Experimental verification of photonic band-tail states and their use for shaping laser properties

Abstract

Shaping light to generate desired optical properties is one of the important topics in optics and photonics that has been studied for a long time. A complete control over the light intensity, polarization, frequency, phase, and even the spatio-temporal distribution of electromagnetic fields is the long-sought primary objective of light shaping, which can be the base technology for applied science and industry that handles the shape of light, leading to advanced optical functionalities and next generation photonic devices. The study of light shaping is considered to be the process of controlling the shape of light by manipulating the spatial and temporal optical properties of material, with understanding of electromagnetic properties of the medium in which light propagates. Historically, material properties of media have been a major methodology, which is represented by the dispersion relation on wavelength, birefringence on polarization, and nonlinearity. However, research on structural properties such as reflection, diffraction, and scattering at the interface, originated from the spatial arrangement of materials, is being actively carried out as well, consisting a large branch of modern photonics including photonic crystals, metamaterials, and topological photonics. The essence of studying light shaping is then to generate the structured light, using these material and structural methodologies, in order to improve an existing optical system and to develop new photonic devices. Thus, within a finite material pool, the issues of shaping light eventually result in a problem of the spatio-temporal arrangement of materials. The structure based on the periodic arrangement is used in many fields due to the intuitive design, but this approach is difficult to apply to optical systems which require complexity, because of the limited structural parameters. On the other hand, an optically disordered system which randomly arranges materials without specific restrictions provides vast degrees of structural freedom that increase according to the system size, but consumes a large amount of resources in order to predict optical properties and to form desired light shapes. That is, the structural degree of freedom and the predictability are complementary. In this thesis, a photonic crystal alloy is proposed as an ideal compromise that can easily predict and design optical properties while ensuring sufficient structural degrees of freedom to shape light with complexity corresponding to the real world. Here, the degree of freedom increases with the diversity of photonic atoms, however, the scattering strength at each lattice site can be controlled individually and independently to design the entire system in a pixelated scheme since the underlying crystalline structure is maintained. The spectral characteristics are then investigated to reveal that the eigenmode of the proposed system is the photonic band-tail state existing in the photonic band-gap. For this state, it is experimentally confirmed that the energy range in which the states are distributed is determined by the crystalline structure and the scattering strength, and that the spatial near-field distribution varies in a wide range, including the weak and strong localization, depending on the energy of the mode and the scattering strength of the structure. These observations prove the localization of the photonic band-tail state, which was theoretically predicted in 1987. In addition, the modal properties of the photonic band-tail states are distributed in a wide spectro-spatial range across the complete band-gap and from only a few lattices to the entire structure, which is a great advantage for light shaping. The band-tail laser, a conceptually novel laser device that uses the band-tail state as a resonant mode, is proposed, and the light shaping within a membrane is demonstrated by realizing the band-tail laser in a slab waveguide embedding InAsP/InP multiple-quantum-well structure. Using only the structural parameters of the photonic crystal alloy, monotonic control of modal density from multi-mode to single-mode, and precise manipulation of both the modal energy and modal extents of a single-mode operating band-tail laser are demonstrated. Furthermore, the near-field profile of a mode can be modulated in various shapes from the fundamental shape to high-order shapes including orbital angular momentum and spiral pattern. The design of light shaping, based on structural degrees of freedom in the band tail laser, is far more intuitive and effective than any disordered system, and the range of controllable modal properties and demonstrated shaping capabilities are superior to any known laser platform. The performance of band tail lasers is also comparable to the modern cavity lasers. Therefore, the photonic band-tail state and the band-tail laser, proposed in this thesis as a light shaping platform, could incorporate the currently known small library of lasing platform and even expand its boundary by realizing elaborate light shaping with various near-field shapes, which contributes to the development of various fields that deal with the shape of light.