Metal Nanotechnology For Advanced Display And Optical Applications

[0001] Metal has always been the most important component material for applications involving electromagnetic (EM) energies. Every significant progress in EM technology evolution, always being driven to a higher frequency (i.e. from kilo, mega, giga to tera hertz), had been correlated with the ability of fabricating metal components at a precision level matching the shorter wavelength scale. Metal has a high density of free electrons and is most effective in producing as well as guiding EM waves. Recent emergence of nanotechnology is likely to further expand the application envelope; advancing from far IR to visible due to the arrival of metal fabrication technology at the nanometer scale.

[0002] In the past few decades, the solid-state technology revolution in electronics, showcased by precise fabrication of semiconductors and semimetals, has led to maneuvering electron transfers among molecular orbital groups. The developments of Light Emitting Diodes (LED) and Organic Light Emitting Diodes (OLED) demonstrated that these molecularly engineered composites could successfully generate EM waves at a precise visible spectrum range (i.e. light emission at a desired frequency range). In addition, the recent discovery of Surface Plasmon Coupled Emission (SPCE) proved that nano-fabricated metallic thin films (30 ~ 50 nm thickness), when located in the near field (100~200 nm) of a LED or an OLED emitting source, could substantially enhance the quantum yield and the radiation output of such a device. ^I1|2] These progresses in generating and guiding EM waves at the optical frequencies have clearly mapped out a technical direction for future application of metallic nanotechnology. Embodiments of the present invention apply and modify these nanotechnology innovations and, in particular, are beneficial to the advancement of optical displaying technologies.

[0003] Replacing CRT monitors with flat panels, originated in the computer industry, has become the major trend in the whole display industry. Markets for flat-panel devices, particularly the large-screen TV market, are growing much bigger and more profitable. Moreover, the rapid market growth in hand-held communication tools (e.g., mobile phones) as well as portable computing equipment requires flat displaying devices

K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai and A. Scherer "Surface-plasmon-enhanced light emitters based on InGaN quantum wells" Nat. Mater. 2004, 3, 601-605.

² D. K. Gifford and D. G. Hall "Extraordinary transmission of organic photoluminescence through an otherwise opaque metal layer via surface plasmon cross coupling" Appl. Phys. Lett, 2002, 80, 3679-3681.

to be lighter, thinner and more power efficient. Consequently, the flat displaying technology is expected to advance substantially in the coming decades. Three displaying technologies, field emission, OLED and LCD are likely to dominant the future optical displaying market. Application of nanotechnology to advance their common, underlying light emission and optical wave guiding technologies, are likely to revolutionize future designs of the displaying panels.

[0004] Embodiments of the present invention apply metallic nanotechnology to incorporate innovations in each segment and/or the integration of them. [0005] Embodiments of the present invention, as illustrated by following examples and design principles, utilize current and new metallic nanotechnologies and compositions of matter, adapted for and useful in light emission designs and devices and/or for optical wave guiding applications and for novel displaying devices utilizing same. In particular, embodiments of the present invention substantially increase the power efficiency of current display devices. Still further, embodiments of the present invention are utilized for designing a new generation of displaying devices which, for example, may be brighter, lighter, and/or thinner than currently available devices and/or may be flexible.

[0006] Embodiments of the present invention may be achieved by controlling the morphology and composition of a metal nanolayer and attaching same to a dielectric coating layer so that the metal's (bulk and surface) plasmons resonance could be coupled inward with a light emission process and outward with the generation of light radiation. By using a metal with a low loss factor, such as silver, embodiments of the present invention guide and reuse the scattered light to substantially improve the devices' power efficiencies. Specifically, in various embodiments of the present invention the nanotechnology design utilizes metal plasmon resonance for enriching polarization, separating (rather than filtering) colors, and enhancing light emission of a backlight source. Current LCD devices tend to lose 50% to the absorptions by a first polarizer, and additionally 66% due to color filtration. In current light emitting devices (LED, OLED, field emissions), the emission efficiency is critically hindered due to light entrapments (by SPR mode) within the thin metal electrodes. The design principles according to embodiments of this invention utilize the coupling with metal plasmon resonance (in and out) to substantially elevate the efficiencies of these devices altogether.

1. Coupling With Surface Plasmon In A Metal Thin Film

[0007] In a previous invention, we disclosed enhancement of an OLED emission by more than 20% with a surface corrugation structure readily incorporated by a simple wet coating process (hereinafter, maybe referred to as "ISTN coatings, "). ^[3) The most effective surface morphology had a structural correlation length similar to the wavelength of visible light. However, such a corrugation structure was not placed within close proximity (near-field range) of the metal film or the emitting layer. When a corrugation is positioned on a metal thin film in the near-field range of an emitting layer, the enhancements in light extraction were reported to be orders of magnitude higher. ^t4 ' ⁵ ' ⁶⁷¹ Embodiments of this invention incorporate the enhancement to achieve further improvements in effects ISTN coatings.

