Self-Exciting Surface Enhanced Raman Spectroscopy

Self-exciting surface enhanced Raman spectroscopy (SERS) employs an integral optical excitation source to provide an excitation signal to provide self-excitation of a SERS structure. The SERS structure includes a plurality of nanofingers having SERS-enhancing nanoparticles disposed adjacent to the nanofingers.

Certain examples have other features that are one of in addition to and in lieu of the features illustrated in the above-referenced figures. These and other features are detailed below with reference to the above-referenced figures.

DETAILED DESCRIPTION

Examples in accordance with the principles described herein provide detecting or sensing the presence of an analyte or a target species using scattering spectroscopy, e.g., surface enhanced Raman spectroscopy (SERS). In particular, examples in according with the principles described herein provide detecting or sensing an analyte with a SERS structure and that employs an integral optical excitation source to generate an optical excitation signal. As such, the surface enhanced Raman spectroscopy is ‘self-exciting,’ according to various examples of the principles described herein. Among other characteristics, surfaced enhanced Raman spectroscopy that is self-exciting may facilitate implementations that are uniquely compact. Further, self-excitation with an integral optical excitation source may provide higher level Raman scattering signals and thus better detection and sensing due to potentially higher localized power levels of the optical excitation signals given the proximity of the optical excitation source and a surface or structure employed in the surface enhanced Raman spectroscopy.

Herein, other applicable forms of scattering spectroscopy that may be used include, but are not limited to, surface enhanced coherent anti-stokes Raman scattering (SECARS), resonant Raman spectroscopy, hyper Raman spectroscopy, various spatially offset and confocal versions of Raman spectroscopy, as well as direct monitoring of plasmonic resonances. SERS may provide detection and identification of the analyte and in some examples, quantification of the analyte. In particular, the detection or sensing may be provided for an analyte that is either adsorbed onto or closely associated with a surface in SERS, according to various examples. Herein, the scattering spectroscopy will generally be described with reference to SERS-based scattering spectroscopy for simplicity of discussion and not by way of specific limitation, unless otherwise indicated.

Raman-scattering optical spectroscopy or simply Raman spectroscopy, as referred to herein, employs a scattering spectrum or spectral components thereof produced by inelastic scattering of photons by an internal structure of a material being illuminated or ‘excited’. These spectral components contained in a response signal (e.g., a Raman scattering signal) produced by the inelastic scattering may facilitate determination of the material characteristics of an analyte species including, but not limited to, identification of the analyte. Surface enhanced Raman spectroscopy (SERS) is a form of Raman spectroscopy that employs a ‘Raman-enhancing’ surface. SERS may significantly enhance a signal level or intensity of the Raman scattering signal produced by a particular analyte species. In particular, in some instances, the Raman-enhancing surface includes a region associated with the tips of nanostructures such as, but not limited to, nanofingers or nanorods. The tips of the nanofingers may serve as nanoantennas to one or both of concentrate a stimulus or excitation field and amplify a Raman emission leading to further enhancement of the strength of the Raman scattering signal, for example.

In some examples of SERS, a SERS surface that includes a plurality of nanofingers is configured to enhance production and emission of the Raman scattering signal from an analyte. Specifically, an electromagnetic field associated with and surrounding the nanofingers (e.g., tips of the nanofingers) in a ‘Raman-enhancing’ configuration may enhance Raman scattering from the analyte, in some examples. A relative location of the nanofingers themselves as well as tips of the nanofingers in the Raman-enhancing configuration may provide enhanced Raman scattering. Concentration of the excitation field and amplification of the Raman scattering signal may be associated with plasmonic modes supported by the nanostructures, according to various examples. The plasmonic modes may provide or produce so-called ‘hotspots’ in a scattering spectroscopy enhancing structure that includes the nanostructures, for example.

Herein, a ‘hotspot’ or more precisely a ‘SERS hotspot’ is defined with respect to scattering spectroscopy as a region or location on a substrate, or more generally within a scattering spectroscopy enhancing structure, that exhibits a spatially localized enhancement of an electromagnetic field. The SERS hotspot may be act as a ‘field concentrator’ to concentrate and locally enhance an incident electromagnetic field, for example. In various examples, the localized enhancement may be associated with one or both of an incident or excitation signal (i.e., incident electromagnetic field) used to stimulate the scattering spectroscopy enhancing structure and the production and subsequent radiation of the scattering signal. In particular, at the SERS hotspot, localized electromagnetic fields are enhanced by characteristics of the scattering spectroscopy enhancing structure. The SERS hotspot may be due to spatially localized surface plasmon resonances associated with the scattering spectroscopy enhancing structure, for example. In some examples, the electromagnetic field enhancement due to the SERS hotspot may result in electromagnetic fields that are orders of magnitude higher in a vicinity of the SERS hotspot than in regions outside of the SERS hotspot as well as in an electromagnetic wave (e.g., optical stimulus or optical excitation signal) used to excite the SERS hotspot. Note that, while a particular structure may represent a SERS hotspot, a SERS hotspot is only ‘hot’ when in the presence of the optical excitation signal, according to various examples.

