The present invention generally pertains to laser plasmonic systems and more particularly to polarization and phase control of surface plasmon waveguiding.
Metallic nanoparticles have been studied extensively throughout recent history in an effort to understand both their unique emissive properties, as seen for example in stained glass, and their more recently discovered ability to localize and enhance electromagnetic fields. It is known that pyridine adsorbed on a roughened silver electrode produced a Raman spectrum 105-106 times greater than would be expected. This effect, now known as surface enhanced Raman scattering (hereinafter “SERS”) is due to nanoscale structures produced by roughening, and their ability to localize surface plasmons into ‘hot spots’ or regions of amplified electromagnetic (hereinafter “EM”) field. The localization of surface plasmons has also been used to enhance multiphoton processes.
This localized surface plasmon (hereinafter “LSP”) resonance has led to a wide variety of applications. It made possible the first room-temperature optical detection and SERS spectroscopy of single molecules, and has allowed for low threshold lasing of dyes. Individual nanoparticles can be used as biosensors with zeptomolar sensitivity, and nanowire bundles can be used to direct analytes into regions with a localized and amplified EM field.
Furthermore, “long-distance” propagation of surface plasmons can occur via surface plasmon waves (hereinafter “SPWs”). Work has been done to design waveguides and develop structures that act as mirrors and beamsplitters for SPWs. SPWs have also been used to mediate fluorescence resonance energy transfer (hereinafter “FRET”) over 120 nm by sandwiching a thin silver film between the donor and acceptor molecules. In dendritic nanoparticle structures, both local and long distance effects are possible because of the two types of surface plasmons: SPWs, which propagate along the metal surface, and LSPs, which are confined to metal nanoparticles. The combination of these two types of surface plasmons allows EM radiation incident on a dendritic structure to propagate away from the focal spot along the surface (via SPWs) and then localize in a particular region (via LSPs). The difficulty in exploiting nanostructures that support both SPWs and LSPs is in controlling how the energy travels and where it localizes.
In accordance with the present invention, a laser plasmonic system is employed. Another aspect of the present invention provides a laser beam and a carrier where an emission occurs downstream of a focal point of the laser beam. An additional aspect of the present invention provides two-photon-induced luminescence in a sample. In another aspect of the present invention, luminescence occurs distant or remotely from the focal point. A further aspect of the present invention uses polarization and phase control of surface plasmon waveguiding. In another aspect of the present invention, the control of two-photon-induced luminescence of silver nanoparticle clusters is provided. By using a femtosecond laser focused down to ˜0.5 μm, emission via surface plasmons should be observable up to 100 μm from the focal spot. The regions of emission can be controlled by changing the polarization of the incident beam and by changing the spectral phase across the spectrum of the laser pulse. Moreover, an aspect of the present invention employs the laser pulse in communications and/or on a microchip. A method of using a laser plasmonic system is also provided.
The experimental realization of the control of the present invention is expected to lead to advancements in surface plasmon-based photonics (hereinafter “plasmonics”). This is advantageous by bridging the gap between optics and electronics through carefully designed wires capable of carrying both electronic and optical signals over centimeter distances. The present invention can selectively control surface plasmon-mediated two-photon-induced luminescence in a dendritic silver nanoparticle system over distances of up to 100 μm. This control is achievable by changing the polarization of the incident beam and by controlling the phase across the spectrum of a femtosecond laser pulse used for excitation. Furthermore, the present invention uses the phase and polarization dependence to address photonically different locations within substantially 100 μm from the focal spot. Additionally, it is expected that the present invention system includes silver particles that will enhance two-photon fluorescence. The present invention laser plasmonic system is also advantageous over prior devices since the present invention is highly controllable and reproducible. The present invention system also provides the ability to measure the spectral phase at various points of localization as well as identify which part of the laser beam the nanoparticle is primarily interacting with. Additional advantages and features of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings.
