3.1.

Photon-Counting TES for the Mid-Infrared

In this section, we describe how a TES operated as a calorimeter, instead of a bolometer, can be used for the MISC-T instrument, reducing the requirements on instrument stability while maximizing effective throughput and overall observing time.

A bolometer is unable to distinguish various external factors from the optical signal and therefore puts more stringent requirements on control of instrumental systematics, stability of the system, and the length of time over which such stability must be maintained. Compared to a calorimeter, a bolometer is more negatively impacted by system drift, stray power coupling into the detector, changes in ambient operating conditions, including the local magnetic field environment, low frequency ($1/f$) noise, and array nonuniformities. These effects manifest as a shift or drift in the baseline signal, the measured current flowing in the TES. Because the drift can be confused with an optical signal (Fig. 2), the burden to account for any drifts rests on the instrument as a whole. Especially as bolometers become more sensitive and therefore slower, commonly employed techniques to push the optical signal into a higher frequency band (e.g., by chopping) are more difficult to execute. By contrast, as long as the intrinsic resolution of the detector is high enough, this shift or drift in the baseline signal has a negligible impact on a calorimeter’s ability to detect a photon event. Moreover, photon-counting observations are more efficient. No external modulation is required and no observation time is spent observing a calibrator. From a sensitivity perspective, it has been shown in several previous studies that TES calorimeters can be realized with sufficient intrinsic energy sensitivity to count THz photons.^40,41

Fig. 2

Illustration of the advantage to photon counting. (a, b) The top frame shows how power is measured bolometrically with an ideal instrument. Incident optical power is manifested as a shift in the “baseline” TES current in the time domain. In the frequency domain, one would observe a higher white noise level due to incident photon noise. (c, d) The center frame illustrates how system drift can be confused with an optical signal; the long time constants of bolometers place stringent requirements on system stability. (e, f) The bottom frame shows how drift is mitigated if individual photon events are resolved (as opposed to just measuring a flux of photons). In this case, system drift has no effect on the measurement.

A TES dark count analog occurs when no photon is incident upon the detector and an anomalous noise trace triggers an event acquisition and application of the optimal filter to this noise record results in the energy of an in-band photon. One finds that if you have a TES in its steady-state operating bias collecting noise records at 50 MHz continuously in the dark with an energy-resolving power of 4 for the lowest photon energy of the band of interest, the dark count event rate is of order one per age of the universe. We use this $\mathcal{R}>4$ criterion for a “noiseless” detector (effectively no detector false positives). In Fig. 3, we show an example for conditions that are just barely satisfying our criterion for noiseless photon counting with $\mathcal{R}=5$. In red and black are TES current time series plotted for one photon event with $\mathcal{R}=5$ (red) and a corresponding record for the TES at its operating point with a no photon record (black). To the right is the distribution of many such records after application of an optimal filter to extract the energy of each record. We see even for only $\mathcal{R}=5$ the population of 0 and 1 photon records are well separated. The likelihood of mistaking a noise record for a photon record is extremely low. This can also be seen as the 0 photon distribution at an energy of 1 is very small. As $\mathcal{R}$ increases beyond $\mathcal{R}=5$, the dark count rate remains negligible. Since the energy resolution $\mathrm{\Delta }{E}_{\mathrm{FWHM}}$ is constant, as the photon energy increases, the height of the pulse signal proportionally increases, leading to a proportionally higher $\mathcal{R}$ measurement.

Fig. 3

Example of our minimal criterion for noiseless single-photon counting. (a) TES model simulated TES current time-series records of a photon pulse with spectral resolving power $\mathcal{R}=5$ (red) and a noise record with no photon (black). (b) Distribution of filtered energy of individual records with 1 or 0 photons. Even with sensitivity of only $\mathcal{R}=5$, it is extremely unlikely to falsely report a 0-photon record (noise record) as an in-band photon.

