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In this technical review we give an introduction to optical spectroscopy for layered materials as a powerful, non-invasive tool to access details of the electronic band structure and crystal quality. Potential applications in photonics and optoelectronics are based on our understanding of the light-matter interaction on an atomic monolayer scale. Here atomically thin transition metal dichalcogenides, such as MoS2 and WSe2, are model systems for layered semiconductors with a bandgap in the visible region of the optical spectrum. They can be assembled to form heterostructures and combine the unique properties of the constituent monolayers. We review the working principles of micro-photoluminescence spectroscopy and optical absorption experiments. We discuss the physical origin of the main absorption and emission features in the optical spectra and how they can be tuned. We explain key-aspects of practical set-ups for performing experiments in different conditions such as variable temperatures or in applied magnetic fields and how parameters such as detection spot size and excitation laser wavelength impact the optical spectra. We describe the important influence of the direct sample environment, such as substrates and encapsulation layers, on the emission and absorption mechanisms. A survey of optical techniques that probe the coupling between layers and analyse carrier polarisation dynamics for spin- and valleytronics is provided.
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Nonlinear optical phenomena are widely used for the study of semiconductor materials. The paper presents an overview of experimental and theoretical studies of excitons by the method of optical second and third harmonics generation in various bulk semiconductors (GaAs, CdTe, ZnSe, ZnO, Cu$_2$O, (Cd,Mn)Te, EuTe, EuSe), and low-dimensional heterostructures ZnSe/BeTe. Particular attention is paid to the role of external electric and magnetic fields that modify the exciton states and induce new mechanisms of optical harmonics generation. Microscopic mechanisms of harmonics generation based on the Stark effect, the spin and orbital Zeeman effects, and on the magneto-Stark effect specific for excitons moving in an external magnetic field are considered. This approach makes it possible to study the properties of excitons and to obtain new information on their energy and spin structure that is not available when the excitons are investigated by linear optical spectroscopy. As a result of these studies, a large amount of information was obtained, which allows us to conclude on the establishing of a new field of research - exciton spectroscopy by the method of optical harmonics generation.
Rare earth doped crystals are promising systems for quantum information processing. In particular paramagnetic rare earths could be used to build coherent interfaces with optical and microwave photons. In addition, isotopes with non zero nuclear spins could provide long lived states for quantum state storage and processing. Yb3+ is particularly interesting in this respect since it is the only paramagnetic rare earth with a spin 1/2 isotope, which corresponds to the simplest possible level structure. In this paper, we report on the optical and magnetic properties of Yb3+ in the two sites of Y2SiO5, a commonly used crystal for quantum applications. We measured optical inhomogeneous linewidths, peak absorption coefficients, oscillator strengths, excited state lifetimes and fuorescence branching ratios. The Zeeman tensors were also determined in the ground and excited states, as well as the ground state hyperfine tensor for the 171Yb3+ (I = 1=2) isotope. These results suggest that Yb3+:Y2SiO5 is a promising material for applications like solid state optical and microwave quantum memories.
We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2 near its exciton resonance. The observed differential reflectance signals exhibit signatures of strong many-body interactions including the exciton-exciton interaction and free carrier induced band gap renormalization. The exciton-exciton interaction results in a resonance blue shift which lasts for the exciton lifetime (several ps), while the band gap renormalization manifests as a resonance red shift with several tens ps lifetime. Our model based on the many-body interactions for the nonlinear optical susceptibility fits well the experimental observations. The power dependence of the spectra shows that with the increase of pump power, the exciton population increases linearly and then saturates, while the free carrier density increases superlinearly, implying that exciton Auger recombination could be the origin of these free carriers. Our model demonstrates a simple but efficient method for quantitatively analyzing the spectra, and indicates the important role of Coulomb interactions in nonlinear optical responses of such 2D materials.
The embedding of a given point set with non-crystallographic symmetry into higher-dimensional space is reviewed, with special emphasis on the Minkowski embedding known from number theory. This is a natural choice that does not require an a priori construction of a lattice in relation to a given symmetry group. Instead, some elementary properties of the point set in physical space are used, and explicit methods are described. This approach works particularly well for the standard symmetries encountered in the practical study of quasicrystalline phases. We also demonstrate this with a recent experimental example, taken from a sample with square-triangle tiling structure and (approximate) twelvefold symmetry.
The sensitive correlation between optical parameters and strain in Mo$S_2$ results in a totally different approach to tune the optical properties. Usually, an external source of strain is employed to monitor the optical and vibrational properties of a material. It is always challenging to have a precise control over the strain and its consequences on material properties. Here, we report the presence of a compressive strain in Mo$S_2$ crystalline powder and nanosheets obtained via the process of ball-milling and probe sonication. The diffraction peaks in the X-ray diffraction pattern shift to higher 2$theta$ value implying a compressive strain that increases with the processing time. The absorption spectra, photoluminescence and Raman modes are blue-shifted w.r.t the bulk unprocessed sample. The observed blue-shift is attributed to the presence of compressive strain in the samples. Whereas in thin nano-sheets of Mo$S_2$, it is very likely that both quantum confinement as well as strain result in the observed blue-shift. These results indicate that by optimizing the processing conditions and/or time, a strain of desired amount and hence tunable shift in optical properties of material can be achieved.
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