*Article* **Application of High-Speed Quantum Cascade Detectors for Mid-Infrared, Broadband, High-Resolution Spectroscopy**

**Tatsuo Dougakiuchi 1,\* and Naota Akikusa <sup>2</sup>**


**Abstract:** Broadband, high-resolution, heterodyne, mid-infrared absorption spectroscopy was performed with a high-speed quantum cascade (QC) detector. By strictly reducing the device capacitance and inductance via air-bridge wiring and a small mesa structure, a 3-dB frequency response over 20 GHz was obtained for the QC detector, which had a 4.6-μm peak wavelength response. In addition to the high-speed, it exhibited low noise characteristics limited only by Johnson–Nyquist noise, bias-free operation without cooling, and photoresponse linearity over a wide dynamic range. In the detector characterization, the noise-equivalent power was 7.7 <sup>×</sup> <sup>10</sup>−<sup>11</sup> W/Hz1/2 at 4.6 <sup>μ</sup>m, and it had good photoresponse linearity up to 250 mW, with respect to the input light power. Broadband and high-accuracy molecular spectroscopy based on heterodyne detection was demonstrated by means of two distributed-feedback 4.5-μm QC lasers. Specifically, several nitrous oxide absorption lines were acquired over a wavelength range of 0.8 cm−<sup>1</sup> with the wide-band QC detector.

**Keywords:** mid-infrared; quantum cascade detector; high-speed operation; heterodyne detection; high-resolution spectroscopy
