Patent No. 6781690 Sensors employing nanoparticles and microcavities

Patent No. 6781690

Sensors employing nanoparticles and microcavities (Armstrong, et al., Aug 24, 2004)

Abstract

An optical sensing enhancing material (and corresponding method of making) comprising: a medium, the medium comprising a plurality of aggregated nanoparticles comprising fractals; and a microcavity, wherein the medium is located in a vicinity of the microcavity. Also an optical sensor and sensing method comprising: providing a doped medium, the medium comprising a plurality of aggregated nanoparticles comprising fractals, with the material; locating the doped medium in the vicinity of a microcavity; exciting the doped medium with a light source; and detecting light reflected from the doped medium.

Notes:

GOVERNMENT RIGHTS

The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of National Science Foundation Contract Nos. DMR 9623663 and DMR 9810183, by ARO under grant DAAG55-98-1-0425 awarded by the U.S. Army, and under New Mexico Universities Collaborative Research (NUCOR) Program grant numbers 9882 and 9964.


CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patent application Ser. No. 09/572,721, entitled "Optical Enhancement With Nanoparticles And Microcavities", filed on May 16, 2000, now U.S. Pat. No. 6,608,716 and the specification thereof is incorporated herein by reference.

That application claimed the benefit of the filing of U.S. Provisional Patent Application Serial No. 60/134,564, entitled "Fractals in Microcavities: Giant Coupled, Multiplicative Enhancement of Optical Responses," filed on May 17, 1999, and of the filing of U.S. Provisional Patent Application Serial No. 60/190,863, entitled "Microcavity Enhanced Optical Processes and Fractal Enhanced Optical Processes," filed on Mar. 20, 2000, and the specifications thereof are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

The present invention relates to enhancing linear and nonlinear optical emission using nanoparticles, wherein the nanoparticles are either non-aggregated or aggregated, and microcavities. The aggregrated nanoparticles comprise fractals. Microcavities are used in combination with nanoparticles for greatly enhanced optical emission.

2. Background Art

Recently, fractal aggregates of gold, silver, and other noble metals have received attention in the field of linear and nonlinear optical research. Fractals comprise aggregates of particles in colloidal solutions, sols and gels, and soot and smoke. Also, most macromolecules exist in the form of fractals. A fractal aggregate is a system of interacting particles, with special scale-invariant geometry. Scale-invariance in particle aggregates manifests itself in spacial scales larger than the sizes of particles forming the cluster and smaller than the size of the whole cluster; therefore, to track the fractal geometry in a single aggregate it must be relatively large. However, an ensemble of small aggregates of particles, with the number of particles on the order of only ten or more, can also manifest the fractal geometry statistically, on average, despite the fact that single clusters do not manifest the fractal geometry when considered individually. Thus, the term fractals comprises an ensemble of large aggregates (the ensemble can be small and consist of few, or even one, cluster), or a large ensemble of small aggregates of particles, which statistically show the fractal (scale-invariant) geometry with some interval of sizes.

Enhanced optical response in metal nanocomposites characterized by fractal geometry and thin metallic films containing nanoscale surface features has been intensively studied. R. K. Chang and T. E. Furtak, Ed., Surface Enhanced Raman Scattering (Plenum Pres, NY, 1982); M. Moskovits, Rev. Mod. Phys. 57, 783 (1985); R. W. Boyd, et al., Pure Appl. Opt. 5, 505 (1996); V. M. Shalaev and M. I. Stockman, Sov. Phys. JETP 65, 287 (1987); V. A. Markel, et al., Phys. Rev. B 43, 8183 (1991); V. M. Shalaev, Phys. Reports 272, 61 (1996); V. A. Markel, et al., Phys. Rev. B 53, 2425 (1996); V. M. Shalaev, et al., Phys. Rev. B 53, 2437 (1996); M. I. Stockman, Phys. Rev. Lett. 79, 4562 (1997); S. G. Rautian, et al., JETP Lett. 47, 243 (1988); V. P. Safonov et al., Phys. Rev. Lett 80, 1102 (1998); V. M. Shalaev et al., J. Nonlinear Optical Physics and Materials 7, 131 (1998). Enhancement in the optical response is associated with the excitation of surface plasmons, collective electromagnetic modes whose characteristics are strongly dependent on the geometrical structure of the metallic component of the medium. Collective optical excitations, such as surface plasmons, are often spatially localized in fractals. This localization leads to the presence of nanometer-scale spatial regions of high local electric fields, "hot spots", and accordingly, to significant enhancement for a variety of optical processes, such as Raman scattering, four-wave mixing, and nonlinear absorption and refraction. In some cases, the local enhancement at a hot spot can be 10.sup.9 greater than the average enhancement resulting from the fractal itself, averaged over the entire surface of the fractal.

