Editorial Feature

Why Are Bolometers Useful for Studying Materials?

Bolometers are instruments used to measure the radiant heat of a material, the electrical resistance of which varies with temperature. Using bolometers in material research has been significant in understanding how a material's resistivity changes with temperature. Here, we offer a comprehensive overview of bolometers and their applications.

bolometer concept represented by wooden letter tiles

Image Credit: shaneinsweden/Shutterstock.com

Bolometer - Design and Working

Invented by an American astronomer in 1878, the bolometer is a highly sensitive instrument that detects the electromagnetic radiation or heat of a material. Initially, a bolometer was integrated with a telescope to quantify infrared radiation.

The resistivity of materials depends on their temperature, which has led to the popularity of bolometric infrared detectors. As a result, selecting materials for bolometric designs is relatively easy.

The radiation absorber of the bolometer should be sufficiently sized to intercept the measured signal, have a high absorptivity within the desired frequency range, and have a low heat capacity. Additionally, the supporting substrate should have a low heat capacity and high thermal conductivity, ensuring isothermal operation during bolometer operation.

The thermometer, with a low heat capacity, low electrical noise, and adequate temperature dependence of electrical resistance, is thermally attached to the radiation or supporting substrate. The thermal link, which connects the thermally active portions of the bolometer to the heat sink, has a low heat capacity and appropriate thermal conductance for the required application.

The bolometer also includes a thin metal layer that acts as an absorptive element and is connected to a heat sink through a thermal link. Upon radiation exposure, the temperature of the metal layer rises above that of the reservoir, owing to its absorption capabilities.

The heat capacity ratio between the absorptive component and reservoir determines the intrinsic thermal time constant. Temperature change is gauged using a resistive thermometer connected to the absorptive component. Occasionally, the resistance of the component is used to evaluate the temperature change.

Recently developed bolometers can achieve high sensitivity without the need for cooling. These bolometers function as thermometers to measure temperature changes in the absorber. They use temperature-dependent resistors or semiconductor devices, such as diodes or transistors, as sensitive thermometers.

Thermoelectric bolometers, also known as thermopiles or thermocouples, are known for their low noise conversion of thermal signals into electrical signals. The main sources of noise are fundamental thermal fluctuation noise and Johnson-Nyquist noise. Additionally, the thermoelectric transduction process does not require any external power for signal generation, making it suitable for low-power operation. Bolometers are designed to operate at room temperature.

Advantages of Bolometers

Bolometers offer several advantages, including the following:

  • Compared with traditional particle detectors, bolometers offer higher sensitivity and energy resolution.
  • Capability to gauge nonionizing particles and ionizing particles and photons.
  • Can operate at room temperature, eliminating the need for cooling. 

Graphene Bolometers for High-Sensitivity Power Detection and Single-Photon Detection 

Researchers have invested significant time and resources into creating advanced detectors utilizing graphene-based bolometers, which leverage the distinctive features of graphene, including weak electron-phonon coupling, low heat capacity, and low resistance.

Theoretical studies have focused on understanding the phonon cooling mechanism from both acoustic and optical phonon modes as well as the effects of doping, temperature, and disorder on electron-phonon scattering.

Previous experimental studies have explored different approaches for measuring electron temperature and achieving a phonon cooling bottleneck. It was found that, at an operating temperature of 0.1 K and with superconducting led to the confinement of hot electrons.

A graphene superconductor tunnel junction bolometer can effectively function as a highly sensitive power detector with a noise equivalent power (NEP) of around 5 x 1020 W/√Hz due to its resistance readout. Nevertheless, it cannot be used as a single-photon detector due to its large surface area.

By using the Johnson noise readout method in the nonlinear regime, a single-photon detector can be realized. In contrast, when operating in the linear regime, the Johnson noise readout technique enables power detection with a noise equivalent power (NEP) of roughly 1.2 × 10–19 W/√Hz. While the sensitivity may not be as high as that of a graphene-superconductor tunnel junction bolometer, the response of the detector to power is significantly more linear.