[0008] A composite structure with spatial periodicity matching the wavelength of visible light (i.e. a photonic structure) substantially modify the dispersion (frequency-wave vector) relationship of visible light in that composite. A corrugated structure at an interface between two media differing in dielectric constants would as well modify the dispersion relationship at the interfacial zone should its periodicity, or structure correlation length, be at a wavelength scale. When such a structure is constructed within a short distance to a metal thin layer, the near- (evanescent) field interactions between the dielectrics and the metal plasmons would affect intensities of both emission and transmission of light from nearby oscillating dipoles.' ⁸¹ To demonstrate this effect, we incorporated such a structure within a system designed for achieving a surface plasmon coupled emission (SPCE). [0009] The SPCE depends on the near-field interactions of the excited fluorophore with the metal surface and the surrounding dielectric layers. For fluorophores

X. Wu, R. Zhang, and A. J. M. Yang "Optical Thin Films with Nanocorrugated Surface Topologies by A Simple Coating Method" US Patent Application 60/656097, 2005, also non-provisional PCT recently filed

⁴ J. M. Lupton, B. J. Matterson, I. D. Samuel, M. J. Jory and W. L. Barnes "Bragg scattering from periodically microstructured light emitting diodes" Appl. Phys. Lett., 2000, 77, 3340- 3342.

⁵ D. E. Grupp, H. J. Lezec, T. W. Ebbesen, K. M. Pellerin and T. Thio "Crucial role of metal surface in enhanced transmission through subwavelength apertures" Appl. Phys. Lett., 2000, 77, 1569-1571.

⁶ W. L. Barnes, A. Dereux and T. W. Ebbesen "Surface plasmon subwavelength optics" Nature, 2003, 424, 824-830.

⁷ S. Wedge and W. L. Barnes "Surface plasmon-polariton mediated light emission through thin metal films" Optics Express, 2004, 12, 3673

⁸ "Topical review: Fluorescence near interfaces: the role of photonic mode density", W. L. Barnes, J. Modern Optics, 1998, vol. 45, no. 4, 661-699

excited in close proximity (20 - 250 nm) to a metal thin film, SPCE occurred at a definite angle and with a narrow angular distribution.' ⁹¹

[0010] In the following examples, we measured SPCE intensities of laser-excited

Rhodamine 6 which was embedded within a corrugated dielectric mixtures layer and spin- coated onto a 45 nm silver film. The coupling of fluorophores excitation with silver film resulted in a highly directional surface plasmon-coupled emission (SPCE) through the silver film and the glass substrate. By creating a corrugated structure within the film layer containing the dye molecules, further and unexpected enhancements of photoluminescence due to surface corrugation were observed. [0011] The dielectric mixture according to embodiment of the invention is composed of fluorinated silica particles and a binding medium. By varying the size of the fluorinated silica nanoparticles we were able to control the surface correlation length scale of the corrugated surface structure. It was found that the coupling efficiency of the directional light emission is strongly correlated to the surface morphology, particularly the surface correlation length, of the corrugated dielectric structure. The following example gives the details of the experimental setup and the results:

1.a. SPCE Experiments: Materials And Methods Sample Preparation

[0012] A Sol-gel coating solution was prepared according to procedures disclosed in the previously mentioned patent application 60/656,097, and in PCT application PCT/US2006/006240,filed February 23,2006, the entire disclosures of which are incorporated herein in their entirety, by reference thereto. The fluorinated silica nanoparticles with a controlled particle size were synthesized using a modified Stό ^' ber process. The particle sizes used in examples below are 12, 150, 330, 482 nm, respectively.

[0013] Glass microscope slides (plain; Corning) were coated by vapor deposition equipment of EMF Co. (Ithaca, NY). A 45-nm-thick layer of silver was deposited on the glass, followed by a 5-nm-thick layer of SiO ₂ to serve as a spacer and protector. Sol-gel coatings embedded with silica nanoparticles in sizes of 12, 150, 330, 482 nm respectively were deposited on the surface by spin coating to create a randomly roughened surface. Fluorophore Rhodamine 6 from Aldrich was then deposited on the roughened surface by spin coating a 100 μM isopropanol solution at 1700 rpm. The actual film thickness was

I. Gryczynski, J. Malicka, K. Nowaczyk, Z. Gryczynski, and J. R. Lakowicz "Effects of sample thickness on the optical properties of surface plasmon coupled emission" J. Phys. Chem. B, 2004, 108, 12073-12083.

measured using Filmetrics 20. The samples' surface morphology was characterized by atomic force microscopy (AFM).

