-D scanning of microfluidic channel

2. A line of nanojet is produced at the side of the microfluidic channel. The nanojet interacts with the particles that flow pass and form backscattering patterns. These patterns are collected by a beam-splitter and a photodetector for analysis. The backscattering of those particles which pass through the line is collected for analysis. This implementation will allow efficient I-D localized scanning of the fluid (Fig. 3). Bio-chip (top view) Backscattering light to camera Beam splitter Beam-splitter Plane waves of laser

Fig. 3 1-D Scanning on side of microfluidic channel

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3. Fig. 4 Subwavelength Optical Probing of Fluidic Microparticles

A spot of nanojet is produced at the spherical probe. The nanojet interacts with the particles that are at the spot to give scattering patterns. These patterns are collected by a photodetector for analysis. The probe can be moved around along the x-y plane easily as a 2-D scan to observe the area of interest (Fig. 4). Plane waves of laser Fiber Scattering to camera Bio-chip Channel

2-D probe scanning of microfluidic channel

Fabrication of Microfluidic Bio-chips Commercially available bio-chips usually have channels that have at least one-dimension of its cross-sectional area less than I mm. Capillary forces will principally govern the velocity of the fluids flowing in the channels; therefore the flow will exhibit special characteristics such as zero velocity at the channel walls. Fig. 5 Lab-on-a-Chip from Agilent [4] In order to prove our theory of applying nanojet on bio-chips, we had fabricated bio-chips using microscope glass plates. The height of the channels was 3mm and the widths were 125um, 400um and 600um. Using optical fiber and syringe needles as masks made these dimensions. Optical fibers with diameter of 125um were put on chips for generating photonic jets.

Department of Electronic & Computer Engineering President's Cup 2007 Final Report Width of 125um, 400um or 600 um 25mm Subwavelength Optical Probing of Fluidic Micropat1icles 75mm Width of 125um, 400um or 600um

Fig. 6 Fabricated glass-plates-made fluidic channel; (a) schematic of cross-section; (b) schematic of top view; (c) fabricated fluidic chip of 600um channel width; (d) fabricated fluidic chip of 400um channel width Fig. 7 Fiber on channel and syringe needle inside channel as viewed under microscope Controlling the flow of fluids inside the channels was proved to be particularly challenging. Commercially available bio-chips use built-in electric field or magnetic field to control the fluidic motion; however, these require complex nanofabrication processes. In our proof of theory, syringes were used instead to pump fluids into the channels. For chips with channel widths 400um and 600um, the syringe needle was put inside the channels to pump the fluids. (Fig. 7) For chips with channel width 125um, the fluid was injected from outside the chip into the channel. Experimental Setup The experimental setup for scanning microfluidic channel had a collimated 1550nm laser source reflected at a beam splitter and then focused onto the microfluidic chip. The backscattering was collected through the beam splitter to an imaging system (Fig. 8) (Fig.9).

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CCD camera Objective lens for imaging Beam splitter Plano-convex lens for focusing Fluidic channel Subwavelength Optical Probing of Fluidic Micropalticles Objective lens for collimation l550nm laser source Fig. 8 Schematic of experimental setup for scanning fluidic channel Imaging White light source Beam splitter Fluidic channel Collimation of 1550nm laser source

Hong Kong University of Science and Technology Department of Electronic & Computer Engineering President's Cup 2007 Final Report CCD camera Imaging lens Beam splitter Laser source x-y translation for fluidic chip CCD camera Imaging lens Beam splitter Focusing lens Fluidic channel x-y translation for fluidic channel Hong Kong University of Science and Technology (c) Subwavelength Optical Probing of Fluidic Microparticles y-z translation for CCD camera x-y-z translation for imaging lens x-y-z translation for focusing lens

Department of Electronic & Computer Engineering President's Cup 2007 Final Report Y translation for syringe pumping Syringe Subwavelength Optical Probing of Fluidic Microparticles Fluidic channel Fig. 9 Experimental setup for fluidic channel scanning Experimental Results The fluidic glass plate was observed with the setup. A white light source was used at the side to help locating the channel-fiber intersection. De-ionized water CDI water) was injected into the 600um-wide channel. As the DI water flowed through the channel, the backscattering from the 1550nm laser illumination markedly decreased in intensity. This was mainly due to the light absorption of the water, which was particularly strong around wavelength 1.4um. The DI water flowing away from the fiber was also reflected by the white light illumination. The water flowing motion was uncontrollably overflowed when the capillary force inside the channel was not enough to push the fluid forward. 125um-wide channel l25um Fiber Glue Hong Kong University of Science and Technology Department of Electronic & Computer Engineering President's Cup 2007 Final Report Glue Backscattering of 1550nm illumination Backscattering of 1550nm illumination