According to various examples, the electromagnetic field enhancement is associated with physical characteristics of the scattering spectroscopy enhancing structure including, but not limited to, a shape of elements (e.g., nanoparticles or nanostructures) that make up the scattering spectroscopy nanostructure, the materials and the material properties (e.g., losses) of the elements, and an arrangement of the elements (e.g., nanoparticles adjacent or nearly adjacent to one another). The electromagnetic field enhancement at the SERS hotspot may also be related to characteristics of the electromagnetic field including, but not limited to, a frequency of and an angle of incidence of an excitation signal used to excite the SERS hotspot. The electromagnetic field enhancement at the SERS hotspot may, in turn, produce an enhancement of a scattering signal produced by an analyte in a vicinity of the SERS hotspot, according to various examples.

A ‘nanorod’ or equivalently a ‘nanofinger’ herein is defined as an elongated, nanoscale structure having a length (or height) that exceeds a nanoscale cross sectional dimension (e.g., width) taken in a plane perpendicular to the length, for example. In some examples, the length may exceed by several times the nanoscale cross sectional dimension. In particular, the length of the nanofinger is generally much greater than the nanofinger width (e.g., length is greater than about 2-3 times the width). In some examples, the length may exceed the cross sectional dimension (or width) by more than a factor of 5 or 10.

For example, the width may be about 40 nanometers (nm) and the height may be about 400 nm. In another example, the nanofinger width or diameter may be between about 100 nm and 200 nm and the length may exceed about 500 nm. For example, the width may be about 130 to about 170 nm and the length may be about 500 to about 800 nm. In yet another example, the width at a base of the nanofinger may range between about 20 nm and about 100 nm and the length may be more than about 1 micrometer (μm). In another example, the nanofinger may be conical with a base having a width ranging from between about 100 nm and about 500 nm and a length that may range between about one half (0.5) μm and several micrometers.

In various examples, nanofingers of the plurality may be grown (i.e., produced by an additive process) or produced by etching or a subtractive process. For example, the nanofingers may be grown as nanowires using a vapor-liquid-solid (VLS) growth process. In other examples, nanowire growth may employ one of a vapor-solid (V-S) growth process and a solution growth process. In yet other examples, growth may be realized through directed or stimulated self-organization techniques such as, but not limited to, focused ion beam (FIB) deposition and laser-induced self assembly. In another example, the nanofingers may be produced by using an etching process such as, but not limited to, reactive ion etching, to remove surrounding material leaving behind the nanofingers. In yet other examples, various forms of imprint lithography including, but not limited to, nanoimprint lithography as well as various techniques used in the fabrication of micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS) are applicable to the fabrication of the nanofingers and various other elements described herein.

A ‘nanoparticle’ herein is defined as a nanoscale structure having substantially similar dimensions of length, width and depth. For example, the shape of a nanoparticle may be a cylinder, a sphere, an ellipsoid, or a faceted sphere or ellipsoid, or a cube, an octahedron, a dodecahedron, or another polygon. The nanoparticle may be a substantially irregular three-dimensional shape, in other examples. The size of the nanoparticle may range from about 5 nm to about 300 nm, for example, in diameter or dimension. In some examples, the nanoparticle dimensions may be within a range of about 50 nm to about 100 nm, or about 25 nm to about 100 nm, or about 100 nm to about 200 nm, or about 10 nm to about 150 nm, or about 20 nm to about 200 nm.

In some examples, a nanoparticle may be a substantially homogeneous structure. For example, the nanoparticle may be a nanoscale metal particle (e.g., a nanoparticle of gold, silver, copper, etc.). In other examples, the nanoparticle may be a core-shell structure that is substantially inhomogeneous, by definition. For example, the nanoparticle may include a core of a first material that is coated by a second material that may be different from the first material. The second material of the coating or shell may be a metal while the first material may be either a conductor or a dielectric material. In another example, the second material may be a dielectric and the first material may be a conductor such as a metal, for example. A nanoparticle that is capable of supporting a plasmon (e.g., either a surface plasmon or a bulk plasmon) is defined as a ‘plasmonic nanoparticle’. For example, a metal nanoparticle or a metal clad nanoparticle may serve as a plasmonic nanoparticle.

By definition herein, ‘nanoscale’ means a dimension that is generally less than about 1000 nanometers (nm). For example, a structure or particle that is about 5 nm to about 300 nm in extent is considered a nanoscale structure. Similarly, a slot having an opening size of between about 5 nm and 100 nm is also considered a nanoscale structure, for example.

