PHOTOLUMINESCENT POINT LIGHT SOURCE

Inventors: Valentyn Liulevych and Gang-yu Liu

BACKGROUND

Field of the Invention

[0001] The present invention relates to high-resolution near-field optical imaging. More specifically, the present invention relates to a method and apparatus for producing a high intensity near-field light source using a photoluminescent point light source.

Related Art

[0002] Near-field optical imaging is a powerful optical engineering technique which has the ability to acquire optical images of samples with spatial resolutions well-below the "diffraction limit." The diffraction limit arises from a phenomenon in conventional "far-field" optical microscopes in which a propagating light may be focused to a spot size with a minimum diameter of roughly one half the wavelength of light. Consequently, when using a conventional diffraction-limited microscope for optical imaging, the highest resolution obtainable is limited by the diffraction limit, which is typically on the order of a couple hundred nanometers.

[0003] Near-field optical imaging is based on the discovery that if a point light source with a dimension which is a fraction of the wavelength (often referred to as "sub-wavelength" dimension) is positioned in the close proximity (typically IOnm-lOOnm) to a subject being studied, an imaging resolution well-below the diffraction limit of the light can be achieved. Note that in contrast to far-field imaging, wherein propagating light is used for illumination, near-field imaging involves using a non-propagating "evanescent" light source to interact with an object.

[0004] A near field scanning optical microscopy (NSOM), which is also referred to as a scanning near-field optical microscopy (SNOM), uses a near-field optical imaging mechanism.

Typically, NSOMs can be classified into two types: "aperture" NSOMs and "apertureless"

NSOMs. An aperture NSOM produces a sub-wavelength light spot by guiding light through an ultra small aperture located at the end of a tapered, metal-coated optical fiber, or at the tip of an atomic force microscope (AFM) probe. In this type of system, an apertured optical fiber or AFM probe operates as a waveguide for both sample illumination and near field optical signal collection. However, the practicality and range of applications of aperture NSOMs is severely limited by the following inherent drawbacks: complexity and cost of probe manufacturing; low light emission from the aperture; and poor reliability and durability of the probe.

[0005] In contrast, an apertureless NSOM does not emit light but instead uses a small sharp probe to perturb an optical field carrying spatial information of a sample being imaged, wherein the sharp probe is used to either scatter the light or to detect the coupling between dipoles in the probe and the sample. The apertureless NSOM has the advantage of using simple low-cost probes such as standard AFM cantilevers, and avoids other drawbacks associated, with the apertures describe-above. Unfortunately, the apertureless NSOM generally requires a complicated detection mechanism to detect useful near-field signals which are inundated by a dominant far-field illumination. Furthermore, the acquired near-field image is often difficult to interpret due to the complicated interaction mechanisms between the probe and sample.

[0006] Hence, what is needed is a method and an apparatus for producing a high-intensity near-field light source without the problems described above.

SUMMARY

[0007] One embodiment of the present invention provides a system that generates a point light source. During operation, the system illuminates a sharp probe using an excitation light, wherein the sharp probe is made of a photo luminescent (PL) material. Illuminating the sharp probe causes a PL emission from the sharp probe. This PL emission generates a point light source at the apex of the sharp probe, wherein the spot size of the point light source is restricted by the radius of curvature of the apex of the sharp probe.

[0008] In a variation on this embodiment, the point light source is a near-field light source, wherein the spot size of the point light source is smaller than the diffraction limit of the PL emission wavelength.

[0009] In a variation on this embodiment, the sharp probe can include: an atomic force microscopy (AFM) probe; a tapered optical fiber probe; and a nanotube probe, which is made of carbon or boron.

[0010] In a variation on this embodiment, the radius of curvature of the apex of the sharp probe is between 0.3 nm and 250 run.

[0011] In a variation on this embodiment, the excitation light can include: a laser beam; a light emitting diode (LED) light source; and a high intensity white light source.

[0012] In a further variation on this embodiment, the system illuminates the sharp probe by focusing the excitation light on the apex of the sharp probe.

