2.1.

Mid-IR Detection

The latest design of the MISC transit spectrometer includes three different wavelength bands covering the range from 2.8 to $20\mu \mathrm{m}$ : TRA-S from 2.8 to $5.5\mu \mathrm{m}$, TRA-M from 5.5 to $11\mu \mathrm{m}$, and TRA-L from 10.5 to $20\mu \mathrm{m}$.² The current baseline is to use HgCdTe detectors for the shorter bands and blocked impurity band detectors (BIBs) for TRA-L. Recent measurements of Si:As BIBs show that they should be able to meet the TRA-L stability requirement of 20 ppm,¹⁵ but no detector to date has demonstrated the stringent <5-ppm stability required for the TRA-S and TRA-M bands. Recent advances in SNSPD technology indicate that it should be possible to achieve saturated internal detection efficiency out to $11\mu \mathrm{m}$ in the near future, making SNSPDs a possible candidate for these bands. A demonstration of SNSPD sensitivity out to $20\mu \mathrm{m}$ has yet to be attempted and remains an open question in the field.

In order to improve the energy sensitivity of the nanowire detectors and extend the photoresponse out to longer wavelengths, there are several possible approaches. One is to decrease the cross-sectional area of the nanowire. This results in a higher probability of a hotspot being generated since the energy per unit area is higher and thermal conductivity along the length of the nanowire is smaller. Another approach is to engineer the material to reduce the superconducting gap energy. Reducing the superconducting gap energy results in a larger number of broken Cooper pairs (quasiparticles) for a given amount of deposited energy in the superconductor. Alternatively, the material can be engineered to reduce the free carrier density or density of states. Reducing the free carrier density or increasing the resistivity of the material results in the deposited energy being divided among fewer quasiparticles, thus increasing the effective temperature of each quasiparticle.

The first approach, reducing the width or thickness of the nanostrip, is challenging since it places more stringent requirements on the nanofabrication tolerances. We recently studied the effects of changing the nanowire dimensions on the quality of fabricated SNSPDs. To quantify SNSPD quality, we used the constriction factor, which is the ratio of the current at which a nanowire ceases to have 0 resistance (the “switching current,” which is limited by defects in the nanowire) to the maximum depairing current.¹⁶ A constriction factor closer to 1 is desirable for sensitivity to low-energy photons. In that particular investigation, it was found that the constriction factor degraded for narrower superconducting strips (widths between 200 and 55 nm were studied), which suggests that the edge roughness plays a significant role in limiting the switching current. In addition, it was found that increasing the temperature above 20% of the critical temperature ($T_c$) also leads to a degradation in the constriction factor, shedding light on the optimum operating temperature for SNSPDs. It is expected that advances in nanostrip fabrication techniques will improve the edge roughness, improving the yield of low-energy sensitive SNSPDs. The second approach, reduction of the superconducting gap energy and thus the transition temperature, requires a lower operating temperature ($\sim 100$ to 300 mK) in order to achieve the optimum sensitivity to long wavelength photons. Reduction of the free carrier density, the third approach to improve the energy sensitivity, is the most attractive avenue to explore because it does not require a reduction of the operating temperature and yield should not be affected.

We fabricated detectors from WSi to have a lower free carrier density by increasing the silicon content of the films. The films are cosputtered from separate W and Si targets, so the composition of the films may be tuned by adjusting the relative sputtering powers. The film thickness was $\sim 3\mathrm{nm}$, and the superconducting transition temperature was 3 K. Each detector is a single $10\text{-}\mu \mathrm{m}$-long nanowire instead of a large-area meander, in order to reduce the probability of fabrication defects or constrictions along the length of the wire.

The SNSPDs were measured at a temperature of 0.9 K and flood illuminated using various quantum cascade lasers also mounted inside the cryostat emitting at different wavelengths. Figure 2(a) shows normalized, background-subtracted photoresponse count rate (PCR) versus bias current curves for these SNSPDs. PCR curves are shown for four different nanowire widths (50, 60, 70, and 80 nm), and measurements were obtained at four wavelengths (1.55, 4.8, 7.4, and $9.9\mu \mathrm{m}$). The nanowires show the onset of a saturation plateau at a wavelength of $7.4\mu \mathrm{m}$ for all four wire widths, but do not saturate at $9.9\mu \mathrm{m}$. By further lowering the $T_c$ of the WSi film, saturated efficiency has recently been demonstrated at $9.9\mu \mathrm{m}$ with a wire width of 50 nm.¹⁷ The limits of SNSPD sensitivity to low-energy photons have not yet been reached or fully understood, thus both experimental and theoretical studies in this direction are active in the field.

