Optical: systems and elements – Single channel simultaneously to or from plural channels – By refraction at beam splitting or combining surface
Reexamination Certificate
2002-09-25
2003-09-09
Shalwala, Bipin (Department: 2673)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By refraction at beam splitting or combining surface
C359S245000, C359S247000, C359S250000, C359S263000, C359S638000, C345S032000
Reexamination Certificate
active
06618204
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light modulator. More particularly, it relates to a light modulator utilizing surface plasmon, a light source and a display apparatus using the light modulator, and a method for driving the light modulator.
2. Description of the Related Art
As a conventional display apparatus, especially, a field sequential display apparatus, there is disclosed an apparatus using a color filter disk (eg., “Color Liquid Crystal Display” written and edited by Shunsuke Kobayashi: published in Dec. 14, 1990, P. 117) With this apparatus, a color filter disc colored in red, green, and blue, i.e., three primary colors of light is placed in front of a monochrome cathode ray tube, and rotates in synchronism with display to enable color display. Similarly, color display is also possible even by placing the color filter disc in front of a white light source, and combining a black shutter (black-and-white shutter type display device) therewith.
Further, as an apparatus of another system, on pages 120 and 121 of the aforesaid document “Color Liquid Crystal Display”, there is shown an apparatus for performing field sequential color display by combining two high-speed liquid crystal display devices referred to as &pgr; cells, and a total of three sheet polarizers and color sheet polarizers in front of a monochrome CRT(cathode ray tube).
Further, there is also shown the technology using a CRT, LED (light emitting diode), or cold cathode fluorescent tube as a backlight, and using a liquid crystal display as a black shutter on pages 122 and 123 in the document “Color Liquid Crystal Display”. With this apparatus, backlights of respective three primary colors are prepared so as to alternately flash. One example thereof is shown as a field sequential full color LCD in “Monthly Published Display”, the July issue, pp. 11-16, (1998). In this example, the cold cathode fluorescence tube backlight which is illumination light of commonly used liquid crystal display apparatus is temporally switched among red, green, and blue.
In recent years, there are proposed a light modulator utilizing an electromagnetic wave supported by the interface between a metal and an insulator (a dielectric material) referred to as a surface plasmon wave (SPW), a display apparatus which permits color display by utilizing this light modulator, and a light source thereof as a display apparatus whereby color display is implemented without using the foregoing color filter and color light source. That is, conductors such as metals can be defined as a gas of electrons in electrostatic equilibrium inside a continuum of positive fixed charges. It can be considered as a “condensed” electron plasma with electron density approximately equal to 10
23
electrons per cm
3
. There exists a longitudinal wave referred to as a surface plasma oscillation in addition to a volume plasma oscillation which is a normal plasma oscillation. The electric field due to the surface plasma oscillation has a periodic wave form in a direction parallel to the metal surface, while having a form of evanescent wave which evanesces exponentially in a direction perpendicular to the metal surface. Plasmons are quanta associated with the plasma oscillation (collective wave excitation of a conductive electronic gas) in the metal. Because of high electron density, quantum effects dominate. The surface plasmon waves can be optically excited by resonant coupling. The condition for resonance is strongly dependent on the refractive indices and thickness of the media near the metal-insulator interface. The intensity of the light wave can be modulated by coupling the light wave with the surface plasmon wave. Generally, if coupling between the surface plasmon wave and the light wave is strong, the attenuation of the emitted light wave is strong, and if coupling is weak, there occurs almost no attenuation of the emitted light wave.
Attenuated total reflection (ATR) effect has been utilized to optically excite surface plasmon waves through a high-index prism. Light, traveling in free-space, is sent toward the metal-insulator interface through the prism with an angle larger than the critical angle, producing an evanescent wave field which may overlap the surface plasmon wave field. If the propagation constant Kev of the evanescent wave is in harmony with the propagation constant Ksp of the surface plasmon, the surface plasmon resonance is excited on the metal surface. Two configurations are mainly used for optically exciting the surface plasmon wave. The first is Otto's ATR configuration. This Otto's configuration is shown in FIG.
1
A. In this Otto's configuration, there exists a small air gap between a metal medium layer
101
stacked on a thick insulator
102
and a high-index prism
103
. A surface plasmon wave
105
is optically excited by the incident light. Further, the second configuration used to optically excite surface plasmon waves is Kretschmann's modified ATR configuration as shown in FIG.
1
B. In this configuration, a thin metallic foil
101
is inserted between the prism
103
and the insulator
102
. Surface plasmon waves
105
are also optically excited by absorbed light which will not be reflected light
106
. This configuration is more practical since there is no air gap. It is noted that the high-index prism
103
for generating the evanescent wave may be a diffraction grating with a period smaller than the wavelength of the incident light, or other optical components.
Here, when a prism is used as an optical component, the propagation constant (wave number) K
ev
of the evanescent wave is represented by the following equation (1):
K
ev
=
n
(
ω
)
·
K
0
(
ω
)
·
sin
θ
=
n
(
ω
)
·
ω
/
c
·
sin
θ
=
n
(
λ
)
·
2
π
/
λ
·
sin
θ
(
1
)
where c is the speed of light in vacuum, &ohgr; is the angular frequency, &lgr; is the wavelength, n(&ohgr;) and n(&lgr;) are the refractive indices of the prism in the case of an angular frequency &ohgr; and a wavelength &lgr;, respectively, K
0
(&ohgr;) is the wave number in the case of an angular frequency &ohgr; in vacuum, and &thgr; is the incident angle of light with respect to the underside of the prism. Therefore, the wave number of the evanescent waves can be harmonized with the propagation constant of the-metal surface plasmon by adjusting the refractive index n(&ohgr;) or n(&lgr;) of the prism and the incident angle &thgr; of light.
On the other hand, the propagation constant Ksp of the surface plasmon is given by the following equation 2, where the angular frequency of the surface plasmon is &ohgr;, and the dielectric indices of the metal and the dielectric indices of the low-index medium in contact with the metal are respectively &egr;m and &egr;0,
k
sp
=
ω
c
·
ϵ
m
(
ω
)
·
ϵ
0
ϵ
m
(
ω
)
+
ϵ
0
=
2
π
λ
·
ϵ
m
(
λ
)
·
ϵ
0
ϵ
m
(
λ
)
+
ϵ
0
(
2
)
where the &egr;m(&ohgr;) and &egr;0(&ohgr;) are the dielectric indices of the metal in the case of the angular frequency &ohgr; and the wavelength &lgr;, respectively. Here, since the &egr;m is a complex number, the propagation constant Ksp is also a complex number. The evanescent waves generated by using a prism when Kev=Ksp generates the surface plasmon. In order to strongly excite the metal surface plasmon, the metal surface plasmon itself must be a wave with a long life. That is, it is required that the imaginary part of the propagation constant Ksp is small, and the attenuation associated with propagation is small.
The imaginary part of the propagation constant Ksp is approximatively solved, assuming that respective complex-numbers are Ksp=Ksp′+iKsp″, and &egr;m=&egr;m&pr
Hayes & Soloway P.C.
Kovalick Vincent E.
NEC Corporation
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