Optical: systems and elements – Light interference – Produced by coating or lamina
Reexamination Certificate
1998-08-31
2001-10-02
Chang, Audrey (Department: 2872)
Optical: systems and elements
Light interference
Produced by coating or lamina
C349S106000, C349S196000, C359S578000, C359S579000
Reexamination Certificate
active
06297907
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wavelength tunable optical filters and, more particularly, to devices based on a surface plasmon tunable filter.
BACKGROUND
An optical wavelength filter is a device that reflects or transmits light of a desired wavelength or within a certain wavelength range. For example, an interference bandpass filter selectively transmits light within a selected wavelength transmission bandwidth while absorbing light of wavelengths outside the transmission bandwidth. Such optical filtering with respect to wavelength provides a means of controlling the energy and spectral composition of light and is widely used in a variety of optical signal processing, detection, and display applications.
Excitation of surface plasmon waves at a metal-dielectric interface has been demonstrated as an efficient way of implementing a spectral filtering mechanism in response to an electrical control signal. See, for example, Wang and Simon, “Electronic Reflection with Surface Plasmon,” Opt. Quantum Electron.25, S925 (1993) and Wang, “Voltage-Induced Color-Selective Absorption with Surface Plasmon”, Appl. Phys. Lett. 67, pp. 2759-2761 (1995). Surface plasmon are oscillations of free electrons caused by resonant absorption of a p-polarized incident optical wave at a metal-dielectric interface when the wavelength and incident angle of the optical wave satisfy a plasmon resonance condition. More specifically, the plasmon resonance condition requires that the component of the optical wave vector along the metal-dielectric interface matches the plasmon wave vector, K
p
:
K
p
=
2
π
λ
ε
1
ε
2
ε
1
+
ε
2
,
(
1
)
where, ∈ is the wavelength of the optical wave, ∈
1
and ∈
2
are the dielectric permittivity constants for the metal and the dielectric material, respectively.
At surface plasmon resonance, the energy of the incident optical wave is strongly absorbed and converted into the energy of oscillating free electrons in the metal. Therefore, the reflected optical wave is strongly attenuated or even vanishes. When the incident angle of the optical wave is fixed at a constant, the optical wavelength &lgr; satisfying the plasmon resonance condition may be changed by varying the dielectric permittivity constant ∈
2
of the dielectric material. If the input optical wave is white light, the color of the reflected optical wave will change with ∈
2
. This phenomena effects a surface plasmon tunable filter in reflection mode.
Therefore, an electronically tunable filter can be formed by using an electro-optic material as the dielectric material. The voltage applied on the electro-optic material changes its index of refraction and thereby changes the wavelength for the surface plasmon resonance.
Wang and Simon disclose color display devices based on a surface plasmon filter using a liquid crystal electro-optic material. U.S. Pat. Nos. 5,451,980 and 5,570,139, which are incorporated herein by reference. The index of the refraction of the liquid crystal is changed by applying a voltage to alter the spectral composition of the reflected light.
SUMMARY
The devices disclosed herein use surface plasmon waves at metal-dielectric interfaces to alter the spectral composition of light having a p-polarized component. The metal material in general has a negative dielectric constant and the dielectric material has a positive dielectric constant. The electrical field of the p-polarized component at non-normal incidence induces electric dipoles in a metallic layer that forms one side of a metal-dielectric interface due to the excitation of the free electrons in the metal. The direction of the induced dipoles is perpendicular to the metal-dielectric interface. The radiation of the dipoles generates a surface plasmon wave with a wave vector parallel to the interface. The strength of the surface plasmon wave is maximal at the metal-dielectric interface and decays exponentially on both sides of the interface.
The energy conversion from the incident light to the surface plasmon wave is maximal when the incident angle, wavelength of the incident light, the dielectric constants of the metal and the dielectric materials satisfy a surface plasmon resonance condition. In general, this resonance condition relates to mode matching between the p-polarized incident light and the surface plasmon wave at a metal-dielectric interface and may vary with the specific incident coupling mechanism and the structure of the interfaces (e.g., a single interface or two closed coupled interfaces).
One embodiment of a surface plasmon filter includes a dielectric layer sandwiched between two metallic layers to form two closely spaced symmetrical metal-dielectric interfaces. The optical thickness of the dielectric layer is configured to allow for excitation of surface plasmon waves on both metal-dielectric interfaces by an input optical wave. The dielectric layer may be less or larger than one wavelength but in general on the order of a wavelength. The coupling between the surface plasmon waves produces a reflected wave and a transmitted wave that have mutually complimentary colors.
The surface plasmon resonance frequency can be tuned by adjusting the optical thickness of the dielectric layer. Either the layer thickness or the index of the refraction of the dielectric layer may be adjusted to change the transmission wavelength. One implementation uses an adjustable air gap as the dielectric layer. Another implementation uses a layer of an electro-optic material to vary the optical thickness by changing the index of refraction with a voltage control signal.
One or more additional metal-dielectric interfaces may be added and coupled to the two metal-dielectric interfaces to form a multilayer surface plasmon filter. Such a multilayer structure can be configured to achieve a desired shape in the transmission spectrum profile. For example, a “notch” filter can be so formed to produce a square-like transition from a transmissive spectral region to a reflective spectral region and to achieve a desired transmissive bandwidth.
The surface plasmon filter can be used to form a wide range of devices. One such device is a tunable Fabry-Perot filter based on an air-gap surface plasmon filter. Various color filters for color display systems such as color LCD displays can be formed based on a surface plasmon filter.
These and other aspects and advantages of the present invention will become more apparent in light of the accompanying drawings, the detailed description, and the claims.
REFERENCES:
patent: 5570139 (1996-10-01), Wang
California Institute of Technology
Chang Audrey
Curtis Craig
Fish & Richardson P.C.
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