Fractals also have another important property--they are subject to surface enhanced Raman scattering (SERS) by adsorbed molecules. Suitable substrates known to exhibit SERS include colloidal metal particles, vacuum deposited films, single crystals, and matrix isolated metal clusters. O. Silman, et al., J. Phys. Chem. 87, 1014-23 (1983). Also, adsorption of dye molecules, e.g., Rhodamine 6G (R6G), on colloidal Ag or Au is known. P. C. Lee and D. Melsel, J. Phys. Chem. 86, 3391-95 (1982). Once adsorbed onto the colloidal particle, the dye may exhibit strong surface enhanced Raman scattering.

Fractal aggregates of metal nano-sized particles can provide dramatic enhancement for various linear and nonlinear optical responses, including Raman Scattering (RS) and Hyper-Raman Scattering (HRS). This occurs because of localization of optical plasmon excitations within small parts of a fractal aggregate, hot spots, smaller than the size of the fractal and often smaller than the wavelength. When sufficiently concentrated, the large electromagnetic fields in the hot spots can result in very large enhancement of optical responses. The small areas, where the fractal optical excitations are localized, have very different local structures and, therefore, they are characterized by different resonant frequencies. Because of the large variety in local geometries of fractal hot spots, the normal modes of a fractal aggregate cover a huge spectral range, from the near ultra-violet to the far-infrared, leading to giant enhancement of optical responses within this large spectral range. Furthermore, since the dielectric constant of metal is negative and increases in magnitude toward the longer wavelengths, the enhancement for optical processes becomes progressively larger toward infrared (IR) wavelengths.

The various nano-scale areas, where the resonant fractal excitations are localized, can be thought of as a set of different optical "nano-resonators", each having different resonance frequencies resonating in the visible and IR spectral ranges. These fractal nano-resonators have large resonance quality-factors (Q), representing the local-field enhancement, that increase from the visible to the IR region of the spectrum.

Large enhancement for SERS can also be obtained in compact structures, such as nano-sized spheroids or small chain-like aggregates of particles. However, a compact structure of a given geometry has very few normal modes, for example, one, in a sphere, and three, in a spheroid, and thus provide enhancement at only a few selected frequencies. In contrast, in random fractals, there are always such configurations of particles (nano-resonators) that resonate at any given wavelength. Thus, the inherent properties of random fractals provide localization of optical excitations which become sensitive to the local structures. In addition, the fractals exist as a large variety of resonating local structures, which leads to a very broad enhancement band from the near ultra-violet to the far-infrared region of the spectrum.

An alternative approach for achieving large enhancement of the optical response involves the excitation of morphology-dependent resonances (MDRs) in dielectric microcavities. R. K. Chang and A. J. Campillo, Ed., Optical Processes in Microcavities, World Scientific, Singapore-New Jersey-London-Hong Kong (1996). These resonances, which may have very high quality factors, Q on the order of 10.sup.5 to 10.sup.9, result from confinement of the radiation within the microcavity by total internal reflection. Light emitted or scattered in the microcavity may couple to the high-Q MDRs lying within its spectral bandwidth, leading to enhancement of both spontaneous and stimulated optical emissions. For example, enhanced fluorescence emission from a dye-doped cylindrical or spherical microcavity occurs when either the laser pump or the fluorescence, or both, couple to microcavity MDRs. J. F. Owen, Phys. Rev. Lett. 47, 1075 (1981). Moreover, the increased feedback produced by MDRs is sufficient to obtain laser emission from a dye-doped microdroplet under both a continuous wave (CW) and pulsed laser excitation. H. M. Tzeng, et al., Opt. Lett. 9, 499 (1984); A. Biswas, et al., Opt. Lett. 14, 214 (1988). The existence of high-Q microcavity modes is also responsible for numerous stimulated nonlinear effects including stimulated Raman and Rayleigh-wing scattering and four-wave parametric oscillation under moderate intensity CW excitation. M. B. Lin and A. J. Campillo, Phys. Rev. Left. 73, 2440 (1994).

