Monolithic system and method for enhanced Raman spectroscopy

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MONOLITHIC SYSTEM AND METHOD FOR
ENHANCED RAMAN SPECTROSCOPY

CROSS REFERENCE TO RELATED

APPLICATIONS 5

The present application is related to concurrently filed U.S. patent application Ser. No. 11/044,676 entitled INTEGRATED MODULAR SYSTEM AND METHOD FOR ENHANCED RAMAN SPECTROSCOPY, and concur- 10 rently filed U.S. patent application Ser. No. 11/044,781 entitled INTEGRATED SYSTEM AND METHOD FOR TRANSVERSAL ENHANCED RAMAN SPECTROSCOPY.

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FIELD OF THE INVENTION

The present invention relates to chemical analysis using Raman spectroscopy. More particularly, the present invention relates to integrated systems for enhancement of Raman 20 spectroscopy combining a radiation source and Raman scattered radiation sensing. In addition, the present invention relates to hyper-Raman, where the excitation wavelength is approximately an integer multiple of the Raman wavelength.

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BACKGROUND OF THE INVENTION

Raman Spectroscopy is a well-known spectroscopic technique for performing chemical analysis in the gas, liquid or solid phase. In conventional Raman Spectroscopy, high 30 intensity monochromatic light from a light source, such as a laser, is directed onto an analyte to be chemically analyzed. The analyte may contain a single species of molecules or mixtures of different molecules. Furthermore, Raman Spectroscopy may be performed on a number of different 35 molecular configurations, such as organic and inorganic molecules in crystalline or amorphous states.

For a system that does not have surface or nanostructure enhancement, the majority of the incident photons of the light are elastically scattered by the analyte molecule. In 40 other words, the scattered photons have the same frequency, and thus the same energy, as the photons that were incident on the analyte. This is known as Rayleigh scattering. However, a small fraction of the photons (i.e., 1 in 107 photons) are inelastically scattered by the un-enhanced analyte mol- 45 ecule at a different optical frequency than the incident photons. The inelastically scattered photons are termed the "Raman scattered radiation" and may be scattered at frequencies greater than, but most are usually scattered at a frequency lower than, the frequency of the incident photons. 50 When the incident photons collide with the molecules and give up some of their energy, the Raman scattered photons emerge with a lower energy and thus at a lower frequency. The lower energy Raman scattered photons are commonly referred to in Raman spectroscopy as the "Stokes radiation." 55 A small fraction of the molecules are already in an energetically excited state and when the incident photons collide with the molecules, the Raman scattered photons emerge at a higher energy and thus at a higher frequency. The higher energy Raman scattered photons are commonly referred to 60 in Raman spectroscopy as the "anti-Stokes radiation." Raman scattering can occur from the rotational, vibrational, or electronic states of the molecules.

The Stokes and the anti-Stokes Raman scattered photons are collected using optics, the different frequencies are 65 dispersed spatially with some type of spectrometer, and the photons are registered by a detector, such as a photomulti

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plier, resulting in a spectral graph of intensity at a corresponding frequency (i.e., proportional to energy) for the Raman scattered photons. By plotting the intensity of the inelastically scattered Raman photons against frequency, a unique Raman spectrum, which corresponds to the particular analyte molecules, is obtained. This Raman spectrum may be used to identify chemical species, as well as other physical properties of the analyte. While conventional Raman Spectroscopy is suitable for bulk chemical analysis, it is not effective for surface studies because the signal from the bulk Raman scattered photons overwhelms any signal from Raman scattered photons near the surface.

In hyper-Raman spectroscopy, when excitation radiation impinges on an analyte molecule, a very small number of photons may be scattered at frequencies corresponding to the higher order harmonics of the excitation radiation, such as the second and third harmonics (i.e., twice or three times the frequency of the excitation radiation). Some of these photons may be Raman scattered photons with a frequency that is Raman-shifted relative to the frequencies corresponding to the higher order harmonics of the excitation radiation. Therefore, in hyper-Raman spectroscopy, the incident excitation photons have approximately V2, lh, or Vi the frequency of the Raman photons.