[0013] In a variation on this embodiment, the system illuminates the sharp probe by illuminating the apex of the sharp probe at a predetermined incident angle to the axis of the sharp probe.

[0014] In a variation on this embodiment, the PL material can include: silicon nitride; a PL metal, which can include gold, silver, platinum, and copper; a PL semiconductor; and any other PL material suitable for fabricating the sharp probe.

[0015] In a variation on this embodiment, the system uses the point light source as a near- field light source in a near-field scanning optical microscopy (NSOM).

[0016] In a further variation on this embodiment, the system obtains the near- field light source for the NSOM by exciting the PL emission in the sharp probe with a grazing incident- angle illumination.

[0017] In a further variation on this embodiment, the NSOM comprises one or more filters configured to separate the illumination light source from the PL emission.

[0018] In a further variation on this embodiment, the system uses the point light source in the NSOM to obtain true color near-field optical images.

[0019] In a variation on this embodiment, the system adjusts the PL emission intensity by varying one or more of the following: probe geometry; probe material composition and

nanoscopic structure; intensity of the excitation light; and incident angle of the excitation light with respect to the axis of the probe.

[0020] Another embodiment of the present invention provides a system that generates a wavelength-tunable point light source. During operation, the system first coats a sharp probe with a layer of photoluminescent (PL) material. The system then illuminates the coated sharp probe using an excitation light to cause a PL emission from the coating material. The PL emission generates a point light source at the apex of the coated sharp probe, wherein the wavelength of the PL emission is tunable by varying the composition of the PL material.

[0021] In a variation on this embodiment, the point light source is a near-field light source, wherein the spot size of the point light source is smaller than the diffraction limit of the PL emission wavelength.

[0022] In a variation on this embodiment, the sharp probe can include: an atomic force microscope (AFM) probe; a tapered optical fiber probe; and a nanotube probe which is made of carbon or boron. [0023] In a variation on this embodiment, the radius of curvature of the apex of the coated sharp probe is between 0.3 nm and 250 nm.

[0024] In a variation on this embodiment, the excitation light can include: a laser beam; a light emitting diode (LED) light source; and a high intensity white light source.

[0025] In a further variation on this embodiment, the system illuminates the coated sharp probe by focusing the excitation light on the apex of the coated sharp probe.

[0026] In a variation on this embodiment, the system illuminates the coated sharp probe by illuminating the apex of the coated sharp probe at a predetermined incident angle to the axis of the coated sharp probe.

[0027] In a variation on this embodiment, the PL material can include: silicon nitride; a PL metal, which can include gold, silver, platinum, and copper; a PL semiconductor; and any other PL material suitable for coating the sharp probe.

[0028] In a variation on this embodiment, the system uses the point light source as a near- field light source in a near-field scanning optical microscopy (NSOM).

[0029] In a further variation on this embodiment, the system obtains the near-field light source for the NSOM by exciting the PL emission in the coated sharp probe with a grazing incident-angle illumination.

[0030] In a further variation on this embodiment, the NSOM comprises one or more filters configured to separate the illumination light source from the PL emission.

[0031] In a further variation on this embodiment, the system uses the point light source in the NSOM to obtain true color near-field optical images.

[0032] In a variation on this embodiment, the system adjusts the PL emission intensity by varying one or more of the following: probe geometry; PL material composition and nanoscopic structure; intensity of the excitation light; and incident angle of the excitation light with respect to the axis of the probe.

[0033] In a variation on this embodiment, the layer of PL material has a thickness between 0.3 nm to 100 nm.

[0034] In a variation on this embodiment, the system tunes the wavelength of the PL emission by: adjusting PL domains size; adjusting PL domain density; or creating color centers.

BRIEF DESCRIPTION OF THE FIGURES

[0035] FIG. IA illustrates a technique for producing a point light source using the PL effect in accordance with an embodiment of the present invention [0036] FIG. IB illustrates a true color photograph of a PL emission from a silicon nitride

AFM tip upon illuminated by an ultraviolet laser in accordance with an embodiment of the present invention.

