Digital-mode bolometer

Abstract

A bolometer may include a sensitive layer configured to provide two distinct electrical impedance states at a temperature above 200K. Electrical impedance state 1 and electrical impedance state 2 may be separated by an energy barrier that is between 5% and 3000% of the average thermal energy in the sensitive layer. Incident infrared electromagnetic radiation is converted into heat, resulting in a change in the sensitive layer from electrical impedance state 1 to electrical impedance state 2. The continued absorption of incident electromagnetic radiation may cause a discrete series of changes in the bolometer response from electrical impedance state 1 to electrical impedance state 2.

Description

DIGITAL-MODE BOLOMETERCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 558,440 filed February 27, 2024, the entirety of which is hereby 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 bolometers and, in particular, digital bolometers.BACKGROUND

[0004] Existing infrared bolometers operating above 200K, including uncooled bolometers struggle with achieving the high sensitivity required for high-speed readout. High speed readout requires a shorter integration time leading to fewer photons and lower incident optical power over the integration window. Generally, this leads to a reduction in the detector’s signal to noise ratio. Therefore, improvements in sensitivity are necessary for high-speed operation of bolometers that operate without significant cooling. These high-speed capabilities are particularly useful in emerging infrared applications like infrared LIDAR, free-space communication, and autonomous automotive sensing.

[0005] The most sensitive cooled infrared bolometer is the superconducting nanowire single photon detector. The output of this device as well as other single photon detectors includes a series of ‘clicks’ or discrete changes in state. The rate of ‘clicks’ is dependenton the amount of incident light present on the detector. Therefore, the response of these devices, which we call digital-mode detectors are fundamentally different from the continuous response of traditional infrared bolometers or photodiodes.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 an example of a digital-mode bolometer.

[0008] FIG. 2 illustrates the response of the digital-mode bolometer to changes in incident infrared electromagnetic radiation.

[0009] FIG. 3 is a diagram illustrating the potential energy landscape of an analog bolometer.

[0010] FIG. 4 is a diagram illustrating the potential energy landscape of a digital-mode bolometer.DETAILED DESCRIPTION

[0011] A bolometer is a device sensitive to electromagnetic radiation which changes electrical impedance in response to a change in temperature. In microbolometers and other standard analog bolometers the change in impedance is a continuous and approximately linear response to the change in temperature. However, in some devices the change in impedance is resisted by an energy barrier in the device’s potential energy landscape. This energy barrier can prevent a linear change in impedance until the device’s thermal energy passes some threshold. Once the energy passes this threshold the device state will change, causing a change in impedance. Afterwards, the device returns to its initial state or something energetically equivalent to its initial state. We call these devices digital-mode bolometers since their discrete states digitize the incoming information into discrete impedance levels.

[0012] A key requirement for digital-mode bolometers is the ability of the device to return to its initial energy state. This return is required for transition events to encode information about intensity and timing of the incident light signal. For such a device, the timing and rate of its discrete state transitions can be collected to gain information about incident electromagnetic radiation. The time for the device to return into its initial state is sometimes called reset time, or dead time. This dead time places a limit on the maximum readout speed of digital-mode bolometers. Typically, the dead time is limited by either thermal transport or nonequilibrium thermal effects.

[0013] At equilibrium, the thermal energy in a bolometer follows a Boltzmann distribution. In other words, the thermal energy in a bolometer fluctuates, with a mean value of kBT. This fluctuation can lead to Johnson-Nyquist noise, or other types of noise depending on the type of bolometer. Thermal noises and Johnson-Nyquist noise are just a couple of the many types of noise present in the absence of light also called dark noise. Typically, analog dark noise is best described by a Gaussian distribution, while digitalmode dark noise is best described by a Poisson distribution. Generally speaking, analog bolometers have a continuous output, so their noise follows a continuous probability distribution. Meanwhile, digital-mode bolometers with their discrete output have noise that follows a discrete distribution. Therefore, the noise in digital-mode and analog bolometers follow different distributions due to the different nature of discrete and continuous random variables. This has a significant effect for sensitivity metrics that rely on the signal to noise ratio since the noise is fundamentally different in the digital-mode case.