The energy sensitivity in the X-ray is used to determine the energy of an incident photon with high accuracy. In this application, the high energy sensitivity of a TES can be used to push noiseless single-photon detection down to lower photon energies and into the MIR for Origins. The combination of high speed, noiseless single-photon detection, and sensitivity is used to unambiguously distinguish in-band photon events from noise and identify and remove out-of-band events caused by cosmic rays or background. In grating-based dispersive spectrometers, the energy sensitivity also enables rejection of higher-order photons that correspond to a different spectral channel, a capability unique to energy-resolving single-photon detectors and not found in other single-photon technologies [e.g., electron-multiplying charge-coupled devices,⁴² superconducting nanowire single-photon detectors,⁴³ avalanche photodiodes,⁴⁴ quantum dot detectors,⁴⁵ etc.].

For the MISC-T instrument, the TES calorimeter design for the lowest photon energy of the band ($\lambda =20\mu \mathrm{m}$) is most challenging. It requires the lowest $\mathrm{\Delta }{E}_{\mathrm{FWHM}}$ for noiseless photon counting while also needing the largest absorbing area for an absorber-coupled strategy [see Sec. 4.3; $2\lambda \times 2\lambda$ is sufficient^46,47]. We therefore focus our discussion on the $\lambda =20\mu \mathrm{m}$ detector, which uses a $40\times 40\mu {\mathrm{m}}^2$ resistive Bi absorber coupled to a TES. We find applying the TES model to our MISC-T TES design achieves the required photon count rates, sensitivity, and absorption efficiency with known materials parameters. In Fig. 4, we plot resolving power $\mathcal{R}$ for $\lambda =20\mu \mathrm{m}$ versus $\log \alpha$ over a range of typical $\alpha$ values. We find that internal thermal fluctuation noise (ITFN) in the absorber is not negligible and departs from the simple analytic single body expression in Eq. (1). The ITFN limits sensitivity and shows that increasing $\alpha$ much above 20 provides a limited improvement in sensitivity. The key take away is that even accounting for ITFN, the sensitivity is greater than the photon-counting criterion (over six times larger). This gives significant sensitivity margin even at the lowest photon energy of the MISC-T band. Noiseless photon counting becomes easier (even greater margin) for the rest of the MISC-T band.

Fig. 4

MISC-T TES spectral resolving power $\mathcal{R}$ at $\lambda =20\mu \mathrm{m}$ versus $\log \alpha$ operating at 70 mK. The TES model predicts this device designed for high absorption efficiency at $\lambda =20\mu \mathrm{m}$ gives sufficient energy sensitivity for noiseless photon counting with margin. Even very modest values of $\alpha$ give sufficient sensitivity to count photons with ample tolerance for system instabilities, which for realistic values only lead to minor degradation of energy resolution (see Chiao et al.⁴⁸).

The thermal recovery time of TES calorimeters designed for the optical is typically of order $1\mu \mathrm{s}$, accommodating count rates of $\sim 1$ million counts per second.^35,49–51 During the thermal recovery time, the TES is still able to receive and detect a photon and therefore has no dead time, a feature unique among single-photon detectors. If an additional absorption event occurs during the detector, recovery signal pile-up occurs. In such a case, the ability to extract the energy of each event is degraded below the maximum resolving power of the detector. For spectroscopy applications with no dispersive optics, such records are tagged as lower resolution events and different processing is employed to extract the event energies. For MISC-T, where the TESs are distributed over a spectral channel, pulse pile-up can be tolerable and the single-photon events are identified by the fast rise of the leading edge provided the detector stays below the saturation energy. Even for $\lambda =20\mu \mathrm{m}$, we see in Fig. 4 that the sensitivity is over six times larger than required to photon count and an increase in rate will not impact the ability to count photons. The energy sensitivity margin increases further as the wavelength decreases. As the rate increases further, the calorimeter operation begins to become more bolometer-like and the advantages of photon counting start to diminish.

