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The extension of Multi-mode Absorption Spectroscopy (MUMAS) to the infra-red spectral region for multi-species gas sensing is reported. A computationally efficient, theoretical model for analysis of MUMAS spectra is presented that avoids approximations used in previous work and treats arbitrary and time-dependent spectral intensity envelopes, thus facilitating the use of commercially available Interband Cascade Lasers (ICLs) and Quantum Cascade Lasers (QCLs). The first use of an ICL for MUMAS is reported using a multi-mode device operating at 3.7 μm to detect CH4 transitions over a range of 30 nm. Mode-linewidths are measured using the pressure-dependent widths of an isolated absorption feature in HCl. Multi- species sensing is demonstrated by measurement of partial pressures of CH4, C2H2 and H2CO in a low-pressure mixture with uncertainties of around 10%. Detection of CH4 in N2 at 1 bar is demonstrated using a shorter-cavity ICL to resolve spectral features in pressure-broadened and congested spectra. The first use of a QCL for MUMAS is reported using a commercially available device operating at 5.3 μm to detect multiple absorption transitions of NO at a partial pressure of 2.79 μbar in N2 buffer gas. The revised model is shown to enable good fits to MUMAS data by accounting for the time-variation of the spectral intensity profile during frequency scanning. Individual mode-linewidths are derived from fits to pressure- dependent MUMAS spectra and features from background interferences due to H2O in laboratory air are distinguished from those of the target species, NO. Data obtained at scan rates up to 10 kHz demonstrate the potential for achieving short measurement times. The development of a balanced ratiometric detection scheme for MUMAS with commercially available multi-mode lasers operating at 1.5 μm is reported for simultaneous detection of CO and CO2 showing improved SNR performance over previous direct transmission methods and suitability for a compact field-employable instrument. In addition, MUMAS spectra of CO2 are used to derive gas temperatures with an uncertainty of 3.2% in the range 300 - 700 K.

Terahertz (THz) quantum cascade lasers (QCLs) are compact electrically pumped unipolar semiconductor laser which can produce a continuous wave radiation of high output power in the range of 1.2 to 5.6 THz. The QC vertical-external-cavity surface-emitting-laser (QC-VECSEL) is an external cavity configuration that supports high-power operation with excellent beam quality and broadband tunability. The key component of the QC-VECSEL is an amplifying reflectarray metasurface, based on a subwavelength array of surface-radiating metal-metal waveguide antenna elements loaded with QC-laser gain material. Despite its importance, up to now the spectral properties of the QC-metasurface have been designed by simulations and have only been verified indirectly through observation of the QC-VECSEL lasing characteristics, or by passive FTIR reflectance measurements at room temperature. Furthermore, design takes place using simulations based upon simplified models for the material loss and the QC-gain, where uncertain Drude model parameters for material losses are used, and the detailed interaction of the intersubband transition with the metasurface is neglected. In the past decade, THz time domain spectroscopy (THz-TDS) has been widely used to investigate gain spectra and laser dynamics of THz QC-lasers based on various ridge waveguide geometries. During my doctoral studies, I designed and built up a reflection-mode THz-TDS system to study amplifying quantum-cascade (QC) metasurface samples as a function of injected current density. The first direct spectral measurements were performed on QC-metasurfaces using reflection-mode THz-TDS. Several different kinds of metasurface were designed that were suitable for study by the THz-TDS system. Extremely strong absorption features for QC-metasurfaces whose resonance frequency designed below 3 THz is measured at zero bias, which is associated with coupling between the metasurface resonance and an intersubband transition within the QC material. In one case, nearly perfect absorption is observed due to the transition from weak to strong light-matter coupling condition. Increase in reflectance are observed as the devices are biased, both due to reduction in intersubband loss and the presence of intersubband gain. Significant phase modulation associated with the metasurface resonance is observed via electrical control for some certain metasurfaces, which may be useful for electrical tuning of QC-VECSEL. These results provide insight into the interaction between the intersubband QC-gain material and the metasurface and modify the design rules for QC-VECSELs for both biased and unbiased regions.