Fluorescence Measurement

[0014] The spin-coated slides were attached to a hemi-cylindrical prism made of

BK7 glass (Scheme 1). This combined sample was positioned on a precise rotary stage, which allows excitation and observation at any desired angle relative to the vertical axis along the cylinder. The sample was excited at a normal incidence from the sample side which has a refractive index lower than that of the prism. This is called the reverse Kretschmann (RK) configuration. Observation of the emission was performed with a 3- mm-diameter fiber bundle, covered with a 200 μM vertical slit, positioned about 15 cm from the sample. This corresponds to an acceptance angle below 0.1°.

Excitation

Scheme 1. Surface plasmon coupled emission with normal excitation from the side opposite the prism (reverse Kretschmann configuration)

[0015] For excitation we used the 514-nm output of a modelocked argon ion laser,

76MHz repetition rate, 120 ps half-width. Scattered light at 514 nm was suppressed by observation through a holographic supernotch-plus filter (Kaiser Optical System, Inc., Ann Arbor, Ml). Emission intensities were observed through a long-wavepass filter LWP 550 in addition to the notch filter. All emission spectra were recorded through the notch filter. Frequency-domain intensity decays were measured with the 10-GHz instrument.

1.b. Results And Correlation With Surface Morphology [0016] All the samples in this study were excited in the RK configuration, for which the incident light cannot directly excite surface plasmons in the metal film. Hence the angle-dependent emission would be primarily due to the near-field interaction of metal

surface plasmons with the excited fluorophores, not with the incident light. We measured the emission intensities for all accessible angles relative to the normal axis. For sample A, illustrated in Figure 1 , which contains the smallest silica nanoparticles of 12 nm and have the lowest surface roughness, there are two angle SPCE peaks with different s and p polarizations at 46° and 71°, respectively. According to previous literature results [9], those peaks were attributed to waveguide modes, where both s (transverse electric, TE) and p (transverse magnetic, TM) modes are known to propagate in nonmetallic dielectric layer because the configuration supports propagating waves of either polarization. The far field fluorescence was also measured as free space intensity on the opposite of prism at the angles between 230 and 270 degrees,

10 30 50 70 90 110 130

Degree

Figure 1. Angular distribution of SPCE for Sample A vs free space intensity

(between 90° and 130°)

[0017] For sample B, in which 150 nm silica nanoparticles were used, it is found in Figure 2 that only one p polarized emission peak was observed at 308° (or 52°). The intensity of the SPCE was compared with the free space intensity at 230 degree. The ratio is found to be 1.3.

230 250 270 290 310 330 350 Degree

Figure 2. Angular distribution of SPCE for Sample B vs free space intensity

(between 230° and 270°)

[0018] For sample C, in which 330 nm silica nanoparticles were used, it is found that still one p polarized emission peak was observed and the angle was at 52 °. The fluorescence spectra are shown in Figure 3. Here again the SPCE is exclusively p polarized light. The intensity of the SPCE was compared with the free space intensity at 230°. The ratio is found to be 2.0.

Figure 3. Angular distribution of SPCE for Sample C vs free space intensity

(between 230 ° and 270°)

[0019] For sample D, in which 482 nm silica nanoparticles were used, the p polarized SPCE was again observed at 52 °, as shown in Figure 4. However, the intensity of SPCE is much weaker in comparison with the free space intensity. The ratio is reduced to 0.35. The change in the surface topography of the dielectric coating definitely affects the out-coupling efficiency of light emission. It is interesting to note though that the surface roughness has not affected the angular distribution of the SPCE.

Degree

Figure 4. Angular distribution of SPCE for Sample D vs free space intensity (between

230° and 270°)

[0020] The surface morphology of similar randomly roughened silica coating has been studied extensively by AFM as disclosed in our prior patent application 60/656,097. [3]. The height image of Sample B is shown in Figure 5. The average roughness Sa and root mean square average roughness Sq can be known from the 5X5 μm image and equal to 20 and 27 nm, respectively. In addition, the degree of roughness was determined by using fast Fourier transform of the 2D height image (Figure 6) and correlated with the correlation length of pair correlation function. It was found that the surface correlation length scale for Sample B is 236 nm. Results obtained from studying a series of coating samples and disclosed in our prior patent application established a linear relationship between the surface correlation length and the particle size, shown in Figure 7. The empirical equation is as follows:

L = 1.83 * X - 20 Equation 1 where L is the surface correlation length determined by AFM along with Fourier transform, X is the silica nanoparticle size measured by dynamic light scattering. According to this equation the surface correlation length scales L for sample A, B, C and D in this disclosure are 2 nm, 2 55nm, 584 nm and 862 nm, respectively. The linear relationship between average surface roughness and the particle size is shown in Figure 8. [0021] For sample A ₁ the surface roughness is very small, which can almost be regarded as a smooth surface, the waveguide modes of SPCE are predominant. The

alternating s and p polarization can be observed at various thickness of the coating. However, for samples B, C and D, the waveguide mode of SPCE is significantly suppressed with the increase in the surface roughness. Only one peak with p polarization can be observed and the angle is consistently appeared at 52°.