Increasing the number of spectral channels increases the spectral information of the measurement and decreases the photon rate per channel. Increasing the event rate per spectral channel much above 1 million counts per second is achievable by: (1) decreasing the pulse recovery time, (2) increase the number of TES pixels per spectral channel (oversampling the point spread function), and (3) implementing photon counting algorithm for high count rates. The faster pulse recovery time is achievable by operating the TES at higher temperatures (see Fig. 5(b)) or by engineering a TES’s thermal coupling. Calkins et al.⁵¹ have demonstrated engineering of the thermal coupling of the TES to achieve a thermal recovery time of less than half a microsecond. In Fig. 5, we show the MISC-T TES response to a $\lambda =20\mu \mathrm{m}$ (60 meV) photon. In this TES model simulation, 60 meV of energy is deposited at nine different locations in a high-efficiency absorber. The simulation on the left is at 70 mK and on the right 100 mK. By raising the temperature, we increase the speed of the detector. The faster pulses at higher temperatures show positional dependence of absorption, which impacts the energy resolution. But even here, the sensitivity is sufficient for noiseless photon counting with margin. The maximum count rate expected for MISC-T is $\sim 2.5\times {10}^5$ photons per second incident on the detector,¹ thus, we do not expect as-demonstrated device speeds to be limiting.

Fig. 5

Simulated time-domain signals of the MISC-T TES microcalorimeter response to $\lambda =20\mu \mathrm{m}$ photons (60 meV). The bath temperature in this simulation is 50 mK, $\alpha =10$, and the absorber consists of a 10 nm-thick Bi film. (a) Series of pulses absorbed at nine different points in a $40\times 40\mu {\mathrm{m}}^2$ absorber at 70 mK. The nine different pulses correspond to 60 meV of energy deposited at nine different positions in the absorber (increasing distance from TES with increasing hue). The different colors show that there is some positional dependence to the pulse shape due to finite thermal conductance of the absorber structure. (b) A similar simulation, but at 100 mK. This speeds up the detector response at the cost of energy resolution.

4.1.

Bolometer Sensitivity

A bolometer’s sensitivity is parametrized by its NEP. In the dark, a bolometer’s NEP is ideally limited by phonon noise ($N_p$), the fundamental thermodynamic noise that arises due to the random exchange of energy across the thermal link coupling the bolometer pixel to the thermal bath (see Fig. 1). In this limit, NEP $\simeq N_p$. Under optical bias, a background-limited detector is limited by photon noise, with $N_p$ as the leading subdominant noise term. Thus, $N_p$ is the figure of merit for a bolometer’s sensitivity (see Mather⁵⁶ for a complete treatment of bolometer noise). The power spectral density of the phonon noise ${|⟨N_p⟩|}^2$ is given by

NEP can be expressed in several ways that represent different measurements and device characteristics. It is important to note the distinctions between them. First is the thermal fluctuation noise NEP (${\mathrm{NEP}}_{\mathrm{TFN}}$). ${\mathrm{NEP}}_{\mathrm{TFN}}$ is calculated from Eq. (4) using a value for $G$ extracted from $I-V$ curve measurements. It represents a theoretical limit, where there is no significant contribution to the noise from other known or unknown noise sources (e.g., Johnson noise, amplifier noise, or excess noise). Next is the electrical NEP (${\mathrm{NEP}}_{\mathrm{el}}$). ${\mathrm{NEP}}_{\mathrm{el}}$ is calculated from the measured current noise spectral density $i_n$ and frequency-dependent electrical responsivity $S_{\mathrm{el}}(\omega )$, where $S_{\mathrm{el}}(\omega )=S_{\mathrm{el}}/(1+i\omega \tau )$.⁵⁷ Here, $S_{\mathrm{el}}$ is the DC electrical responsivity to dissipated Joule power, extracted from the $I-V$ curve, and $\tau$ is the time constant of the device. Finally, there is the optical NEP (${\mathrm{NEP}}_{\mathrm{opt}}$). In contrast to ${\mathrm{NEP}}_{\mathrm{TFN}}$ and ${\mathrm{NEP}}_{\mathrm{el}}$, ${\mathrm{NEP}}_{\mathrm{opt}}$ is necessarily derived from optical measurements and is calculated from the measured current noise spectral density $i_n$ and the device’s responsivity to incident optical power $S_{\mathrm{opt}}$. In a background-limited detector, ${\mathrm{NEP}}_{\mathrm{opt}}$ would be dominated by photon noise. Both ${\mathrm{NEP}}_{\mathrm{el}}$ and ${\mathrm{NEP}}_{\mathrm{opt}}$ account for all noise sources, but only ${\mathrm{NEP}}_{\mathrm{opt}}$ accounts for potential inefficiencies and loss mechanisms associated with photon absorption.

