MCT (HgCdTe) LN2 Infrared Detectors for FTIR and Broadband IR Detection

Cooling an MCT detector to liquid nitrogen temperature (~77 K) dramatically reduces its thermal noise and dark current. At room temperature, the HgCdTe sensor would generate significant noise currents that mask low-level IR signals. By using LN₂ cooling, the detector operates in a background-limited regime, achieving much higher detectivity (D*) and enabling the detection of very weak infrared signals. In essence, liquid nitrogen cooling unlocks the full sensitivity potential of the MCT material, far beyond what is possible at ambient or thermoelectric-cooled temperatures.

The FTIR series refers to MCT detectors that are optimized for Fourier Transform Infrared (FTIR) spectrometers, covering an extended spectral range (approximately 2 µm to 22–24 µm). These detectors have a longer wavelength cut-off (up to ~24 µm) compared to standard 2–5 µm or 2–13 µm MCT detectors. This allows them to capture the broad infrared spectrum used in many FTIR applications. The FTIR series detectors typically have slightly lower peak D* (since longer wavelength detection is more challenging and noise-limited) but are designed to provide optimum performance (high sensitivity and fast response) across the entire mid- to far-IR range needed for broadband spectroscopy. They also feature wedged windows (e.g. ZnSe or KRS-5) to minimize interference fringes over that wide spectral band.

The choice of element size is a trade-off between sensitivity, field of view, and speed. Smaller element sizes (e.g. 0.1 mm or 0.25 mm) have higher electrical resistance and lower detector capacitance, which generally yields higher D and responsivity (i.e. better sensitivity) along with faster response times. However, a very small detector captures a smaller optical spot or field of view, which means it collects less total infrared energy unless your incoming beam is tightly focused. Larger element sizes* (e.g. 1 mm or 2 mm) can intercept more light from diffuse or large-area sources and have a wider acceptance angle, which is beneficial if your optical setup isn’t tightly focused. The trade-off is that larger detectors have lower impedance and higher device capacitance, so their detectivity is somewhat lower and their response can be slower (though still in the microsecond range). In summary, use the smallest detector that still comfortably captures your optical beam: for a collimated or small-spot FTIR beam, a 0.25 mm detector might be ideal for maximum sensitivity, whereas for a divergent source or large-area measurement, a 1 mm detector could be more suitable.

These LN₂-cooled MCT detectors are photoconductive devices. Unlike photovoltaic (PV) detectors which generate a voltage or current directly under illumination, photoconductive MCT detectors change their electrical conductivity in response to IR light and require an external bias voltage during operation. In practice, the detector is integrated into a bias circuit and usually connected to a low-noise transimpedance preamplifier. When infrared radiation hits the HgCdTe sensor, its conductivity increases, causing a change in the current flowing (under the applied bias). The preamplifier then converts this small current change into a measurable voltage signal. Photoconductive detectors often provide very high responsivity, but they do require this bias/preamplifier setup. The manufacturer offers dedicated preamplifier modules and recommends using them to achieve the best performance from the MCT detector.

The choice of window material depends on the detector’s spectral range, ensuring high transmission over that range. For example, standard 2–5 µm MCT detectors use a Sapphire window (good transparency in the near to mid-IR), while 2–13 µm detectors use Zinc Selenide (ZnSe) windows which transmit well up to ~14 µm. The longest-wavelength FTIR series detectors (up to 24 µm) often use KRS-5 (Thallium Bromoiodide) windows, since KRS-5 remains transparent into the far-infrared. All these windows are made slightly wedged (at a small angle, rather than perfectly parallel faces). The wedge is critical to prevent etalon effects – that is, interference fringes caused by multiple internal reflections between two flat parallel surfaces. By using a wedged window, any reflections are deflected and do not coherently interfere with the incoming signal, thus avoiding unwanted spectral ripples or baseline distortions in sensitive measurements.

The hold time on a single LN₂ fill depends on the dewar design. Standard packaging for these detectors includes 8-hour and 12-hour dewars as common options (often referred to as MSL-8, MDL-8 for 8 h and MSL-12, MDL-12 for 12 h). There are also larger dewars available (e.g. 24-hour hold time) for applications that require continuous overnight operation without refilling. In an 8 h dewar, the liquid nitrogen will gradually evaporate and warm up after about eight hours, so the detector needs topping up or will slowly lose cooling performance beyond that. The naming “side-looking” (MSL) vs “down-looking” (MDL) refers to how the infrared window is oriented: side-looking dewars have the window on the side of the cryostat, useful for horizontal optical paths, whereas down-looking dewars have the window at the top (the detector looks upward through the top window), which can be convenient for bench-top or upward-facing setups. By choosing the appropriate dewar (8h, 12h, or 24h and side- or down-looking), you can match the detector packaging to your experiment’s physical layout and the required duration of continuous operation. Keep in mind that if truly long-term 24/7 operation is needed without any user intervention, an alternative cooling method (such as an integrated Stirling cooler) might be recommended, but for most laboratory setups the LN₂ dewars are a practical and cost-effective solution.

Yes. One of the advantages of MCT (HgCdTe) technology is that its bandgap – and thus the detector’s cut-off wavelength – can be tuned by adjusting the alloy composition of mercury, cadmium, and telluride. The standard detectors cover popular ranges (5 µm, 13 µm, 16 µm, 22 µm cut-offs, etc.), but if your application requires a different cut-off (for example, perhaps you need peak sensitivity around 10 µm or a cut-off at 18 µm), the manufacturer can produce custom MCT detectors to meet those specifications. Custom orders can define the target wavelength range or the peak λ_p, and the material will be engineered accordingly. Additionally, packaging can be customized – for instance, if you need a different window material, a specific dewar form factor, or integration into a third-party cryocooler, those modifications can be discussed. Keep in mind that custom detectors may have longer lead times and certain minimum order requirements, but it is certainly possible to obtain an MCT detector precisely tailored to your spectral and mechanical needs.

LN₂-cooled MCT detectors are used whenever extremely sensitive IR detection is required over the mid to far-infrared range. A classic application is in FTIR (Fourier Transform Infrared) spectrometers, where an MCT detector can significantly increase the signal-to-noise ratio and allow fast scan speeds in the 2–16 µm region (for example, in analytical chemistry or materials identification). They are also employed in infrared microscopy and imaging setups (as single-point detectors for scanning microscopes), in gas analysis instruments (to detect absorption features of gases in the mid-IR), and in thermal emission measurements of materials (e.g. characterizing the infrared emissivity/reflectivity of surfaces). Scientists and engineers in fields like environmental monitoring, defense (IR signature analysis), and semiconductor research use these detectors for broadband IR spectroscopy and radiometry. In any scenario where you need to measure very faint IR signals or very small changes in infrared intensity – such as trace gas spectroscopy, low-concentration pollutant detection, or studying weak thermal radiation – an LN₂-cooled MCT detector is often the go-to solution for its superior sensitivity.