Figure 5: AFM contour height image of Sample B

Figure 6: Fourier Transform of AFM height image of Sample B

800

0

100 200 300 400

Particle Size (inn)

Figure 7: linear relationship between silica nanoparticles and correlation distance of the coating surface topography [L=1.83x-20 (nm)]

0 100 200 300 400 500

Particle Size (nm)

Figure 8: linear relationship between average surface roughness of the coating surface vs size of silica nanoparticles.

I.e. Discussion of SPCE Enhancement By ISTN Coating

[0022] The enhancement of the SPR coupled emission may come from three different origins: (a) Enhancement of dye emission, (b) Enhancement of SPR coupling, (c) Enhancement of light extraction from SPR. Preferred enhancements of the ISTN's coating are composed of fluorinated silica particles of a definite size. Because of the fluorination, the refractive index of the particle can be lowered below that of the dye resin mixture. When the particle size is near the wavelength scale, the differences in refractive indexes (i.e., difference in dielectric constants) resulted in effects resembling those of a photonic structure. Light may be reflected among wavelength scale particles and enhance the local EM field around dye molecules. This will amplify the emission of the dye molecules. In addition, this variation in dielectric constants at wavelength scale could enhance the light coupling with the SPR mode similar to Bragg scattering (SPR coupling in). Furthermore, a periodical variation of dielectric constants (such as a grating structure) near the interface of a thin metal film is known to substantially increase the radiation from the SPR mode (SPR coupling out).

Fig. 9 A dielectric contrast layer composed of domains of dielectric constant differing from that of the continuum is attached to a metal thin film. This composition and structure could accomplish coupling light emission in with SPR (enhancing spontaneous emission), coupling SPR out to radiation, entrapping light within dielectrics (enhancing induced emission).

[0023] The amplifications of dye emission may have originated from the spontaneous and/or stimulated (induced) energy transfers among two quantum states. The induced emission is amplified by increased field density while the spontaneous emission is amplified by mode coupling with metal SPR which has a high mode density in the visible spectrum domain. The mechanisms of coupling in and coupling out with SPR by having periodical dielectric contrast at a distance and correlation length scale

corresponding to the relevant wavelength are essential for the enhancements of the spontaneous emission.

AF - F - F = hv

A _mn : Spontaneous emission

B _mn : Stimulated (induced) emission

E _n

Einstein 's law of radiation:

Bmn = B _nm p(y) - hv p(v) : Mode density

[0024] Strictly speaking, coating layers according to embodiments of this invention, although containing periodical variations in dielectric constant, do not constitute a photonic material of conventional definition though the dielectric layer may be referred to herein as a photonic dielectric contrast layer. The ordering structure in coatings according to embodiments of the present invention are only short-ranged, while a regular photonic material has a long-range ordering. In embodiments of the ISTN coating, if using a high concentration of particles, the volume exclusion effects would lead to a pair correlation function peaking at a distance determined by the particle diameter. This would lead to a short-ranged photonic structure at a length scale set by the particle size. Furthermore, the fluorinated particles, having a very low surface free energy due to the fluorine atoms, tend to accumulate at the resin-air interface, leading to a dense and, consequently, a higher ordering structure at the interface. Further experiments can quantify each mode of enhancement contributions. The optimized coating formulation (i.e. the particle size, fluorine content and correlation length) regarding SPCE enhancement maybe determined empirically.

[0025] In general, some embodiments of the invention demonstrate that unexpectedly the position of the dielectric contrast layer relative to the metal thin film relative may significantly affect SPCE and the resulting enhancements to brightness and color. Specifically, the inventors have discovered that when the layers are positioned within near-filed range of each other and a light emission source, SPCE, brightness and color may be enhanced at least three fold.

[0026] Accordingly, in some embodiments some structures according to the present invention may any of the following general structures.

Figure 10: Structure with dye layer on top of and permeating dielectric layer

Figure 11 : Structure with dye layer permeated within the dielectric layer.

Domain with Different

Figure 12: Structure with dye layer on to of spacer layer and beneath dielectric layer

Dielectric layer Domain with Different Dielectric Constant

Figure 13: Structure with dye layer incorporated therein.