Reductions in operating temperature $T$ and thermal conductance $G$ to achieve higher sensitivity impact the bandwidth of a bolometer. Recall that the response time of a bolometer is proportional to $C/G$. At low temperatures, both $C$ and $G$ scale with temperature: $C\propto T$ and $G\propto T^n$. Bolometers are engineered to achieve a specific $n$ value depending on the physics of the heat transfer. Typical values include $n=3$, 4, or 5, corresponding to ballistic phonon conduction, Kapitza conduction, or electron–phonon conduction, respectively. In a realized bolometer, the measured $n$ value may take on intermediate values if there are multiple series/parallel conduction paths through differing means or if there are few propagating modes.^58,59 Figure 6 illustrates the natural tradeoffs that occur between operating temperature, time constant, and NEP. In short, a more sensitive bolometer is also slower, thus introducing more stringent requirements on overall instrument stability.

Fig. 6

(a) Illustration of the relationship between a bolometer’s operating temperature, time constant, and NEP when $G$ is dominated by electron-phonon conduction ($n=5$, solid), Kapitza conduction ($n=4$, dashed), and phonon conduction ($n=3$, dotted). The blue lines correspond to the axis on the left, and the red lines to the axis on the right. This plot assumes that each bolometer has the same NEP and time constant at 300 mK. Real bolometers typically have exponents that take a value between 3 and 5, indicating multiple conduction mechanisms. (b) The impact on time constant if the most sensitive bolometer demonstrated to date⁶⁰ was modified to meet Origins requirements by simply reducing $G$. To make the device the required 10× more sensitive requires a $100\times$ reduction in $G$, which, in turn, makes the device $100\times$ slower.

Engineering $G$ to achieve a certain value also impacts the saturation power of a bolometer $P_{\mathrm{sat}}$, defined as the power required to heat the bolometer to a temperature $T_{\mathrm{sat}}$ above which the responsivity of the device falls below some critical level. The relationship between $G$ and $P_{\mathrm{sat}}$ can be understood from the power balance requirement of steady-state operation, where the power flowing out the thermal link equals the sum of absorbed optical power and Joule power dissipated by the TES resistance. Note that stray power can manifest itself as either optical power or Joule power. For $G=A{T}^n$, where $A$ is a constant and $n$ is the exponent of the thermal conductance, $P_{\mathrm{sat}}$ is given by

Thus, for a given bath temperature and sensor transition temperature ($T_c\simeq T_{\mathrm{sat}}$), the saturation power of the device scales with the thermal conductance. As $G$ is reduced to achieve higher sensitivity, the stray power requirements of the experimental platform—electrical and optical—become proportionally more stringent. In particular, implementation of techniques to mitigate stray power like filtered connectors at the cryostat vacuum feedthroughs, thermal blocking and powder filters in close proximity to the detectors, and single-point grounding is necessary to achieve a sufficiently quiet experimental space.

In the dark, the most sensitive bolometer demonstrated to date was developed by SRON and is described by Ridder et al.,⁶⁰ Khosropanah et al.,⁶¹ and Suzuki et al.⁶² It uses a TiAu TES on a leg-isolated SiN membrane, achieving ${\mathrm{NEP}}_{\mathrm{TFN}}$ of $<1\times {10}^{-19}\mathrm{W}/\sqrt{\mathrm{Hz}}$ and ${\mathrm{NEP}}_{\mathrm{el}}$ of $\sim 2\times {10}^{-19}\mathrm{W}/\sqrt{\mathrm{Hz}}$, with a phonon noise-limited bandwidth of $\sim 50\mathrm{Hz}$ and a saturation power below a few femtowatts. Figure 6 illustrates the impact on this detector’s time constant if $G$ were reduced by the necessary factor of $\sim 100$ to meet Origins sensitivity requirements. Under optical bias, Karasik and Cantor measured ${\mathrm{NEP}}_{\mathrm{opt}}$ of $3\times {10}^{-19}\mathrm{W}/\sqrt{\mathrm{Hz}}$ using a Ti hot electron bolometer, the lowest optical NEP demonstrated to date in a TES bolometer.⁶³

4.2.

