Bolometric imaging system

Abstract

A bolometric imaging device may include an array of plasmonic nanoantennas which convert electromagnetic radiation in the infrared into heat. Each plasmonic nanoantenna may include a first metallic layer which simultaneously acts as an electrode for each nano-pixel, a first dielectric layer vertically disposed above to the metallic layer, and a metallic structure on the first dielectric layer. The bolometric imaging device may further include an array of nano-pixels where each nano-pixel includes a fixed magnetic polarity layer (Fixed Layer) in a first magnetic direction, a selective magnetic polarity layer (Free Layer) configured to switch magnetic polarity in response to heat from the array of plasmonic nanoantennas, and a barrier layer dispose between the fixed layer and the free layer. The bolometric imaging device may further include an electrode layer vertically disposed below the Fixed Layer.

Description

BOLOMETRIC IMAGING SYSTEMCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 558,498 filed February 27, 2024, the entirety of which is incorporated by reference.GOVERNMENT FUNDING

[0002] This invention was made with government support under 1654676 DMR and 1641101 EFMA awarded by the National Science Foundation. The government has certain rights in the invention.TECHNICAL FIELD

[0003] This disclosure relates to imaging and, in particular, to bolometric imaging.BACKGROUND

[0004] The infrared imaging market has steadily grown in recent years. There are a wide variety of existing uses including non-destructive testing, medical diagnostics, night vision, gas detection, and material characterization. The majority of the growth in commercial use has been in uncooled cameras which have much smaller size weight and power (SWaP) compared to infrared cameras with active cooling. However, existing uncooled cameras have significant limitations in readout speed and sensitivity. Infrared detectors require improvements in these areas for emerging commercial applications like infrared LIDAR, high resolution spectral imaging, night-time autonomous driving, and free space communications.

[0005] Currently, advanced detectors in thermal wavelengths include vanadium oxide (VOx) microbolometers, mercury cadmium telluride (MCT) photodiodes, and superconducting nanowire single photon detectors (SNSPDs). Vanadium oxide detectorsare a type of bolometer, i.e. a device which changes it’s resistance due to a temperature change induced by absorbed infrared radiation. The VOx microbolometer is currently the most sensitive room temperature thermal imager and is commonly used in commercial applications. Another important technology is the MCT photodiode which is based on a combination of ll-VI semiconductors. This combination is used to achieve a small bandgap capable of detecting infrared light. However, the small bandgap in an MCT photodiode produces significant noise, requiring cryogenic cooling at liquid nitrogen temperatures. Currently MCTs are commonly used in military applications due to their high sensitivity and high resolution. Lastly, SNSPDs are an emerging technology capable of single photon detection in the infrared. They are based on a small superconducting nanowire which acts as a high-speed bolometer and can detect infrared photons at subKelvin temperatures. Bolometers have demonstrated success in existing and emerging thermal imaging applications and therefore show promise for future detector technologies.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The embodiments may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale. Moreover, in the figures, like-referenced numerals designate corresponding parts throughout the different views.

[0007] FIG. 1 illustrates a perspective view of a plasmonic nanoantenna.

[0008] FIG. 2 illustrates a perspective view of a plasmonic nanoantenna array.

[0009] FIG. 3 illustrates a perspective view of a microlens is shown.

[0010] FIG. 4 illustrates a perspective view of a microlens array.

[0011] FIG. 5 illustrates a perspective view of an optical stack is shown.

[0012] FIG. 6 illustrates a perspective view of the array of nano-pixels.

[0013] FIG. 7 illustrates a side view of a bolometric imaging device.

[0014] FIG. 8 illustrates a perspective view of a bolometric imaging device.

[0015] FIG. 9 illustrates an example of a bolometric system including a readout circuit.DETAILED DESCRIPTION

[0016] Recently, detectors which respond to light with a series of ‘clicks’ have found significant usage in research and in commercial applications such as LIDAR. Detectors that display discrete readout are often called ‘digital-mode detectors’. There are many examples of these detectors in visible wavelengths, but only recently have digital-mode detectors been developed in the infrared (e.g. SNSPDs, and MCT avalanche photodiodes). However, these detectors require cooling to operate. Recently an uncooled infrared digital-mode bolometer has been developed called the spintronic ultrafast nanoscale (SUN) bolometer. This device is capable of relatively sensitive high-speed readout. However, the active area of the device is extremely small. This reduces the amount of light collected by the detector and can therefore have a significant effect on the sensitivity.