a-d are electron microscopy images showing nanoparticles used with the preferred embodiment laser plasmonic system;
a and b are graphs showing expected two-photon luminescence of silver nanoparticles used with the preferred embodiment laser plasmonic system;
a-d are illustrations expected from characterizing the polarization properties of remote emissions used with the preferred embodiment laser plasmonic system;
a and 12b are graphs showing simulations of integrated second harmonic intensities used with the preferred embodiment laser plasmonic system;
c and 12d are graphs showing expected intensities of emission used with the preferred embodiment laser plasmonic system;
Referring to
Sample 31 includes dendritic nanowires on a cover slip. The nanowires are preferably made from silver nanoparticles which are synthesized by a citrate reduction, and cluster formation is induced by the addition of fumaric acid. The clusters precipitate onto the quartz cover slips over 60 hours. The cover slips are then removed from the solution, rinsed in MilliQ water, and allowed to dry. Electron microscopy images are obtained in order to characterize the nanoparticle samples.
For thin films of silver nanoparticle aggregates, the intensity of the excitation beam must be high enough to induce observable emission, but low enough that the sample is not irreparably damaged. Empirically, average powers as low as 2 μW (˜707 W/cm2 average power; 7.28×109 W/cm2 peak power) are capable of causing irreversible damage to the sample after even one scan. For samples with good plasmonic waveguiding, powers are reduced to ˜1 mW and well tolerated.
Excitation of these films of silver nanoparticles results in intense luminescence due to LSP resonance. The luminescence appears consistent with two-photon-induced surface-plasmon mediated fluorescence of silver oxide on the surface of the silver nanoparticle films. This is supported by a power study of silver nanoparticles in solution, in which a quadratic dependence of the signal on pulse intensity is shown in
Thin films of silver nanoparticle clusters should amplify the two-photon-induced fluorescence emission from a dye-doped polymer thin film. When performing a similar experiment utilizing a rhodamine 590-doped thin film of poly(vinyl alcohol), the amplification of the rhodamine-590 emission can be attributed directly to the luminescence of the silver nanoparticle clusters themselves. That is, through comparison of average peak intensity of a film of rhodamine-590, a film of silver nanoparticle clusters, and a film of the two together, a simple addition of the signal from the two former leads to the signal of the latter. In other words, any increase in signal when comparing rhodamine and rhodamine with silver nanoparticles can be attributed to luminescence directly from the silver nanoparticles and not to an amplification of rhodamine fluorescence.
When exciting the thin films of silver nanoparticle clusters at a single point, with a focal spot diameter of ˜0.5 μm, relatively intense, highly localized emission should be observable tens of microns away. This is illustrated in
A number of methods to control nonlinear optical excitation based on phase-shaped femtosecond laser pulses can be used. The influence of phase-shaped pulses on the two-photon remote luminescence exhibited by dendritic silver nanoparticles will be disclosed hereinafter. Referring to
Regions of remote emission can be controlled by the spectral phase of the ultrashort laser pulses used for excitation.
d shows the intensity of emission from these same three regions when γ is fixed to 15 fs and δ is scanned from 0-4π. Again, each point shows different behaviors. The solid squares illustrate different relative peak intensities, for example, and other points show asymmetries in the various peaks as well as variations in the peak spacing. The open circles in
The expected presence of remote emission, and the ability to control it via polarization and phase indicates that it is possible to control plasmonic propagation and emission over distances far greater than previously possible. With a laser focused to a 0.25 μm2 region, localized emission over a 103 μm area can be controlled. This four orders-of-magnitude control achieved with phase-shaped and polarized pulses may have great significance in the field of electronics, in which miniaturization of computer microprocessor or memory chips is limited by the size of (and subsequent heat loss due to) the wires used to transport electronic information. The development of plasmonic waveguides will allow for controlled transport of optical information along nanowires, lowering the size barrier currently faced.