Thermal Isolation Techniques

There are several thermal isolation techniques that show promise toward enabling Origins-type sensitivities in bolometers. In no particular order, these fall into four basic categories (Fig. 7): isolation via long (diffusive) legs, short (ballistic) legs, phononic filter/bandgap legs, and weak electron–phonon (e-p) coupling in the sensor material (a “hot electron” bolometer).

Fig. 7

Examples of bolometers that use the thermal isolation techniques most likely to meet Origins sensitivity requirements. (a) Long-legged bolometer with micromachined SiN legs. Here, the bolometer pixel is $\sim 50\times 50\mu {\mathrm{m}}^2$, and the required per-pixel area is $\sim 500\times 500\mu {\mathrm{m}}^2$. Devices like this have achieved excellent sensitivity but suffer from a low fill fraction. (b) Ballistic leg bolometer developed for the HIRMES instrument (image credit: A. Brown). Compared to the long-legged bolometer at left, the fill fraction offered by this type of bolometer is much higher. The membrane is micromachined from Si and is $\sim 680\times 480\mu {\mathrm{m}}^2$ in area. (c) Phononic leg supporting a bolometer membrane (image credit: K. Denis and K. Rostem). A realized phononic filter bolometer resembles the ballistic leg bolometer at the panel (b), except nanomachined legs like the one pictured replace simpler dielectric beams. (d) Hot electron solid-substrate bolometer that accomplishes thermal isolation via weak e-p coupling in the W sensor film. This device is $\sim 2\times 2\mu {\mathrm{m}}^2$ in area. This device was designed for dark measurements and is not coupled to an antenna, however, antenna coupling would be appropriate for optical characterization (see Sec. 4.3).

“Long (diffusive) leg bolometers” generate thermal isolation via diffuse phonon transport across the legs that support a membrane-isolated bolometer. The legs and the membrane are typically micromachined from Si or SiN. Conduction via diffuse phonon transport follows Fourier’s law of thermal conduction: $G=\kappa A/\ell$, where $\kappa$ is the material’s bulk thermal conductivity, $A$ is the cross-sectional area of the leg, and $\ell$ is the length of the leg. As has been pointed out in the literature,^64,65 the Fourier limit is an oversimplification of physics. $\kappa$ is not a bulk property, but rather a property that depends on the details of the leg’s fabrication process, which can lead to nonuniform conductance across an array or between fabrication runs. In addition, the $G\propto {\ell }^{-1}$ behavior only holds for legs longer than a certain threshold; $\ell >400\mu \mathrm{m}$ has been reported.⁶⁵ Bolometers that use long-leg isolation are exemplified in practice by the SPICA/SAFARI bolometers developed by SRON^60–62 (see Sec. 4.1). A representative optical micrograph of a long-leg bolometer is shown in Fig. 7. Despite the excellent achieved sensitivity of long-legged bolometers, an obvious limitation is the low fill fraction achieved. By area, the SPICA/SARARI bolometers have a fill fraction of $\sim 4\%$. SPICA/SAFARI has three bands between 600 and 2000 pixels per band, yielding far fewer pixels than the $\sim \mathrm{60,000}$ required for OSS. With $G\propto A/\ell$, reducing the conductance by 100× to meet OSS requirements would necessarily reduce the fill fraction. For FIP, however, these bolometers already meet the sensitivity requirement and the array’s size is similar to SPICA/SAFARI. An added complication to longer legs is that as the leg length increases, the heat capacity of the legs can become non-negligible, leading to measurable ITFN degrading device performance. Long-legged bolometers lend themselves most naturally to resistive absorber coupling. Antenna coupling would likely have to incorporate the legs themselves, and further investigation is required to determine whether the electromagnetic requirements of an antenna could be consistent with the thermal requirements of the legs.