[0014] The noise equivalent power (NEP) is a sensitivity metric for the power at which the detector’s signal to noise ratio is equal to one. Due to the fundamental difference in statistics, analog and digital-mode detectors have different definitions of NEP. Using the standard definition for analog NEP we haveSvNEPa= a 31

[0015] where Svis the noise current spectral density, and 5? is the responsivity. For digital-mode detectors we use the definition from the single photon detector community

[0016] where h is Planck’s constant, f is the frequency of light, D is the dark count rate, and SDE is the system detection efficiency. SDE can also be thought of as the signal per photon. Similarly, we can find the SDE or the signal per photon in the analog case, keeping in mind that signals below the noise floor are not detected in the output. In this case, we find that the SDE is given by

[0017] where tintis the detector integration time. Note, that the SDE in the digital case can be rewritten as hf D SDEd= - - d NEP

[0018] If we compare the SDE (i.e. signal per photon) at fixed NEP, we find that the SDE is significantly higher in the digital-mode case. In other words, there is a discrepancy in standard NEP definitions which leads to better-than-expected performance in digitalmode detectors compared to analog detectors. Increased SDE for the same NEP has a direct effect on the noise equivalent differential temperature (NEDT), which means that the NEDT is significantly better than expected in digital-mode detectors. NEDT is the most commonly used metric for sensitivity in commercial detectors because it better captures the nuances of thermal imaging compared to metrics like NEP. Taking each of these factors into consideration, digital-mode bolometers have significant potential for increased sensitivity in thermal imaging.

[0019] FIG. 1 illustrates a perspective view of an example of a bolometer 100 according to the technical advancements described herein. The terms “vertical,” “top,” “bottom,” “above,” and “below” are oriented with respect to the Y reference line shown in FIG. 1 . The terms “horizontal” and “side” are oriented with respect to the X reference line shown in FIG. 1. The bolometer includes a sensitive layer 102 on which infrared electromagnetic radiation is incident 104. The sensitive layer 102 is connected to a readout circuit through a series of electrodes 106. In some embodiments, a thermally insulating layer 108 is horizontally next to to the sensitive layer 102. In some embodiments, a layer reflective to electromagnetic radiation 1 10 is below the sensitivelayer 102. In some embodiments, an antireflective layer 112 is placed above the sensitive layer 102.

[0020] The sensitive layer 102 is a layer which produces a series of electrical impedance changes based on the temperature of the sensitive layer 102. The material is selected such that linear electrical impedance changes are prevented by an energy barrier between the two impedance states. Some examples of such effects include ferromagnetism, ferroelectricity, Mott insulators and materials that experience metalinsulator thermodynamic transitions. Some possible materials for the sensitive layer include VO2, VOx, CoFe, CoFeB, and Hfo.5Zro.5O2. The volume and dimensions of the device are selected such that the potential energy barrier between two impedance states is small. For example, the thickness of the sensitive layer 102 may be between 20nm and about 200nm. The purpose of the sensitive layer 102 is to respond to heat from incident infrared electromagnetic radiation by increasing the rate of electrical impedance changes. The rate of the electrical impedance changes is the signal of the digital-mode bolometer. The electrodes 106 are horizontally next to the sensitive layer 102. The thermally insulating layers 108 are horizontally next to the sensitive layer 102.

[0021] Several bolometers and detectors have previously been demonstrated that generate discrete changes in electrical impedance states from infrared light. Superconducting based detectors such as the superconducting nanowire single photon detector, have demonstrated a discrete response to infrared radiation. Photodiodes such as the mercury cadmium telluride-based electron avalanche photodiode have also produced a discrete response to incident infrared radiation. However, these existing technologies require cryogenic cooling, typically with liquid nitrogen or liquid helium, and therefore have operating temperatures significantly below 200K. Instead of cryogenic cooling, the sensitive layer achieves digital operation by reducing the size of it’s barrier through control of the device volume. Thermal activation over the potential energy barrier is limited by the scale of the energy barrier, which is proportional to the volume of the sensitive layer. By controlling the scale of the sensitive layer the device can be designed to respond to small changes in temperature induced from incident infrared light. A potential barrier with a scale between 5% and 3000% of the average thermal energy inthe sensitive layer can be achieved through careful control of the volume and through material choice. Many previous devices have not demonstrated such a barrier or have not reduced the size of the barrier to the scale of average thermal energy in the device. The presence of a barrier in a particular material indicates an energy cost between two distinct states of the material, such as an energy cost between two distinct magnetization orientations, or an energy cost between two distinct charge polarizations.