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Subwavelength Optical Probing of Fluidic Microparticles 600um-wide fluidic channel 125um optical fiber 600um-wide fluidic channel Syringe needle Fig. 10 Fiber on channel as observed with white light; (a) 125um-wide channel with l25um fiber under white light illumination (b) 1550nm laser illumination at 600llm-wide channel-fiber intersection; (c) l550nm laser illumination and syringe needle inside 600um-wide channel under white light illumination

Backscattering from 1550nm illumination Backscattering from 1550nm illumination Subwavelength Optical Probing of Fluidic Micropm1ic1es 600um-wide channel Reflected white light from flowing DI water 600um-wide channel Fig. 11 Injecting DI water into fluidic channel; (a) Backscattering from 1550nm illumination before injecting DI water; (b) Backscattering from 1550nm illumination after injecting DI water

Hong Kong University of Science and Technology Department of Electronic & Computer Engineering President's Cup 2007 Final Report Backscattering from 1550nm illumination Subwavelength Optical Probing of Fluidic Microparticles

Overflowed DI water 600um-wide channel Fig. 12 Overflowed 600um-wide channel Coke was then injected into the 400um-wide channel. The flowing motion was captured in serial images with time intervals of 20ms. As Coke flowed through the channel, the backscattering from the 1550nm laser illumination varied in intensity. This variation could be contributed to the carbonated bubbles inside Coke. The variation in backscattering intensity is as shown in Fig. 13. Experiment

Time (20ms) Fig. 13 Backscattering light intensity variation as Coke Zero passes through the photonic jet

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Generating Photonic Nanojets To generate nanojet, previous research groups have suggested striking plane waves at circular dielectric of diameter smaller than 10um, which are difficult to handle in experiment. Our group has instead been using single-mode optical fiber (diameter of 125um) as the dielectric, which is then tapered down in a stepwise manner to approach 10um in diameter as suggested by the computer simulation previously stated. The photonic jets generated by these fibers of different diameter were then measured using Shadow Image Measurement and Knife-edge Scanning Measurement, which are to be discussed in later sections. Tapering was done by taking advantage of the localized heating from fusion splicer which is originally used for fusing optical fibers. The sparks generated by strong electric fields inside the machine locally heat up a point on the fiber and then pull at the two ends of the fiber. By carefully repeating this process along the length of the fiber, we could get tapered fibers with various diameters.

Subwavelength Optical Probing of Fluidic Microparticles

Fig. 14 Fiber tapering process; (a) fiber put inside the fusion splicer was shown on display; (b) localized heating; (c) localized heating was done over the length of the fiber; (d), (e) curvature at the end of the tapering Hong Kong University of Science and Technology Department of Electronic & Computer Engineering President's Cup 2007 Final Report

Experimental Setup The experimental setup for generating the photonic jets had a collimated 1550nm laser source passing through an iris and then focused at a piece of optical fiber. The fiber was positioned inside the focal volume of the focused beam to make sure the light striking the fiber is well-collimated and most focused. The image at the shadow side of the fiber was then captured by an imaging system for the Shadow Image Measurement. A knife-edge was added right behind the fiber for the Knife-edge Scanning Measurement (Fig. 15). 1550nm laser Objective lens for collimation Iris Optical fiber Objective lens Knife-edge Focusing plano-convex lens CCD Camera

Fig. 15 Schematic of experimental setup for generating and measuring photonic jets

Shadow Image Scanning Measurement A shadow image of an illuminated fiber (diameter of 67um) is shown in Fig. 16. The patterns at the two sides are the side walls and the central line of light is Laser source collimation the focused photonic jet. Because the diameter of the fiber is known, by taking the ratio of the side walls to photonic jet we can estimate the waist of the photonic jet. Although the accuracy of this indirect measurement method was largely limited by the sensitivity of the CCD camera used, it can nevertheless give a quick and easy approximation of the real beam waist. The measured Full Width Half Maximum (FWHM) beam waists are as shown in Fig. 18, Fig. 19 and Fig. 20.