[0027] In some embodiments, the structures of Figures 10-13 may be used to enhance emissions, brightness and/or color in various devices. The structure may be arranged on a substrate, which may be a glass substrate, and may include a metal thin film, a spacer layer, a dielectric layer and a dye layer. The metal thin film may be placed

directly on a substrate. The spacer layer, which is optional, may be a silca-based protective layer that may protect the metal thin film from oxidation and other degradation. The dielectric layer (which may be referred to as a photonic dielectric contrast layer) may be a continuum having domains that have a different dielectric constant than the dielectric constant of the continuum material. The domains may exist as part of a bi-continuous phase with the continuum material, or may be nanoparticles sized in the range from 10 to about 600 nm embedded within the continuum material. The dielectric layer may be placed directly on the metal thin film layer or separated from it with a spacer layer. The dye layer may be placed on the dielectric layer or on the spacer layer. The dye layer may also be incorporated within the spacer layer. Because the dielectric layer is porous in some embodiments, the dye material may permeate into the dielectric layer during the layering process. Accordingly, as shown in Figures 10 and 11 the dye may fully or partially permeate the dielectric layer. The layers are generally arranged such that the distance between the metal layer and the outermost layer is within a near field range, for example from approximately 0-600 nm.

[0028] In some embodiments, these general structures may be used to form microlenses and optical cavities. The structures may be modified to increase efficiency and or to permit re-use of reflected light. The structures are not necessarily linear as shown in the Figures and may be shaped in any useful shape as needed.

1.d Applications Of SPCE In Display Device Design

[0029] Metal nanotechnology, demonstrated by SPCE and by the inventive coating enhancement technology, has several novel applications in optical displaying devices according to embodiments of the present inventor. For LCD displays, metal thin films may be used to greatly enhance the light output. Current LCD devices lose 50% of light to polarization (adsorbed by iodine polarizer), 66% to color filters; with eventually less than 10% light used for the bright state. The SPCE mode of light transmission was highly (P-) polarized and with a sharp distribution at an angle determined by the optical frequency. Thus, a thin metal film, with a designed corrugation structure according to enhancement of the present invention, guides and separates differently colored lights coming from a common white light source. The replacement of color filtration by color separation in an LCD is extremely important as, by eliminating color filtration due to absorption, the colored light output may be increased up to three times.

[0030] In embodiments of the present invention, the metal thin film can be structured as microlense to first separate and then guide (focus) the three prime colors (RGB) to the respective color pixels, as shown in the following schematic sketch:

Metal thin film

White liεftt Color pixels

Dielectric coatings Color Se ^p aration by Microlense

[0031] The design scheme may be based on the variations of the color angle in terms of the dielectric composition as well the metal thin film geometry. The detailed microlense structure may be device specific and can be constructed readily by one having ordinary skill in the art.

[0032] According to another embodiment of the invention, and based on the above experiments the polarization ratio of a light source in LCD is enriched. Thus, as demonstrated by Examples C and D, the transmitted lights are predominantly P polarized. Integrating SPCE with the design of a microlense may substantially increase the polarization ratio of the light output from a backlight source in favor of the plarization in perpendicular to the absorption axis of the first polarizer. This will in turn minimize the light loss to absorption by the first polarizer.

[0033] The use of metal nanotechnology to concentrate one polarization in output and utilizing the other polarization to enhance stimulated emission in backlight emission source provides significant impacts in the design of new LCD devices according to embodiment of the present invention. Generating color pixels by separating rather than filtering the complementary color components alone shall improve the light utilization by up to three fold.

[0034] The SPCE, because of the substantial enhancement achieved by embodiment of the present invention may be utilized for OLED devices as well. Conventional OLEDs consist of a transparent conductive anode and an opaque (Al or Ag) electrode on top. The emitted light must come out through the bottom anode, making the 'on-chip' OLED integration with silicon driver electronics rather difficult. A device capable of emitting light from the top cathode allows all circuitry components such as wiring and

transistor to be placed at the bottom to avoid interfering with light output. Therefore, there has been an increasing demand in top-emitting OLED (TOLED) for active-matrix OLED display. Embodiments of the invention disclosed herein may enhance the light emission and transmission through a thin silver layer and thus be integrated with the design of top Ag electrode in a TOLED.

[0035] Recent advancements in LED lighting technology use roughened metal films to enhance quantum yields. Thus, it is an indication that, by including embodiments of the present invention, the lighting efficiency of LED may also be substantially increased. The extension of our invention from photoluminescent to electroluminescent may, therefore, extend the applications of coating enhanced SPCE from LCD displays to OLED, LED and field emission devices.