[0017] Many other types of technologies have attempted to address small sensor volume by increasing the intensity of light in the absorption layer. This can be achieved by utilizing plasmonic effects to increase the intensity of light near metallic elements. Heat from these regions can then be transported into the device’s sensitive layer. Metamaterial structures with plasmonic nanoantennas have generated interest as an improvement to detector design since they can simultaneously increase intensity and absorption. To increase absorption in thermal bands like MWIR and LWIR the dimensions of the nanoantenna are optimized along with the thickness of a dielectric and metallic layer. Nanoantennas have an additional benefit since their design can be used to filter light of specific frequencies and polarizations. This feature is extremely important in material identification, gas detection, spectral imaging, and autonomous driving. Using the nanoantennas as narrowband filters, pixel position can be used to determine spectral information similar to existing visible light cameras. Furthermore, the use of a microlens array can increase the effective area of the device, which is extremely important in detecting thermal signals from black bodies. Additionally, a microlens array will also increase the intensity on the absorptive plasmonic nanoantennas. The addition of a microlens array and plasmonic nanoantennas will have a significant impact on thecapabilities and sensitivity of SUN bolometers, helping them go beyond what is currently achieved in the state of the art.

[0018] The figures associated with these drawings include lines referring to the X, Y, and Z directions. The terms above, below, top, bottom, and vertical are used to specify location with respect to the Y axis. The terms horizontal are used to specify location with respect to the X and / or Z axis.

[0019] FIG 1 illustrates a perspective view of a plasmonic nanoantenna 100. The plasmonic nanoantenna 100 includes a first metallic layer 102 which absorbs incident infrared radiation and simultaneously acts as an electrode to an optical stack (see FIG. 5 for an example of an optical stack and FIGs 7-8 for an example of the orientation of the first metallic layer 102 with respect to the optical stack). The plasmonic nanoantenna 100 may further include a dielectric layer 104 above the first metallic layer 102. The dielectric layer 104 may enhance absorption and enhance the plasmonic resonance to increase the intensity of light absorbed in the first metallic layer 102.

[0020] The plasmonic nanoantenna 100 may include a metallic structure 106. In some examples, the metallic structure may be a cylindrical shape that protrudes form a surface of the first dielectric layer 104. The size and shape of a metallic structure 106 may be configured to improve electromagnetic absorption and intensity in the infrared through plasmonic effects.

[0021] In some embodiments, the plasmonic antenna 100 may include a second metallic layer 108 to further improve the absorption in the infrared. The second metallic layer may be positioned between the first metallic layer 102 and the dielectric layer 104.

[0022] FIG. 2 illustrates a perspective view of a plasmonic nanoantenna array 110. The plasmonic nanoantenna 100 (example shown in FIG 1 ) is repeated periodically in an array to enhance absorption. Accordingly, the plasmonic nanoantenna array 110 may include a plurality of metallic structures 106. The metallic structures 106 may protrude from the dielectric layer 104. The dielectric layer may be positioned above the first metallic layer 102. In some examples, a second metallic layer 108 may be positioned between the first metallic layer 102 and the dielectric layer 104.

[0023] FIG. 3 illustrates a perspective view of a microlens 112 is shown. The microlens 112 includes a dielectric layer 114, and a curved dielectric element 116. The curved dielectric element 116 may focus incident light onto the plasmonic nanoantenna 100 (shown in FIGs. 1 , 7, and 8). The curved dielectric element 116 may protrude from a surface of the dielectric layer 114. In some embodiments, an antireflective coating 118 is adjacent the dielectric layer 114 to improve transmission. For example, the antireflective coating 118 may be applied to the bottom side of the dielectric layer, which is the side opposite where the microlens 112 protrudes from.

[0024] FIG. 4 illustrates a perspective view of a microlens array 120. The microlens array 120 further increases the intensity of light on the first metallic layer 102 and simultaneously increases the effective area of the pixel (See FIGs. 7-8 for the orientation of the microlens array with respect to the first metallic layer 102 and pixel). These features along with the nanoantennas are used to improve the detection efficiency of the SUN bolometer.

[0025] FIG. 5 illustrates a perspective view of an optical stack 122. The optical stack may alternatively be referred to as a nano pixel. The optical stack 122 includes a fixed magnetic polarity layer (Fixed Layer) 124, a barrier layer 126, a selective magnetic polarity layer (Free Layer) 128, and an electrode layer 130 (or a portion thereof when the electrode layer is shared in an array of optical stacks). The optical stack 122 is configured such that the selective magnetic polarity layer 128 changes magnetic direction in response to heat from the plasmonic nanoantenna 100. This change in magnetic direction results in a change in electrical impedance due to tunnel magnetoresistance effects from the two magnetic layers. In some embodiments, a reflective layer 132 is included above the selective magnetic polarity layer 128 to enhance reflection into the plasmonic nanoantenna array. In some embodiments, the optical stack may include a thermally insulating layer 134.