Further, as used herein, the article ‘a’ is intended to have its ordinary meaning in the patent arts, namely ‘one or more’. For example, ‘a nanofinger’ means one or more nanofingers and as such, ‘the nanofinger’ means ‘the nanofinger(s)’ herein. Also, any reference herein to ‘top’, ‘bottom’, ‘upper’, ‘lower’, ‘up’, ‘down’, ‘front’, back′, ‘left’ or ‘right’ is not intended to be a limitation herein. Herein, the term ‘about’ when applied to a value generally means within the tolerance range of the equipment used to produce the value, or in some examples, means plus or minus 10%, or plus or minus 5%, or plus or minus 1%, unless otherwise expressly specified. Moreover, examples herein are intended to be illustrative only and are presented for discussion purposes and not by way of limitation.

FIG. 1Aillustrates a cross sectional view of a self-exciting surface enhanced Raman spectroscopy (SERS) structure100, according to an example consistent with the principles described herein.FIG. 1Billustrates a perspective view of the self-exciting SERS structure100ofFIG. 1A, according to an example consistent with the principles described herein. As illustrated, the self-exciting SERS structure100is configured to sense an analyte in a vicinity of the self-exciting SERS structure100. For example, the analyte may be suspended in and carried by a fluid that flows through or past the self-exciting SERS structure100, for example. According to various examples, the self-exciting SERS structure100senses the analyte by producing a Raman scattering signal through an inelastic interaction between an excitation signal and the analyte. Furthermore, the excitation signal is produced by the self-exciting SERS structure100itself (i.e., providing self-excitation of the Raman scattering signal), according to various examples. In particular, the self-exciting SERS structure100includes an integral optical excitation source, according to various examples.

As illustrated, the self-exciting SERS structure100includes a plurality of nanofingers110that includes one or more nanolasers112. In particular, a nanofinger110of the plurality includes an optical gain material to provide stimulated emission (e.g., of photons) and an optical cavity to provide optical feedback. The combination of the optical gain material and the optical cavity yield a nanolaser112, according to various examples. In some examples, each nanofinger110of the plurality is so configured as nanolasers112. The nanolaser112of the nanofinger110is configured to provide an optical excitation signal. In particular, the optical excitation signal is provided through light amplification by stimulated emission of radiation (i.e., photons produced by lasing) within the optical cavity employing the optical feedback, according to various examples. In some examples, the excitation signal is, or at least includes, the optical excitation signal produced by the nanolasers112of the respective nanofingers110. As such, the nanolasers112are the integral optical excitation source, according to various examples.

In some examples, the nanofingers110have a distal or free end longitudinally opposite to another end that is attached to or otherwise supported by (e.g., a ‘fixed’ end) a supporting substrate114. For example, the nanofinger110may be rigidly attached to the supporting substrate114at the fixed end. In other examples, the nanofingers110of the plurality may be indirectly attached to the supporting substrate114through an intermediate material or layer, for example. In yet other examples (not illustrated inFIGS. 1A-1B), the nanofingers are not attached to the supporting substrate at either longitudinally opposite ends (e.g., both ends are free). For example, the nanofingers110may be distributed or laying in a substantially horizontal configuration on the supporting substrate114, while not being attached at either of the ends of the nanofinger110.

In various examples, the nanofinger110or a portion thereof may be configured to preferentially capture or retain the analyte in a vicinity of the nanofinger110. For example, a surface of the nanofinger110may adsorb or bind the analyte. In some examples, the nanofingers110or a portion thereof may be functionalized to preferentially bind or provide selective adsorption of the analyte. In some examples, the nanofingers110may actively capture or trap the analyte (e.g., by a motion of the nanofingers110).

In some examples, the optical gain material of the nanofinger110may include a semiconductor. In particular, any semiconductor or hybrid semiconductor combination (e.g., various metal-semiconductor combinations) that provides optical gain may be employed. For example, the semiconductor may be or include a doped or undoped (i.e., substantially intrinsic or unintentionally doped) semiconductor such as various III-V and II-VI compound semiconductors including, but not limited to, one or more of gallium arsenide (GaAs), indium gallium arsenide (InGaAs), gallium nitride (GaN), gallium antimonide (GaSb) indium phosphide (InP). In other examples, the nanofinger110may include a solid host material doped with an impurity. The solid host material may include, but is not limited to, yttrium aluminum garnet (YAG), yttrium lithium fluoride (YLF), sapphire (aluminum oxide), and various glasses, while the impurity dopant may include, but is not limited to, chromium (Cr), neodymium (Nd), erbium (Er) and titanium (Ti). In yet other examples, the nanofinger110may be or include a plastic or a polymer such as, but not limited to, polyfluorene or polymethyl methacrylate (PMMA) doped with a perylimide dye, and related plastics and plastic/dye combinations that exhibit stimulated emission when pumped by an optical pump, for example.

According to various examples, the self-exciting SERS structure100further includes a nanoparticle120disposed adjacent to the nanofingers110. In some examples (e.g., as illustrated), the adjacent nanoparticle120is disposed on the free end of the nanofinger110. For example, the nanoparticle120may be attached to the free end of the nanofinger110opposite to the fixed end that is attached to the supporting substrate, as illustrated inFIGS. 1A and 1B. In other examples (not illustrated), the nanoparticle may be attached or otherwise located or associated with a longitudinal side of the nanofinger.

