Terahertz imaging and quantum cascade laser based devices

Abstract

The terahertz (THz) frequency range (f=0.3-10 THz, [lambda]=30-1000 lam) is much less technologically developed that the adjacent microwave and infrared frequency ranges, but offers several advantages for imaging applications: THz wavelengths offer better spatial resolutions than microwave frequencies, and THz radiation is able to penetrate materials that are opaque at infrared frequencies (e.g. packaging, plastics, paints and semiconductors). These features, combined with the unique THz spectral signatures of chemicals have lead to the development of terahertz imaging systems for non-destructive test. However, the weak radiation sources in these existing systems result in single pixel scanning architectures requiring minutes to acquire images or enhanced speed at the expense of signal to noise ratio (SNR). In this thesis, a system for real-time imaging is demonstrated using recently developed terahertz quantum-cascade laser (QCL) sources, along with commercial, focal plane array thermal detectors. The system uses a high power (48 mW) 4.3-THz QCL, which is also used to characterize the focal plane array, resulting in a noise equivalent power (NEP) of 320 pW/Hz. The source and detector are used in a synchronous detection scheme, resulting in an SNR of ~25 dB/pixel at a 20-Hz frame rate. This represents a two order of magnitude improvement in speed over previous systems at comparable SNRs. Real-time imaging over a 25-m distance is described, using a QCL adjusted for emission in the narrow 4.9 THz atmospheric transmission window.

(cont.) The challenges posed by the long THz wavelengths in QCL waveguide design leads to a tradeoff between high temperature operation (<186K) and high power/good beam patterns (248 mW peak, l2deg FWHM). To mitigate these tradeoffs, a technique for buttcoupling a metal-metal waveguide QCL to an index matched lens is developed. The resulting device achieves the highest reported power for a MM waveguide (145 mW peak) and while retaining a high operating temperature (160 K) and achieving a narrow beam pattern (<5deg). The lens coupling technique is also used to add spectroscopic capability to the system, through the development of an external cavity QCL. The butt-coupling of an antireflection coated lens to a semi-insulating surface plasmon waveguide QCL results in increased optical losses and suppression of lasing. Lasing is recovered using an external optical system with a reflective grating for frequency selective feedback. A device is characterized showing 4% tuning range at ~4.4 THz, and is among the first demonstrations of tunable THz QLCs.