[0036] The light emission enhancements by SPCE have a broad impact in optical displaying technologies. By separating a polarization and/or colors, part of the light energy can be recycled to increase the power efficiency. However, the scheme outlined here goes a step further by guiding the returned light back in a photonic cavity to accomplish higher stimulated emission. The presence of metal surfaces at sub-micron dimensions may enhance spontaneous emission as well. These two practices can be used with LCD backlight design following the principles described here. We describe below how to include both metal thin films and nanoparticles in design of displaying devices.

2. Extensions From Metal Thin Films To Nanoparticles [0037] This invention clearly demonstrates a strategy of applying metal nanotechnology for the design of a new generation of optical displaying devices. This strategy emphasizes the integration of wave guiding structure, made by metal and dielectric nanotechnology, with the backlight source to accomplish the following:

(1) Using SPCE to enhance spontaneous emission,

(2) Separating and focusing three prime colors to respective color pixels,

(3) Concentrating one polarization (P) and

(4) Entrapping the other polarization (S) within the emitting zone to enhance the stimulated emission.

[0038] These methods for enhancing both spontaneous and stimulated emission shall have broad impact on many other optical applications beyond making displaying devices. For example, LED lighting efficiency may be enhanced to the level that a total

replacement of traditional lighting sources is feasible. Further, the enhanced interactions among photons and metal surface plasmons may be used to increase the photon utilization in solid-state solar panels as well. The interactions of radiation photons and metal plasmons are a general phenomenon that can be broadly applied to electroluminescence, photo- luminescence, chemo-luminescence, photoelectric and thermoelectric devices.

[0039] All these unique characteristics demonstrated by metal nanofilms can be further amplified by using metal nanorods with controlled aspect ratio and length. We know these unique properties originated from the movements of free electrons in a metal. In a bulk metal film, we cannot separately control the surface and bulk modes of plasmon oscillations. In fact the movements of surface electrons would shield bulk electrons from EM radiation (the skin depth). A metal nanorod can be made adequately thin and long so that all the bulk electrons are under EM radiation while the long interfaces in the longitude direction may still support surface plasmon modes. Consequently, there are more combinations of options to accomplish scattering amplification, color separation and polarization enrichment.

3. Scattering By Silver Nanorods

[0040] Edwin Land first proposed polarizers based on aligned metallic thin wires. ¹¹⁰¹ The concept has already been used for polarizing electromagnetic waves at a much longer wavelength (radio waves). Aligning silver needles in a polymer matrix already produced a crude polarizer ^[11l in the infrared range. Because of the large size of the needles (several microns), the polarizer was not suitable for the visible range. In order to make a polarizer for visible light, the width and spacing of the silver rods must be made much smaller than the wavelength of light. Because of nanotechnology, what used to be a technical challenge now becomes an opportunity to greatly elevate the light polarization technology. Using aligned silver nanorods to make a polarizer could revolutionize this product. Significant benefits such as better resistance to heat and moisture, no need of thick protection layers, easier and faster processing may be realized because of emerging metal nanotechnology.

[0041] Fine-tuning the aspect ratio and the length of silver nanorods at visible wavelength scale may generate many new optical-electronic applications. The following

¹⁰ E. H. Land, "Some aspects of the development of sheet polarizers", J. Opt. Soc. Am. 41, 957 (1951)

¹¹ "Heat- and moisture-resistant thin-film polarizer", V I Studenov, and M G Tomilin, J Opt. Technol. 66 (6), June 1999, pp 550-553

theoretical derivation is believed to explain the results obtained from experiments disclosed further below and meantime provide grounds for new applications based on our results.

3.a Theoretical Basis Of Plasmon Resonance In Metal Nanoparticles [0042] There are numerous references in literature discussing the enhanced light absorption and scattering by metallic nanoparticles. The dielectric constant of metal is normally calculated according to the following equations based on the Drude model:

ε _m where ω _p is referred as the plasmon frequency, T is inversely proportional to its DC conductivity, NA/ is the electron density, q is the charge and m _q is the mass of an electron respectively.

