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Artificial crystals are two-dimensional systems with an artificially created periodic potential. Control over this potential allows one to design the optical and electronic properties of a material. This thesis provides theoretical predictions for new effects in artificial crystals as well as proposals for future experiments. Some of these effects are currently being investigated experimentally. There are two kinds of problems that we study theoretically. Both are relevant to artificial crystals created lithographically. The first kind of problem covers band-structure for non-interacting particles. This includes a study of flat bands, and of magnetic-field effects following from Hofstadter's butterfly. The second kind of problem is related to electron-electron correlations. Typically, correlations are stronger in artificial crystals than in natural crystals, and we investigate ways to take advantage of this fact. An important point is that the correlations in artificial crystals are tunable, allowing one to drive the system through different quantum phase transitions. Artificial crystals are a remarkably flexible and rich platform for studying many-body physics, and our work demonstrates this. This thesis studies three different kinds of artificial crystals. Two electronic systems: semiconductor-based artificial graphene and patterned bilayer graphene. And one optical system: the artificial polaritonic crystal. In particular, the following single-body problems are addressed. In semiconductor-based artificial graphene we study the quantum Hall effect and current distributions in Hofstadter's butterfly, magnetic breakdown, emergent kagome physics, and the kagome flat band. In patterned bilayer graphene we study a tunable and robust flat band. In artificial optical crystals we study the formation of a polaritonic band-structure. I also address the following many-body problems. Mott transitions and charge ordering in the semiconductor-based kagome flat band. Many-body Coulomb screening and driven Mott transitions in patterned bilayer graphene.

When a quantum impurity is immersed in a bath with which it interacts, it becomes dressed by the excitations of the bath. The dressed quantum impurity can then be described as a quasiparticle with renormalised properties, commonly called the polaron-- an important concept first suggested by Landau in 1933. In this thesis, we seek to understand the modifications of the interactions experienced by an exciton-polariton impurity when it is immersed in a bath of electrons or a bath of opposite-spin exciton-polaritons. Exciton-polaritons are formed from the hybridisation of photons and excitons. Therefore, they inherit the light mass from the former and the interaction properties from the latter. The motivation of our research is twofold. Firstly, polarons have been proposed to play a crucial role in physical phenomena with potential applications such as high-Tc superconductivity and itinerant ferromagnetism. Thus, the understanding of polarons has become a highly sought after goal in physics today. Secondly, polariton interactions are a crucial ingredient for many proposed applications. These include both classical and quantum optical communication technology which the world will become highly-dependent on in the forseeable future. Our experiments are performed on a type of 2D semiconductor known as monolayer molybdenum diselenide (MoSe2) which is strongly coupled to a zero-dimensional (0D) open fiber cavity at cryogenic temperatures (T = 4 K). In order to ensure strong-coupling between the excitons and the photons, the van der Waals (vdW) semiconductor heterostructures and the distributed Bragg reflector (DBR) mirror coatings are designed after optimisation with transfer matrix simulations. The concave dimpled surfaces on fibers are fabricated with low surface roughness using carbon dioxide (CO2) laser ablation. We investigate the system in two regimes with time-resolved pump-probe spectroscopy: a quantum impurity in a fermionic bath of electrons and in a bosonic bath of opposite-spin exciton-polaritons. In the fermionic case, we show that the impurity-impurity interactions are sizably enhanced-- in one device by a factor of 50 when itinerant electrons are injected into the system such that the exciton-polariton impurities become dressed by electron-hole pair excitations out of the Fermi sea. The enhancement comes from a phase-space filling effect due to the saturation of the bath. Furthermore, we observe optical amplification of the lower-polariton mode in the presence of the electrons. There is a clear dependence of the amplification magnitude on the polarisation of the pumped and probed polariton modes. In the bosonic case, we demonstrate the formation of Bose polarons by immersing a minority spin-down exciton-polariton population into a bath of spin-up exciton-polaritons. The optical spectrum shows clear signatures of the splitting of a single polariton branch into an attractive Bose polaron and a repulsive Bose polaron, corresponding to a redshift and blueshift respectively. These are formed due to the dressing of the spin-down exciton-polariton with the Bogoliubov excitations of the resonantly pumped non-equilibrium condensate of spin-up exciton-polaritons. We furthermore investigate the impurity-impurity interactions by increasing the impurity density beyond the single impurity limit. We demonstrate the tunability of the magnitude as well as sign of both the impurity-bath and impurity-impurity interactions.