Optical microcavities are resonators that have at least one dimension, on the order of a single or at most a small integral number of optical wavelengths. See Dodabalapur, et al., U.S. Pat. No. 5,405,710, entitled "Article Comprising Microcavity Light Sources." The specific geometry of the microcavity and the boundary conditions on the interface of the dielectric-to-air impose selective normal modes on the optical microcavity. Typical microcavities have diameters of 100 microns or less. Such microcavities have shown technological promise for constructing novel light emitting devices. Possible applications of microcavities devices include flat panel displays, optical interconnects, optical fiber communications, and light emitting diode (LED) printing. For example, in a display application, a device may consist of three microcavities, each microcavity emitting in the blue, green, and red regions of the visible spectrum. Further, resonant microcavities have the advantage of emitting light in a highly directional manner as a result of their inherent geometry.

As described briefly above fractal aggregates and resonating microcavities are known to cause large enhancements of optical emissions. The present invention uses the properties of nanoparticles, fractals, and microcavities to enhance optical emissions for a variety of apparatuses and methods. The present invention further combines the properties of these optical enhancement processes by placing nanoparticles and/or fractal aggregates within a high-Q microcavity. Overall, the observed optical enhancement of the invention is multiplicative rather than additive of the two processes. Results demonstrate the unique potential of such devices in the development of ultra-low threshold microlasers, nonlinear-optical devices for photonics, as well as new opportunities of micro-analysis, including spectroscopy of single molecules.

SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)

The present invention is a light emitting apparatus that is comprised of at least one light source, such as a laser, and a medium that is made up of nanoparticles. These nanoparticles can either be non-aggregated nanoparticles and/or aggregated nanoparticles, wherein the aggregated nanoparticles comprise fractals. Preferably, each fractal comprises at least ten aggregated nanoparticles, and furthermore each fractal comprises a dimension less than that of the embedding space. The apparatus can further comprise a microcavity. The medium is then located in the vicinity of the microcavity in order to enhance the optical emission. To be in the vicinity of the microcavity, the medium is located within a light wavelength of the surface of the microcavity or within the boundaries of the microcavity. The microcavity can be either solid or hollow. When the microcavity is solid, the medium can either be located on a surface of the microcavity or embedded within the microcavity. When the microcavity is hollow, the medium can either be located within the hollow microcavity or on a surface of the hollow microcavity. The microcavity can be either cylindrical, spherical, spheroidal, polyhedral, or an optical wave guide microcavity. The exterior dimension of the microcavity is preferably at least twice that of the optical wavelength of interest.

The medium can further be contained within a liquid suspension, gel matrix, or solid matrix. The medium itself can be of metal, semi-metal, and/or a semiconductor. Metals that can be used for the medium can be either silver, gold, platinum, copper, aluminum, or magnesium. The semi-metal can be graphite. Any of Group IV, Group III-V, or Group II-VI semiconductors can be used.

Preferably the average diameter of each individual nanoparticle is less than that of the optical wavelength of interest. The light source, such as a pump laser, for the present invention preferably emits light of wavelengths between approximately 200 and 100,000 nanometers, more preferably between approximately 300 and 2,000 nanometers. The light source also emits light, having between approximately 1 nanowatt and 100 watts of power.

Optionally, at least one optically active organic and/or inorganic molecule is adsorbed on a surface of the nanoparticles. For example, laser dye or sodium citrate molecules can be adsorbed on a surface of the nanoparticles. The laser dye can be a xanthene, coumarin, pyrromethene, styryl, cyanine, carbon-bridged, naphthofluorescein-type, acridone, quinalone derivative, p-terphenyl, p-quaterphenyl, or a 9-aminoacridine hydrochloride dye. In the alternative, at least one optically active organic and/or inorganic molecule is located within the light wavelength of the surface of the nanoparticles. Again, such a molecule can be either a laser dye or sodium citrate molecules.

The present invention is also a method of enhancing the optical emission of a material and comprises the steps of doping a medium, wherein the medium comprises a plurality of nanoparticles, either non-aggregated nanoparticles and/or aggregated nanoparticles, and exciting the doped medium with at least one light source. The aggregated nanoparticles are fractals. The medium can be doped with at least one material from the materials including a single molecule, a plurality of molecules, a nanocrystal, a solid matrix, DNA, DNA fragments, amino acids, antigen, antibodies, bacteria, bacterial spores, and viruses. The method can further include the step of locating the doped medium in the vicinity of a microcavity. Locating can comprise locating the medium on a surface of a solid microcavity or embedding the medium within a solid microcavity. Alternatively, the locating step can comprise locating the medium within a hollow microcavity, or alternatively locating the medium on a surface of a hollow microcavity.