[0037] FIG. 2A illustrates a technique for producing a wavelength tunable point light source in accordance with an embodiment of the present invention [0038] FIG. 2B illustrates a true color photograph of a PL emission from an Ag coated

AFM tip upon illuminated by an ultraviolet laser in accordance with an embodiment of the present invention.

[0039] FIG. 3 illustrates typical PL emission spectra of a bare silicon nitride probe and an Ag coated AFM probe in accordance with an embodiment of the present invention.

[0040] FIG. 4 illustrates a schematic diagram of an NSOM system which uses a PL point light source in accordance with an embodiment of the present invention.

[0041] FIG. 5A illustrates a PL NSOM image which is acquired simultaneously with an AFM topography image for gold nanoislands deposited on mica in accordance with an embodiment of the present invention.

[0042] FIG. 5B illustrates a representative PL NSOM cursor profile of a nano-gap at the edge of a gold nanoisland in FIG. 5 A in accordance with an embodiment of the present invention.

[0043] FIG. 6 illustrates a true color NSOM optical image of photo luminescent silver islands with three color-channel cross-sections in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0044] The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the claims.

Overview

[0045] The present invention provides a technique for producing a high intensity point light source from a sharp probe made of a photolurninescent (PL) material or alternatively from a sharp probe coated with a PL material. Such a point light source can be used in a NSOM system as the near-field light source to achieve sub-wavelength imaging resolution.

[0046] More specifically, a focused light beam (e.g. an ultraviolet laser beam) is utilized to induce a PL emission from the tip of a PL material-based probe by exciting the PL material in the tip of the probe. This PL emission is confined to the tip of the probe, which results in a point light source. Such a point light source has the intrinsic properties of high stability, long

durability and high emission intensity. Furthermore, by varying the PL material composition or by treating the PL material in the probe, a wide range of emission wavelengths can be obtained, which can subsequently be used to image broad categories of samples.

Photoluminescent Effect

[0047] The photoluminescenct (PL) effect results from a process wherein an atom or a cluster in a single element material or a compound absorbs photon energy, transitions to a higher energy state, and subsequently emits a photon to return to a lower energy state. The PL effect has been observed in many different materials, which include semiconductor materials (e.g., gallium arsenide, silicon nitride, porous silicon), metals (e.g., gold and silver), chemical compounds and crystals (e.g. rare earth elements), and organic materials (e.g., organic polymers, π-electron systems). In addition to using the PL material, producing a PL emission also requires an external light source to illuminate the PL material. This external light source can be either a broadband light source containing a wide spectrum of wavelengths, or it can be a narrowband or single wavelength source. Generally, to excite a particular energy transition in a PL material, photon energy supplied by the light source has to be equal or higher than the photon energy characteristic to the PL emission. The PL process can be extremely fast, and the time between absorption and emission can be on the order of nanoseconds.

Producing a Point Light Source Using PL Effect

[0048] FIG. IA illustrates a technique for producing a point light source by utilizing the PL effect in accordance with an embodiment of the present invention.

[0049] In this example, the excitation light source is a laser 102, which can be a pulse laser, or a continuous-power laser. Furthermore, laser 102 can be a semiconductor laser, a solid state laser, a gas laser, or any other types of laser now known or later developed. Alternatively, the external light source can include high energy white light sources, such as a mercury lamp, high energy light emitting diodes (LED), or any other light source suitable for exciting the PL energy transitions in a PL material.

[0050] Excitation light 104 from laser 102 radiates upon a sharp atomic force microscope (AFM) probe 106, so that the tip of the probe is illuminated by the incident light. Note that the

AFM probe is made of a PL material, such as silicon nitride. Note that the probe material can alternatively include PL metals or PL semiconductors. The sharpness of AFM probe 106 is typically defined by the radius of curvature at the apex of the probe. This radius of curvature of a typical AFM probe can be between 0.3 nm to 250 nm. Note that the present invention is not limited to using AFM probes. Other probes that can be used to produce a point light source include a taper optical fiber probe, a nanotube probe made of carbon or boron, or any other sharp probes that are microfabricated or nanofabricated from a PL material.

