Surface plasmon optical modulator element

Details Surface plasmon optical modulator element Surface plasmon optical modulator element

2000-02-07

2003-08-26

Lester, Evelyn (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave directional modulation

C359S244000, C359S262000, C359S263000, C359S321000, C252S582000, C430S581000

Reexamination Certificate

active

06611367

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a surface plasmon optical modulator element which modulates light on the basis of generation of surface plasmon.
2. Description of the Related Art
In metal, free electrons vibrate in a group to generate compression wave called plasma wave. The compression wave generated in a surface of metal is quantized into surface plasmon.
There has been investigated a surface plasmon optical modulator element which modulates light on the basis of the phenomenon that the surface plasmon is generated by light wave as disclosed, for instance, in “Ultrafast wide range all-optical switch using complex refractive-index changes in a composite film of silver and polymer containing photochromic dye” by Kyoichi Sasaki and Toshihiko Nakamura, Journal of Applied Physics, vol. 83, No.6, pp. 72 to 78 (1998).
FIG. 4
shows the basic arrangement of a surface plasmon optical modulator element which has been conventionally conceived (will be referred to as “the conventional surface plasmon optical modulator element”, hereinbelow). As shown in
FIG. 4
, the conventional surface plasmon optical modulator element basically comprises a dielectric material block
11
disposed so that light-to-be-modulated modulated
10
travels through the interior of the dielectric material block
11
and impinges upon one surface
11
a
thereof at an angle of total reflection, a metal film
12
formed on the surface
11
a
of the dielectric material block
11
, a photo-functional film
13
which is formed on the metal film
12
and whose refractive index is changed upon exposure to light, and a modulating light source
15
which projects modulating light
14
onto the photo-functional film
13
.
In the surface plasmon optical modulator element, the angle of incidence &thgr; of the light-to-be-modulated
10
to the surface
11
a
of the dielectric material block
11
is set, for instance, so that relatively strong surface plasmon resonance is generated in the metal film
12
when the modulating light
14
is projected onto the photo-functional film
13
and relatively weak or no surface plasmon resonance is generated in the metal film
12
when the modulating light
14
is not projected onto the photo-functional film
13
.
With this arrangement, the light-to-be-modulated
10
is reflected in total reflection at the interface between the dielectric material block
11
and the metal film
12
and travels in the direction of arrow A in a state where the modulating light
14
is not projected onto the photo-functional film
13
. To the contrast, when the modulating light
14
is projected onto the photo-functional film
13
, surface plasmon resonance is generated to cancel the total reflection and the amount of light-to-be-modulated
10
reflected from the interface is greatly reduced or nullified. Thus, by controlling projection of the modulating light
14
onto the photo-functional film
13
, the light-to-be-modulated
10
traveling in the direction of arrow A can be modulated.
Conventionally, the metal film
12
is generally of gold. Properties of a gold film are not apt to be changed by oxidation, which ensures stable modulation.
In the conventional surface plasmon optical modulator element in which a gold film is employed, there has been a problem that a sufficient degree of modulation cannot be obtained unless the refractive index of the photo-functional film
13
is greatly changed.
This will be described in detail with reference to
FIG. 2
, hereinbelow.
FIG. 2
shows the relation between the angle of incidence &thgr; of the light-to-be-modulated
10
to the surface
11
a
of the dielectric material block
11
and the reflectance of the surface
11
a
of the dielectric material block
11
for different conditions under which surface plasmon resonance is generated. In
FIG. 2
, characteristic curve a represents the relation when the condition under which surface plasmon resonance is generated (will be referred to as “the surface plasmon resonance generating condition”, hereinbelow) is relatively slack and characteristic curve b represents the relation when the surface plasmon resonance generating condition is relatively strict. The characteristic curves a and b are translated substantially in the direction of the abscissa when the refractive index of the photo-functional film
13
on the metal film
12
changes according to whether the modulating light
14
is projected onto the photo-functional film
13
. As can be understood from the characteristic curve a, when the surface plasmon resonance generating condition is slack, the difference between the reflectance when the modulating light
14
is projected and that when the modulating light
14
is not projected, i.e., the degree of modulation, cannot be large unless the difference between the refractive indexes of the photo-functional film
13
when the modulating light
14
is projected onto the photo-functional film
13
and when the modulating light
14
is not projected onto the photo-functional film
13
is extremely large. The refractive index of the photo-functional film
13
, the reflectance of the surface
11
a
and the like when the modulating light
14
is projected onto the photo-functional film
13
will be referred to as the refractive index of the photo-functional film
13
, the reflectance of the surface
11
a
and the like with projection of the modulating light
14
, hereinbelow. Similarly the refractive index of the photo-functional film
13
, the reflectance of the surface
11
a
and the like when the modulating light
14
is not projected onto the photo-functional film
13
will be referred to as the refractive indexes of the photo-functional film
13
, the reflectance of the surface
11
a
and the like without projection of the modulating light
14
, hereinbelow.
To the contrast, as can be understood from the characteristic curve b, when the surface plasmon resonance generating condition is strict, the difference between the reflectance with projection of the modulating light
14
and that without projection of the modulating light
14
can be large and a large degree of modulation can be obtained even if the difference between the refractive indexes with and without projection of the modulating light
14
is small.
When the metal film
12
is of gold, the surface plasmon resonance generating condition is relatively slack as represented by the characteristic curve a in FIG.
2
and accordingly it is difficult to obtain a sufficient degree of modulation.
When a metal film
12
of silver is employed, the surface plasmon resonance generating condition becomes more strict. However properties of a silver film are apt to be changed by oxidation and accordingly, it is difficult to obtain stable modulation when a silver film is employed.
As disclosed in the above identified paper, the photo-functional film
13
has been conceived to be formed by use of dye comprising spiropyran series photochromic compound. Further the photo-functional film
13
is conceived to be formed by use of phthalocyanine dye as disclosed in the arguments in a public lecture, “Frontier Technology in Molecular Photonics” (Dec. 11, 1997) pp. 181 to 187, published by Shizuoka University Electronics Laboratory.
However, the photo-functional film
13
of spiropyran series photochromic dye is disadvantageous in that it is slow in response to light and accordingly is unsuitable for high speed modulation. Further, the photo-functional film
13
of phthalocyanine dye is disadvantageous in that its light absorption wavelength is limited and accordingly it can only serve to modulation of light at limited wavelengths.
Further, there has been a problem in the conventional surface plasmon optical modulator element that it is difficult to correctly position relatively to each other the light-to-be-modulated
10
and the modulating light
14
which are caused to impinge upon the metal film on opposite sides of the metal film. That is, since the light-to-be-modulated
10
is generally a thin beam and the modulating light

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