Thermal imager using metamaterials
View Patent ↗An apparatus and method are disclosed for detecting terahertz radiation at room temperature. A detecting pixel includes a sub-wavelength split-ring resonator, and is mechanically coupled to (but thermally decoupled from) a substrate via a cantilever formed from two materials that have a significant mismatch in their thermal expansion coefficients. Incident radiation causes the split-ring resonator to resonate, thereby generating heat that is transferred to the cantilever, causing the cantilever to flex. An optical readout system includes a secondary light source, such as a laser, that shines on a reflective surface on the pixel, whereby a photodiode detects the reflected light and permits calculation of a relative deflection of the pixel in the nanometer range. An exemplary detector has a noise equivalent power rating of approximately 60 pW/√Hz.
1. An apparatus for detecting terahertz radiation at room temperature, the apparatus comprising:
a substrate; and
a detection structure including (a) an optically reflective surface, and (b) a radiation absorption pad having formed thereon a split-ring resonator, the split-ring resonator having a characteristic dimension, and an electrical resonance frequency corresponding to a wavelength of the terahertz radiation that is greater than the characteristic dimension;
the detection structure being coupled to the substrate by a flexible cantilever comprising (a) a plurality of actuation legs formed from two materials having substantially different coefficients of thermal expansion, and (b) a plurality of thermal isolation legs having substantially lower thermal conductivity than the actuation legs.
2. An apparatus according to claim 1 , wherein the substrate comprises a silicon (Si) wafer or a gallium arsenide (GaAs) wafer.
3. An apparatus according to claim 1 , wherein the detection structure comprises a silicon nitride (SiN x ) thin film.
4. An apparatus according to claim 1 , wherein the optically reflective surface is formed from gold (Au).
5. An apparatus according to claim 1 , wherein the plurality of actuation legs are formed from layers of silicon nitride (SiN x ) and gold (Au).
6. An apparatus according to claim 1 , wherein the plurality of thermal isolation legs are formed from silicon nitride (SiN x ).
7. An apparatus according to claim 1 , wherein the wavelength of the terahertz radiation is between 30 μm and 3000 μm.
8. An apparatus according to claim 1 , wherein the characteristic dimension of the split-ring resonator is between 5 μm and 500 μm.
9. An apparatus according to claim 1 , wherein the radiation absorption pad has formed thereon a plurality of split-ring resonators.
10. A method of imaging an object, the method comprising:
illuminating the object with terahertz radiation having a wavelength; and
absorbing, in a detector, a portion of the terahertz radiation that has interacted with the object, the detector comprising a plurality of pixels that move when heated, each pixel having formed thereon a split-ring resonator, at least one of the split-ring resonators having a characteristic dimension that is less than the wavelength of the terahertz radiation and an electrical resonance frequency that corresponds to the wavelength of the terahertz radiation;
wherein absorption of the portion of radiation causes the at least one split-ring resonator to generate sufficient heat to cause movement of the pixel on which it is formed.
11. A method according to claim 10 , wherein the wavelength of the terahertz radiation is between 30 μm and 3000 μm.
12. A method according to claim 10 , wherein the characteristic dimension of the split-ring resonator is between 5 μm and 500 μm.
13. A method according to claim 10 , wherein the detector further comprises a fixed substrate, and wherein each pixel in the plurality of pixels is attached to the substrate by a cantilever that flexes when heated.
14. A method according to claim 13 , wherein each cantilever comprises (a) a plurality of actuation legs formed from two materials having substantially different coefficients of thermal expansion, and (b) a plurality of thermal isolation legs having substantially lower thermal conductivity than the actuation legs.
15. A method according to claim 10 , wherein a given pixel has formed thereon a plurality of split-ring resonators, and absorption of the portion of radiation causes the plurality of split-ring resonators to generate sufficient heat to cause movement of the given pixel.
16. A method according to claim 10 , further comprising:
illuminating the detector pixels with a secondary light source; and
measuring light, from the secondary light source, that is reflected from the surface of a pixel to determine a relative deflection of the pixel.
17. A method according to claim 16 , wherein the secondary light source is a laser.
18. A method according to claim 10 , wherein each pixel absorbs a different portion of the terahertz radiation that interacted with the object, thereby causing non-uniform deflection of the pixels.
19. A method according to claim 18 , further comprising producing a raster image having pixel data, each pixel datum being a function of the relative deflection of a corresponding detector pixel.