[0051] Illuminated sharp probe 106 then produces a responsive PL emission spectrum with a peak wavelength determined by the specific PL material. In one embodiment of the present invention, laser 102 emits ultraviolet (UV) light which causes a PL emission spectrum from a silicon nitride probe with a peak intensity at 650nm (i.e., orange light).

[0052J In one embodiment, excitation light 104 illuminates probe 106 at a predetermined incident angle 108 to cause the PL emission from probe 106 to have a desired light intensity. Furthermore, excitation light 104 illuminates probe 106 at a power level suitable for causing a PL emission from probe 106 without resulting in damage to the probe 106. Note that although not shown in FIG. 1, a focusing mechanism, for example an objective lens, is typically used to focus excitation light 104 to a small spot with the high power density on the probe.

[0053] PL emission 110 from probe 106 is confined to the apex of the probe. The spot size of the PL emission is roughly equal to or slight larger than the apex of the probe, which is defined by the radius of curvature of the apex. This is because the sub-wavelength dimension of the apex results in an exponential decay of the PL emission intensity away from the apex (decay length <250 nm), which keeps the emitted PL light localized around the apex.

[0054] For example, using a 30 nm radius probe, a light spot size of about 50 nm can be obtained, which is only about 1/10 of the 650 nm emission wavelength. Hence, the apex of sharp probe 106 effectively becomes a point light source upon illumination. Because the obtained spot size can be significantly smaller than the diffraction limit of the light, the point light source can be used as a near-field light source in near-field optical systems.

[0055] FIG. IB illustrates a true color photograph of a PL emission from a silicon nitride AFM tip upon illuminated by an UV laser in accordance with an embodiment of the present

invention. The tiny orange light spot indicates the optical confinement of the PL emission.

Producing a Wavelength Tunable Point Light Source Using PL Effect

[0056J In one embodiment of the present invention, a sharp probe in combination with the PL effect can be used to obtain a wavelength tunable point light source.

[0057] FIG. 2A illustrates a technique for producing a wavelength tunable point light source in accordance with an embodiment of the present invention.

[0058] In FIG. 2A, sharp probe 202 can generally include any type of probe with a sub- micro size probe tip, including, an AFM probe, a tapered optical fiber, a carbon nano-tube, or any other sharp probes. Note that in contrast to sharp probe 106 in FIG. IA, sharp probe 202 can be made of either a PL material or a non-PL material, because probe 202 itself is not used for producing a PL emission. In one embodiment, sharp probe 202 is made of glass (SiO ₂ ).

[0059] Sharp probe 202 is additionally coated with a layer of PL material, which is referred to as PL coating 204. PL coating 204 can include any PL material, such as a PL semiconductor, a PL metal, or a mixture of different PL materials. PL coating 204 is formed on the surface of sharp probe 202 using one or more of the thin film deposition techniques, which can include, but not limited to: electron beam evaporation; thermal resistive evaporation; molecular beam epitaxy (MBE); low pressure chemical vapor evaporation (LPCVD); metal orgainc chemical vapor evaporation (MOCVD); radio-frequency (RF) magnetron sputtering deposition; direct-current (DC) magnetron sputtering deposition; ion beam sputtering deposition; and electroplating. In one embodiment of the present invention, a 10 nm thick silver (Ag) is coated over a bare AFM tip by electron beam evaporation.

[0060] PL coating 204 provides uniform coverage over sharp probe 202, including on all side surfaces and the apex. For example, for a typical pyramidal AFM probe, PL material 204 is coated over all four surfaces of the pyramid and over the tip of the probe.