Referring to
Scanning a series of well-characterized spectral phase functions across the spectrum of a transform-limited excitation laser pulse and collecting two-photon-induced fluorescence from a thin, rough film of silver nanoparticles allow quantification of the second- and third-order dispersions introduced by the sample using far-field detection. While this approach uses spectral data for measuring the phase distortions inherent in a femtosecond laser pulse for the purpose of compensating those distortions via the MIIPS method, the integrated intensity of the regions of remote emission as a function of spectral phase can be used to simultaneously determine the second- and third-order phase distortions at each point in the sample.
As can be observed in
A microchip 155 or the like is connected to fiber optic cable 149. Conductive nanowire carriers 157, created by a chain of adjacent nanoparticles, act as a nanoplasmonic waveguide. Nanowires 157 are preferably made of a silver based material placed onto or etched in a microchip, such as through e-beam lithography, with a single layer of magnesium oxide or similar dielectric material coating thereon to prevent oxidation. Each wire 157 is preferably 10-1,000 nm in thickness, 1-100 nm in width and less than 1 mm long. The shaped laser beam pulse will propagate as a surface plasmon wave but will cause an emission downstream of a focal point, located at an emitting nanoparticle of the carrier which is separated from the immediately upstream portion of the nanowire by a gap. The emitter gap or discontinuity is less than about one wavelength, such as less than 10 nm. In other words, the system excites and thereby creates a surface “wave” on the conductive nanoparticles forming the electrical/photonic circuit. The controller software and shaper control the nanoplasmonic waveguide emission location based on at least one or more of the following parameters: (a) input polarization and/or phase of the pulse; (b) wire dispersion characteristics, for example, positive or negative second- or third-order dispersion; (c) size, orientation and/or resonant frequency of the nanoparticle emitter; and (d) material composition of the nanoparticle emitter, for example, gold or silver.
More specifically,
Referring to
Functionally, although the plasmonic wave signal travels in all direction, the emitter nanoparticle downstream of the focal point in the nanowire luminesces or activates according to phase and polarization characteristics of the pulse. Emission luminescence is being induced in the visible range starting with near-infrared light. It is believed that the process is created by two-photon excitation, or it is a nonlinear up-conversion via second harmonic generation and then one photo absorption. Furthermore, the plasmonic waveguide network delivers signals across several tens of micrometers to different discrete locations with nanometer precision. Again, the network or circuit is addressed at a single point using shaped femtosecond pulse in the near-IR, and changes in the phase and/or polarization of the shaped pulses are used to address each separate location wherein there is a local emission of visible light. The system is further operable to serve as a multiplexing encoder/decoder.
Additionally, MIIPS software in the controller measures the chromatic (second and third order phase) dispersion occurring due to the propagation of surface plasmon wave transmission through plasmonic waveguides. For this purpose, a sinusoidal phase is scanned on the incoming laser field and the integrated LSPR emission, after the signal propagates on the waveguide, is detected and collected as a function of the different phase functions. The MIIPS software then automatically analyzes the results and makes the necessary distortion corrections accordingly for subsequently shaped pulses. Moreover, tailored surface plasmon wave waveguides are created that have specific second- and third-order dispersions. Therefore, remote, localized plasmon resonance emissions, microns away from the source, are advantageously created, controlled and applied, by way of example but not limitation, to communications.
While various embodiments have been disclosed herein, it should be appreciated that other modifications may be made that are covered by the system and methods of the present invention. For example, alternate lasers, optics, software, controllers and conductive carriers can be employed as long as they function as described although various advantages of the present invention may not be achieved. Furthermore, a microchip-mounted laser may alternately be used in place of the separate oscillator and fiberoptic cable. The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/832,032, filed on Jul. 20, 2006, which is incorporated by reference herein.
A portion of this invention was made with U.S. Government support under Contract No. 61-3237 awarded by the U.S. Department of Energy. The U.S. Government may have certain rights in this invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US07/16274 | 7/18/2007 | WO | 00 | 1/15/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60832032 | Jul 2006 | US |