Due to the deficiencies with performing surface studies using conventional Raman Spectroscopy, another Raman Spectroscopy technique called Surface Enhanced Raman Spectroscopy (SERS), which is effective for performing surface studies, has been developed. In SERS, a monolayer or sub-monolayer amount of the molecules to be analyzed is adsorbed onto a specially roughened metal surface. Typically, the metal surface is made from gold, silver, copper, lithium, sodium, or potassium. Raman Spectroscopy has also been used employing metallic nanoparticles or nanowires for the metal surface, as opposed to a roughened metallic surface, which is hereinafter referred to as NanoEnhanced Raman Spectroscopy (NERS). The intensity of the Raman scattered photons from a collection of molecules adsorbed on such a surface is typically about 104-106 greater than conventional Raman Spectroscopy from a similar number of molecules in a bulk specimen, and can be as high as 108-1014 for a single molecule adsorbed near two or more metal nanoparticles. Although not thoroughly understood, the selectivity of the surface Raman signal results from the presence of surface enhancement mechanisms and is mainly attributed to two primary mechanisms: electromagnetic enhancement and chemical enhancement, with the electromagnetic enhancement being the dominant mechanism. The enhanced electromagnetic field is highly dependent on the surface roughness features of the enhancement surface. The chemical enhancement is believed to be dependent on the altered electronic structure of the enhancement surface due to adsorption of the analyte. The enhanced electromagnetic field of the enhancement surface, which is adjacent to the analyte, irradiates the analyte, producing an enhanced Raman signal having a strength that is, in part, proportional to the square of the enhanced electromagnetic field. Thus, Raman spectroscopy may be used to study monolayers of materials adsorbed on metals, and even single molecules adsorbed near an appropriate metal nanostructure (i.e., NERS).

In a conventional SERS system, a spectrometer collects all radiation, including the Stokes radiation, anti-Stokes radiation, and the elastically scattered radiation, and provides a spectrum of the scattered radiation. The spectrum may then be used to identify the chemical species, as well as other physical properties, of the analyte. Conventionally, the 3

spectrometer may include various optical elements such as lenses, gratings, photomultipliers, and filters. While this arrangement may be flexible, it is often bulky, expensive, and requires significant user interaction to analyze a chemical species. 5

An integrated monolithic Raman spectroscopy analysis system, which integrates a laser light source and a scattered radiation dispersing and detection element, may be a smaller and less costly system. This integrated Raman spectroscopy analysis system may be easier to use and may be adaptable 10 to detection of a predetermined set of molecules. In addition, a monolithic Raman spectroscopy analysis system may integrate the Raman scattering enhancement structure (surface or nanostructure), creating a self-contained integrated Raman spectroscopy analysis system. 15

BRIEF SUMMARY OF THE INVENTION

The present invention, in a number of embodiments, includes devices for generating Raman scattered radiation, 20 Surface Enhanced Raman Spectroscopy (SERS) systems, Nano-Enhanced Raman Spectroscopy (NERS) systems (NERS), and methods of performing SERS and NERS.

In one aspect of the present invention, a molecular analysis device comprises a substrate and a laser source disposed 25 on the substrate and configured to generate laser radiation. The molecular analysis device also includes a waveguide disposed on the substrate and configured for substantially containing the laser radiation. The waveguide includes an opening formed in a first surface of the waveguide. A Raman 30 enhancement structure may be disposed in the opening of the waveguide and configured to generate a Raman scattered radiation when the laser source irradiates an analyte disposed on the Raman enhancement structure. The molecular analysis device also includes a wavelength demultiplexer 35 disposed on the substrate and configured for dispersing the Raman scattered radiation into at least two unique wavelengths. At least two radiation sensors disposed on the substrate are configured for sensing one of the at least two unique wavelengths. 40

Another embodiment of the present invention comprises a molecular analysis device. The molecular analysis device includes a substrate, a laser source disposed on the substrate and configured to generate a laser radiation, and a chirped photonic crystal. The chirped photonic crystal comprises a 45 root waveguide configured for receiving and guiding the laser radiation and configured for guiding a Raman scattered radiation. A Raman enhancement structure may be disposed in an opening of the root waveguide and configured to generate the Raman scattered radiation when the laser 50 source irradiates an analyte disposed on the Raman enhancement structure. The chirped photonic crystal also includes at least two wavelength-specific waveguides, each wavelength-specific waveguide disposed at a position along a chirped axis correlated with one of at least two unique 55 wavelengths, and wherein each wavelength-specific waveguide is configured for guiding one of the at least two unique wavelengths. The molecular analysis device also includes at least two radiation sensors disposed on the substrate are configured for sensing one of the at least two 60 unique wavelengths.