The terahertz (THz) quantum cascade lasers based on intra-cavity difference frequency generation (DFG) in mid-infrared (mid-IR) quantum cascade lasers (QCLs), so-called THz DFG-QCLs, are the only electrically pumped, compact, and mass-producible THz semiconductor sources operating at room temperature. The Cherenkov phase matching scheme enables efficient out-coupling of THz radiation from the active region of the THz DFG-QCLs. However, the use of semi-insulating (S.I.) InP substrate and lateral current injection limit further improvements. This dissertation focuses on out-coupling methods for THz DFG-QCLs to overcome the limitations of Cherenkov THz DFG-QCLs. An application of Ⅲ-Ⅴ-on-silicon hybrid semiconductor laser concept to Cherenkov THz DFG-QCLs improves THz out-coupling efficiency owing to the smaller refractive index and THz absorption of float-zone high-resistivity silicon (FZ HR Si) substrate used in these devices than S.I. InP substrate. The transfer-printing method using a SU-8 adhesion layer is introduced to replace the original S.I. InP substrate with FZ HR Si substrate. These devices showed 5 times higher out-coupling efficiency than the reference Cherenkov THz DFG-QCLs on S.I. InP substrate at 3.5 THz. The use of wire grid polarizer pattern (WGP) instead of highly doped semiconductor current injection layer in the Cherenkov THz DFG-QCLs reduces the resistance of the devices with minimized output power decrease. Our proof of concept device with WGP sandwiched by SU-8 adhesive layer used for bonding FZ HR Si substrate and the waveguide of THz DFG-QCL demonstrated significant decrease of applied voltage while showing 1.8 times higher THz power output than the reference Cherenkov THz DFG-QCL devices at 3 THz. The double-metal waveguide THz DFG-QCL devices with surface grating out-coupler on top of the waveguide emitting the THz radiation in forward direction were also demonstrated in this dissertation. The surface grating out-coupler enables THz extraction from the whole length of the waveguide, therefore, efficient out-coupling can be achieved. Our device represents at least a factor of 2 improvement in the performance compared to the best Cherenkov DFG-QCLs operating below 2 THz

To date, quantum cascade lasers are the only semiconductor laser sources that can cover a broad range of wavelength from mid-infrared down to terahertz frequency. The operation physics of quantum cascade lasers is qualitatively different from other conventional lasers; the lasing action is based on the unipolar characteristics, i.e. intersubband transition, giving rise to an atomic-like gain transition, and the superlattice transport, relaxation channel for hot electrons, which can not be found any analogues in any conventional laser systems. In this thesis, the nature of electronic transport and the femtosecond pulse propagation in quantum cascade lasers have been addressed by using ultrafast optical techniques. In particular, the electronic transport in quantum cascade lasers was extensively investigated by using femtosecond time-resolved pump-probe techniques. Sub-picosecond resonant tunneling injection from the quantum cascade laser injector ground state into the upper lasing state was found to be incoherent due to strong dephasing in the active subband. The few-picosecond gain recovery due to transport through superlattice was observed and interpreted in terms of dielectric relaxation within the superlattice miniband. We also observed the strong coupling of the electronic transport to the intra-cavity photon density, which we term photon-driven transport. By using an ultrafast upconversion method, the dispersion of the active waveguide was characterized by measuring the wavelength-dependent propagation delay. Contributions from material dispersion, waveguide dispersion, and small-signal gain dispersion were separated, and compared to the experimentally measured pulse broadening as for a self-consistency check. Pulse re-shaping and possible contribution of coherent effects on pulse propagation and gain dynamics were investigated by comparing weak versus strong pulse injection for various bias conditions. In order to support the experimentally measured data, several quantum transport models were used: density-matrix formalism for the resonant tunneling process, Monte-Carlo simulation for the superlattice relaxation, and Maxwell-Bloch equation for the pulse propagation. All theoretical models strongly support the experimental results.