Terahertz (THz) radiation has become increasingly important in many scientific and commercial fields in recent years. It possesses many remarkable features resulting in an increased use of THz radiation for various applications, like biomedical imaging, security screening, and industrial quality control. The capability of these applications depends directly on the availability of powerful THz sources and high-responsivity, fast THz detectors. Current commercial products used to detect THz radiation, like Golay cells and pyroelectric detectors, have only slow detection rates and poor sensitivities. Other commercial THz detectors, like bolometers, are more sensitive but require liquid helium cooling. In this thesis, two types of room-temperature high-responsivity graphene-based THz detectors are presented, relying on the unique properties of graphene and the function of plasmonic antenna arrays which boost the interaction between THz waves and graphene. Graphene has been demonstrated as a promising material for THz detection. However, the challenge is its insufficient light absorption that largely limits the responsivity. The first design is based on an array of planar antennas arranged in series and shorted by graphene squares. Highly efficient photodetection can be achieved by using the metallic antenna to simultaneously improve both light absorption, as resonant elements, and photocarrier collection, as electrodes. The device has been characterized with quantum cascade lasers, yielding a maximum responsivity of ~2 mA/W at 2 THz. The second detector is based on an array of interdigitated bow-tie antennas connected in parallel and shunted by graphene squares. The arms of the bow-tie antennas were made of two metals with different work functions to create a built-in electric field and improve the responsivity. The device has been characterized and yields a maximum responsivity of ∼34 μA/W at 2 THz. Efficient THz imaging is presented by integrating the detector in a QCL-based THz imaging system.

The Raman Injection Laser is a new type of laser which is based on triply resonant stimulated Raman scattering between quantum confined states within the active region of a Quantum Cascade Laser that serves as an internal optical pump. The Raman Injection Laser is driven electrically and no external laser pump is required. Triple resonance leads to an enhancement of orders of magnitude in the Raman gain, high conversion efficiency and low threshold. We studied this new type of laser and conclude some basic equations. With reasonable experimental parameters, we calculated the laser gain, losses and the output power of the Raman Injection Laser by using Mathematica and FEMLab. Finally we compared the theoretical and experimental results.

Quantum cascade laser (QCL), as a unipolar semiconductor laser based on intersubband transitions in quantum wells, covers a large portion of the Mid and Far Infrared electromagnetic spectrum. The frequency of the optical transition can be determined by engineering the layer sequence of the heterostructure. The focus of this work is on Terahertz (THz) frequency range (frequency of 1 - 10 THz and photon energy of ~ 4 - 40 meV), which is lacking of high power, coherent, and efficient narrowband radiation sources. THz QCL, demonstrated in 2002, as a perfect candidate of coherent THz source, is still suffering from the empirical operating temperature limiting factor of T ≈ ħω/kB, which allows this source to work only under a cryogenic system. Most of high performance THz QCLs, including the world record design which lased up to ~ 200 K, are based on a resonant phonon (RP) scheme, whose population inversion is always less than 50%. The indirectly-pumped (IDP) QCL, nicely implemented in MIR frequency, starts to be a good candidate to overcome the aforementioned limiting factor of RP-QCL. A rate equation (RE) formalism, which includes both coherent and incoherent transport process, will be introduced to model the carrier transport of all presented structures in this thesis. The second order tunneling which employed the intrasubband roughness and impurity scattering, was implemented in our model to nicely predict the behavior of the QCL designs. This model, which is easy to implement and fast to calculate, could help us to engineer the electron wavefunctions of the structure with optimization tools. We developed a new design scheme which employs the phonon scattering mechanism for both injecting carrier to the upper lasing state and extracting carrier from lower lasing state. Since there is no injection/extraction state to be in resonance with lasing states, this simple design scheme does not suffer from broadening due to the tunneling. Finally, three different THz IDP-QCLs, based on phonon-photon-phonon (3P) scheme were designed, grown, fabricated, and characterized. The performance of those structures in terms of operating temperature, threshold current density, maximum current density, output optical power, lasing frequency, differential resistance at threshold, intermediate resonant current before threshold, and kBT/ħω factor will be compared. We could improve the kBT/ħω factor of the 3P-QCL design from 0.9 in first iteration to 1.3 and the output optical power of the structure from 0.9 mW in first design to 3.4 mW. The performance of the structure in terms of intermediate resonant current and the change in differential resistance at threshold was improved.