This dissertation covers a series of experiments that realized, investigated, and controlled exciton-trion-polaritons in transition-metal dichalcogenides (TMDs). First, theoretical and experimental research that led to the established understanding of trions are reviewed. Then, recent theoretical developments that led to a complete reimagining of excitons, trions, and their interactions with photons is discussed. With the theoretical background established, detailed measurements of optical conductivity of TMD MoSe$_2$ are combined with quantitative analysis to determine the nature of strong Coulomb interactions between excitons and bound trions in TMDs. Then, design and simulations of, and measurements on a hybrid photonic-crystal-TMD structure is discussed whereupon remarkable agreement between experiment and theory evidences the existence of coherent exciton-trion-polariton formation. Initial experimental forays into electronic control of the optical properties of this polariton system is then reviewed. In the final chapter of this dissertation, a non-exhaustive investigation of potential device applications based on the understanding of exciton-trion-polaritons is conducted.

Transition Metal Dichalcogenides (TMDCs) have recently become a very popular topic of research due to their wide range of novel optical properties. Many of these materials feature excitons with large oscillator strengths and binding energies, making them an attractive choice as a material for exploring exciton-polariton physics. TMDCs are also very versatile, and can be combined with each other to form arbitrary combinations of materials that can have interesting, tunable properties that change depending on how the heterostructures are constructed. In this thesis, we will look at several different projects concerning light-matter coupling in TMDCs. First, we numerically calculate the properties of photonic crystals designed to exhibit guided-mode resonances for the purposes of achieving strong coupling with evanescently coupled group VI TMDC monolayers. We detail the optimization of these devices, and show that particular attention needs to be paid to the thickness of the device to ensure maximum exciton-photon coupling. We also demonstrate that the in-plane patterning makes these devices suitable for on-chip integration, and that they have more interesting dispersion properties compared to standard optical cavities. These numerical results of strong coupling are corroborated by experimental results which show that this type of platform can indeed support polaritons at both cryogenic and room temperatures, making this the first demonstration of all-dielectric photonic crystal polaritons created with TMDCs. This is followed by numerical results detailing a dual-band photonic cavity designed to have independently tunable resonances, for coupling to multiple different emitters in a single layer of active material, in contrast to standard types of cavities whose modes are not independently tunable. The cavity combines a traditional Fabry-Perot type cavity with a guided-mode resonance type cavity, and the two modes both have electric field distributions that strongly couple to the active material at the center of the cavity, a result which is generally not possible with standard optical cavities. Then, we present experimental results showing highly valley-polarized emission from hBN encapsulated MoSe2/WSe2 heterobilayers. The emission has two prominent peaks, one of which has the standard optical selection rule and one of which has a flipped optical selection rule, and both exhibit high valley polarization. These results are presented alongside some numerical calculations about the electronic and optical properties of TMDC heterobilayers. Finally, we present experimental results, supported by numerical calculations, which demonstrate for the first time that the group VII material ReS2 is capable of supporting self-hybridized polaritons, which manifest as a splitting of the exciton absorption. The high background index of the material results in an effective optical cavity that is formed by the ReS2/Air and ReS2/substrate (Au or Sapphire) interfaces. We fully characterize the polarization resolved absorption, and present transfer matrix calculations which are in excellent agreement with the experimental results, making this the first demonstration of self-hybridized polaritons in group VII materials.

In this dissertation, we explore the interplay between strongly correlated electrons and optical polaritons confined to two dimensions. This is implemented experimentally with gallium arsenide quantum wells hosting a two-dimensional electron gas (2DEG). The structure is further embedded in an optical microcavity to reach the strong light-matter coupling regime. Optically created excitons in the quantum well act as impurities in the surrounding electron gas, forming collective excitations termed Fermi polarons. The resulting polaron-polariton modes are used to study the quantum Hall effect and allow tailoring photonic properties via control over the electrons. The electronic ground states in the integer and fractional quantum Hall regimes are investigated using polariton spectroscopy. This tool allows probing the spin polarization of the 2DEG by optical means. Many-body spin textures are studied around integer filling, in particular for the case of vanishing g-factor which is expected to favor large skyrmions. Using an optimized device structure, we advance polariton spectroscopy by drastically reducing unwanted modifications of the electron density upon optical illumination. We observe coupling of the polaron-polaritons to different fractional quantum Hall states as the filling factor is varied. In a second part, a polariton Hall bar device is fabricated to investigate the connection between electronic and polariton transport in two dimensions. We demonstrate acceleration of polaritons by shaping the electron density with external electric and magnetic fields. For a spin polarized electron gas, we demonstrate the creation of spin density gradients. They are used to route polaritons on the Hall bar device in a spin selective manner, reminiscent of an optical spin Hall effect. In the last part, we present four-wave mixing experiments performed with polaron-polaritons. The nonlinear optical response of polaritons is measured while they are coupled to different quantum Hall states. A surprising increase in nonlinear signal is found for the specific filling fractions 2/3 and 2/5. These results demonstrate enhanced polariton-polariton interactions both compared to other fillings and to the case of exciton-polaritons without 2DEG. This constitutes a step towards polariton blockade and suggests that nonlinear optics may allow us to extract properties of the correlated electronic states beyond linear spectroscopy.

