Phd Defence by Sushil Tandukar

Title: Hybrid micro-cavity lasers based on high-Q grating resonator


Principal supervisor:  Prof. Il-Sug Chung
Co-supervisor: Dr. Luisa Ottaviano

Evaluation Board
Senior Researcher Lars H. Frandsen, Fotonik
Professor Mattias Hammar, Royal Institute of Technology in Stockholm
Research Scientist Guilhem Almuneau, CNRS,

Master of the Ceremony
Elizaveta Semenova

Si-photonics based optical interconnects is one promising candidate to overcome the bottleneck posed by electrical interconnects for future on-and off-chip data communication in high performance computing system. One key issue with Si-photonic based optical interconnects, however, is that an energy-efficient, high-bandwidth, and high-speed laser is missing. This challenge has motivated researchers around the world to demonstrate various innovative Si-on-chip light sources. Within this realm, we have experimentally demonstrated high speed Si-on-chip long wavelength lasers based on the high-Q resonance mode supported by high contast grating (HCG) and hybrid grating (HG). These lasers are named as HCG-resonator laser and HG-resonator laser, respectively. Apart from surface emission these lasers can also have functionality of outputting light laterally into an Si-waveguide for on-chip interconnect application. In addition, by simply changing the grating parameter, different lasing wavelength emission can be achieved for both designs. This property is beneficial for wavelength-division-multiplexing (WDM) technique, where different optical signal with slightly different wavelengths are combined in a single optical waveguide. As a result high bandwidth data transmission can be achieved. With such properties of HCG- and HG-resonator, both lasers are promising light source candidates for future optical interconnects application.

The HCG-resonator laser is experimentally demonstrated by integrating submicron III-V membrane including active layers onto the SOI substrate using an ultra-thin BCB bonding process. The grating on the III-V laser is patterned using e-beam lithography and dry etching. Here, the III-V grating itself acts as a mirror-less resonator cavity and the surface lasing emission achieved under optical pumped operation has been tuned from 1546nm to 1591nm by simply changing the III-V grating bar width. The laser emitting at 1568nm showed the lowest lasing threshold at absorbed pump power of 36 μW corresponding to an approximated current injection of 0.27 mA. Also, key signature of demonstrated HCG-resonator lasers, is that it supports closely spaced multiple lasing transverse modes with similar output power level with wavelength spacing of around 0.4–0.5 nm. During small signal analysis, an additional resonance peak, known as photon-photon resonance (PPR), lying close to 40GHz is observed. This is due to coupling between very closely spaced lasing sub-transverse modes. Such higher order resonance peak can boost the direct modulation bandwidth of the lasers beyond the limitations set by carrier-photon resonance (CPR).

The HG-resonator laser is experimentally demonstrated by bonding sub-micron III-V membrane with active layer onto the bottom Si grating pattern formed on SOI substrate using low temperature direct wafer bonding (DWB). Here, the combination of the III-V membrane and Si grating forms the resonator cavity. Here, the bottom Si grating can also enable routing of the light laterally into the adjoined Si-waveguide. The surface lasing emission achieved under optical pumping operation is single mode and is tuned from wavelength of 1529nm to 1554nm by simply changing the Si grating bar width. The laser emitting at 1539nm showed the smallest lasing threshold at absorbed pumped power of 238 μW, corresponding to an estimated current injection level of 1.85 mA. During small single analysis, PPR resonance peak at around 35 GHz is observed which is expected due to the optical feedback from reflection at the III-V terminating section close to the waveguide. By butt-coupling a multi mode fibre onto the cleave Si-waveguide an inplane emission is also measured.


tor 28 mar 19
13:30 - 16:30


DTU Fotonik



Lyngby Campus
Building 308, Auditorium 13