Fig. 2

(a) Normalized PCR versus bias current curves for SNSPDs fabricated from a WSi film with a higher silicon content (sputtering power on silicon target increased from 180 to 260 W). PCR curves are shown for four different nanowire widths (50, 60, 70, and 80 nm) and measurements were obtained at four wavelengths (1.55, 4.8, 7.4, and $9.9\mu \mathrm{m}$). The SNSPDs become sensitive to longer wavelengths at higher bias currents. The efficiency is observed to saturate at 1.55, 4.8, and $\text{7.4\hspace{0.17em}\hspace{0.17em}}\mu \mathrm{m}$ for all wire widths, but not at $9.9\mu \mathrm{m}$. (b) Simulation of optical absorption efficiency with an optimized optical cavity. The top image shows a schematic of the cross section of a simple optical stack based around a Fabry–Pérot cavity. Light is incident from the top of the device. The bottom plot shows modeled spectra of absorption into the WSi nanowires for different thicknesses of the lower Ge layer (H3). The model indicates that absorption above 50% is possible.

As described in the introduction, the presence of a saturation plateau in the photoresponse versus bias curve is not only necessary for ultrastable operation, but also indicates 100% internal detection efficiency. The total detection efficiency of an SNSPD depends both on this internal efficiency and on the optical absorption into the nanowire layer. The absorption efficiency is enhanced by embedding the nanowire in an optical stack that maximizes the electric field at the nanowire. Figure 2(b) shows a schematic of a simple optical cavity stack that could be used for mid-IR detectors. A 100-nm gold layer is coated as a reflection mirror on a silicon substrate to reflect normal incident light. A 2.5-nm-thick WSi nanowire meander with 150-nm-wide wires on a 300-nm pitch is embedded in Ge layers, creating a Fabry–Pérot cavity. We utilized a rigorous coupled-wave analysis model to calculate light absorption in the layer of WSi nanowires as a function of the thickness of the bottom Ge layer. The peak absorption efficiency in WSi nanowires is $\sim 55\%$ in the mid-IR region from 2 to $10\mu \mathrm{m}$. Based on this simulation result, we can predict that mid-IR SNSPDs with a total detection efficiency of $\sim 50\%$ are achievable. Higher absorption and broader spectral bandwidths may be possible with different materials or a more complex stack design. We are currently assessing additional dielectric materials for this purpose.

2.2.

Kilopixel Arrays

The TRA-S and TRA-M bands of the MISC instrument will have a spectral resolving power of $R=50$ to 100, which corresponds to $\sim 50$ spectral bins per band. The minimal detector array for each of these bands should have a few pixels per spectral bin and several pixels in the spatial dimension, leading to an array of $\sim 500$ pixels per band. Additionally, the Origins Science and Technology Definition Team is exploring the use of an innovative densified pupil design to enhance the optical stability of MISC,¹⁸ so several subarrays will be required per wavelength band. Overall, between 1 and 10 thousand pixels will be needed at a minimum.

A variety of multiplexing schemes have been demonstrated for SNSPDs, which could be scaled to kilopixel-class arrays.¹⁹ SNSPDs are compatible with frequency domain multiplexing schemes similar to MKID readout, and 16-pixel SNSPD arrays have been demonstrated with both dc and ac biasing schemes.^20,21 SNSPDs’ high timing resolution and low velocity of propagation can be converted into position information by reading out the time difference between pulses leaving both ends of the device: the closer the position of the absorbed photon to one end of the nanowire, the longer the delay between when the pulse reaches the closer end of the nanowire and when it reaches the farther end of the nanowire. This time-of-flight information has been used to construct an imager with 590 effective pixels out of a single nanowire element.²² Superconducting logic has also been employed to read out SNSPD arrays, with up to 64 pixels addressed individually²³ and 16 pixels multiplexed in the row-column architecture described below.²⁴

The most mature multiplexing scheme for SNSPDs is arguably the “row-column” approach,^25,26 which we have recently demonstrated for a kilopixel SNSPD array.¹⁴ This technique allows multiplexing of $N^2$ pixels using only $2N$ readout lines. The principle of operation is shown schematically in Fig. 3(a). Each row or column of the array has a bias tee to apply a dc bias current to the pixels and to read out ac signals. Bias current is introduced through each row and divided equally among the pixels in the row using a resistor in series with each pixel. The current through each pixel in the row flows out through a separate column line, where it is connected to ground through a resistor or a large inductor. When a photon is detected in a pixel, the pixel becomes resistive and diverts its current to the row readout amplifier, generating a positive pulse. The current flowing through the column corresponding to that pixel is now lower due to the pixel’s diverted current, generating a negative pulse in the column amplifier. The correlation between a positive row pulse and a negative column pulse determines which pixel produced the detection event.