“Short (ballistic) leg bolometers” achieve thermal isolation by minimizing the solid angle between the radiating sensor and legs that support a membrane. The ballistic limit occurs when the length of the leg is shorter than the phonon mean free path ${\ell }_{\mathrm{mfp}}$ in the material, so it is assumed that phonons that pass through the leg aperture escape to the bath. For very small cross-sectional areas, ballistic legs have been shown to approach the so-called “quantum limit” of thermal conductance, where only four phonon modes can propagate.⁶⁶ The bolometers developed for the HIRMES instrument⁵⁸ used this technique to achieve kilopixel arrays with ${\mathrm{NEP}}_{\mathrm{el}}<1.8\times {10}^{-18}\mathrm{W}/\sqrt{\mathrm{Hz}}$,⁶⁷ in good agreement with the targeted value of NEP $<3\times {10}^{-18}\mathrm{W}/\sqrt{\mathrm{Hz}}$ for background-limited operation. Compared to long-legged bolometers, ballistic leg bolometers can achieve much higher array fill fractions, and the achieved thermal conductance of realized legs is both calculable from real bulk material properties and is much more tolerant of fabrication nonidealities and nonuniformities.^66,68,69

“Phononic filter bolometers” use nanomachined legs to generate coherent phonon scattering that suppresses propagated phonon modes to below the quantum limit.^34,68–70 Like ballistic leg bolometers, phononic filter bolometers promise short legs that enable high array filling fractions but with the additional advantage of reduced thermal conductivity relative to ballistic leg designs. Theoretical work by Rostem et al.⁶⁸ indicates that a factor of 5 improvement over the quantum limit is possible with phononic filter legs that are just $10\mu \mathrm{m}$ long and are machinable using known techniques. Devices presented by Williams et al.⁶⁹ achieved a factor of $\sim 2$ reduction in thermal conductance below the quantum limit using phononic filter legs. Fabrication of phononic filters is challenging as submicrometer lithography techniques are required, but recent work by Denis et al.⁷¹ demonstrates the reliable and robust fabrication of phononic leg-isolated TESs.

“Hot electron bolometers (HEBs)” use the inherent decoupling of electrons and phonons in certain metals at low temperatures for thermal isolation. The conductance between electrons and phonons at low temperature is given by

4.3.

Radiation Coupling Techniques

Antenna coupling can be used to define the sensor’s angular acceptance and coupling to the electromagnetic radiation field. It converts the incident fields (photons) into electronic excitations in the absorber media, which are dissipated and subsequently detected as heat. To achieve high antenna coupling, efficiency requires transforming the modal symmetry and impedance scales encountered by the wave in free space to that present in the circuit elements employed to absorb the radiation. A wide variety of techniques exist to carry out these functions; however, due to the need to realize large arrays of sensors, in particular, methods amendable to implementation as planar lithographic structures are primarily of interest. Consideration of the overlap in the angular response of the individual antenna elements in a multibeam array can provide insights into the interpixel isolation and maximum achievable power coupling.⁷⁴ Similarly, care is warranted in providing isolation between the detector bias/readout and the optical signal paths. Rejection of out-of-band radiation sources, which can potentially load a bolometric sensor and degrade the achievable response, also needs to be realized. These functions are typically achieved through the use of choke circuits on the sensor wafer and cooled thermal blocking filters at appropriate locations in the instrument system.