Subwavelength Optical Probing of Fluidic Microparticles

Fig. 16 Shadow image of fiber (diameter of 67um) Imaging System Focusing plano-convex lens Optical fiber Fig. 17 Nanojet system setup with Shadow Image Measurement

Hong Kong University of Science and Technology Department of Electronic & Computer Engineering President's Cup 2007 Final Report Subwavelength Optical Probing of Fluidic Microparticles

Fig, 18 Transmitted graph of line of photonic jet produced by striking optical fiber (diameter of 125um) with 1550nm illumination and its intensity profile

Fig.19 Pixel Transmitted graph of line of photonic jet produced by striking optical fiber (diameter of 67um) with 1550nm illumination and its intensity profile

Pixel Fig. 20 Transmitted graph of line of photonic jet produced by striking optical fiber (diameter of 40/lm) with 1550nm illumination and its intensity profile

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Knife-edge Scanning Measurement The knife-edge scanning technique is a direct measurement method to measure the size of a focused spot. A knife-edge was made to move across the spot in interest in a stepwise manner, blocking some of the light in every step (Fig. 21). The transmitted light intensity at every step was measured (Fig. 22). By differentiating the measured intensity as a function of scanning position, the light intensity profile can be obtained. Light spot Knife-edge Fig. 21 Schematic of knife-edge scanning of a light spot Fig. 22 Knife-edge Scanning of fiber (diameter of 125um) at 3um intervals

Experimental Setup An experimental setup for the knife-edge scanning was integrated to the existing Shadow Image measurement setup. A small silicon chip coated with 1000A. of gold was used as the knife-edge (Fig. 23a). The coating was used because silicon is transparent to the 1550 source. The coating thickness of 1000A was used because it was thick enough to block the light effectively but still able to give a reasonably sharp edge. A microscope was setup at the side to monitor the distance between the knife-edge and the optical fiber. Several fibers were firmly mounted on a C-clamp with wax, which was purposely aligned in a diagonal manner so that the switching between the delicate fibers of different diameters can be easily done. Hong Kong University of Science and Technology Subwavelength Optical Probing of Fluidic Microparticles (a) (b) Fig.23 (a) 1000A gold-coated silicon chip; (b) a fiber (diameter of 40um) mounted on C-clamp as viewed under microscope 1550nm laser Iris Optical fiber Objective lens for imaging Objective lens for collimation Knife-edge Focusing plano-convex lens CCD Camera

Fig. 24 Schematic of knife-edge scanning experimental setup

Department of Electronic & Computer Engineering President 's Cup 2007 Final Report Collimated laser source Tilting Focusing lens x-y translation stage Subwavelength Optical Probing of Fluidic Micropartic1es Microscope Imaging lens Optical fiber on C-mount Knife-edge Imaging lens Optical fibers Knife-edge Fig. 25 Knife-edge scanning experimental setup

Hong Kong University of Science and Technology Department of Electronic & Computer Engineering President's Cup 2007 Final Report Subwavelength Optical Probing of Fluidic Microparticles Fig. 26 Knife-edge scanning across an optical fiber as observed under microscope Experiment Results The knife-edge was carefully tilted such that it was parallel to the fiber and was moved across the fiber with steps of 0.5um. The step size and thus the accuracy of the measurement were limited by the translation stage knot which has smallest intervals of 0.5um. The knife-edge scanning measurement was done on photonic jets focused by optical fibers with diameter of 125um, 67um and 40um. Their FWHM spot sizes were then calculated from the graphs. Since the generated photonic jet will diverge in a Gaussian manner, the knife-edge had to scan across the jet at the narrowest position to get the real spot waist. This position is found by repeating the scanning measurement along the y direction near the fiber. The measurement with the smallest value is the waist of the photonic jet.

Raw Data Polynomial Fitting x-position (0.5um)

Plane waves Gaussian divergence Knife-edge scanning position x Fig. 27 Knife-edge scanning at different point of the beam The knife-edge scanning measurements of the photonic jets generated with fibers of different diameter are as shown in Fig. 28, Fig. 29 and Fig. 30. Calculated Intensity Profile 7.0um x-position (O.5um) Fig. 28 Transmitted intensity graph of knife-edge scanning of photonic jet produced by striking optical fiber (diameter of I 25flm) with 1550nm illumination and its calculated intensity profile Hong Kong University of Science and Technology Department of Electronic & Computer Engineering President's Cup 2007 Final Report Subwavelength Optical Probing of Fluidic Microparticles

Raw Data Polynomial Filling

x-position (O.5um) Calculated Intensity Profile 4.5um x-position (O.5um)

Fig. 29 Transmitted intensity graph of knife-edge scanning of photonic jet produced by striking optical fiber (diameter of 67um) with 1550nm illumination and its calculated intensity profile Raw Data Polynomial Filling

x-position (O.5um)