The advent of efficient light-confining platforms marked the burgeoning of the field of Cavity Quantum Electrodynamics (Cavity QED), which brought along the realization of exotic phenomena that emerges under extreme light-matter interaction regimes.On the other hand, in chemistry, the interaction between light and matter is at the core of invaluable spectroscopic techniques and a number of photo-induced chemical processes. From a theoretical standpoint, light-matter interaction in those scenarios can be treated perturbatively, such that molecules and light preserve their individual identity. This picture breaks down when we consider one photonic mode interacting with molecules such that the energy of this interaction surpasses their respective linewidths. This so-called strong coupling (SC) regime has been accomplished in microsetups which can sustain confinement comparable to the wavelength of light that promotes molecular excitations. Under the SC regime, the excitations sustained by the device are no longer of purely material character but rather exhibit a hybrid molecular and photonic nature, and are usually known as polaritons. The recent observation of the slowing down of chemical reactions in the SC regime has incentivized communities of different backgrounds to converge in order to understand the range of possibilities the regime has to offer on the control of molecular processes. In this dissertation, a combination of quantum optics and chemical rate theory is used to understand the emergent dynamics of molecules embedded in an strongly confined electromagnetic environment. More specifically, an analysis of the tunability of chemical reactivity of realistic polariton setups in the dark (i.e. in the absence of any driving source) is presented. Later on, it is shown that Singlet Fission and Reverse-Intersystem Crossing, two photophysical processes of technological relevance, represent an ideal testing ground to explore the rich dynamics afforded under the polariton regime. In the latter phenomena, special emphasis is cast upon the fleeting nature of the electro- magnetic environment, as well as on its implications on the degree of control on molecular processes.

In this dissertation, the application of advanced spectroscopy, such as two-dimensional infrared (2D IR) spectroscopy to vibrational-polaritons challenges and advances our understanding in both fields, as mentioned detailly in the introduction. In the fourth chapter, 2D IR uniquely resolves excitation of hybrid light-matter polaritons and unexpected dark states in a state-selective manner, revealing otherwise hidden interactions between them. Moreover, 2D IR signals highlight the impact of molecular anharmonicities which are applicable to virtually all molecular systems. Besides revealing the polariton-dark modes interactions, the spectroscopy tools has also been employed to study the optical nonlinearities of the molecular vibrational polaritons. In the fifth chapter, the control of vibrational polariton coherent nonlinearities has been fulfilled by manipulation of macroscopic parameters such as cavity longitudinal length or molecular concentration. In chapter six, we studied the long-lived dynamics of vibrational polaritons in various solvent environments. While the relaxation from upper polariton (UP) to dark modes is always fast (

There have been many great advances in the generation and manipulation of optics in the visible and near infrared (IR) range over the past decade. This is largely due to plasmonic enhancement, which has led to new technology in biosensing and molecule detection, solid-state lighting, and solar energy harvesting. The field of plasmonics uses quanta of plasma oscillations, plasmons, formed from the interaction between electromagnetic radiation and free electrons to enhance optical near field magnitudes. However, there is still a large region of the electromagnetic spectrum, covering the mid-infrared (MIR) and terahertz (THz) regions, ranging from 3 μm to 1 mm, that has not benefitted from this research. Polaritonics have recently gained attention from the scientific community to access this region and enhance current technologies. Polaritons are an IR alternative to plasmons; these quasiparticles are formed from light coupling to an elementary material excitation, such as phonons, plasmons, magnons, or excitons. In this project, we look at the past, present, and future of terahertz technology and the role phonon polaritons play in their advancement. A hexagonal boron-nitride/gallium nitride superlattice is examined through simulated reflection experiments to identify phonon polariton modes.