The Raman Injection Laser is a new type of laser which is based on triply resonant stimulated Raman scattering between quantum confined states within the active region of a Quantum Cascade Laser that serves as an internal optical pump. The Raman Injection Laser is driven electrically and no external laser pump is required. Triple resonance leads to an enhancement of orders of magnitude in the Raman gain, high conversion efficiency and low threshold. We studied this new type of laser and conclude some basic equations. With reasonable experimental parameters, we calculated the laser gain, losses and the output power of the Raman Injection Laser by using Mathematica and FEMLab. Finally we compared the theoretical and experimental results.

Quantum cascade laser (QCL), as a unipolar semiconductor laser based on intersubband transitions in quantum wells, covers a large portion of the Mid and Far Infrared electromagnetic spectrum. The frequency of the optical transition can be determined by engineering the layer sequence of the heterostructure. The focus of this work is on Terahertz (THz) frequency range (frequency of 1 - 10 THz and photon energy of ~ 4 - 40 meV), which is lacking of high power, coherent, and efficient narrowband radiation sources. THz QCL, demonstrated in 2002, as a perfect candidate of coherent THz source, is still suffering from the empirical operating temperature limiting factor of T ≈ ħω/kB, which allows this source to work only under a cryogenic system. Most of high performance THz QCLs, including the world record design which lased up to ~ 200 K, are based on a resonant phonon (RP) scheme, whose population inversion is always less than 50%. The indirectly-pumped (IDP) QCL, nicely implemented in MIR frequency, starts to be a good candidate to overcome the aforementioned limiting factor of RP-QCL. A rate equation (RE) formalism, which includes both coherent and incoherent transport process, will be introduced to model the carrier transport of all presented structures in this thesis. The second order tunneling which employed the intrasubband roughness and impurity scattering, was implemented in our model to nicely predict the behavior of the QCL designs. This model, which is easy to implement and fast to calculate, could help us to engineer the electron wavefunctions of the structure with optimization tools. We developed a new design scheme which employs the phonon scattering mechanism for both injecting carrier to the upper lasing state and extracting carrier from lower lasing state. Since there is no injection/extraction state to be in resonance with lasing states, this simple design scheme does not suffer from broadening due to the tunneling. Finally, three different THz IDP-QCLs, based on phonon-photon-phonon (3P) scheme were designed, grown, fabricated, and characterized. The performance of those structures in terms of operating temperature, threshold current density, maximum current density, output optical power, lasing frequency, differential resistance at threshold, intermediate resonant current before threshold, and kBT/ħω factor will be compared. We could improve the kBT/ħω factor of the 3P-QCL design from 0.9 in first iteration to 1.3 and the output optical power of the structure from 0.9 mW in first design to 3.4 mW. The performance of the structure in terms of intermediate resonant current and the change in differential resistance at threshold was improved.

High-fidelity chemical kinetic models for low-temperature combustion processes require high-fidelity data from fundamental experiments conducted in idealized transient reactors, such as shock tubes and rapid compression machines (RCM). Non-intrusive laser absorption diagnostics, in particular quantum cascade lasers (QCL) in the mid-infrared wavelength region, provide a unique opportunity to obtain quantitative, time-resolved species concentration and temperature from these reactive systems. In this work, three novel laser absorption diagnostics in the mid-infrared wavelength region are presented for three different experimental applications. The first diagnostic was developed for measuring CO2 concentration using an external cavity QCL centered in the ν3 fundamental vibrational band of CO2. Absorption cross-sections were measured in a shock tube, at a fixed wavelength for the R(32) line centered at 2371.42 cm-1 (4.217 µm) over 700 – 2900 K and nominal pressures of 1, 5 and 10 bar. The diagnostic was used to measure rate coefficients for the reaction between carbon monoxide and hydroxyl radical over 700 – 1230 K and 1.2 – 9.8 bar using highly dilute mixtures. The second diagnostic was developed for measuring CO concentration using a pulsed QCL centered at 2046.28 cm-1 (4.887 µm) and an off-axis cavity implemented on the RCM. The duty cycle and pulse repetition rate of the laser were optimized for increased tuning range, high chirp rate and increased line-width to achieve effective laser-cavity coupling. A gain factor of 133 and time resolution of 10 μs were demonstrated. CO concentration-time profiles during the oxidation of highly dilute n-heptane/air mixtures were recorded and compared with chemical kinetic models. This represents the first application of a cavity-enhanced absorption diagnostic in an RCM. Finally, a calibration-free temperature diagnostic based on a pair of pulsed QCLs centered at 2196.66 cm-1 and 2046.28 cm-1 was implemented on the RCM. The down-chirp phenomenon resulted in large spectral tuning (∆v ~ 2.8 cm-1) within a single pulse of each laser at a high pulse repetition frequency (100 kHz). The diagnostic for was used to measure the temperature rise during first-stage ignition of n-pentane at nominal pressures of 10 and 15 bar for the first time.

