PhD Defence by Alireza Taghizadeh

Theoretical Investigation of Subwavelength Gratings and Vertical Cavity Lasers Employing Grating Structures



Assoc. Prof. Il-Sug Chung, DTU Fotonik



Prof. Jesper Mørk, DTU Fotonik


Evaluation Board

Assoc. Prof. Andrei Lavrinenko, DTU Fotonik

Prof. Geert Morthier, University of Gent, Belgium

Prof. Mattias Hammar, KTH, Sweden


Master of the Ceremony

Assoc. Prof. Sanshui Xiao, DTU Fotonik



This thesis deals with theoretical investigations of a newly proposed grating structure, referred to as hybrid grating (HG) as well as vertical cavity lasers based on the grating reflectors. The HG consists of a near-subwavelength grating layer and an un-patterned high-refractive-index cap layer.  Though both sides of the grating layer are not surrounded by low refractive-index materials as in high-index-contrast gratings (HCGs), the HG can provide a near-unity reflectivity over a broader wavelength range than HCGs, or work as a resonator with a quality (Q) factor as high as 109. The physics behind these reflector and resonator properties are studied thoroughly. A HG structure comprising a III-V cap layer with a gain material and a Si grating layer enables the realization of a compact vertical cavity laser integrated on Si platform, which has a superior thermal property and fabrication feasibility than the HCG-based ones.  Furthermore, the concept of cavity dispersion in vertical cavities is introduced and its importance in the modal properties is numerically investigated. The dispersion curvature of a cavity mode is interpreted as the effective photon mass of the cavity mode. In a vertical cavity based on a HCG or HG reflector, this effective photon mass can be engineered by changing the grating parameters, which is not the case in a vertical cavity based on distributed Bragg reflectors. This engineering capability enables us to form various photonic heterostructures in lateral directions, which is analogous to electronic quantum wells in conduction or valence bands. Several interesting configurations of heterostructures have been investigated and their potential in fundamental physics study and applications are discussed.


Tue 26 Apr 16
13:30 - 16:30


DTU Fotonik


Lyngby Campus
Building 306, auditorium 38