Long wave infrared detection using probabilistic spintronic bolometer arrays

Abstract: The use of probabilistic spintronic devices for infrared radiation detection has introduced a shift in approach to thermal imaging. The integration of probabilistic magnetic tunnel junctions with infrared plasmonic nano-antennas achieves high-sensitivity digital-mode infrared sensors at room temperature. Here, we present a scalable approach towards multipixel plasmonic-spintronic bolometer array fabrication and readout. We fabricate proof-of-concept 2x2 row-column multiplexed probabilistic plasmonic sprintronic arrays and show their response to long-wave infrared radiation (8-14um) with high readout speeds (10K-1M counts per second). These spintronic, ultrafast, nanoscale (SUN) bolometers can result in novel high-pixel density CMOS compatible infrared detection platforms. Our work provides a broadband (9kHz to 3GHz) readout platform for future digital probabilistic detector applications. Furthermore, our approach addresses a key challenge associated with scaling infrared pixel sizes that can drive progress towards high pixel density detector arrays for infrared sensing and microscopy applications.

• The paper presents a new digital mode approach for LWIR detection using stochastic MTJs that enables high-speed, room-temperature operation.
• It employs plasmonic nano-antennas with a CMOS-compatible, row-column multiplexed readout to achieve sub-micron pixel density and effective signal isolation.
• Performance is validated through noise-equivalent differential temperature (NEDT) measurements, showing promise against conventional microbolometer technology.

Probabilistic Spintronic Bolometer Arrays for Long-Wave Infrared Detection

Introduction

This paper presents a scalable approach to long-wave infrared (LWIR) detection using arrays of probabilistic spintronic bolometers, specifically leveraging stochastic magnetic tunnel junctions (MTJs) integrated with plasmonic nano-antennas. The work addresses the limitations of conventional LWIR detectors—such as HgCdTe photodiodes and VOx-based microbolometers—by demonstrating high-sensitivity, high-speed, room-temperature operation in a digital-mode architecture. The authors fabricate and characterize a proof-of-concept 2x2 row-column multiplexed array, establishing a platform for high pixel density, CMOS-compatible infrared imaging.

Device Architecture and Operating Principle

The core detection mechanism utilizes stochastic MTJs, which exhibit thermally activated transitions between parallel and anti-parallel magnetization states. These transitions, or "counts," are induced by incident infrared radiation absorbed and concentrated by plasmonic nano-antennas atop the MTJ pillar. The energy barrier of the MTJ sensing layer is engineered ($E_b \sim 161$ meV) to be lower than the energy of LWIR photons, enabling stochastic switching at room temperature. The count rate follows a Poisson process, as confirmed by interarrival time histograms, and increases with the temperature rise in the sensing layer due to incident IR radiation.

The digital-mode nature of the device—where bistable magnetization flips are read out as resistance state transitions—contrasts with the analog response of conventional bolometers. This digital response is inherently robust to background fluctuations and supports ultrafast readout speeds ($10^4$–$10^6$ counts per second).

Array Fabrication and Readout Architecture

The authors implement a row-column multiplexed architecture for the SUN bolometer arrays, enabling individual pixel addressing and scalable readout. Each device terminal is connected to a row or column line, with RF switches and bias-tees used to select and read out individual pixels while minimizing crosstalk. Non-selected devices are AC grounded on both terminals, effectively isolating their signals. This architecture supports both sequential and simultaneous row-wise readout, facilitating high pixel density and efficient utilization of readout circuitry.

A thresholding algorithm is employed to extract ultrafast transition events from the readout waveform, enabling accurate count rate determination. The readout platform operates over a broadband frequency range (9 kHz to 3 GHz), supporting the high-speed nature of the stochastic MTJ devices.

Performance Characterization

The sensitivity of the SUN bolometer array is quantified using Noise-Equivalent Differential Temperature (NEDT) measurements. NEDT is a critical metric for thermal imaging, representing the minimum detectable temperature change. The authors report device-level NEDT values with a best-case performance of 103 mK at 25 Hz for single-pixel devices, and demonstrate linear response and low standard deviation across four devices in the array. The measurement setup is benchmarked against a commercial VOx-based microbolometer imager, validating the experimental methodology.

Device-to-device variation in NEDT is attributed to fabrication inconsistencies, with prospects for improved uniformity through process optimization. The sub-micron device sizes (100–300 nm) enable oversampled, high-density arrays, with pixel pitches well below the diffraction limit ($<1\,\mu$m).

Scaling, Background-Limited Performance, and Readout Trade-offs

The paper addresses the challenges of pixel scaling in IR FPAs, noting that conventional technologies suffer from increased noise and reduced sensitivity as pixel sizes decrease. Spintronic bolometers, by contrast, maintain performance at sub-wavelength scales and are CMOS-compatible, facilitating integration with existing ROICs.

In high-background scenarios, the digital detection mechanism of SUN bolometers offers superior SNR scaling ($SNR \propto 1/\sqrt{\phi_B}$) compared to analog detectors ($SNR \propto 1/\phi_B$), where $\phi_B$ is the background photon flux. This property is particularly advantageous for room-temperature operation, where background noise dominates.

The row-column multiplexed readout architecture decouples well capacity from pixel area, allowing for higher amplification and integration capacity than pixel-level ROIC architectures. This enables ultra-high pixel densities without compromising sensitivity, a key requirement for near-field IR sensing and microscopy.

Implications and Future Directions

The demonstration of multi-pixel probabilistic spintronic bolometer arrays establishes a new paradigm for digital-mode infrared detection. The architecture supports high-speed, high-resolution imaging with sub-micron pixel pitches, opening avenues for near-field IR applications, hyperspectral microscopy, and other domains requiring ultra-high pixel density.

The digital nature of the detection process, combined with scalable readout, positions spintronic bolometers as promising candidates for next-generation IR FPAs. Future work may focus on further miniaturization, improved fabrication uniformity, and integration with advanced signal processing techniques to exploit pixel correlations and background rejection.

Conclusion

This work demonstrates the feasibility of scalable, high-sensitivity, high-speed LWIR detection using probabilistic spintronic bolometer arrays. The row-column multiplexed architecture enables ultra-high pixel densities and efficient readout, overcoming key limitations of conventional IR detectors. The results suggest significant potential for spintronic digital detectors in advanced thermal imaging, near-field sensing, and microscopy applications, with further developments likely to enhance performance and integration capabilities.