[0026] The thermally insulating layer 134 may be positioned horizontally next to the fixed magnetic polarity layer 124, the barrier layer 126, the selective magnetic polarity layer 128, the reflective layer 103, or a combination thereof. For example the thermally insulating layer may extend from the electrode layer 130 and along the aforementionedlayers. The thermally insulating layer 134 may improve heat transport through the optical stack. Materials for the thermally insulating layer may include, for example, SiO2, or Si3N4.

[0027] FIG. 6 illustrates a perspective view of the array of nano-pixels 136. Each nanopixel 122 includes the optical stack 122. The nano-pixels protrude from the electrode layer 130 .

[0028] FIG. 7 illustrates a side view of a bolometric imaging device 140 where incident infrared radiation 142 is shown. The bolometric imaging device 140 includes an array of plasmonic nanoantennas 110, a microlens array 120, and an array of nanopixels 136. The microlens array 120 is positioned vertically above the array of plasmonic nanoantennas 110. The array of plasmonic nanoantennas 110 is positioned vertically between above the microlens array 120 and the array of nanopixels 136. The array of nanopixels 136 positioned vertically below the array of plasmonic nanoatennas 110.

[0029] The array of plasmonic nanoantennas 110 and microlens array 120 optimize the collection of light and absorption of light while simultaneously increasing the intensity of light in the absorption layers. Together, these factors improve each nanopixel’s sensitivity to incident infrared radiation. The nanopixels include a spintronic ultrafast nanoscale (SUN) bolometer designed such that the selective magnetic polarity layer flips magnetic orientation in response to increased heat. This change in the magnetic orientation is converted into a change in resistance through magnetoresistance effects with the nearby fixed magnetic polarity layer. Thereby, the incident heat is converted into a series of discrete changes in the nanopixel resistance.

[0030] The plasmonic nanoantenna array 120 may include a first metallic layer 102 which simultaneously acts as an electrode for each nano-pixel 122 and a first dielectric layer 104. The first dialectic layer 104 is vertically disposed above the metallic layer 102. The first metallic layer 102 may contact the first dielectric layer 104. Alternatively, a second metallic layer 108 may be disposed between the first metallic layer 102 and dielectric layer 104 In some examples, the material of the first metallic layer 102 and / or second metallic layer 108 may be Au, Ti, Ge, TiN, WSi, and / or NbN. The material of the first dielectric layer 104 may be Ge, SiO2, AI2O3, MgF2, CaF2, ZnSe, ZnS, and / or S.The thickness of the first dielectric layer 104 may be between about 10 nm and about 1 um.

[0031] Each plasmonic nanoantenna 100 may include a metallic structure 106 which may be vertically disposed adjacent to the dielectric layer 104. The material of the metallic structure 106 may Au, Ti, Ge, TiN, WSi, and / or NbN. In some examples, the scale and shape of the metallic structure 106 is configured to increase absorption in a particular wavelength band. For example, the width of the metallic structure may be between about 20nm and about 10um., and the height may be between 5 and 200nm.

[0032] The scale and shape of the metallic structure may be varied for different nanopixels across the same array. The shape and scale of the metallic structure can be used to selectively filter light of a particular wavelength or polarization. Using different filters is useful for collecting the full spectrum of light, or for selectively filtering out particular absorptions bands. This is useful in infrared where the material being imaged often has a unique absorption spectrum. Therefore this can provide additional information about the objects being imaged.

[0033] The plasmonic nanoantenna in the above bolometric imaging device 140 may include a second metallic layer 108 vertically disposed between the first dielectric layer 104 and first metallic layer 102.

[0034] The bolometric imaging device 140 may further include a microlens array 120. Each microlens may include a second dielectric layer 114 and a curved dielectric element 116. The curvature of the dielectric element 116 is configured to focus incident electromagnetic radiation. In some examples, the material of the microlens (an components thereof) may include Ge, SiO2, AI2O3, MgF2, CaF2, ZnSe, ZnS, and / or S. In some examples, the microlens array may further include an anti-reflection coating 118 outwardly disposed on the second dielectric layer 114.

