Apparatus and methods for sensing or imaging using stacked thin films
Technologies are generally described for methods and systems for sensing or imaging. The apparatus includes a stack of a plurality of thin films, such as polymer thin films. The stack has a substantially imaginary total reflectance coefficient.
1. An apparatus for sensing or imaging with light at a wavelength λ, the apparatus comprising:
a stack including a plurality of thin films;
wherein the stack has a substantially imaginary total reflectance coefficient for the light at the wavelength λ,
wherein the substantially imaginary total reflectance coefficient is a total reflectance coefficient for which the real part of the reflectance is substantially zero, and the imaginary part of the reflectance is substantially nonzero.
2. The apparatus of claim 1 , wherein the plurality of thin films comprise polymer thin films.
3. The apparatus of claim 1 , wherein the plurality of thin films include a plurality of bi-layers, wherein each of the plurality of bi-layers comprises a layer of high refractive index and a layer of low refractive index.
4. The apparatus of claim 3 , wherein the stack further includes:
a top layer; and
a substrate.
5. The apparatus of claim 4 , wherein the top layer has a refractive index (n l ), a thickness (d l ), and a reflectance (r l ) selected such that they satisfy the following equations:
r
s
′
=
r
s
-
r
l
1
-
r
s
r
l
and
r
net
=
r
l
+
r
s
′
ⅇ
-
j
2
ϕ
l
1
+
r
l
r
s
′
ⅇ
-
j
2
ϕ
l
=
±
j
β
[
r
s
′
(
1
+
r
l
2
)
]
2
-
[
r
l
(
1
+
r
s
′
2
)
]
2
(
1
-
r
l
2
r
s
′
2
)
2
=
β
2
cos
(
4
π
n
l
d
l
λ
)
=
-
(
r
l
r
s
′
)
(
1
+
r
s
′
2
1
+
r
l
2
)
wherein β is an amplitude of a total reflectance coefficient r net of the stack including the top layer, wherein the total reflectance coefficient is imaginary and has a form of ±jβ, wherein j is the imaginary unit and ±j=√−1, and r s and r s ′ are the reflectance of the substrate and the plurality of bi-layers combined, calculated with air and the top layer as the ambient medium, respectively.
6. The apparatus of claim 5 , wherein the plurality of thin films include a plurality of bi-layers, wherein each of the plurality of bi-layers comprises a layer of high refractive index and a layer of low refractive index.
7. The apparatus of claim 6 , wherein the high refractive index is about 1.45-1.65, and the low refractive index is lower than the high refractive index.
8. The apparatus of claim 6 , wherein the layer of high refractive index and the layer of low refractive index each have a thickness of approximately ¼ of the wavelength λ.
9. The apparatus of claim 1 , wherein the apparatus is configured to sense or image a sample disposed over the stack through a change in reflectance caused by the presence of the sample.
10. The apparatus of claim 9 , wherein the sample comprises one of a biological sample, a chemical material, or an optical material, and wherein the apparatus is configured as one of a biosensor, a chemical sensor, or a color sensor.
11. A method of making a sensor or an imager, the method comprising:
selecting materials for a top layer, a plurality of thin films, and a substrate; and
forming a stack with the top layer, the plurality of thin films, and the substrate,
wherein the materials are selected such that stack has a substantially imaginary total reflectance for light at a wavelength λ used for imaging or sensing, and
wherein the substantially imaginary total reflectance coefficient is a total reflectance coefficient for which the real part of the reflectance is substantially zero, and the imaginary part of the reflectance is substantially nonzero.
12. The method of claim 11 , wherein the plurality of thin films comprise polymer thin films.
13. The method of claim 11 , wherein the forming comprises disposing the plurality of thin films as a plurality of bi-layers, wherein each of the plurality of bi-layers comprises a layer of high refractive index and a layer of low refractive index.
14. The method of claim 13 , wherein the layer of high refractive index and the layer of low refractive index each have a thickness of approximately ¼ of the wavelength of the light used for sensing in the respective layer.
15. The method of claim 11 , wherein the selecting comprises selecting a refractive index (n l ), a thickness (d l ), and a reflectance (r l ) of the top layer such that they satisfy the following equations:
r
s
′
=
r
s
-
r
l
1
-
r
s
r
l
and
r
net
=
r
l
+
r
s
′
ⅇ
-
j
2
ϕ
l
1
+
r
l
r
s
′
ⅇ
-
j
2
ϕ
l
=
±
j
β
[
r
s
′
(
1
+
r
l
2
)
]
2
-
[
r
l
(
1
+
r
s
′
2
)
]
2
(
1
-
r
l
2
r
s
′
2
)
2
=
β
2
cos
(
4
π
n
l
d
l
λ
)
=
-
(
r
l
r
s
′
)
(
1
+
r
s
′
2
1
+
r
l
2
)
wherein β is an amplitude of a total reflectance coefficient r net of the stack including the top layer, wherein the total reflectance coefficient is imaginary and has a form of ±jβ, wherein j is the imaginary unit and ±j=√−1, and r s and r s ′ are the reflectance of the substrate and the plurality of bi-layers combined, calculated with air and the top layer as the ambient medium, respectively, and wherein λ is the wavelength of the light used for imaging or sensing.
16. A method of sensing or imaging a sample, the method comprising:
providing a stack comprising a plurality of polymer thin films, wherein the stack has a substantially imaginary total reflectance for light at a wavelength λ used for imaging or sensing, wherein the substantially imaginary total reflectance coefficient is a total reflectance coefficient for which the real part of the reflectance is substantially zero, and the imaginary part of the reflectance is substantially nonzero;
directing a beam of light toward the stack in the absence of the sample;
measuring the reflectance of the stack in the absence of the sample;
disposing a sample over the stack;
measuring the reflectance of the stack in the presence of the sample; and
obtaining the difference in reflectance of the stack in the presence and absence of the sample.
17. The method of claim 16 , wherein the plurality of thin films include a plurality of bi-layers, wherein each of the plurality of bi-layers comprises a layer of high refractive index and a layer of low refractive index.
18. The method of claim 17 , wherein the layer of high refractive index and the layer of low refractive index each have a thickness of approximately ¼ of the wavelength λ of the light used for imaging or sensing in the respective layer.