Quantum cascade laser (QCL) diodes have potential applications in many areas including emissions analysis and explosives detection, but like many solid-state devices they suer from degraded performance at higher temperatures. To alleviate this drawback, the thermal properties of the QCL diodes must be better understood. Using molecular dynamics (MD) and photothermal radiometry (PTR), the thermal conductivity of a representative QCL diode is computed and measured respectively. The MD results demonstrate that size eects are present in the simulated systems, but if these are accounted for by normalization to experimental results the thermal conductivity of the QCL can be reasonably obtained. The cross-plane conductivity is found to be in the range of 1.8 to 4.3 W=m K, while the in-plane results are in the range of 3.7 to 4.0 W=m K. These values compare well with experimental results from the literature for both QCL materials and for AlInAs and GaInAs, which the QCL is composed of. The cross-plane conductivity results are lower than those of either AlInAs or GaInAs, which demonstrates the phonon scattering at the interfaces. The in-plane results are between AlInAs and GaInAs, which is to be expected. The PTR results are less concrete, as there seem to be heat transfer eects active in the samples which are not included in the models used to t the frequency scans. These effects are not 2D heat transfer artifacts nor are they the result of volumetric absorption. It is possible that they are the results of plasmon induction, but this is only supposition. As the data stand, the PTR and MD results are within an order of magnitude of each other and follow reasonable trends, which suggests that both results are not too far o from reality. While the experimental results are not entirely conclusive, the simulations and experiments corroborate each other suciently to warrant further investigation using these techniques. Additionally, the simulations present sucient internal consistency so as to be useful for thermal property investigation independent of the PTR results.

Quantum cascade lasers (QCLs) are a candidate for calibration sources in space-based remote sensing applications. However, the output beam from a QCL has some characteristics that are undesirable in a calibration source. The output beam from a QCL is polarized both temporally and spatially coherent, and has a non-uniform bivariate Gaussian profile. These characteristics need to be mitigated before QCLs can be used as calibration sources. This study presents the design and implementation of an optical system that manipulates the output beam from a QCL so that it is spatially and angularly uniform with reduced coherence and polarization.

Since their initial demonstration in 1994, quantum cascade lasers (QCLs) have become prominent sources of mid-infrared radiation. Over the years, a large scientific and engineering effort has led to a dramatic improvement in their efficiency and power output, with continuous wave operation at room temperature and Watt-level output power now standard. However, beyond this progress, new functionalities and capabilities need to be added to this compact source to enable its integration into consumer-ready systems. Two main areas of development are particularly relevant from an application standpoint and were pursued during the course of this thesis: wavelength control and wavefront engineering of QCLs. The first research direction, wavelength control, is mainly driven by spectroscopic applications of QCLs, such as trace gas sensing, process monitoring or explosive detection. We demonstrated three different capabilities, corresponding to different potential spectroscopic measurement techniques: widely tunable single longitudinal mode lasing, simultaneous lasing on multiple well-defined longitudinal modes, and simultaneous lasing over a broad and continuous range of the spectrum. The second research direction, wavefront engineering of QCLs, i.e. the improvement of their beam quality, is relevant for applications necessitating transmission of the QCL output over a large distance, for example for remote sensing or military countermeasures. To address this issue, we developed plasmonic lenses directly integrated on the facets of QCLs. The plasmonic structures designed are analogous to antenna arrays imparting directionality to the QCLs, as well as providing means for polarization control. Finally, a research interest in plasmonics led us to design passive flat optical elements using plasmonic antennas. All these projects are tied together by the involvement of Fourier analysis as an essential design tool to predict the interaction of light with various gratings and periodic arrays of grooves and scatterers. / Engineering and Applied Sciences