[0061] The illumination technique for a PL-coated sharp probe is substantially the same as in FIG. IA. In the embodiment of FIG. 2 A, a laser 206 is used to illuminate the probe tip at a predetermined incident angle. However, the PL emission is produced by PL coating 204 instead of the probe material. Because PL coating 204 is sufficiently thin, the radius of the probe tip

before and after the coating is not significantly increased. This is illustrated in a zoom-in cross- sectional view of coated apex 208. In one embodiment, the thickness of PL coating 204 can be in a range of 0.3 nm to 100 nm. It is desirable to maintain a small radius at the probe apex, which defines the spot size of the PL emission. [0062] Because different PL materials have different PL emission spectra, one embodiment of the present invention produces a desired PL emission wavelength by selectively depositing a PL material. For example, a gold (Au) coating can produce a yellow light, a copper (Cu) produces a red light, and a Ag coating gives rise to a blue light. Hence, by varying the composition of the PL coating material, a range of emission wavelengths may be obtained. [0063] FIG. 2B illustrates a true color photograph of a PL emission 210 from an Ag coated AFM tip upon illuminated by an UV laser in accordance with an embodiment of the present invention. The tiny blue light spot indicates a similar confinement to the orange PL emission in FIG. IB.

[0064] FIG. 3 illustrates typical PL emission spectra of a bare silicon nitride (Si ₃ N ₄ ) probe and an Ag coated AFM probe in accordance with an embodiment of the present invention. The S13N4 spectrum 302 has a broad peak which is maximized at 600 nm (i.e., 2.1 eV). The general feature of spectrum 302 is consistent with PL spectra of silicon nitride and oxynitride of various compositions which have typical PL maximum emission in the range of 2.2 eV- 2.8 eV. Studies have shown that the PL emission of a silicon nitride AFM tip with a peak at 2.1 eV corresponds to the photoluminescence of amorphous Si clusters with an average size of about 2 nm. Ag spectrum 304 has a main peak at 504 nm, and the overall spectrum is narrower in comparison to spectrum 302.

[0065] In one embodiment of the present invention, the PL emission wavelength may be varied by performing one or more post-treatments of PL coating 204. More specifically, such post-treatments can adjust material properties of PL coating 204 by changing PL domains size, varying PL domain density, or creating color centers. For example, it has been observed that using high-energy plasma to treat the PL coating can induce changes in the crystalline structure of the PL material, which causes a peak shift in the PL emission spectrum.

Using a PL Light Source in a NSOM System (PL NSOM System)

[0066] The point light source produced from PL emission of a sharp probe can be used as a near-field light source in a near-field optical microscope to achieve sub-wavelength optical imaging resolution as is described below.

System Setup

[0067] FIG. 4 illustrates a schematic diagram of an NSOM system 400 which uses a PL point light source in accordance with an embodiment of the present invention.

[0068] PL NSOM system 400 includes an AFM scanner (MFP-3D, Asylum Research Corp., not shown in FIG. 4) which is mounted on an inverted optical microscope (IX-50, Olympus America, only objective lens 402 of the microscope is shown). AFM scanner is equipped with a commercial silicon nitride AFM cantilever. AFM cantilever is integrated with a pyramidal silicon nitride AFM probe 404, which is suspended above an optically transparent sample 406. Note that besides using a silicon nitride probe, AFM probe 404 can also include other PL material-based probes without PL coating, or any commercially available sharp probe with a PL coating as earlier described.

[0069] A 405 run, 4 mW diode laser 408 (World Start Tech, Canada) is used as the excitation light to excite a PL emission from the apex of AFM probe 404. Specifically, the output beam from laser 408 is first reflected by prism 410, and then focused onto the tip of AFM probe 404 through objective lens 402. Objective lens 402 is a high numerical aperture (NA), total internal reflection fluorescent (TIRF) oil immersion lens, which can focus a light beam to a spot size between 1 μm - 5 μm. High NA objective lens 402 also provides a high optical magnification of the AFM tip and the ability to use a high angle of refraction (relative to the normal), which is described below. [0070] Illumination on AFM probe 404 induces a PL emission 412 which is localized to the apex of AFM probe 404, thereby generating a sub- wavelength point light source with a dimension similar to the apex radius. When using this sub-wavelength point light source to illuminate sample 406 placed in the close proximity, a near-field interaction between the point light source and the sample allows imaging the sample with sub-wavelength resolution.