[0043] When metal nanoparticles are embedded within a medium with dielectric constant of ε _d , the intensity of light absorption peaks at a frequency m _q at which ε _m (ω _o )+2 ε _d = 0. This peak frequency, ω. _o , was generally referred as the plasmon resonance frequency of that particle. Differing from the surface plasmon resonance, this is in fact a resonance phenomenon of bulk plasmons. We developed the following physical model to explain this resonance phenomenon. For a metal sphere much smaller than the wavelength, i.e. a sphere of radius « 400 nm, we may assume that it subject to an oscillatory electrical field with a constant amplitude(E _o e ^"ιωt ):

Surface plasmon resonance Bulk plasmon resonance

[0044] The oscillation of free electrons in metal particles, besides driven by the force of the external field, is also subject to a return force resulting from the accumulated boundary charges; for a sphere in vacuum the return force is equivalent to a free oscillation frequency of the magnitude: _ 4πNg ² _^ a> _p

⁰ Z Vm _q * 3

The polarizability of the metal sphere becomes:

Our model adequately explained the absorption peak of silver nanospheres at 350 nm. ^[121 Although having only approximation terms no higher than which of dipole interactions, this model provided the essential physical picture of bulk plasmons resonance and can readily be generated to shapes other than a sphere. Using dielectric theories worked out in an elliptical coordinates,' ¹³¹ we can obtain the polarizability for an ellipsoidal shaped particle (with principal axes of a, b, and c):

abc ^∞ ds δ _n = — J- , where the following pictures give a good

² 0 (5 + α ² )Vθ + a ² )(s + b ² )(s + c ² )

0 9 estimate of resonance frequencies for different shapes: CO ₀ = S _s ω _p

δ _s = 1/3 1 δ _s ~ 0 δ _s ~ 0.5

[0045] Among all metals, silver has the best overall properties for applications in the optical region. (It is not coincidental that silver is the best metal for making mirrors.) Silver's plasmon frequency, ω _p , is approximately 1.36x10 ¹⁶ hertz while the visible light frequencies is 3 - 4x10 ¹⁵ hertz. This means the bulk plasmon resonance frequencies of silver nanoparticles may be adjusted easily within the entire visible spectrum by varying a rod's aspect ratio. Furthermore, silver's damping frequency r, a measure of loss to Joule heating, is also the lowest among all metals at 2.63x10 ¹³ Hertz. Thus, color generation by silver nanorods would be mostly from light scattering (by dipole oscillation), not from color absorption (heat loss). Silver nanorods are ideal for accomplishing color separation in replacement of color filtration.

12

"The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment", K. Lance Kelly, Eduardo Coronado, Lin Lin Zhao, and George C. Schatz, J. Phys. Chem. B 2003, 107, 668-677

13 "Electromagnetic Theory", J A Stratton, McGraw-Hill Book Company, 1941 , pp. 207-217

[0046] When silver nanorods are embedded within a low-loss dielectric layer, the nanocomposite shall be a highly efficient reflective color separator. The colored lights are either transmitted through or scattered back to the light source where they could be effectively recycled and reused (like a selective silver mirror). The efficiency of this color generation could be three times higher than which obtained from color filters. Furthermore, when the embedded silver nanorods are aligned into one direction, the composite shall generated linearly polarized, colored lights. [0047] The following examples are designed to prove this principle. Silver nanorods of various aspect ratios are mixed with PVA solution and cast into a thin film. After casting and aging, the film is heated to 90° ~120° C on a hot plate and stretched with clamps to a desired elongation ratio (2-10 times). Absorbance in visible spectrum of polarized light in the directions parallel and perpendicular to the drawing direction are measured for each sample, respectively.

3.b Silver Nanoparticles For Generating Polarized, Color Lights [0048] Ag nanorod/Polyvinyl alcohol(PVA) composite film was prepared by casting Ag nanorod suspension of PVA water solution. The silver nanorods were synthesized with cationic surfactant Cetyltrimethylammonium bromide (CTAB) as a soft template (C. J. Murphy & N. R. Jana; Adv. Mater. 14, 80, 2002). The aspect ratio of the nanorod is controlled by adjusting Ag seed amount added during the nanorod synthesis process. Before dispersing in the PVA solution, the Ag nanorods were purified and concentrated by centrifugation. The suspension was applied onto a flat glass with a coating blade. A tinted thin film with good mechanic strength was obtained after evaporation of water at room temperate for 24 hours. In the final cast composite film, the silver nanorod content is about 0.5wt%. The Ag nanorod/PVA film with Ag nanorod orientated with PVA molecular chain was made by stretching the cast composite film at 9O ⁰ C by heating with a hotplate. The draw ratio is 5.

[0049] Absorbance spectra of Ag nanorod PVA film with different aspect ratio of the nanorod are shown in following figures. All the films showed strong dichroic effect after stretching. The dichroic ratio, which depended on the aspect ratio of Ag nanorods and the drawing ratio of the film, changed with wavelength. For these stretched films, the absorbance peaks appeared at different wavelength when applied a polarized light with polarization direction parallel or perpendicular to the direction of stretching.