[0022] The thermally insulating layer 108 is a layer that exhibits low thermal conductivity to thereby reduce the leakage of heat into the surrounding structure. Thus, the thermally insulating layer 108 increases the change in temperature of the sensitive layer due to incident infrared electromagnetic radiation. Some possible materials for the thermally insulating layer include SiO2, Ge, Si, air, or vacuum. The thermally insulating layer 108 is adjacent to the sensitive layer 102. Alternatively or in addition, the thermal insulation layer 108 may be positioned on both sides of the electrode 106. In some embodiments the thermally insulating layer 108 surrounds the bolometer 100, or portions thereof.

[0023] The reflective layer 110 reflects light transmitted through the sensitive layer 102 back into the sensitive layer 102. The reflective layer 1 10 thereby increases the absorption of infrared electromagnetic radiation by the bolometer 100. Some possible materials for the reflective layer include Au, Ag, Ti, or W. The reflective layer 1 10 is inwardly facing and vertically disposed on the sensitive layer 102. The placement is such that transmitted radiation is reflected back into the sensitive layer 102.

[0024] The anti-reflective layer 112 reduces reflection from the top surface of the bolometer 100 upon which infrared electromagnetic radiation is incident. The anti- reflective layer 112 thereby increases the absorption of infrared electromagnetic radiation by the bolometer 100. Some possible materials for the anti-reflective layer 1 12 include ZnS, PbFk, commercially available diamond-like coatings, or commercially available broadband AR coatings. The anti-reflective layer 1 12 is outwardly facing and vertically disposed above the sensitive layer 102.

[0025] The absorptive layer 1 14 increases the absorption of the bolometer 100 by absorbing some of the incident infrared electromagnetic radiation. Some possiblematerials for the absorptive layer include Au, Ti, black Au, Ge, TiN, WSi, or NbN. The absorptive layer 1 14 is vertically disposed above to the sensitive layer 102. In some examples, the absorptive layer 114 is disposed between the antireflective layer 1 12 and the sensitive layer 102.

[0026] The sensitive layer 102 is next to electrodes 106. In some embodiments the electrodes 106 are placed horizontally next to the sensitive layer 102. In some embodiments the electrodes 106 are placed vertically next to the sensitive layer 102. In either case, the electrodes 106 may contact the sensitive layer 102. The thermally insulating layer 108 is adjacent to the sensitive layer 102. In some embodiments the thermally insulating layer 108 is simultaneously adjacent to the electrodes 106. In some embodiments the thermally insulating layer 108 is simultaneously between the reflective layer 1 10 and the anti-reflective layer 112 (or between the reflective layer 110 and the absorptive layer 1 14). The anti-reflective layer 112 is vertically disposed above the sensitive layer 102 and outwardly facing. In some embodiments, the anti-reflective layer 112 is vertically disposed above to the absorptive layer 1 14 and outwardly facing. The reflective layer 1 10 is vertically disposed below the sensitive layer 102 and inwardly facing. In some embodiments, the reflective layer 110 is vertically disposed below the absorptive layer 114 and inwardly facing. The absorptive layer 114 is vertically disposed adjacent to the sensitive layer 102. In some embodiments the absorptive layer 114 is vertically disposed adjacent to the thermally insulating layer 108, and the reflective layer 110. In some embodiments the reflective layer 110 and antireflective layer 1 12 can be vertically disposed between to the electrodes 106 with the reflective layer 1 10 inwardly facing and the anti-reflective layer 1 12 outwardly facing. In some embodiments the reflective layer 110 and antireflective layer 112 can be vertically disposed adjacent to the thermally insulating layer 108 with the reflective layer 110 inwardly facing and the antireflective layer 112 outwardly facing.

[0027] The antireflective layer 112 being outward facing means that incident light would arrive at the antireflective layer first before traveling towards the following layers. The purpose of this layer is to improve coupling of light towards the absorptive layer 114.

[0028] The reflective layer 1 10 being inward facing means that incident light would reach the reflective layer last after traveling through the other layers. The purpose of this layer is to reflect transmitted light back into the absorptive layer 1 14 to further improve absorption.

[0029] FIG. 2A-B illustrates an equivalent circuit for the bolometer and the digital bolometer response to changes in incident infrared electromagnetic radiation. FIG. 2A shows Vout with respect to the electrodes 106 shown in FIG. 1. As illustrated in FIG. 2B, the digital bolometer changes the rate of transitions between two impedance states in response to changes in incident infrared electromagnetic radiation. This type of signal is fundamentally different from analog bolometers and results in noise determined by discrete random variables.