[0035] The bolometric imaging device 140 may further include an array of nano-pixels 136. Each nano-pixel 122 may include an optical stack. In some examples, the width of the nano-pixel 122 may be sized between about 10 nm and about 1 um. The height of the nano-pixel may be between 1 mm and 1 urn. The nano-pixels may be sensitive to infrared light when the nano-pixel 122 is between about 20 nm and about 500 nm.

[0036] The optical stack 122 may include a fixed magnetic polarity layer (Fixed Layer) in a first magnetic direction. The optical stack may further include a barrier layer vertically disposed adjacent to the Fixed Layer.

[0037] The optical stack my further include a selective magnetic polarity layer (Free Layer) vertically disposed next to the barrier layer and simultaneously vertically disposed between to the metallic layer 102 of the plasmonic nanoantenna array 110 and the electrode layer 130. The first metallic layer 102 or second metallic layer 108 of the plasmonic nanoantenna array 110 may act as a top electrode, while electrode layer 130 acts as a bottom electrode. When the magnetic polarity of the Free Layer is along the first magnetic direction, the optical stack is in a parallel (P) configuration whereby the optical stack presents an electrical impedance to current flow below an impedance threshold. When the magnetic polarity of the Free Layer is opposite the first direction, the optical stack is in an anti-parallel (AP) configuration whereby the optical stack presents an electrical impedance to current flow higher than the impedance threshold. An electrode layer may be vertically disposed adjacent the Fixed Layer.

[0038] The material of the magnetic Fixed Layer and the Free Layer may be CoFeB, GdFeCo, TbFeCo, CoFe, Co, Pt, and / or Pd. In some examples, the barrier layer may include MgO.

[0039] In some examples, the optical stack may comprise a light reflective layer vertically disposed adjacent the first metallic layer. In some examples, the nano-pixel further comprises a heat insulating layer horizontally disposed adjacent the optical stack 122.

[0040] The array of plasmonic nanoantennas 110 may be vertically disposed above the array of nano-pixels 136. The microlens array 120 may be vertically disposed above the array of plasmonic nanoantennas 110. Photons absorbed by the plasmonic nanoantennas 110 are converted into heat to thereby switch the magnetic polarity of the Free Layer. The switch in magnetic polarity does not require the optical stack to be reset.

[0041] In some examples, the bolometric imaging device 140 may include a third dielectric layer 144 vertically disposed adjacent to the array of plasmonic nanoantenna110 and simultaneously vertically disposed adjacent to the microlens array 120. This dielectric layer is configured to further improve absorption in the plasmonic nanoantenna.

[0042] FIG. 8 illustrates a perspective view of the bolometric imaging device 140. The microlens array 120 sits above the array of plasmonic nanoantennas 110, which likewise sits above the array of nano-pixels 136. In some embodiments, a third dielectric layer 144 joins the microlens array and the array of plasmonic nanoantennas.

[0043] FIG. 9 illustrates an example of a bolometric imaging system 900 having the bolometric imaging device 140 connected to a readout circuit 902. The readout circuit 902 is connected through either the first 102 or second metallic layer 108 which acts as the top electrode. The bottom electrode is the electrode layer 130, which may be connected to ground. The readout circuit 902 may include a biasing circuit 904, a passive differentiator 906, a diode bridge 908, and / or a comparator 910. Additional and alternative circuits are possible. The readout circuit 902 may generate a digital output signal corresponding to a change in electrical impedance of the nanopixel array 136 . The purpose of the biasing circuit 904 is to provide a controlled current bias, such that changes in the resistance of the bolometric imaging device 140 lead to voltage signals which are readout electrically through the rest of the readout circuit 902. The purpose of the passive differentiator 906 is to convert changes in voltage levels into sharp pulses of positive and negative voltage. The purpose of the diode bridge 908 is to take the sharp pulses and align the pulses along the same voltage direction (either positive or negative). The purpose of the comparator 910 is to convert the sharp pulses into a digital signal between two standard voltage levels.

[0044] While various embodiments have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible. Accordingly, the embodiments described herein are examples, not the only possible embodiments and implementations.

[0045] A second action may be said to be "in response to" a first action independent of whether the second action results directly or indirectly from the first action. The second action may occur at a substantially later time than the first action and still be in response to the first action. Similarly, the second action may be said to be in response to the firstaction even if intervening actions take place between the first action and the second action, and even if one or more of the intervening actions directly cause the second action to be performed. For example, a second action may be in response to a first action if the first action sets a flag and a third action later initiates the second action whenever the flag is set.