[0071] PL emission 412 from AFM probe 404 passes through sample 406 during a scanning imaging process, and the transmitted light is collected by the same high NA objective lens 402. The collected light signal passes through an optical filter 414 (HQ430LP, Choma Optical) which blocks the far-field component at the illumination wavelength (405nm), while the near-field signal at the PL emission wavelength is detected by a photomultiplier tube (PMT) 416. The signal received by PMT 416 is recorded simultaneously with AFM topography and deflection and the PL emission spectrum is recorded with an Ocean Optics USB2000 spectrometer. Note that generally one or more optical filters may be used in place of optical filter 414. [0072] Note that although we describe using the PL point light source in a transmission detection NSOM system, this PL point light source can alternatively be used in other types of NSOM systems with different detection schemes.

Grazing Illumination [0073] In one embodiment of the present invention, the light path 418 of the excitation beam is configured to exit sample 406 at a predetermined "grazing" angle 420 (i.e., near parallel to the sample plane). The grazing incident beam which strikes the probe tip at grazing angle 420 can facilitate achieving a high PL emission intensity. This is because a p-polarized excitation beam propagating at this grazing angle contains a predominant electrical field E 422 in the normal direction of the sample plane (i.e., parallel to the probe axis). Electrical field E 422 excites dipoles in the probe tip, wherein the excited dipoles are oriented parallel to the field and hence to the tip axis, and therefore causes a charge accumulation at the tip end. The charge accumulation subsequently induces a strong electrical field at the tip end, which is referred to as a "tip-induced field enhancement". Consequently, the "grazing" excitation technique enhances the interaction between the incident light and the probe tip, and thereby enhances PL emission intensity.

[0074] In one embodiment of the present invention, PL emission intensity of probe 404 can be tuned by varying graze angle 420. For example, the PL emission intensity can be reduced when grazing angle 420 is increased. Note that in addition to controlling the excitation beam

angle, varying the probe tip geometry (e.g., changing the apex angle), the probe composition, or the illumination intensity can also adjust the PL emission intensity.

[0075] Note that the proposed grazing illumination is clearly distinguishable from an evanescent excitation technique because excitation beam in FIG. 4 does not undergo a total internal reflection (TIR), and the excitation beam on the probe tip is a refracted beam. In one embodiment of the present invention, the angle of p-polarized excitation beam is 5-10° below the critical angle of a TIR.

Properties of the PL Point Light Source

" Intensity and Durability

[0076] For the configuration of PL NSOM system 400, a 2.5 nW PL emission intensity at the tip-sample interface can be estimated based on an average spectral PMT sensitivity of 2 V/nW. More generally, the intensity of the PL emission used for imaging can vary between 1-5 nW, which depends on the AFM probe's specific composition and geometry, as well as the alignment of the excitation beam. Assuming that a laser beam with an intensity of 4 mW was uniformly focused on a 2 μm x 2 μm spot, the portion of the light intensity within a 20 nm x 20 nm area (approximation of the tip emission area) would be about 0.4 μW. Comparing this value with 2.5 nW PL emission yields a PL efficiency of ~10 ^"2 , which is a typical value for amorphous semiconductors PL at room temperature.

[0077] The PL point light source has shown long term (years) stability and durability (continuous operation without tip damage, intensity degradation or photobleaching), making it a reliable light source in any NSOM system.

■ Evanescent Wave

[0078] For PL NSOM system 400, it is observed that when the probe tip is moved away from the sample surface, the PL intensity decreases exponentially with a decay length of 55±6 nm. This observation indicates that PL emission at the probe tip is an evanescent wave which is localized to the apex of the probe, and does not propagate to the far field.

Samples for NSOM Resolution Test

[0079] For PL NSOM contrast and resolution tests, reflective gold nanoislands on transparent mica were prepared using a particle lithography technique. Specifically, an ordered monolayer of 800 nm polystyrene spheres (Duke Scientific, CA) was first made by depositing 37 μl of a nanosphere suspension for 30 minutes over 3.75 cm ² of freshly cleaved mica. The resulting array of spheres served as a mask for the subsequent gold deposition. Next, an adhesion layer of Cr (20 nm) was evaporated onto the latex sphere covered mica surface, followed by an evaporation of 40 nm of Au. The nanospheres were then removed by sonication in tetrahydrofuran (THF) for 20 minutes. Afterwards, the sample was rinsed with deionized water and exposed to a hydrogen flame to remove any remaining latex particles and to improve the gold morphology.