According to various examples, the optical excitation signal provided by the nanofingers110that include nanolasers112is configured to illuminate the nanoparticles120. In particular, according to some examples, the nanofinger110acting as a nanolaser112may preferentially emit the optical excitation signal through the ends of the nanofinger110, including the free end at which the nanoparticle120is located. The optical excitation signal may be substantially concentrated in a vicinity of the nanoparticle120located at the end of the nanofinger110, for example. When the nanoparticle120is located at the side of the nanofinger110, the optical excitation signal emitted from an end of the nanofinger110may illuminate a nanoparticle120of an adjacent nanofinger110, for example. In another example, an evanescent field of the optical excitation signal within the nanofinger110may couple to and thus ‘illuminate’ the nanoparticle120at the side of the nanofinger110.

According to various examples, the nanoparticle120includes a SERS-enhancing material. In particular, a material of the nanoparticle120is configured to enhance Raman scattering from an adjacent analyte. The SERS-enhancing material may be substantially any material that supports a surface plasmon, according to various examples. In other examples, the nanoparticles120may support a bulk plasmon. In some examples, the SERS-enhancing material of the nanoparticle120is a conductive material such as a metal. For example, the metal may include, but is not limited to, gold, silver, platinum, other noble metals, aluminum, copper, as well as an alloy or a mixture of any of these metals with each other or another metal. The nanoparticle120may be a gold catalyst nanoparticle used to grow the nanofinger110using VLS growth, for example.

In some examples, the nanoparticle120may include substantially only the conductive material (e.g., the metal). For example, the nanoparticles120may be metal nanoparticles120. In other examples, the conductor material (e.g., the metal) may be used to form a surface of the nanoparticles120. For example, the nanoparticles120may include a metal shell surrounding a core of another material such as, but not limited to, a semiconductor or a dielectric. In some examples, the nanofinger110itself may also support one or both of surface plasmons and bulk plasmons (e.g., when the nanofinger110is or includes an electrically conductive material such as metal). For example, the nanofinger110may include a metallic surface either along an entire length of the nanofinger110or in a vicinity of the tip at the free end. The presence of surface plasmons and bulk plasmons may be responsible for rendering one or both of the nanofinger110and the nanoparticle120Raman-enhancing, according to various examples.

In some examples, the nanoparticle120may be functionalized to preferentially adsorb the analyte. For example, the surface of a metal nanoparticle120may be functionalized to preferentially bind with particular analyte species. The surface functionalization may be provided by a metal-oligonucleotide conjugate to preferentially bind various molecules such as, but not limited to, DNA, RNA, or segments of either thereof, for example.

According to some examples, the plurality of nanofingers110with nanoparticles120disposed on the nanofingers110are arranged as an ordered group or array on the supporting substrate114. In some examples, the ordered array includes a ‘multimer’ of nanofingers110with attached nanoparticles120. A nanofinger110of the multimer includes a nanolaser112, or in some examples, each nanofinger110of the multimer includes a nanolaser112. The ordered array including a multimer of nanofingers110is configured to provide a SERS hotspot between adjacent ones of the nanoparticles120disposed on the free ends of the nanofingers110in the multimer, according to some examples.

According to various examples, the multimer may include a group of two, three, four, five, six or more nanofingers110. A multimer having two nanofingers110may be referred to as a ‘dimer,’ a multimer having three nanofingers110may be referred to as ‘trimer,’ a multimer having four nanofingers110may be referred to as a ‘tetramer,’ and so on. In some examples, the nanofingers110of the multimer may be arranged such that at least the free ends of the nanofingers110with attached nanoparticles120are located at vertices of a polygon (e.g., a digon, a trigon, a tetragon, a pentagon, a hexagon, and so on). The polygon may be a regular polygon, in some examples.

FIG. 2Aillustrates a top view of a multimer116having two nanofingers110with attached nanoparticles120arranged as a dimer, according to an example consistent with the principles described herein.FIG. 2Billustrates top view of a multimer116having three nanofingers110with attached nanoparticles120arranged as a trimer, according to an example consistent with the principles described herein.FIG. 2Cillustrates top view of a multimer116having four nanofingers110with attached nanoparticles120arranged as a tetramer, according to an example consistent with the principles described herein.

According to various examples, the nanoparticles120of the multimer116may be either touching one another or spaced apart from one another. For example, the nanoparticles120on the tips of the nanofingers110in the multimer116may be substantially touching or in close proximity, such that they are separated by a gap of about a few nanometers or less. Further, the nanofingers110in the multimer116may be tilted such that their tips with the attached nanoparticles120lean toward one another. The tilting may facilitate contact between the nanoparticles120on the tips of the nanofingers110, for example.