Another embodiment of the present invention includes a method for performing Raman spectroscopy. The method of performing Raman spectroscopy comprises generating a laser radiation from a laser source disposed on a substrate, 65 confining the laser radiation in a waveguide disposed on the substrate, and disposing an analyte over at least a portion of

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a Raman enhancement structure disposed in an opening in the waveguide. The method further includes irradiating the analyte and the Raman enhancement structure with the laser radiation to generate a Raman scattered radiation, dispersing the Raman scattered radiation into at least two unique wavelengths, and sensing the at least two unique wavelengths with at least two radiation sensors disposed on the substrate.

Another embodiment of the present invention includes a method for performing Raman spectroscopy. The method of performing Raman spectroscopy comprises generating a laser radiation from a laser source disposed on a substrate, confining the laser radiation in a root waveguide formed in a chirped photonic crystal, and disposing an analyte over at least a portion of a Raman enhancement structure disposed in an opening in the root waveguide. The method further includes irradiating the analyte and the Raman enhancement structure with the laser radiation to generate a Raman scattered radiation, dispersing the Raman scattered radiation into at least two unique wavelengths, and sensing the at least two unique wavelengths with at least two radiation sensors disposed on the substrate.

BRIEF DESCRIPTION OF THE SEVERAL
VIEWS OF THE DRAWINGS

While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which:

FIG. 1A is a top view of an exemplary molecular analysis device including a Raman enhancement structure disposed in an opening of a waveguide;

FIG. IB is a side view of the exemplary molecular analysis device of FIG. 1A;

FIG. 2 is a top view of an exemplary wavelength demultiplexer employing an arrayed waveguide grating; and

FIG. 3 is a top view of an exemplary wavelength demultiplexer employing a chirped photonic crystal; and

FIG. 4 is a top view of another exemplary molecular analysis device including a Raman enhancement structure disposed in an opening of a waveguide, wherein the waveguide is formed in a chirped photonic crystal.

DETAILED DESCRIPTION OF THE
INVENTION

The present invention, in a number of embodiments, includes structures and devices for use in Surface Enhanced Raman Spectroscopy (SERS) systems, Nano-Enhanced Raman Spectroscopy (NERS) systems and methods of performing SERS and NERS. Systems that may incorporate SERS and NERS techniques are generally referred to herein as employing Enhanced Raman Spectroscopy (ERS). As shown in the figures and described in further detail below, molecular analysis structures and molecular analysis devices of the present invention incorporate a laser source, a waveguide, a wavelength demultiplexer, and at least two radiation sensors integrated onto a substrate. A Raman enhancement structure may be disposed in an opening in the waveguide, where the Raman enhancement structure and an analyte disposed on the Raman enhancement structure may be exposed to laser radiation from the laser source. In addition, the sensitivity of Raman spectroscopy systems of the present invention may be enhanced by the type of surface 5

on the Raman enhancement structure to enable more accurate and sensitive detection of chemical species and other physical properties of the sample being analyzed, including single molecule detection. Furthermore, the Raman spectroscopy systems of the present invention may be employed 5 as an analytical instrument for detecting specific chemicals.

FIGS. 1A and IB illustrate an exemplary embodiment of a molecular analysis device 100 employing ERS. The molecular analysis device 100 includes the substrate 110, the laser source 120 disposed on the substrate 110, the 10 waveguide 130 disposed on the substrate 110, the wavelength demultiplexer 240 disposed on the substrate 110, and at least two radiation sensors 290 disposed on the substrate 110.

This embodiment of FIGS. 1A and IB may be used as a 15 self-contained ERS analysis system. A Raman enhancement structure 300 may be disposed in an opening 138 of the waveguide 130, an analyte 310 may then be disposed on the Raman enhancement structure 300. In operation, the laser source 120 emits a laser radiation 125 to irradiate the analyte 20 310 on the Raman enhancement structure 300. The combined irradiation of the Raman enhancement structure 300 and the analyte 310 may generate Raman scattered radiation 160. The Raman scattered radiation 160 may be detected as a variety of wavelengths by the radiation sensors 290, as 25 explained more fully below.

The waveguide 130 may be configured for containing the laser radiation 125 emanating from the laser source 120 and the Raman scattered radiation 160 emanating from the analyte 310 on the Raman enhancement structure 300. The 30 waveguide 130 may be formed to include a resonant cavity 134 formed between a first reflector 132 and a second reflector 136. In addition, if the resonant cavity 134, second reflector 136, and first reflector 132 are configured for a specific wavelength, the waveguide 130 may also amplify 35 the laser radiation 125 substantially near that specific wavelength. An opening 138 may be formed through the first reflector 132 (also referred to as a first surface) such that the Raman enhancement structure 300 may be disposed within the resonant cavity 134 and on a surface of the second 40 reflector 136 as shown in FIG. 1A.

