The quantum cascade laser (QCL) is a novel semiconductor device emitting radiation in the mid-infrared, a spectral region traditionally accessed using complex nonlinear optical techniques. QCLs provide compact, high-power (>100 mW) mid-IR sources. A low bandwidth (<MHz), wide tuning range, and room temperature operation for wavelengths >4.3 μm mean that QCLs are ideal sources for both pump and probe lasers. When a polarised, vibrationally-excited sample interferes with the chirped probe laser on a timescale that is shorter than the typical relaxation processes for the system, rapid passage signals are observed. Recently, continuous wave (cw) QCLS have been developed that can operate near room temperature, enhancing the versatility of these devices for a range of applications, including use in medical diagnostics.

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