[0050] In following figures (Fig. 14-18), the samples A, B, C were prepared with the aspect ratio in ascending order (estimated to be from 3 ~ 10 from samples A to C). The peaks of the spectra red shifted with the increase of the aspect ratio. In figure 19, the correlation of peak wavelength to aspect ratio is shown.

Spectra of Ag/PVA Film A

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Wavelength (nm )

Figure 14. The absorbance spectra of Ag nanorod/PVA film A with and without stretching. The dichroic effect is obtained by measuring absorbance of polarized light with polarization direction parallel or perpendicular to the stretch direct of the film.

Spectra of Ag/PVA Film B

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Wavelength (nm)

Figure 15. The absorbance spectra of Ag nanorod/PVA film B with and without stretching. The dichroic effect is obtained by measuring absorbance of polarized light with polarization direction parallel or perpendicular to the stretch direct of the film.

Spectra of Ag/PVA Film C

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Wavelength (nm )

Figure 16. The absorbance spectra of Ag nanorod/PVA film C with and without stretching. The dichroic effect is obtained by measuring absorbance of polarized light with polarization direction parallel or perpendicular to the stretch direct of the film.

[0051] To verify that the colors were due to scattering, not absorption, we measured the reflectance spectra of the samples. This backward scattering did show a complementary color which could be recycled in an optical design. The percentage of the

light scattered backward can be monitored by Perkin Elmer UV-Vis spectrometer with an integrating sphere. The integrating sphere can collect both diffuse and specular reflectance. With a specific configuration, we obtained only the diffuse reflectance which is the light scattered back by Ag nanorod in the composite film. Following figures show the diffuse reflectance of the Ag nanorod films with and without stretching. We recorded up to 14% of light in the diffuse reflection from Ag nanorod embedded films. This percentage can be increased with higher concentrations of rods and/or better alignment order parameter.

Backward Scattered Light

400 500 600 700 800 900 Wavelength (nm)

Figure 17. The diffuse reflectance of the Ag nanorod/PVA films without stretching. The film A, B, C, correspond to the films showed in Figure 14, 15 and 16.

Backward Scattered Light

400 500 600 700 800 900 Wavelength (nm)

Figure 18. The diffuse reflectance of the stretched Ag nanorod/PVA films. The film A, B, C, correspond to the films showed in Figure 14, 15 and 16.

3 5 7 9 11 13 15

Aspect Ratio

Figure 19. Dependence of the peak wavelength to Ag rod's aspect ratio

4.Alignment Of Metal Nanorods

[0052] The examples given here used only mechanical stretching to align silver nanorods. Because the purity of silver nanorods is not high enough, we did not attempt to determine the order parameter of the aligned silver nanorod composite. To compete with current polarizers, the dichroic ratio, and consequently, the order parameter must be very high. One way to effectively align metal nanorods is to use liquid crystalline molecules. We estimate that the ideal silver rods for a polarizer product would have a dimension of 5 nm x 100 nm. For this dimension and aspect ratio, there is an ideal liquid crystalline molecule, i.e. cellulose fibril, for the alignment of silver rods. The diameter of cellulose fibrils is in the range of 3 - 10 nm with length ~100 nm ^[14] which are very close to the dimension of the silver nanorods we desire. Experiments are under way to demonstrate this alignment method and determine the effective order parameter.

5. Summary Of Innovations From Metal Nanotechnology

[0053] In this work, we proved the following principles that could be utilized with metal nanotechnology to substantially advance the Optical Displaying Designs.

1. A dielectric contrast layer with the domain correlation scale in the optical wavelength range and deposited near a metal thin film could substantially enhance the Surface Plasmon Coupled light Emission (SPCE) from dye molecules embedded within.

2. This dielectric contrast layer can be made by dispersing a high amount of nanoparticles having a definite size in wavelength range, in a continuum with a different dielectric constant to obtain a structure with a short-ranged correlation resembling that of a photonic structure.

3. SPCE and a dielectric contrast layer can be microengineered to accomplish (a) Enhancing the spontaneous emission by Surface Plasmon Coupling (coupling in), (b) Enhancing stimulated light emission by entrapping and recycling light scattered within the structure, (c) Enhancing light extraction from SPR (coupling out).

4. SPCE and the dielectric contrast structure can be utilized to enrich the separation of polarization as well as colors from an incoherent light source

¹⁴ "Synthesis of mesoporous silica by sol-gel mineralisation of cellulose nanorod nematic suspensions", Erik Dujardin, Matthew Blaseby and Stephen Mann, J. Mater. Chem., 2003, 13, 696-699.

and the light returned by scattering from this structure can be recycled to further stimulate and enhance the light emission.

5. The scattering from aligned silver nanorods can be used to generate polarized color light. The back-scattered light can be recycled and utilized for stimulating and enhancing additional light emission.