Fig. 3

(a) Row–column multiplexing architecture. When a pixel detects a photon, voltage pulses are observed on the pixel’s row and column readout lines. $N^2$ pixels can be addressed using $2N$ readout lines. (b) Optical micrograph of a 1024-pixel row-column SNSPD array. (c) Count rate versus average pixel bias current for each row of the kilopixel array under 1550-nm flood illumination.¹⁴ (d) Log-scale count rate image of a laser spot taken with the array biased at $3\mu \mathrm{A}$, showing dead and hot pixels.¹⁴

The $32\times 32$ row–column array presented by Wollman et al.¹⁴ was fabricated with $30\mu \mathrm{m}$ pixels on a $50\text{-}\mu \mathrm{m}$ pitch. The overall array size was a 1.6-mm square, of which $960\times 960\mu {\mathrm{m}}^2$ consisted of active nanowire regions [Fig. 3(b)]. At its design wavelength of 1550 nm, the array had near-saturated internal detection efficiency, as shown in Fig. 3(c). At higher bias currents, more pixels exhibit excessive count rates due to defects in the nanowires.²⁷ Figure 3(d) shows an example of imaging with the array biased at a current of $3\mu \mathrm{A}$ per pixel. Four dead pixels are evident, along with two hot pixels and three suppressed pixels, giving an overall yield above 99%. At higher bias currents, the emergence of additional hot pixels degrades the effective yield. The measurements presented in Fig. 3 were taken with a single array from the first kilopixel fabrication run and without prior cryogenic screening to preselect devices for yield, so we expect overall yield and performance to improve with additional optimization of the fabrication process and with prescreening.

Row–column arrays are known to suffer from attenuation of their output signals due to current redistribution: when a pixel fires, some of its current is directed into other pixels in its row and column instead of through the readout amplifiers. This effect will be particularly harmful for mid-IR detectors, which have lower operating currents than detectors for shorter wavelengths. We have recently developed an alternative row–column multiplexing scheme that avoids this problem. In a “thermal row–column” (TRC) array, each row or column consists of a single long nanowire element. The rows and columns are fabricated separately on two layers with a thin dielectric spacer between them. When a photon is detected on one layer, some of the phonons produced by Joule heating of the resistive region propagate through the spacer and trigger an event on the other layer directly beneath the hotspot. Coincidences between row and column events, therefore, provide spatial information about the location of the detection. TRC arrays do not require pixel resistors to distribute the current uniformly between pixels, so they are able to achieve high optical fill factors. This scheme has been demonstrated recently in a $4\times 4$ array optimized for 1550 nm.²⁸ At sufficiently high bias currents, the thermal coupling efficiency between the two layers was found to be close to 100%. Kilopixel-scale TRC arrays are currently under development, and we are investigating whether the thermal coupling can be optimized to work with the lower superconducting gap nanowires needed for mid-IR arrays.

2.3.

Large Active Areas

The current optical design for the MISC instrument disperses light across areas of approximately $0.5\times 5{\mathrm{mm}}^2$ at the focal plane for each subpupil of the TRA-S band and $\sim 1\times 10{\mathrm{mm}}^2$ at the focal plane for each subpupil of the TRA-M band.² SNSPD arrays must incorporate a large length of superconducting wire to cover such active areas, whether or not it is broken up into one or many pixels. The total nanowire length is subject to the typical limitations on yield, namely: material inhomogeneity (contaminants, grain boundaries, etc.), geometrical imperfections from lithography and patterning, and topographical disruptions to the film continuity (such as particles pre-existing on the substrate prior to superconducting film deposition). However, recent developments in superconducting detector design and fabrication^29–31 have enabled few-micron-wide superconducting detectors that are potentially much more robust to many of these issues. The large dimensions of these devices are amenable to fabrication with conventional photolithography, facilitating the parallel production of many devices at low cost. We have begun to explore fabrication of WSi-based single-pixel devices (wire width of 900 nm) up to $3.1\times 3.1{\mathrm{mm}}^2$ in area (Fig. 4) and have observed switching current densities close to the expected value required for saturated internal detection efficiency at 1550 nm. Further testing will reveal more about the relative role of different yield factors, as well as how such detectors can be modified for longer wavelengths.

Fig. 4

Optical micrograph of several $3.1\times 3.1{\mathrm{mm}}^2$ single-pixel detectors fabricated with conventional photolithography to explore SNSPD yield across larger active areas. These detectors are the largest SNSPDs fabricated to date.