Terahertz Quantum Cascade Lasers - The Past, Present, and Potential Future

Since their first demonstration in 2002, the development of terahertz frequency quantum cascade lasers has been extremely rapid. We overview some of the advances that have taken place and which have made the terahertz quantum cascade laser such a ubiquitous source. We also consider potential future directions for terahertz quantum cascade laser technology, including its use in satellite-borne instrumentation for future Earth observation and planetary science missions

Effect of a current blocking barrier on a 2–6 μm p-GaAs/AlGaAs heterojunction infrared detector

We report the performance of a 30 period p-GaAs/AlxGa1 − xAs heterojunction photovoltaic infrared detector, with graded barriers, operating in the 2–6 μm wavelength range. Implementation of a current blocking barrier increases the specific detectivity (D*) under dark conditions by two orders of magnitude to ∼1.9 × 1011 Jones at 2.7 μm, at 77 K. Furthermore, at zero bias, the resistance-area product (R 0 A) attains a value of ∼7.2 × 108 Ω cm2, a five orders enhancement due to the current blocking barrier, with the responsivity reduced by only a factor of ∼1.5

Mid-infrared detection in p-GaAs/AlGaAs heterostructures with a current blocking barrier

For the infrared detection in the 3-5 μm range, p-GaAs/AlxGa1-xAs heterojunction is an attractive material system due to light hole/heavy hole and spin-orbit split-off intra-valance band transitions in this wavelength range. Varying the Al mole fraction (x) provides the tuning for the wavelength threshold, while graded AlxGa1-xAs potential barriers create an asymmetry to allow a photovoltaic operation. The photovoltaic mode of operation offers the advantage of thermal noise limited performance. In our preliminary work, a 2 - 6 μm photovoltaic detector was studied. Implementation of an additional current blocking barrier improved the specific detectivity (D∗) by two orders of magnitude, to 1.9×1011 Jones at 2.7 μm, at 77K. At zero bias, the resistance-area product (R0A) had a value of ∼ 7.2×108 Ω cm2, which is five orders higher in magnitude (with a corresponding reduction of the responsivity by only a factor of ∼ 1.5), compared to the R0A value without the blocking barrier. A photoresponse was observed up to 130K

Multimode, Aperiodic Terahertz Surface-Emitting Laser Resonators

Quasi-crystal structures are conventionally built following deterministic generation rules although they do not present a full spatial periodicity. If used as laser resonators, they open up intriguing design possibilities that are simply not possible in conventional periodic photonic crystals: the distinction between symmetric (vertically radiative but low quality factor Q) and anti-symmetric (non-radiative, high Q) modes is indeed here fully overcome, offering a concrete perspective of highly efficient vertical emitting resonators. We here exploit electrically pumped terahertz quantum cascade heterostructures to devise two-dimensional seven-fold quasi-crystal resonators, exploiting rotational order or irregularly distributed defects. By lithographically tuning the lattice quasi-periodicity and/or the hole radius of the imprinted patterns, efficient multimode surface emission with a rich sequence of spectral lines distributed over a 2.9–3.4 THz bandwidth was reached. We demonstrated multicolor emission with 67 mW of peak optical power, slope efficiencies up to ≈70 mW/A, 0.14% wall plug efficiencies and beam profile results of the rich quasi-crystal Fourier spectrum that, in the case of larger rotational order, can reach very low divergence

Extended wavelength infrared photodetectors

Extension of the wavelength threshold of an infrared detector beyond λt=hc/Δ is demonstrated, without reducing the minimum energy gap (Δ) of the material. Specifically, a photodetector designed with Δ=0.40  eV, and a corresponding λt=3.1  μm, was shown to have an extended threshold of ∼45  μm at 5.3 K, at zero bias. Under negative and positive applied bias, this range was further extended to ∼60 and ∼68  μm, respectively, with the photoresponse becoming stronger at increased biases, but the spectral threshold remained relatively constant. The observed wavelength extension arises from an offset between the two potential barriers in the device. Without the offset, another detector with Δ=0.30  eV showed a photoresponse with the expected wavelength threshold of ∼4  μm

Estimation of Spectroscopic Uncertainty and Correlation in Terahertz Time Domain Spectroscopy

We present a method of calculating the measurement variance-covariance matrix of a spectroscopic sample’s complex refractive index from time-domain statistics in order to estimate uncertainty of a measurement. We compare this method to a numerical analysis and previously derived methodology, and show that our time-based estimate is both accurate and adaptable to complex extraction models

Multilayer Extraction of Complex Refractive Index in Broadband Transmission Terahertz Time-Domain Spectroscopy

In terahertz spectroscopy, multi-layered samples often need to be measured, for instance in a liquid flow cell, and this complicates the extraction of material parameters. We present a spectroscopic parameter extraction algorithm for multilayer samples that can also be used to extract the thickness of an unknown sample laye