In some examples, the self-exciting SERS structure100may include a plurality of multimers116. For example, the plurality of multimers116may include several, tens, hundreds, or more multimers116. The multimers116may be spaced apart from one another across the support substrate114, for example. In other examples, the multimers116may be immediately adjacent or even touching one another (e.g., a nanoparticle120of a first multimer116may touch a nanoparticle120of an adjacent SERS multimer116). A spacing between the SERS multimers116of the plurality when spaced apart may be either regular (i.e., a periodic spacing) or irregular (e.g., a substantially random spacing), according to various examples.

In particular, in some examples, the plurality of multimers116may be arranged in a particular repeating ordered pattern or an ‘array’ of multimers116. The array of multimers116, including both small arrays (e.g., bundles) and large arrays, may include, but is not limited to, a linear array or one-dimensional (1-D) array or a two-dimensional (2-D) array (e.g., a rectilinear array, a circular array, etc.). For example, a plurality of multimers116may be arranged in a row for a 1-D array. A plurality of 1-D arrays or rows of multimers116may be arranged next to one another to form a 2-D rectilinear array of multimers116, for example. Various other 2-D arrays may be employed including, but not limited to, polygonal arrays and circular arrays.

Referring again toFIG. 1Aand as mentioned above, the nanolaser112includes an optical cavity in the nanofinger110according to various examples. For example, the optical cavity may include the entire nanofinger110bounded by the longitudinal ends of the nanofinger110. The ends of the nanofinger110may represent a material discontinuity (e.g., a change in dielectric constant) between the nanofinger110and air or a material of the supporting substrate114, for example. The material discontinuity at two opposite ends may act as a pair of opposing mirrors to define the optical cavity, for example. In another example, the nanoparticle120on the free end of the nanofinger110may provide optical reflection and thus form one end of the optical cavity. A second end of the optical cavity may be formed at an interface between the supporting substrate114and the nanofinger110(e.g., due to a material discontinuity). In yet other examples, the nanofinger110may include one or more mirrors such as, but not limited to, Bragg mirrors to provide the optical cavity.

FIG. 3Aillustrates a cross sectional view of nanofingers110including a Bragg mirror118, according to an example consistent with the principles described herein. In particular, the nanofingers110include a Bragg mirror118adjacent to the free end and the nanoparticle120at the free end, as illustrated. The optical cavity may be provided or created between the Bragg mirror118and the supporting substrate114, for example. The Bragg mirror118may be formed during growth of the nanofinger110by varying growth conditions, for example.

FIG. 3Billustrates a cross sectional view of nanofingers110including a Bragg mirror118, according to another example consistent with the principles described herein. In particular,FIG. 3Billustrates the nanofingers110including two Bragg mirrors118. A first Bragg mirror118is located adjacent to the free end of the nanofingers110and a second Bragg mirror118′ is locate adjacent to an end of the nanofinger110that is adjacent to the supporting substrate114(e.g., the fixed end). The optical cavity is provided or created between the two Bragg mirrors118,118′, as illustrated.

FIG. 3Cillustrates a cross sectional view of nanofingers110including a Bragg mirror118, according to another example consistent with the principles described herein. In particular,FIG. 3Cillustrates the nanofingers110including a first Bragg mirrors118to establish a first end of the optical cavity. A second Bragg mirror118′ that forms a second end of the optical cavity is located in the supporting substrate114, as illustrated. For example, the second Bragg mirror118′ may be deposited on or formed in a surface of the supporting substrate114prior to attaching the nanofingers110. In yet other examples (not illustrated), a surface of the supporting substrate114may include a reflective coating (e.g., a metal layer) that provides reflection to form the second end of the optical cavity. In each ofFIGS. 3A-3C, the nanolaser112may be provided by the optical cavity associated with the Bragg mirrors, for example.

Referring again toFIGS. 1A and 1B, the self-exciting SERS structure100is configured to produce the optical excitation signal by ‘lasing’ in the optical material in conjunction with optical feedback provided by the optical cavity of the nanolasers112of the nanofingers110. In some examples, the lasing may be provided by optical pumping, while in other examples, the lasing may be provided by electrical pumping. A pump source provides either the optical pumping or the electrical pumping, according to various examples.

In particular, in some examples, the nanolasers112of the nanofingers110are configured to be optically pumped by an optical pump source. The optical pump source may be a ‘light’ source such as a laser or a light emitting diode (LED) that illuminates the nanofingers110, for example. Optical pumping produces photons by stimulated emission within the optical gain material. The photons then resonate within the optical cavity eventually producing the optical excitation signal, according to some examples. In some examples, the optical pump source includes a vertical cavity surface-emitting laser (VCSEL). The plurality of nanofingers110may be disposed on a surface of an output aperture of the VCSEL, for example. Light emitted by the VCSEL may provide optical pumping, according to some examples.

FIG. 4Aillustrates a perspective view of a self-exciting SERS structure100configured to be optically pumped, according to an example consistent with the principles described herein. In particular, the self-exciting SERS structure100illustrated inFIG. 4Aincludes a plurality of nanofingers110disposed on a surface of an output aperture132of an optical pump source130, e.g., a VCSEL. As illustrated, the optical pump source or VCSEL130is also the supporting substrate114. Also as illustrated, the nanofingers110of the plurality are tilted toward one another in a configuration that may enhance trapping of an analyte102, for example.