Recent Studies

A study published in Science Advances described a vanadium dioxide (VO2) radiometer that can detect uncooled infrared radiation, which is in high demand for security and temperature monitoring. In this study, bolometer arrays with excellent thermal insulation and high absorbance were fabricated using a one-step rolling process to fabricate VO2 nanomembranes. 

Results revealed that the tubular geometry of the bolometer enhanced its light absorption, thermal insulation, and temperature sensitivity. The fabricated tubular VO2 bolometer demonstrated a detectivity of approximately 2 × 108 cm Hz1/2 W−1 across a broad infrared spectrum with a response time of approximately 2.0 ms. The calculated noise-equivalent temperature difference was found to be 64.5 mK. 

This device presented a feasible structural design for polarization-sensitive omnidirectional light-coupling bolometers. The overall performance of tubular bolometers suggested that they have the potential to bridge the gap between thermal and photon detectors in terms of cost and broad applications.

Another article recently published in IEEE Access explored Ge1xPbxOy , a germanium-lead compound, as a representative material for temperature-sensing layers in uncooled microbolometers. The germanium-lead compound-based thin films were deposited on silicon substrates with varying oxygence concentrations using radiofrequency (RF) and direct current (DC) sputtering techniques. 

The prepared samples, when observed using energy dispersive X-ray spectroscopy (EDX), showed various oxygen concentrations, and atomic force microscopy (AFM) analysis showed roughness in the range of 0.6995–0.8660 nm. 

Conclusion 

Overall, bolometers are essential for understanding the thermal behavior and radiation detection of materials. Originally developed for measuring infrared radiation with telescopes, they have evolved into sophisticated instruments that assess changes in material resistance with temperature.

The intricate design of bolometers incorporating radiation absorbers, thermometers, and heat sinks highlights the importance of precise engineering to achieve optimal performance. These instruments exhibit excellent sensitivity and reliability, particularly for detecting nonionizing particles and photons, surpassing traditional particle detectors. In addition, their ability to operate at room temperature eliminates the need for cooling systems, making them ideal for practical applications.

Bolometers are versatile instruments that continue to advance through innovative designs and material exploration. Their ability to bridge the gap between thermal and photon detectors holds potential for diverse applications in security, temperature monitoring, and beyond, making them indispensable tools for material research and radiation detection.

See More: Why use MCT Over Bolometers, Thermopiles or Pyroelectric Sensors?

References and Further Reading

Bahaidra, E., et al. (2023). Optical and Temperature-Dependent Electrical Properties of Ge 1-x Pb x O y Thin Films for Microbolometer Applications. IEEE Access. Available at: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=10188403 

 What is a Bolometer? Accessed on 5 January 2023 

Du, X.,et al. (2013). Graphene-based bolometers. arXiv preprint arXiv:1308.4065. Available at: https://arxiv.org/abs/1308.4065 

Wu, B. et al. (2023). One-step rolling fabrication of VO2 tubular bolometers with polarization-sensitive and omnidirectional detection. Science Advances, 9(42). Available at: https://www.science.org/doi/full/10.1126/sciadv.adi7805 

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Bhavna Kaveti

Written by

Bhavna Kaveti

Bhavna Kaveti is a science writer based in Hyderabad, India. She has a Masters in Pharmaceutical Chemistry from Vellore Institute of Technology, India, and a Ph.D. in Organic and Medicinal Chemistry from Universidad de Guanajuato, Mexico. Her research work involved designing and synthesizing heterocycle-based bioactive molecules, where she had exposure to both multistep and multicomponent synthesis. During her doctoral studies, she worked on synthesizing various linked and fused heterocycle-based peptidomimetic molecules that are anticipated to have a bioactive potential for further functionalization. While working on her thesis and research papers, she explored her passion for scientific writing and communications.

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