[0030] The bolometer is operated by maintaining the operational temperature at a particular set temperature between 200K and 500K to thereby optimize the discrete series of changes in the bolometer response from electrical impedance state 1 to electrical impedance state 2. Electrical impedance state 1 and electrical impedance state 2 are separated by some energy barrier such that the energy barrier is not significantly smaller than the average thermal energy in the sensitive layer. Incident infrared electromagnetic radiation is converted into heat, resulting in a change in the sensitive layer from electrical impedance state 1 to electrical impedance state 2. Continued absorption of incident electromagnetic radiation leads to a discrete series of changes in the bolometer response from electrical impedance state 1 to electrical impedance state 2. Further optimizing the discrete series of changes in the bolometer response from electrical impedance state 1 to electrical impedance state 2 may be achieved by operating the bolometer at a particular set current between 1 uA and 1 A or by applying a magnetic field at a particular set magnetic field between 0.1 mT and 1 T.

[0031] The sensitive layer 102 is designed with a small energy barrier between electrical impedance states. Incident infrared electromagnetic radiation increases the heat in the sensitive layer 102 causing an increase in the rate of electrical impedance changes. Electrodes 106 connecting to the sensitive layer 102 allow for voltage readout of the electrical impedance states upon biasing with current signals. In someembodiments current readout of the electrical impedance is performed upon biasing with a voltage signal. The thermally insulating layer 108, reflective layer 110, anti-reflective layer 1 12, and absorptive layer 114 increase the bolometer 100 sensitivity to incident infrared electromagnetic radiation by increasing the absorption.

[0032] FIG. 3 is a diagram illustrating the potential energy landscape of an analog bolometer according to the present disclosure. Referring to Fig. 3 the analog bolometer potential energy states are approximately linear with resistance. This linear dependence means that as thermal energy increases, the resistance also increases in an approximately linear fashion.

[0033] FIG. 4 is a diagram illustrating the potential energy landscape of a digital-mode bolometer according to the present disclosure. Referring to FIG. 4, the digital-mode bolometer potential energy presents an energy barrier. This energy barrier resists an increase in resistance until the thermal energy passes some threshold. After this threshold is crossed, the resistance changes sharply. Later, the device thermalizes and returns to its initial resistance state.

[0034] 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.

[0035] 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 first action 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.

[0036] 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 combinationsthereof" 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 bolometer, comprising: a sensitive layer configured to provide two distinct electrical impedance states at a temperature above 200K, wherein electrical impedance state 1 and electrical impedance state 2 are separated by an energy barrier that is between 5% and 3000% of the average thermal energy in the sensitive layer, wherein incident infrared electromagnetic radiation is converted into heat, resulting in a change in the sensitive layer from electrical impedance state 1 to electrical impedance state 2, wherein the continued absorption of incident electromagnetic radiation causes a discrete series of changes in the bolometer response from electrical impedance state 1 to electrical impedance state 2; and a series of metallic electrodes coupled to the sensitive layer.

2. The bolometer of claim 1 , wherein the bolometer further comprises an electromagnetic absorptive layer disposed vertically above the sensitive layer, which causes incident electromagnetic radiation to be converted into heat.

3. The bolometer of claim 2, wherein the material of the electromagnetic absorptive layer comprises Au, Ti, Ge, TiN, WSi, NbN, or a combination thereof.

4. The bolometer of claim 1 , wherein the bolometer further comprises an electromagnetic reflective layer vertically below to the sensitive layer.

5. The bolometer of claim 1 , wherein the bolometer further comprises an antireflection layer vertically above the sensitive layer, wherein the anti-reflection layer defines an outer surface of the bolometer.

6. The bolometer of claim 1 , wherein the bolometer further comprises a thermally insulating layer vertically above the sensitive layer.

7. The bolometer of claim 1 , wherein the thickness of the sensitive layer is between 20nm and 200nm.

8. The bolometer of claim 1 , further comprising a reflective layer positioned on a first side of the sensitive layer; and a anti-reflective layer on a second side of the sensitive layer;9. The bolometer of claim 1 , further comprising a readout circuit coupled to the metallic electrodes configured to provide an electrical signal when the infrared bolometer impedance changes.

10. The bolometer of claim 1 , wherein the readout circuit is configured to digitize the electrical signal.

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