Calculated Intensity Profile 1.5um x-position (O,5um) Fig, 30 Transmitted intensity graph of knife-edge scanning of photonic jet produced by striking optical fiber (diameter of 40um) with 1550nm illumination and its calculated intensity profile Experiment Error Fiber diameter (um) Fig. 31 Knife-edge Scanning Measurement results and its errors

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Shadow Image Measurement Knife-edge Scanning Measurement Polynomial Fitting 1.5um Polynomial Fitting Fiber diameter (um) Fig. 32 Spot waist measured as a function of fiber diameter using Shadow Image Measurement and Knife-edge Scanning Measurement The measurement results here were compared to the ones from Shadow Image Measurement. As seen in the graph below, the two sets of results follow the same trend and approach the diffraction limit when the fiber diameter got narrower. The FWHM spot waist of the photonic jet decreased sharply as the fiber diameter approached 10um, i.e. the limit where the photonic jet can break the diffraction limit and have its waist get down to less than half the wavelength. However the trend was not linear and this contradicted the Rayleigh Criterion, which stated that the spot waist is inversely proportional to the lens' diameter. This can be explained by that the light focused by such a small lens (fiber) was affected by diffraction at the lens surface as its diameter approaches the wavelength of the light [5]. This non-linear trend was consistent with the simulations done by the group at Northwestern University. Measuring Working Distance of Photonic Jet The working distance of the photonic jet is defined by the Rayleigh range ZR. It is the distance which a Gaussian beam has its cross-sectional area doubled. The focal volume b of the beam is defined as 2 times the Rayleigh range. Within this range the beam remains well focused and collimated.

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Taking 7.0um as the diameter of the smallest spot, the Rayleigh range ZR can be calculated as followed: where n is the refractive index, W0 is the minimum Gaussian beam spot size, A. is the wavelength of the illumination. From the above equation,

Fig. 33 Gaussian beam and its Rayleigh range To verify this calculation, the FWHM spot sizes generated from fiber with 125um diameter at different fiber-knife-edge distances were measured.

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Raw Data Calculated Intensity Profile Polynomial Fitting x-position (0.5um) x-position (0.5um)

Raw Data 1.2 Calculated Intensity Profile Polynomial Fitting

x-position (O.5um) x-position (0.5um)

Fig. 34 Transmitted intensity graph of knife-edge scanning of photonic jet produced by striking optical fiber (diameter of 125um) with 1550nm illumination at different fiber-knife-edge distances (a) 40um; (b) 80um and their corresponding calculated intensity profile Taking these measurements, the Rayleigh range can be estimated in another way as the spot size is a function of distance z from the beam waist. From the above equation, The Rayleigh ranges calculated using the two methods were reasonably consistent with each other. The beam divergence is therefore following a Gaussian manner and the working distance of the photonic jet is around 25/lm.

Limitations of Knife-edge Scanning Measurement

The accuracy of the Knife-edge Scanning Measurement is largely affected by the alignment of the knife-edge and fiber. If the knife-edge is not vertically parallel to the fiber, the measurement would be skewed and the calculated value would be enlarged (Fig. 35) (Fig. 36).

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Plane waves Knife-edge motion

Fig. 35 Knife-edge making an angle with the fiber

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Raw data Polynomial Fitting

x-position (O.5um) Subwavelength Optical Probing of Fluidic Microparticles Calculated Intensity Profile x-position (0.5um)

Fig. 36 Skewed Knife-edge Scanning Measurement resulted from non-parallel fiber and knife-edge Tapered fiber Plane waves Knife-edge scanning motion Knife-edge position

Tapered fiber tip Fig. 37 Knife-edge scanning of tapered fiber; (a) Fiber-knife-edge distance limited by curvature of tapered fiber; (b) Knife-edge scanning across a tapered fiber tip

The alignment process became more difficult as the fiber diameter got smaller. The knife-edge had to get closer to the fiber but the length of the curved part of the tapered fiber limited its distance from the fiber surface. In view of this we had tapered the fiber until it is broken into two pieces. The lower tip of the knife-edge can be made to scan across the upper tip of the fiber. With this method the alignment job was made easier and the knife-edge can get close enough to the fiber (Fig. 37). Fig. 38 Integrating different part of the transmitted light intensity from fiber

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In calculating the FWHM, only a part of the fiber-transmitted intensity was integrated to give the graphs for measurement. However, the measurement results were found to be unaffected by the dimensions of the part of image being integrated. This accuracy can be attributed to the fact that the Knife-edge Scanning Measurement relies on the change in transmitted light intensity instead of the light intensity value itself.

