Designing subwavelength-structured light sources

Abstract

The laser has long been established as the best possible optical source for fundamental studies and applications requiring high field intensity, single mode operation, a high degree of coherence, a narrow linewidth and short pulses. There are many applications that require lasers of varying frequencies, powers, and far-field properties. In science, the laser is a useful source in spectroscopy and microscopy, for investigating nonlinear optics phenomena and nuclear fusion. More commonly, we find them in barcode readers, laser pointers, and printers. They are also widely used for military, medical, and industrial applications. This thesis is focused on achieving new understanding of the principles and phenomena involved in the interaction of light with a variety of material systems, which will in turn guide the designs of compact lasers with feedback structures having features at the subwavelength-scale. The thesis begins by describing the interaction of light with an arbitrary complex material system, and implementing them into the electromagnetic model using two different theoretical techniques suitable for analyzing microstructured lasers: exact finite-difference time-domain calculations and a semi-analytic coupled-mode formalism. These methods are first applied to analyze lasing action in the photonic crystal (PhC) slabs. This class of lasers, commonly referred to as the photonic crystal surface-emitting lasers (PCSELs), can be integrated on-chip and is essentially the two-dimensional (2D) versions of the second-order distributed feedback lasers, where the higher quality factor lasing mode (dark Fano resonance) is selected through the symmetry mismatch to the free-space modes. The PCSELs have not only achieved the highest surface-emitting single-mode power but also the ability to control the shapes, polarizations and directions of their far-fields. However, as in all laser cavities, the lasing areas of PCSELs are limited by two fundamental constraints; a large area tends to promote both multi-mode and multi-area lasing. We propose to overcome both constraints to achieve single-mode PCSELs of larger areas, and thus higher output powers, by tuning the regular lasing bandedges of quadratic dispersions in typical PCSELs to form a single accidental Dirac cone of linear dispersion at the Brillouin zone center. Moreover, an additional frequency-locking phenomenon at the accidental point, with potentially high density of states, is analyzed. We demonstrate and distinguish experimentally the existence of the dark Fano resonances in a macroscopic 2D silicon nitride PhC slab consisting of a square array of holes. We characterize the passive PhC slab in terms of its resonant frequencies and radiation behaviors using temporal coupled-mode theory and symmetry considerations. We also realize lasing at a dark Fano resonance with diluted solutions of R6G molecules as the gain medium. Next, we turn our attention to the organic dye lasers whose high tunability in the visible wavelengths has attracted interests for many years due to their low-cost processing, flexible choice of substrates, and large emission cross sections that can cover the spectral region from ultraviolet to the near infrared. We investigate the laser dynamics in systems of sub-wavelength photonic structures consisting of organic dye molecules, including their photobleaching effects. Our analysis considers both the chemical properties of the dyes and optical properties of the cavities. We also systematically studied the feasibility of lasing under continuous-wave excitations in optically pumped monolithic organic dye lasers. This study suggests routes to realize an organic laser that can potentially lase with a threshold of only a few W/cm² Lastly, we investigate far-infrared (FIR) (~ 0.2 - 2 THz) laser emission from optically-pumped rotationally excited molecular gases confined in a metallic cavity. Terahertz radiation has already been used in packaging inspection for quality control, chemical composition analysis, and security screening. Submillimeter spatial resolution imaging and incredibly specific molecular recognition are other compelling uses for terahertz radiation. To apply terahertz radiation beyond laboratory or close range (< 10 m) applications, more powerful (> 100 mW) and efficient sources are required to see through highly attenuating obscurants (including the atmosphere). The fundamental limitations in the performances of FIR molecular gas lasers reside in the molecular gas physics due to the so-called vibrational bottleneck. We seek to overcome the resulting challenges through novel optical designs of the feedback structures. To undertake this task, we generalize previous works to allow for a realistic description of the spatio-temporal dynamics characterizing the molecular collisional and diffusion processes. This work expands the current understanding of lasing action in FIR gas lasers and, thus, could contribute to the development of a new class of terahertz sources able to operate efficiently at room temperature. The advent of quantum cascade lasers to replace CO₂ pump lasers may combine to produce truly compact submillimeter-wave laser sources in the near future.