Quantum electrodynamics predicts a non-trivial ground state for an electromagnetic mode. In the absence of photons, the so called zero-point energy of $\frac{1}{2}\hbar\omega$ remains. It gives rise to vacuum electric field fluctuations and important physical effects such as the spontaneous emission, the Lamb shift and the Casimir effect. Experimentally, it remains difficult to tune vacuum field modes and directly observe their physical consequences. By engineering vacuum fields in cavities one can reach a peculiar situation: an electronic excitation of matter can be revived after its decay by photon emission. In this so called strong light-matter coupling regime, the hybrid light-matter excitations (polaritons) are mostly probed with photonic excitations. Such an approach hides, that the coupling arises already from the vacuum field fluctuations in absence of photons. In this work, we develop an experimental platform allowing to probe the electronic part of the polaritonic ground state. Intriguingly, we can tune vacuum field modes, while observing the response in the matter part. It is implemented with a cavity-embedded 2D electron gas in the ultrastrong coupling regime and probed by magneto-transport. Transport - depending on virtual transitions to excited states - is modified, as these transitions become the polaritons in presence of a vacuum Rabi splitting. After a theoretical discussion, we experimentally show that few polariton excitations and also vacuum fields alone modify transport. This opens the way to vacuum-field-controlled many-body states in quantum Hall systems.

This dissertation treats a quantum impurity problem in a semiconductor system. Quantum impurity problems describe the interaction between a single quantum object and a complex environment. They are ubiquitous in physical systems and represent a fundamental eld of research in many-body physics. Prominent examples are the Anderson orthogonality catastrophe and the Kondo effect. In both cases, the impurity is much heavier than the constituents of the interacting environment. If the mass of the impurity is comparable to the surrounding particles, we have a mobile impurity. These systems are usually harder to solve, as evident in the case of lattice polarons, which were rst proposed in 1933 by Lev Landau. A complex but accurate description was found years later in 1955 by Richard Feynman. In recent years, strong coupling between single, mobile quantum impurities and a fermionic bath was realized in cold atoms. The interaction results in the formation of new quasiparticles called Fermi polarons. In contrast to other mobile quantum impurities such as lattice polarons, Fermi polarons can be described with a simple and quantitatively accurate model, which renders them an especially attractive eld of research of many-body physics. In this work, we report the observation of Fermi polarons in a solid state environment, namely a new class of semiconductors called transition metal dichalcogenides (TMDs). TMDs consisting of the transition metal Tungsten or Molybdenum and the chalcogenide Sulphur or Selenium are semiconductors. In the monolayer limit, they feature a direct bandgap, a large Coulomb interaction and a large effective electron and hole mass as compared to GaAs. In combination with the two-dimensional con nement, these result in a large binding energy of the exciton. As a consequence, the exciton remains a rigid particle even when it is surrounded by a two-dimensional electron system (2DES) with a large electron density. When the exciton is surrounded by a 2DES, a second resonance emerges in the optical spectrum. Previously, this resonance was attributed to the trion, a bound state of two electrons and a hole. In this dissertation, we demonstrate that this emerging red-shifted resonance has to be described as a Fermi polaron. Thanks to the large binding energy of excitons in TMDs, we can test the predictions of our model qualitatively and quantitatively for a large range of electron densities. For our experimental investigations, we employ cavity quantum electrodynamics in a zero-dimensional, tunable micro-cavity to investigate the optical spectrum of the TMD monolayer for different electron densities. The possibility to reduce the cavity length to a few wavelengths allows the formation of polaron-polariton modes. The strong light-matter coupling cannot be explained with the trion model, and provides solid evidence for the validity of the Fermi polaron model to describe optical resonances in a 2DES.