A qualitative survey of commonly encountered planar antennas, such as resonant and traveling-wave line- and slot-structures, is provided in Fig. 8. The angular response (or “antenna pattern”) and the impedance scales of complementary line- and slot-like aerial structures are linked by Babinet’s principle in its vector form.⁷⁵ Extension of these symmetry concepts to antenna geometries that are only a function of the angle enables the realization of frequency-independent antenna structures.⁷⁶ Examples of frequency-independent traveling-wave structures include planar logarithmic spiral and log-periodic antennas, which have continuous and discrete scaling symmetries, respectively. In practice, such antennas exhibit deviations from frequency independence from truncation at the inner and outer length scales (i.e., set by the feed network and the maximum extent of the aerials). While this tends to simplify impedance matching to the detector’s absorber circuit, these structures tend to have a low filling fraction when used in arrays over a wide spectral range. The operation of planar antenna arrays in an immersion lens configuration^77–79 has enabled greater control over electromagnetic substrate losses and has allowed a natural separation of the radiation and readout functions of the focal plane.

Fig. 8

Planar slot aerials and their relation to complementary wire aerials (Babinet’s principle).⁷⁵

While antenna-coupled arrays have found utility in instrument applications, where high spatial-sampling presents a driving consideration,⁸⁰ an alternative approach to achieve full sampling is presented by “absorber-coupled” sensor arrays.⁸¹ In this limit, the “antenna response” of the sensor is essentially uniform over the range of interest and the telescope optics in concert with a cold Lyot stop is used to specify and limit radiation presented to the focal plane.⁸² In this configuration, a homogeneous thin film for each pixel’s absorber has resonant (quarter-wave) or frequency-independent back-termination^83,84 depending on the desired spectral range and coupling. Incorporation of frequency-selective surfaces (FSSs) in the absorber structure^85,86 provides an alternative path to achieving the desired impedance scales for high in-band-coupling and improved out-of-band rejection in bolometric sensor settings.

Prior discussions of antenna coupling for bolometer implementations can be found in the literature.^72,87,88 It is important to note that for a bolometric sensor, the detailed thermal mode of operation can place practical constraints on how the device is electromagnetically coupled to the radiation field. For example, in hot-electron bolometers, the antenna has direct electronic coupling and appropriate material selection enables operation of the device. Similarly, in phononic filter, ballistic leg, and diffusive leg bolometers, direct electrical contact would thermally short the detector and reactively electromagnetically coupling the antenna to a resistive absorber element on a thermally isolated element is typically employed. Either approach can be employed for calorimeter applications, though the device we present in Sec. 3 uses a resistive absorber element. In all cases, the choice of absorber strategy should reflect that array fill fraction is a parameter which should be maximized in large array applications like Origins. For completeness, related antenna-coupled sensors at infrared wavelengths⁸⁹ employing other physical detection mechanisms are also conveyed. These noncryogenic implementations have included integrated dipole antennas,⁹⁰ bowtie antennas,^91,92 log-periodic/spiral antennas,⁹¹ microstrip patch antennas,⁹³ microstrip dipole antennas,⁹⁴ and Yagi-Uda arrays.⁹⁵

4.4.

Susceptibility to Cosmic Rays

The interaction between energetic charged particles and the materials used in the detector system leads to a stochastic background of energetic events observed by the sensors in the focal plane over the course of a space mission. Consider, for example, the Planck Surveyor mission, which reported a rate of 80 cosmic ray-related events per minute for the High Frequency Instrument array during operations in its L2 orbit. Even with template fitting of its radiometric data, a science data loss of $\sim 10-15\%$ was experienced due to these “glitch”-like temporal events.^96,97 Beyond impacting observational efficiency, nonideal detector responses associated with these energetic events can lead to instrumental stability and calibration issues, which reduce imaging fidelity if unmitigated. Origins will require higher sensitivity and, by extension, a lower focal plane operating temperature than used in previously deployed systems and thus increase the relative importance of the sensor’s thermal bus implementation on minimizing the impact of cosmic rays. Extension of the detector design techniques employed for calorimetry^98–101 provides a viable path to address this instrumentation need. Mitigating cosmic rays is one particular advantage of a device known as the ideal integrating bolometer,^102–105 which combines leg isolation (diffusive, ballistic, or phononic) with a switchable thermal short. Upon an upset (e.g., cosmic ray hit), the device can be reset quickly to mitigate the data losses that Planck experienced.