When exciting the medium, the exciting step can comprise exciting the doped medium to result in at least one type of optical process, such as photoluminescence, Raman scattering, hyper-Raman scattering, Broullion scattering, harmonic generation, sum frequency generation, difference frequency generation, optical parametric processes, multi-photon absorption, optical Kerr effect, four-wave mixing, and phase conjugation. Optionally, the method further comprises containing the medium within a substance such as a liquid suspension, a gel matrix, or a solid matrix. The doped medium can comprise metal, semi-metal, and/or a semiconductor. Examples of metals include silver, gold, platinum, copper, aluminum, and magnesium. The semi-metal can comprise graphite, and the semiconductor can be any of either Group IV, Group III-V, or Group II-VI semiconductors.

The exciting step preferably comprises emitting light of wavelengths between approximately 200 and 100,000 nanometers, more preferably between 300 and 2,000 nanometers, and wherein the light emitted has between anywhere from approximately 1 nanowatt to 100 watts of power.

The doping step can further comprise doping with at least one optically active organic and/or inorganic molecule located within the light wavelengths of the surface of the medium. These molecules can be, for example, laser dye or sodium citrate molecules.

Furthermore, the present invention provides a wavelength translation apparatus, wherein the apparatus comprises at least one light source and a medium made up of a plurality of nanoparticles, wherein the nanoparticles are either non-aggregated nanoparticles and/or fractals comprised of aggregated nanoparticles. The wavelength translation apparatus can further include a microcavity and a medium, wherein the medium is located in the vicinity of the microcavity to enhance optical emission. The methodology for wavelength translation comprises the steps of providing the medium having a plurality of nanoparticles, be it either non-aggregated and/or aggregated nanoparticles, and exciting the medium with a light source, such as a laser. The method can further include the step of locating the medium in the vicinity of a microcavity to amplify optical emission. Locating the medium in the vicinity of a microcavity to amplify optical emission further comprises amplifying the optical emission via at least one of the following processes: stimulated emission of photons, stimulated Raman scattering, stimulated hyper-Raman scattering, stimulated Broullion scattering, optical parametric amplification, multi-photon emission, four-wave mixing, and phase conjugation.

The present invention further provides an amplifying apparatus having a gain greater than 1.2 and consists of at least one light source, a microcavity, and a medium made up of a plurality of nanoparticles, being either non-aggregated and/or aggregated nanoparticles, and wherein the medium is located in the vicinity of the microcavity to enhance optical emission. The method of amplification comprises providing the medium and locating it within the vicinity of a microcavity to amplify the optical emission, as well as exciting the medium with at least one light source, such as a laser.

The present invention further provides for an optical parametric oscillator comprising at least one light source, a cavity, and a medium wherein the medium comprises a plurality of nanoparticles. The nanoparticles can be non-aggregated nanoparticles and/or aggregated nanoparticles. The aggregated nanoparticles comprise fractals. The medium is located in the vicinity of the cavity to enhance optical emission. Preferably the cavity comprises a microcavity.

The present invention further provides for a light detection and ranging system comprising a transmitter light source; a receiver to receive light produced from the interaction of the transmitter light with constituents; and a medium. The medium comprises a plurality of nanoparticles and the nanoparticles can be non-aggregated and/or aggregated nanoparticles. The light detection and ranging system further comprises a microcavity to receive light from the receiver, wherein the medium is located in the vicinity of the microcavity to amplify the received light.

The present invention still further provides a method of optical data storage and comprises the steps of providing a medium, wherein the medium comprises a plurality of nanoparticles. The nanoparticles can be non-aggregated nanoparticles and/or aggregated nanoparticles. The method further includes the steps of irradiating the medium with polychromatic light and generating hot spots in the medium due to intensity differences of different wavelengths, and spectral hole burning the medium due to photomodification, thereby creating high density storage capabilities. The method for optical data storage can further comprise the step of locating the medium in the vicinity of a microcavity to amplify optical emission.

