CMOS Compatible Optical Leaky Wave Antennas (OLWAs) and Devices

Sponsored by NSF – ECCS

MOTIVATION

Ø What are Leaky Wave Antennas?

o Leaky wave antennas (LW antennas) are travelling wave antennas that make use of the fast guided waves to provide radiation. The LW radiates as it propagates through the waveguide

o Fast-waves are also named leaky waves, which mean the phase constant of the guided wave is smaller than the free-space wave number. This type of waves ‘leak’ out of the wave guide and can be utilized to design very directive antennas

Ø Why “Optical”?

o The optical frequency range covers the electromagnetic spectrum extending from infrared (IR) to ultraviolet (UV)

o We are focusing on radiation at 1550 nm, since it is the optical communication wavelength

Ø Why SOI / CMOS Compatibility?

o It is basic chip fabrication technology in use, accessible and cheap

o It allows for reduced parasitic capacitance and good mode confinement with low optical loss

o It is compatible for integration with CMOS electronics and can be tuned electronically to control far field radiation

BASIC ANTENNA GEOMETRY AND RADIATION CAPABILITIES

	Proposed OLWA	Directive radiation for a specific design [2]
Silicon nitride waveguides comprising periodic Si perturbations for the generation of leaky wave optical radiation. Image taken from [2]		Normalized far field pattern (agreement between theory and full-wave simulations.) Image taken from [2]

Very directive radiation is achieved. Beam width and beam angle are controlled by the LW phase propagation and attenuation constants [2]

TWO DIFFERENT WAYS FOR CONTROLLING THE RADIATION

Electronic Control Image taken from [2-3]	Optical Control Image taken from [4]
Ø Design and simulations of OLWA with electrical controlling capabilities Ø Carrier transportation model in PIN junction for low injection case and high injection case	Ø Design and simulations of OLWA with optical controlling capabilities Ø A pump LASER is utilized to generate carriers inside the Si perturbations (i.e. LASER wavelength will be in correspondence of non-negligible absorption spectrum of Si)

Image taken from [4]

This simple mechanism currently does not provide great control of the radiated power at a single direction. We are currently considering inserting the OLWA into resonators to amplify the control

ENHANCEMENT OF RADIATION CONTROL BY INTEGRATING OLWA WITH A RESONATOR

Integrating OLWAs with an optical resonator (ring or a linear cavity resonator) is one way of achieving enhanced radiation control. In both cases, the variation of the refractive index and losses will imply less coupling to the cavity and also a weaker field in the cavity [5-7].

FABRICATION

Fabrication of antennas on silicon nitride on silicon: waveguides with strained dielectrics and strained semiconductors has been fabricated. Currently nitride thickness is being optimized for proposed antenna design.

Before we start patterning with e-beam lithography, we developed the fabrication steps for strained thick waveguide materials. In particular we use silicon on insulator, silicon on sapphire and silicon nitride on silicon to control etch profiles and waveguide smoothness for low loss operation. The difficulty of the process arises due to strain level that leads to uneven etching and high side wall roughness. Currently 4-step etching process has been adopted to overcome this limitation. In the worst case scenario of highly strained silicon on sapphire we achieve RMS surface roughness less than 25nm, Fig. 9. In the following steps we optimize the growth of waveguide grade silicon nitride on silicon to achieve 1μm thickness.