Comparing the Two Measurement Methods The Knife-edge Scanning Measurement is a more accurate when compared to the Shadow Image Scanning Measurement because it is a direct measurement method. The Knife-edge Scanning Measurement does not depend on any other equipments but the knife-edge motion. The beam width is directly resulted from the spatial displacement of the knife-edge. On the contrary the Shadow Image Scanning Measurement has to depend on the resolution of the CCD camera used. The narrower the photonic jet, the less accurate the measurement is. Yet the Knife-edge Scanning Measurement becomes more difficult as the fiber diameter become smaller.

Department of Electronic & Computer Engineering President's Cup 2007 Final Report The waist of the generated photonic jet becomes smaller and the measurement is severely affected by the alignment of the knife-edge and the fiber. At the present stage, the narrowest fiber we had successfully measured is the 40um fiber. Further work will be done on measuring the photonic jets generated by narrower fibers . Integrating Photonic Jet on Microfluidic Channel The fluid motion in the fluidic channel was successfully observed as a variation in backscattering intensity of the photonic jet. Yet the channel depth (1mm) is much larger than the measured working distance of the photonic nanojet, which is on the order of a few microns. This makes it uncertain that whether the backscattering was due to the interaction of photonic jet with the particles or due to reflection from the channel walls. We can verify if the variation in backscattering is coming from the objects by having a control imaging system that synchronically monitor the fluid flow.

Besides our current implementation of using optical fiber as dielectric to generate photonic jets, we suggest another possible implementation using polystyrene beads with diameters on the order of micron and nanometer, which are now commercially available in both liquid form and dry form. A polystyrene bead can be hold at the tip of an optical fiber through electrostatic forces and light can be shined through the fiber to reach the polystyrene bead. The resulting focused photonic jet can be probed to wherever places to detect the micro-objects in concern (Fig. 39). This method allows more flexible probing of micro-objects and the photonic jet waist can be easily adjusted by using polystyrene beads of different diameter. Polystyrene beads of 1um in diameter were purchased and are to be tested in the near future (Fig. 40). Incoming light Polystyrene bead of micron- sized Optical fiber Optical fiber core Fiber motion Generated photonic jet Substrate for micro-objects Fig. 39 Suggested optical probing with light striking polystyrene bead Hong Kong University of Science and Technology Subwavelength Optical Probing of Fluidic Microparticles Fig. 40 Purchased liquid form polystyrene beads (diameter of 1um) from Polysciences, Inc. Conclusion Photonic jets with waists on the order of wavelength were generated with optical fibers of different diameters as micro-lenses. They were applied onto the scanning of fluidic motion in microfludic biochips. Variation in backscattering light intensity was observed as the fluid flows. The scanning of microfluidic motion and micro-objects flowing in the fluid can be further investigated by adding a synchronized imaging system as control. The photonic jet waists generated by fibers of different were measured using the Shadow Image Measurement and the Knife-edge Scanning Measurement. The jet waists for photonic jets generated by 125u m, 67u m and 40u m fibers are 7.0um, 4.5um and 1.5um respectively as determined by the Knife-edge Scanning Measurement, which is a direct measurement evidence for the existence of photonic jet of one-wavelength in size. The two measurement methods showed very similar trend of jet waists decreasing sharply as fiber diameters are narrowed down to tenths of microns. Further work can be done on generating photonic jets using narrower optical fibers to attempt breaking the theoretical diffraction limit.

References [1] XU Li, Zhigang Chen, Allen Taflove and Vadim Backman, 'Optical analysis of nanopm1icles via enhanced backscattering facilitated by 3-D photonic nanojets,' Opt. Exp. 13, No.2 (24 January, 2005). [2] Zhigang Chen, Allen Taflove and Vadim Backman, 'Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique', Northwestern University, Evanston, Illinois 60208, 5 April 2004 / Vol. 12, No. 7 / OPTICS EXPRESS 1214 [3] Alexander Heifetz, Kevin Huang, Alan V. Sahakian, Xu Li, Allen Taflove and Vadim Backman, 'Experimental confirmation of backscattering enhancement induced by a photonic jet', APPLIED PHYSICS LETTERS 89, 221118 (2006) [4] Agilent Technologies. http://www.agilent.com/ [5] D. A. Fletcher, K. E. Goodson and G S. Kino, 'Focusing in microlenses close to a wavelength in diameter', Opt. Lett. 26, 399-401 (2001) [6] Lee Yi Ho and Wong Wai Kuen, 'Photonic Nanojet Scanning Microscopy', Final Year Project Report, HKUST, 2004

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