The present invention still further provides for near-field optical spectroscopy. This method provides for spatial resolution on the order of 1 nanometer. One method for detecting materials with near-field optics comprises locating the material within a distance shorter than the light wavelength from a tapered end of an optical fiber and detecting the light emitted from the material through the optical fiber. A second method for detecting a material with near-field optics comprises locating a tapered end of an optical fiber within a distance shorter than the light wavelength from the material to the material in order to illuminate the material. A third method of detecting a material using near-field optics comprises locating a sharp tip of a vibrating metal wire within a distance shorter than the light wavelength from the material, and detecting the light emitted from the material with a lock-in method. In all of these methodologies, the material to be detected is located within a distance shorter than the light wavelength from either a tapered end of an optical fiber or a sharp tip of a vibrating metal wire.

Near-field optical spectroscopy is a near-field optical spectroscopic method for detecting chemical compounds and biological materials through their spectroscopic signatures. The present invention is further of near-field optical spectroscopy by increasing the ability to detect any of the following materials: a single molecule, a plurality of molecules, a nanocrystal, DNA, DNA fragments, amino acids, antigen, antibodies, bacteria, bacterial spores, or viruses. The method further comprises obtaining spectroscopic signatures such as electronic vibrational or rotational spectroscopic signatures. The method can further include an optical process such as photoluminescence, Raman scattering, hyper-Raman scattering, Broullion scattering, harmonic generation, sum frequency generation, difference frequency generation, and optical Kerr effect.

Near-field optical signals can be enhanced by the nanoparticles of the present invention, be they non-aggregated nanoparticles and/or aggregated nanoparticles. By doping the material to be detected onto a medium that comprises the nanoparticles, near-field optical signals are enhanced. In the method where the light signal is detected through an optical fiber, the medium can be instead deposited onto the input end of the optical fiber. Furthermore, the microcavity of the present invention can enhance near-field optical spectroscopy of a material when that material is located in the vicinity of the microcavity. By combining the doped medium with the microcavity and locating the medium in the vicinity of the microcavity, near-field optical spectroscopy can also be enhanced. In the case where the light signal is detected through the optical fiber, the medium is instead deposited onto the input end of the optical fiber rather than doped onto the medium.

The invention is additionally of an optical sensing enhancing material (and corresponding method of making) comprising: a medium, the medium comprising a plurality of aggregated nanoparticles comprising fractals; and a microcavity, wherein the medium is located in a vicinity of the microcavity. In the preferred embodiment, the invention additionally comprises an analyte deposited with the medium in the vicinity of the microcavity by laser ablation, particle deposition, or lithography, and a non-reactive surface coating is placed over the analyte and the medium.

The invention is further of an optical sensor and sensing method comprising: providing a doped medium, the medium comprising a plurality of aggregated nanoparticles comprising fractals, with the material; locating the doped medium in the vicinity of a microcavity; exciting the doped medium with a light source; and detecting light reflected from the doped medium. In the preferred embodiment, Raman signals are detected. Analytes may be placed in direct contact with the doped medium or located remotely from the medium. The lights source can comprise two counterpropogating light sources. The microcavity is preferably a quartz tube or quartz rod.

The invention is yet further of a method of detecting a material comprising: exciting both the material and a medium in a vicinity of a microcavity, the medium comprising a plurality of aggregated nanoparticles comprising fractals, with at least one light source; and detecting spectroscopic data of the material. In the preferred embodiment, Raman signals are detected and the material is any one or more of the following items: chemical and biological warfare agents, chemical and biological contaminants of the environment, explosive agents, controlled substances, chemical and biological agents in manufacturing process streams, and chemical and biological agents in a substrate selected from the group consisting of blood, blood byproducts, urine, saliva, cerebral spinal fluid, tears, semen, uterine secretions, fecal matter, respiratory gases, lung secretions, skin, and aqueous humor of the eye.

A primary object of the present invention is to enhance optical emission of molecules by placing such molecules on or near fractals, and locating them within or on a surface of a microcavity for still further enhancement.

Another object of the present invention is to enhance optical emission of nano-sized particles, quantum dots, by placing the nanoparticles on or near fractals located within or on a surface of a microcavity.

A primary advantage of the present invention is the observed enhanced optical emission and lasing of molecules, or nanoparticles, placed on or near fractals located within or on a surface of a microcavity.

Another advantage of the present invention is the observed surface-enhanced Raman scattering, and other linear and non-linear optical processes of molecules, or quantum dots, placed on or near fractals located within or on a surface of a microcavity.

Another advantage of the present invention is the enhanced optical emission of dye molecules placed on or near fractals located within or on a surface of a microcavity.

Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.