The first reflector 132 and second reflector 136 may be structures, such as, for example, distributed Bragg reflectors (also referred to as Bragg Mirrors) or distributed Bragg gratings. Bragg reflectors may be formed in a number of 45 ways using a variety of materials configured as alternating layers; one layer with a low refractive index and the other layer with a high refractive index. Each layer is configured with a thickness of about one quarter of an odd multiple of the specific wavelength to be confined by the waveguide 50 130. For example, the layers may be configured with a thickness of lM of a wavelength or % of a wavelength. The resulting Bragg reflector may also be referred to as a quarter-wave stack. As an example, a Bragg reflector may be formed from alternating layers of GaAs (gallium arsenide) 55 and AlGaAs (aluminum gallium arsenide). Another suitable material combination for forming Bragg reflectors is alternating layers formed respectively from silicon and silicon dioxide. Implementing a larger number of alternating pairs in a Bragg reflector results in a higher refractive index. 60 About thirty layers may create a reflectivity as high as 99.99%.

The Raman enhancement structure 300 disposed in the opening 138 may be used to effect the electromagnetic enhancement of the Raman signal, chemical enhancement of 65 the Raman signal, or both. The Raman enhancement structure 300 may be associated with the second reflector 136 by

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chemically bonding or merely disposing the same on the second reflector 136, followed by weak bonding thereto or, alternatively, no bonding at all.

As used herein, the term "Raman enhancement structure" means any structure configured and formed of a material that may produce chemical enhancement of the Raman signal, electromagnetic enhancement of the Raman signal, or both. Exemplary materials for formation of the Raman enhancement structure 300 are metallic materials, which include gold, silver, copper, aluminum, chromium, lithium, sodium, potassium. However, any other suitable material that may produce chemical enhancement of the Raman signal, electromagnetic enhancement of the Raman signal, or both, is contemplated as within the scope of the present invention.

It has been shown that a relatively rough surface enhances the amount of Raman scattered radiation 160 that may emanate from the Raman enhancement structure 300 when an analyte 310 disposed on the Raman enhancement structure 300 is irradiated. As a result, the Raman enhancement structure 300 may be formed as a single monolithic layer, possibly with a roughened surface, or the Raman enhancement structure 300 may comprise one or more discrete particles.

As one or more discrete particles, the Raman enhancement structure 300 may have a variety of exemplary configurations, such as, nanowires (e.g., a rod shaped configuration), nanodots, nanoparticles (including employing a single nanoparticle), or metallic particles in a colloidal suspension. It is currently preferred to employ at least two silver nanoparticles spaced apart, such that an analyte 310 molecule may be draped therebetween. The aforementioned nanostructures for the Raman enhancement structure 300 may be formed by chemical vapor deposition (CVD), molecular beam epitaxy (MBE), atomic layer deposition (ALD), or any other known technique for depositing the Raman enhancement structure 300 on a surface.

The substrate 110 may be any suitable substrate for integrating the laser source 120, the wavelength demultiplexer 240, and the radiation sensors 290. In addition, if optional components 190 are present, the substrate 110 may be selected for its properties of directing and modifying either optical signals, electrical signals, or both optical signals and electrical signals. By way of example, conventional silicon substrates as well as Group III-V materials, such as gallium arsenide, and indium phosphide may be desirable.

It may be desirable to include other optional components 190 to create an enhanced Raman spectroscopy analysis system. By way of example, Application Specific Integrated Circuits (ASIC), microprocessors, microcontrollers, digital signal processors, and discrete components may be useful. In addition, optical components such as collimators, lenses, filters, waveguides and resonant cavities may be useful. Furthermore, nanowire-type sensor arrays may be desirable for verifying the presence and composition of some analytes 310.

The laser source 120 may be a conventional laser diode, which emits a laser radiation 125 with a wavelength that is effective for ERS analysis. By way of example, an edgeemitting laser diode is illustrated in the exemplary embodiments. However, it is understood that other laser diode implementations are possible and encompassed by the scope of the invention, such as, for example, a Vertical Cavity Surface Emitting Laser (VCSEL). In addition, the laser diode may be integrated into the substrate 110 as part of a monolithic integration, including the substrate 110 and, possibly, other optional components 190.