[0046] To clarify the use of and to hereby provide notice to the public, the phrases "at least one of , , ... and <N>" or "at least one of , , ... <N>, or combinations thereof" or ", , ... and / or <N>" are defined by the Applicant in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted by the Applicant to the contrary, to mean one or more elements selected from the group comprising A, B, ... and N. In other words, the phrases mean any combination of one or more of the elements A, B, ... or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed.

Claims

CLAIMSWhat is claimed is:1 . A bolometric imaging device, comprising: an array of plasmonic nanoantennas configured to convert electromagnetic radiation in the infrared into heat, each plasmonic nanoantenna comprising: a first metallic layer which simultaneously acts as an electrode for each nano-pixel, a first dielectric layer vertically disposed above to the metallic layer, and a metallic structure on the first dielectric layer; an array of nano-pixels, each nano-pixel comprising: a fixed magnetic polarity layer (Fixed Layer) in a first magnetic direction, a selective magnetic polarity layer (Free Layer) configured to switch magnetic polarity in response to heat from the array of plasmonic nanoantennas, and a barrier layer dispose between the fixed layer and the free layer; and an electrode layer vertically disposed below the array of nano-pixels.

2. The bolometric imaging device of claim 1 , wherein when the magnetic polarity of the Free Layer is along the first magnetic direction, the optical stack is in a parallel (P) configuration whereby the optical stack presents an electrical impedance to current flow below an impedance threshold, and when the magnetic polarity of the Free Layer is opposite the magnetic first direction, the optical stack is in an anti-parallel (AP) configuration whereby theoptical stack presents an electrical impedance to current flow higher than the impedance threshold.

3. The bolometric imaging device of claim 1 , further comprising a microlens array, each microlens comprising: a second dielectric layer and a curved dielectric element, wherein the curvature of the dielectric element is configured to focus incident electromagnetic radiation.

4. The bolometric imaging device of claim 3, wherein the bolometric imaging device further comprises a third dielectric layer disposed between the array of plasmonic nanoantenna and the microlens array.

5. The bolometric imaging device of claim 3, wherein the material of the microlens includes at least one of Ge, SiO2, AI2O3, MgF2, CaF2, ZnSe, ZnS, S, or a combination thereof.

6. The bolometric imaging device of claim 3, wherein the microlens array further comprises an anti-reflection coating outwardly disposed on the dielectric layer.

7. The bolometric imaging device of claim 1 , wherein the plasmonic nanoantenna further comprises a second metallic layer vertically disposed adjacent to the first dielectric layer8. The bolometric imaging device of claim 1 , wherein the material of the first dielectric layer includes at least one of Ge, SiO2, AI2O3, MgF2, CaF2, ZnSe, ZnS, S, or a combination thereof.

9. The bolometric imaging device of claim 1 , wherein the material of the first metallic layer includes at least one of Au, Ti, Ge, TiN, WSi, NbN, or a combination thereof.

10. The bolometric imaging device of claim 1 , wherein the material of the metallic structure includes at least one of Au, Ti, Ge, TiN, WSi, NbN, or a combination thereof.1 1 . The bolometric imaging device of claim 1 , wherein the thickness of the dielectric layer is between about 10 nm and about 1 um.

12. The bolometric imaging device of claim 1 , wherein the size of the metallic structure is between about 20nm and about 10um.

13. The bolometric imaging device of claim 1 , wherein the scale and shape of the metallic structure is configured to increase absorption in a particular wavelength band.

14. The bolometric imaging device of claim 1 , wherein the scale and shape of the metallic structure is varied for different nano-pixels.

15. The bolometric imaging device of claim 1 , wherein the material of the magnetic Fixed Layer and the Free Layer includes at least one of CoFeB, GdFeCo, TbFeCo, CoFe, Co, Pt, Pd, or a combinations thereof.

16. The bolometric imaging device of claim 1 , wherein the barrier layer includes MgO.

17. The bolometric imaging device of claim 1 , wherein each nano-pixel is sized between about 10 nm and about 1 um.

18. The bolometric imaging device of claim 1 , wherein optical polarization sensitivity is based on a patterning direction of the metallic element.

19. The bolometric imaging device of claim 1 , wherein the nano-pixel is sensitive to infrared light when the optical stack is between about 20 nm and about 500 nm.

20. The bolometric imaging device of claim 1 , wherein the optical stack further comprises a light reflective layer vertically disposed adjacent the first metallic layer.21 . The bolometric imaging device of claim 1 , wherein the optical stack further comprises a heat insulating layer horizontally disposed adjacent the optical stack.

Images