Quantum Cascade Lasers for Terahertz Applications

The mid-infrared (λ ∼ 3 – 30 μm) and terahertz (THz) or far-infrared (λ ∼ 30 – 300 μm) regions of the electromagnetic spectrum offer unique applications in spectroscopy, sensing, and imaging. However, these longer wavelengths (photon energies ≤ 0.4 eV) are difficult to generate with compact solid-state devices owing to the lack of naturally occurring materials with small bandgaps. Quantum-cascade lasers (QCLs) are unipolar devices based on semiconductor superlattices, in which radiation occurs due to intersubband (rather than interband) optical transitions. After their first demonstration almost two decades ago, mid-infrared QCLs are now available commercially and are the brightest mid-infrared solid-state sources available. Development of THz QCLs is still in research stages, with important progress being made in past several years. THz QCLs are now poised to bridge the so-called "terahertz gap" in the spectrum, which has long remained underdeveloped due to the difficulty in generation of terahertz radiation.After the first demonstration of THz QCL radiating at 4.4 THz and operating up to ∼50 K in 2001, there was a race between different research groups to demonstrate better temperature performance of such lasers. Today, significant improvements have been made in the performance of THz QCLs, which can now cover frequencies from 1.2 THz to 5.4 THz by optimum design of semiconductor superlattices. Operation above 140 K has been realized in GaAs/AlGaAs based QCLs for frequencies ranging from 1.8 – 4.4 THz.This thesis reports several milestones achieved to implement THz QCLs for practical applications. The primary objective of this thesis is to explore novel active region designs from ∼1 – 6 THz. The effect of interface roughness in the semiconductor heterostructures on the performance of such devices are studied and our approach has led to the reduction of threshold current densities by almost a factor of four, while operating above 150 K, as compared to the state-of-art THz QCLs. Such low threshold lasers are desirable for operation of such lasers in relatively compact and electrically operated cryocoolers.QCLs can be designed to emit at two different frequencies when biased with opposing electrical polarities. THz QCLs with bidirectional operation are developed to achieve broadband lasing from the same semiconductor chip that operates at significantly higher temperatures than previous broadband QCLs reported in literature. Such QCLs with broadband will play an important role for sensing and spectroscopy since then, distributed-feedback schemes could be utilized to produce laser arrays on a single semiconductor chip with wide spectral coverage. Finally, approaches toward development of a novel THz spectroscopy scheme is introduced based on QCLs, which eliminates the need of complex and expensive optical set-ups and will potentially eliminate the need of expensive THz detectors and spectrometers. Preliminary results for the feasibility of such a sensor are demonstrated.