Molecular polaritons, dressed light-molecular exciton quasiparticles, have different optical properties compared to excitons. Over the past few decades there has been extensive theoretical understanding and experimental demonstration on how the physical and chemical behaviour of exciton are modified in confined optical cavity. Qualitative and quantitative understanding of nonlinear polariton interactions are at the heart of open questions in the field. These nonlinearities manifests themselves as polariton-polaritons scattering matrices in the lowest order. Nonlinear spectroscopic techniques, like double quantum coherence (DQC), can be used to study these nonlinearities such as vibrational anharmonicities, on-site interactions etc. A quasiparticle representation of coherent multidimensional optical signals for molecular polaritons in confined cavity (in both time and frequency domain) is developed allowing for direct observations of these polariton-polariton scattering terms that are explicitly present in the nonlinear signals. In spirit of understanding anharmonicities, sum over state (SOS) method is employed to study modification of vibrational structure of ground electronic states for Amide-I and II motifs of N-methylacetamide due to varying cavity coupling strengths is done. Again, DQC signals are studied for illustrations. Coherent pumping of molecular polaritons introduces several complications in extrating full structural informaion due to loss of strong coupling, non-equilibrium thermal excitations, phase-space filling factors (causing the polaritons to deviate from usual bosonic nature) and deviations from Hopfield coefficients, to name a few. Compelling remedies to the last two difficultes is proposed using theoretical framework to redefine the interacting polariton problem into a interacting deformed polaritons form. Discrepancies due to both phase-filling factors and deviations from Hopfield coefficients are encoded in a single deformation parameter $q$. A modified DQC is derived and an unique measure for effective enhancement of this signal is presented as a function of number of polaritons. This is used to investigate collective effects due to polariton-polariton scattering. Applications are made for varying number of chromophores (for $N_p \in \{1,2,\cdots,15\}$) picked from a monomer of Light Harvesting Complex II (LHCII). It is shown that polariton-polariton scattering for non-bosonic polaritons could bring the middle polariton branches (MPBs), which are usually embedded deep within the so called ``dark-state", to light.

The confinement of a semiconductor material to an optical microcavity leads to an inherent coupling between light and matter. Depending on the lifetime of the excited state of the semiconductor (the exciton) and the cavity photon, two distinct regimes of interaction are possible. The system is said to be weakly coupled if either the exciton or the cavity photon decay before the two species interact. Weak exciton-photon coupling results in a modification of the exciton lifetime, the spectral shape, and the angular dispersion of emission from the microcavity. Conversely, when the lifetimes of the exciton and cavity photon are long enough so that an interaction occurs prior to either state decaying, the regime of strong exciton-photon coupling is realized. The timescale for coupling is the Rabi period, which depends on exciton and cavity parameters including the exciton oscillator strength and transition linewidths. The eigenstates of the strongly coupled system are known as microcavity polaritons. Microcavity polaritons have unique properties arising from their mixed exciton-photon character, permitting the realization of novel optoelectronic devices. Organic semiconductors are attractive for application in strongly coupled systems due to their large exciton binding energy (~1 eV), which permits a robust coupled state that is stable at room temperature and under electrical excitation. In addition, organic semiconductors exhibit large exciton oscillator strengths (~1015 cm-2) resulting in a strong interaction between the cavity photon and the exciton. We aim to better the understanding of polaritons in organic semiconductor microcavities to push the field towards novel optoelectronic devices.

Metal-insulator-metal (MIM) plasmonic waveguides have been proposed for highly integrated subwavelength structures. In this work, waveguiding and coupling of surface plasmon polaritons (SPPs) within finite planar MIM plasmonic waveguides are examined both theoretically and experimentally. Gain (dye-doped polymer)-assisted MIM waveguides and terahertz quantum cascade laser MIM waveguides are numerically analyzed. The numerical analysis of finite planar MIM waveguides using the transfer matrix formalism reveals both bound and leaky SP modes: three lowest energy bound modes and the highest energy mode consisting of non-radiative (bound) and radiative (leaky) portions separated by a spectral gap at the light line. The leaky regime is further divided into antenna and reactive mode regions. Spatial dispersion effect on the SH mode yields a reduced wave vector in its dispersion curve and an increased propagation loss. The MIM radiative SPPs are probed using attenuated total reflection in the Kretschmann configuration and using free space coupling. Both single- and double-sided leaky waves are analyzed. The leaky wave dispersion relation and its antenna mode radiation pattern are determined through both angle- and wavelength-dependent reflectance of TM polarized free space incident light. The inclusion of a dye-doped polymer into realistic finite MIM plasmonic waveguides is analyzed. The propagation of three bound SP modes, each within respective optimized symmetric glass-Ag-Rh6G/PMMA-Ag-glass waveguides, is calculated for core material exhibiting optical gain at 594 nm. The critical gain coefficients for lossless propagation of these three bound SP modes are determined. Only lossless propagation of the SH mode is predicted. For MIM structure with gain in adjacent medium in ATR geometry, the reflectance and energy flux distribution at resonance conditions versus gain coefficients are examined. The waveguide loss, confinement factor and threshold gain for terahertz quantum cascade laser SP waveguides are modeled from 2 - 7 THz. The effects of plasma layer thickness, plasma doping and substrate thickness and the effects of active region thickness are investigated for semi-insulating surface-plasmon and metal-metal waveguides, respectively. A surface emitting quantum cascade laser SP leaky waveguide are proposed, with emission properties controlled by varying plasma layer thickness.