In other examples, the nanolasers112of the nanofingers110are configured to be electrically pumped by an electrical pump source130to provide the optical excitation signal. For example, the nanofingers110of the plurality may include a semiconductor junction. The electrical pump source130may include a voltage source connected across the semiconductor junction, for example. In various examples, the semiconductor junction may include, but is not limited to, a p-n junction, a p-i-n junction, a heterojunction. For example, the nanolaser112of the nanofinger110may be a double heterojunction laser or a quantum well laser that employs a heterojunction. In another example, a semiconductor junction may be formed between the nanofinger110and the supporting substrate114.

FIG. 4Billustrates a cross sectional view of a self-exciting SERS structure100configured to be electrically pumped, according to an example consistent with the principles described herein. In particular, the self-exciting SERS structure100illustrated inFIG. 4Aincludes nanofingers110that include a p-n junction within the optical gain material (e.g., GaAs) of the nanolaser112within the nanofingers110. Also illustrated is a voltage source V connected across the p-n junction to serve as the electrical pump source130. The supporting substrate114may act as an electrical connection to the fixed end or a base of the nanofinger110, for example, as illustrated. Note that the doping of the semiconductor junction may be reversed in the nanofingers110and still be within the scope of the example herein.

Although not explicitly illustrated inFIG. 4B, the connection of the voltage source across the semiconductor junction (e.g., the p-n junction) may be provided in any of a number of ways. For example, the self-exciting SERS structure100may further include a conductive layer (not illustrated) on top of the nanofingers110. The conductive layer may be attached to the nanoparticles120, for example. The voltage source V may be connected to the conductive layer, for example. In some examples, the conductive layer may be porous to enable an analyte (e.g., suspended in a fluid) to interact with (e.g., be adsorbed by) the nanoparticles120. The porous conductive layer may include, but is not limited to, a conductive permeable membrane and a conductive mesh, for example. In another example, the conductive layer may be substantially nonporous (e.g., a solid conductor layer). When the conductive layer is substantially nonporous, the analyte may be introduced below the conductive layer in a space between the conductive layer and the supporting substrate114, in some examples. For example, a fluid carrying the analyte may be introduced and caused to flow through the SERS structure from an end or an edge thereof between the supporting substrate114and the substantially nonporous conductive layer. In yet other examples, the nanoparticles120including a conductive material (e.g., the SERS-enhancing material) may be sufficiently close to one another to form a conduction path (i.e., the conductive layer) by themselves. Electrical connection may be made to an edge of the conductive layer formed by the nanoparticles120, for example. In yet another example, the nanofingers110may be insulated (e.g., with an insulating shell) and a conductive fluid may be employed to carry the analyte. In this example, the voltage source V may be connected across the semiconductor junction using the conductive fluid.

According to some examples of the principles described herein, a self-exciting surface enhanced Raman spectroscopy (SERS) sensor is provided.FIG. 5illustrates a block diagram of a self-exciting surface enhanced Raman spectroscopy (SERS) sensor200, according to an example consistent with the principles described herein.FIG. 6Aillustrates a perspective view of the self-exciting SERS sensor200illustrated inFIG. 5, according to an example consistent with the principles described herein.FIG. 6Billustrates a perspective view of the self-exciting SERS sensor200illustrated inFIG. 5, according to another example consistent with the principles described herein. According to various examples, the self-exciting SERS sensor200is configured to produce a Raman scattering signal from an analyte using an optical excitation signal provided by the self-exciting SERS sensor200itself.

As illustrated, the self-exciting SERS sensor200includes a SERS structure210. In some examples (e.g.,FIG. 6A), the SERS structure210includes a plurality of SERS-enhancing nanorods212. The SERS-enhancing nanorods212include a SERS-enhancing material, according to various examples. For example, the SERS-enhancing nanorods212may include metal such as, but not limited to, gold, silver, platinum, other noble metals, aluminum, copper, as well as an alloy or a mixture of any of these metals with each other or another metal. In another example, the SERS-enhancing nanorods212may be metal nanorods or may be non-metal nanorods coated with the metal. In other examples, the SERS-enhancing nanorods212may include non-metal nanorods mixed together with nanoparticles that include a SERS-enhancing material. In yet other examples, the SERS-enhancing nanorods212may be a combination of one or both of metal nanorods and metal-coated non-metal nanorods along with nanoparticles that include a SERS-enhancing material.

In other examples (e.g.,FIG. 6B), the SERS structure210may include a plurality of nanofingers214having SERS-enhancing nanoparticles216disposed at an end (e.g., a free end) of the nanofingers214. According to various examples, the SERS-enhancing nanoparticles216include a SERS-enhancing material. In some examples, the SERS-enhancing nanoparticles216are substantially similar to the nanoparticles120described above with respect to the self-exciting SERS structure100.