[0080] To demonstrate PL NSOM's ability to image soft dielectric materials, periodical arrays of the protein nanostructures were produced based on a previously reported procedure of latex particle lithography. Specifically, Bovine Serum Albumin (BSA, fraction V, 98% purity, Sigma Biochemicals) was diluted in milliQ deionized water to a concentration of 2 mg/ml and then mixed with a suspension of 500 nm polysterene latex spheres (BSA:latex = 28000:1). The latex particles were washed by deionized water through a centrifugation process. A drop of nanospheres and BSA solution were deposited and dried on the freshly prepared mica (0001) surface in a closed container. After drying, nanospheres were removed by rinsing with milliQ water. The protein patterns were visible under a 6Ox objective of an optical microscope (Olympus America).

PL NSOM Resolutions on Gold Nanoislands and Protein Nanostructures [0081] FIG. 5A illustrates a PL NSOM image (left) acquired simultaneously with an

AFM topography image (right) for gold nanoislands deposited on mica in accordance with an embodiment of the present invention. Note that using optical-filter 414 to separate illumination light from near-field signal in PL NSOM system 400 enables such simultaneous optical imaging with AFM topography imaging.

[0082] To obtain the PL NSOM imaging resolution, systematic cursor profiles are taken along various directions in the NSOM image to quantify surface features in FIG. 5A. Typically, three nearest neighbor lines are averaged to avoid contribution from random noise. FIG. 5B illustrates a representative PL NSOM cursor profile of a nano-gap at the edge of a gold nanoisland in FIG. 5 A in accordance with an embodiment of the present invention. The NSOM resolutions obtained for this feature yields 10±2 nm and 2±1 nm for lateral and normal directions, respectively.

[0083] PL NSOM images of protein (BSA) nanostructures are also obtained. Periodical features of the protein aggregates are resolved in the NSOM images. Using the same cursor profile technique for gold nanoislands, NSOM resolutions in protein features are measured to be 8±2 nm and 2±1 nm for lateral and normal directions, respectively. The high-resolution nondestructive image of protein indicates potential application of PL NSOM to biological research.

[0084] The above high-resolution PL NSOM images were recorded under a high scanning speed (5 Hz) and at room temperature without any signal filtering or integration below 2 kHz. Such a high speed NSOM image acquisition process is possible mainly due to the high PL emission intensity of the silicon nitride AFM tip as a NSOM light source.

Acquiring True Color NSOM Image [0085] Using PL NSOM system 400, true color near-field images below diffraction limit can be obtained. For example, FIG. 6 illustrates a true color NSOM optical image of photoluminescent silver islands with three color-channel cross-sections in accordance with an embodiment of the present invention. Note that the three separate color channel profiles were obtained using three channel detection configuration (such as using primary colors of Red, Green and Blue).

Conclusion

[0086] The present invention describes a simple technique for generating a nano-point light source by utilizing a PL emission from a sharp probe made of PL material or a sharp probe coated with PL material. In addition to wavelength tunability and power adjustability, this nano- point light source also has high emission intensity, localized emission spatial profile, and long probe durability, which facilitate high-resolution NSOM imaging of a wide range of samples with high imaging acquisition speed.

[0087] Note that in addition to the applications in near-field high-resolution optical imaging systems, the PL point light source may also be used for many other optical applications which require a high intensity nano-light source. These optical applications may include, but not limited to: optical communication systems, such as optical detectors, transmitters, or receivers; optical data storage systems, such as optical readers or writers; optical computing systems, such as signal transmitters or receivers; and in micro/nano chips as nano-light sources.

[0088] The foregoing descriptions of embodiments of the present invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.