"There is plenty of room at the bottom." This bold and prophetic statement from Nobel laureate Richard Feynman back in 1950s at Cal Tech launched the Nano Age and predicted, quite accurately, the explosion in nanoscience and nanotechnology. Now this is a fast developing area in both science and technology. Many think this would bring the greatest technological revolution in the history of mankind. To understand electronic and optical properties of nanostructures, the following problems have been studied. In particular, intensity of mid-infrared light transmitted through a metallic diffraction grating has been theoretically studied. It has been shown that for s-polarized light the enhancement of the transmitted light is much stronger than for p-polarized light. By tuning the parameters of the diffraction grating enhancement can be increased by a few orders of magnitude. The spatial distribution of the transmitted light is highly nonuniform with very sharp peaks, which have the spatial widths about 10 nm. Furthermore, under the ultra fast response in nanostructures, the following two related goals have been proved: (a) the two-photon coherent control allows one to dynamically control electron emission from randomly rough surfaces, which is localized within a few nanometers. (b) the photoelectron emission from metal nanostructures in the strong-field (quasistationary) regime allows coherent control with extremely high contrast, suitable for nanoelectronics applications. To investigate the electron transport properties of two dimensional carbon called graphene, a localization of an electron in a graphene quantum dot with a sharp boundary has been considered. It has been found that if the parameters of the confinement potential satisfy a special condition then the electron can be strongly localized in such quantum dot. Also the energy spectra of an electron in a graphene quantum ring has been analyzed. Furthermore, it has been shown that in a double dot system some energy states becomes strongly localized with an infinite trapping time. Such states are achieved only at one value of the inter-dot separation. Also a periodic array of quantum dots in graphene have been considered. In this case the states with infinitely large trapping time are realized at all values of inter-dot separation smaller than some critical value.

The theory of impulsive stimulated Raman scattering (ISRS) is extended to include the generation and detection of coherent phonons in the opaque regime. A quantum mechanical calculation yields an expression for the frequency-dependent Raman susceptibility appropriate for impulsive excitation. Expressions for the time-dependent phonon amplitude and relative change in transmissivity DeltaT/T and reflectivity Delta R/R are derived for two-band processes and important differences between stimulated and spontaneous Raman scattering are discussed. Experiments in which impulsively-stimulated coherent phonons were resonantly excited are presented. Continuously tunable pulses produced by an optical parametric amplifier were used in a pump-probe geometry. Changes in the reflectivity induced by the A1g phonon in the vicinity of the E'2 gap at 1.95 eV in the semimetal Sb and changes in the transmissivity induced by the A'1 phonon in the vicinity of the fundamental gap at 2.1 eV in the semiconductor GaSe were measured. Calculations of Delta R/R and DeltaT/T based on the theory of ISRS compare well with the experimental results. An investigation of coherent polaritons in the optically isotropic semiconductor ZnSe at 10 K is presented. The polaritons were generated with a single ultrafast pulse and their dispersion was measured for wave vectors in the range 1370 cm-1 to 2780 cm-1 by varying the central pulse energy in the range 1.97 eV to 2.3 eV. Phase matching was achieved due to the large dispersion in the index of refraction. The observed monotonic increase in decay rate as the wave vector decreases is attributed to inhomogeneous broadening and propagation effects. Observations of several inverted peaks and plateaus in the transmissivity as a function of delay in the semiconductor AS2Se3 are discussed. Interactions between the incident probe pulse and co- and counter-propagating internally-reflected pump pulses account for the inverted peaks and plateaus, respectively. Based on phase matching considerations and the relative strength of various nonlinear processes, it is determined that four-wave mixing, two-photon absorption (TPA) and photon-photon scattering (PPS) contribute to the inverted peaks and that TPA and PPS are responsible for the plateaus. A calculation of the change in transmissivity associated with the plateaus agrees well with the data.