According to various examples, the nanofingers214may include one or both of a SERS-enhancing material (e.g., metal) and a substantially non-SERS-enhancing material. For example, the nanofinger214may include a semiconductor. The semiconductor may be doped or undoped (i.e., substantially intrinsic) silicon (Si), germanium (Ge) or an alloy of Si and Ge, for example. In other examples, the semiconductor may include one or more of gallium arsenide (GaAs), indium gallium arsenide (InGaAs), gallium nitride (GaN), or various other III-V, II-VI, and IV-VI compound semiconductors. In other examples, the nanofinger214may be or include a plastic or a polymer such as, but not limited to, polyurethane, poly(tert-butyl methacrylate) (P(tBMA)), polymethylmethacrylate (PMMA), polystyrene, polycarbonate or related plastics. In yet other examples, the nanofinger112may include a metal such as, but not limited to, gold, silver, platinum, other noble metals, aluminum copper, or an alloy or a combination of two or more metals.

As illustrated in FIGS.5and6A-6B, the self-exciting SERS sensor200further includes a vertical cavity surface-emitting laser220(VCSEL). The VCSEL220is configured to produce an optical excitation signal222at an output aperture224of the VCSEL220. According to various examples, the SERS structure210is disposed on a surface of the output aperture224of the VCSEL220.

For example, as illustrated inFIG. 6A, the SERS-enhancing nanorods212may be disposed on the output aperture224. In some examples, the SERS-enhancing nanorods212may be distributed in a substantially random fashion on the output aperture224(e.g., as illustrated). For example, the SERS-enhancing nanorods212may be suspended in a carrier liquid and deposited on the output aperture224using an inkjet printer or similar means. In other examples (not illustrated), the SERS-enhancing nanorods212may be distributed on the output aperture224in a substantially ordered fashion (e.g., as an ordered array). For example, the SERS-enhancing nanorods212may be attached to the surface of the output aperture224at a first end of the SERS-enhancing nanorods212with a second or opposite end extending generally away from the surface. The SERS-enhancing nanorods212may be arranged in an ordered array that includes multimers substantially similar to the multimers116described above with respect to the self-exciting SERS structure100, for example.

Similarly, as illustrated inFIG. 6B, the plurality of nanofingers214may be disposed on the surface of the output aperture224of the VCSEL220. In particular, the nanofingers214having SERS-enhancing nanoparticles216disposed at the free ends thereof may be attached to the output aperture surface at an end opposite the free end, as illustrated. In such a configuration, the nanofingers214may extend generally away from the output aperture surface.

According to various examples, the nanofingers214may be arranged in an ordered array on the output aperture surface. In some examples, the ordered array may include a multimer of adjacent nanofingers214to provide a SERS hotspot between adjacent ones of the nanoparticles216disposed on the free ends of the nanofingers214in the multimer.FIG. 6Billustrates a plurality of multimers having four nanofingers214, for example. In other examples (not illustrated), the nanofingers214may be arranged in an ordered array that includes multimers substantially similar to the multimers116described above with respect to the self-exciting SERS structure100. In some examples, the nanofingers214may be tilted (e.g., tilted toward one another), as illustrated inFIG. 4A.

In some examples, the nanofingers214may serve as a light guide to guide the optical excitation signal from the output aperture224to the nanoparticle216. For example, the nanofingers214may include a dielectric material that serves as the light guide. For example, the nanofingers214may guide the optical excitation signal by total internal reflection. Alternatively, the nanofingers214may guide the optical excitation signal by another means including, but not limited, a sub wavelength optical guide or a surface plasmon waveguide, for example.

FIG. 7illustrates a cross section of the self-exciting SERS sensor200, according to an example consistent with the principles described herein. In particular,FIG. 7illustrates the nanofingers214guiding the optical excitation signal222from the output aperture224of the VCSEL220to the nanoparticle216. As illustrated, the optical excitation signal222is guided by total internal reflection.

Referring again toFIG. 5, in some examples, the self-exciting SERS sensor200may further include a filter230between the SERS structure210and the VCSEL220. The filter230may be a thin film filter, for example. In some examples, the thin film filter230serves as the output aperture surface of the VCSEL220. For example, the thin film filter230may be incorporated into a surface layer of the VCSEL220. Alternatively, the thin film filter230may be deposited on the surface of the output aperture to provide a new surface on which the SERS structure210is disposed.

According to various examples, the filter230may be either a short-pass or bandpass filter configured to reduce or even substantially block optical signals from the VCSEL that may overlap in wavelength with the Raman scattering signal, for example. For example, the filter230may include Bragg layers that allow passage of the optical excitation signal but substantially block other (e.g., spontaneous emission) optical signals produced by the VCSEL220.