Over the last ten years, terahertz (THz or T-ray) biomedical imaging has become a modality of interest due to its ability to simultaneously acquire both image and spectral information. Terahertz imaging systems are being commercialized, with increasing trials performed in a biomedical setting. Advanced digital image processing algorithms are greatly need to assist screening, diagnosis, and treatment. Pattern recognition algorithms play a critical role in the accurate and automatic process of detecting abnormalities when applied to biomedical imaging. This goal requires classification of meaningful physical contrast and identification of information in images, for example, distinguishing between different biological tissues or materials. T-ray tomographic imaging and detection technology contributes especially to our ability to discriminate opaque objects with clear boundaries and makes possible significant potential applications in both in vivo and ex vivo environments. The Thesis consists of a number of Chapters, which can be grouped in to three parts. The first part provides a review of the state-of-the-art regarding THz sources and detectors, THz imaging modes, and THz imaging analysis. Pattern recognition forms the second part of this Thesis, which is represented via combining several basic operations: wavelet transforms and wavelet based signal filtering, feature extraction and selection, along with classification schemes for THz applications. Signal filtering in this Thesis is achieved via wavelet based de-noising. The ultrafast pulses generated terahertz time-domain spectroscopy (THz-TDS), which is demonstrated to justify their decomposition in the wavelet domain as it can provide better de-noising performance. Feature extraction and selection of the terahertz measurements rely on observed changes in pulse amplitude and phase, as well as scattering characteristics of several different types of powder samples under study. Additionally, three signal processing algorithms are adopted for the evaluation of the complex insertion loss function of such samples as lactose, mandelic acid, and dl-mandelic acid: (i) standard evaluation by ratioing the sample with the background spectra, (ii) a subspace identification algorithm, and (iii) a novel wavelet packet identification procedure. These system identification algorithms enable THz measurements to be transformed to features for THz pattern recognition. Meanwhile, a novel feature extraction method involving the use of Auto Regressive (AR) and Auto Regressive Moving Average (ARMA)models on the wavelet transforms of measured T-ray pulse responses of ex vivo osteosarcoma cells as well as other biomedical materials is presented. Classification schemes are carried out via simple and robust schemes, such as the linear Mahalanobis distance classifier, and the non-linear Support Vector Machine (SVM) classifier. In particular, SVMs are used as a learning scheme to achieve the identification of two classes of RNA samples and multiple classes of powered materials. Coherent terahertz detection hardware—THz time-domain spectroscopy (THz-TDS)—is used to obtain all the data for validation of these classification schemes. The past decade has witnessed the tremendous development of terahertz instruments for detecting, storing, analysing, and displaying images. Terahertz time-domain spectroscopy (THz-TDS) is a broadband technique that generates and detects THz radiation in a synchronous and coherent manner. By contrast, the newly developed THz quantum cascade laser is a narrow-band radiation source that provides potential for realising compact systems; they produce image data with higher average power levels. The third part of this Thesis discusses methods to improve the capability of both broad and narrow-band terahertz imaging, driven by computer-aided analytical techniques. A wavelet based reconstruction algorithm for terahertz computed tomography is represented to show how this algorithm can be used to rapidly reconstruct the region of interest (ROI) with a reduction in the measurements of terahertz responses, compared with a standard filtered back-projection technique. These reconstruction algorithms are applied to the analysis of acquired experimental data and to locally recover the two dimensional (2D) and three-dimensional (3D) structures of several optically opaque objects. Moreover, a segmentation technique based on two dimensional wavelet transforms is investigated for the identification of different materials from the reconstructed CT image. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1352839 / Thesis (Ph.D.) - University of Adelaide, School of Electrical and Electronic Engineering, 2009