According to some examples of the principles described herein, a self-exciting SERS system is provided.FIG. 8illustrates a block diagram of a self-exciting SERS system300, according to an example consistent with the principles described herein. As illustrated, the self-exciting SERS system300includes an integral optical excitation source310. The integral optical excitation source310is configured to provide an optical excitation signal312. The self-exciting SERS system300further includes a SERS-enhancing structure320. The SERS-enhancing structure320is configured to be illuminated by the optical excitation signal312from the integral optical excitation source310, according to various examples. In some examples, the illumination produces a Raman scattering signal322from an analyte in a vicinity of the SERS-enhancing structure320. Together, the integral optical excitation source310and SERS-enhancing structure320provide a self-exciting SERS assembly302that may produce the Raman scattering signal322when exposed to the analyte, according to various examples.

In some examples, the SERS structure320includes a SERS-enhancing nanoparticle disposed on a free end of a nanofinger nanolaser. According to various examples, the SERS-enhancing nanoparticle includes a SERS-enhancing material. In some examples, the SERS-enhancing nanoparticle may be substantially similar to the nanoparticle120described above with respect to the self-exciting SERS structure100. In particular, the SERS-enhancing material may include a metal such as, but is not limited to, gold, silver, platinum, other noble metals, aluminum, copper, as well as an alloy or a mixture of any of these metals with each other or another metal. The SERS-enhancing nanoparticle may include substantially only the SERS-enhancing material or may include another material that is coated with the SERS-enhancing material, for example.

According to various examples, the SERS-enhancing nanoparticle is disposed on a free end of a nanofinger nanolaser. The nanofinger nanolaser includes an optical gain material and an optical cavity. The optical gain material provides stimulated emission of radiation (e.g., photons) while the optical cavity is configured to provide optical feedback. Together the optical gain material and the optical cavity support light amplification by stimulated emission of radiation such that the nanofinger nanolaser is configured to produce the optical excitation signal312through lasing. According to various examples, the nanofinger nanolaser is the integral optical excitation source310and provides the optical excitation signal312through the lasing of the optical gain material within the optical cavity of the nanofinger nanolaser.

In some examples, the nanofinger nanolaser is substantially similar to the nanofinger110including a nanolaser described above with respect to the self-exciting SERS structure100. In particular, the nanofinger nanolaser may be configured to produce the optical excitation signal312through either of optical pumping or electrical pumping by a pump source. Further, the nanofinger nanolaser with the nanoparticle disposed on the free end of the nanofinger nanolaser may be arranged in an ordered array of nanofinger nanolasers. The ordered array may be arranged on a supporting substrate, for example. In some examples, the ordered array of nanofinger nanolasers may include a multimer of nanofinger nanolasers. The multimer is configured to provide a SERS hotspot between adjacent ones of the nanoparticles disposed on the nanofinger nanolasers of the multimer. The multimer may be substantially similar to the multimer116described above with respect to the self-exciting SERS structure100, for example. Optical pumping may be provided by an external optical pump source such as, but not limited to, a light emitting diode or a vertical cavity surface-emitting laser, for example.

In other examples, the SERS-enhancing structure320includes a plurality of nanofingers disposed on an output aperture of a vertical cavity surface-emitting laser (VCSEL). The nanofingers are disposed to provide a SERS hotspot when the SERS-enhancing structure320is illuminated by the optical excitation signal312, for example. In these examples, the integral optical excitation source310includes the VCSEL. For example, the VCSEL may be configured to produce the optical excitation signal312and illuminate the plurality of nanofingers disposed on the VCSEL output aperture. In some examples, the SERS-enhancing structure320with the VCSEL acting as the integral optical excitation source310is substantially similar to the self-exciting SERS sensor200, described above.

In particular, in some examples, the nanofingers include a SERS-enhancing material. For example, the nanofingers may include a metal. The nanofingers including a SERS-enhancing material may be substantially similar to the SERS-enhancing nanorods212described above with respect to the self-exciting SERS sensor200, for example. Further, in some examples, the nanofingers have SERS-enhancing nanoparticles disposed on an end of the nanofingers. The nanofingers having the end-disposed SERS-enhancing nanoparticles may or may not also include a SERS-enhancing material, according to some examples. In some examples, the nanofingers having the end-disposed SERS-enhancing nanoparticles may be substantially similar to the nanofingers214having SERS-enhancing nanoparticles216disposed at an end of the nanofingers214, as described above with respect to the self-exciting SERS sensor200, for example.

Referring again toFIG. 8, the self-exciting SERS system300may further include a detector330. The detector330may be configured to detect a Raman scattering signal322produced by an interaction between the SERS-enhancing structure320and an analyte in the vicinity of the SERS hotspot associated with illuminated SERS-enhancing structure320. The detector320may be a spectrometer, for example.

Thus, there have been described examples of a self-exciting SERS substrate, a self-exciting SERS sensor and a self-exciting SERS system that include an integral optical excitation signal source to provide self-excitation of a Raman scattering signal. It should be understood that the above-described examples are merely illustrative of some of the many specific examples that represent the principles described herein. Clearly, those skilled in the art can readily devise numerous other arrangements without departing from the scope as defined by the following claims.