Refractive Index Engineering

Refractive index engineering (RI_Eng) is a generic and comprehensive methodology for constructing integrated photonic circuits in which electrooptic devices, optical components, and photonic structures are interconnected by a mesh of waveguides, and operate in unison to implement complex functions.

RI_Eng exploits the fact that light ions that are incident on a substrate of oxygen perovskites form within the depth of the substrate an amorphous layer with reduced refractive index. The depth of the layer depends on the energy of the implanted ions and the change in the refractive index depends on type of ions, their fluence, and the density of the substrate (Figure 1). An overview of underlying principles of RI_Eng can be found in [REF8].

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Figure 1: left: the amorphous layer and the qualitative representation of the respective refractive index profile produced in KLTN by the implantation of a dosage of 10^16 cm^-2 of alpha particles at 2.3 MeV; right: the view of the output plane of the sample implanted by carbons at two discrete energies of 30 and 40 MeV while the input plane was illuminated by the white light source of the microscope vs. while the light at 670 nm coupled directly into the waveguide cladded by the two resulting amorphous layers.

The essence of the method is to harness spatially selective amorphization processes for constructing complex 3D pre-designed structures with sub-wavelength features and reduced refractive index within the volume of the substrate. These structures constitute the basic building blocks for fabricating electrooptic devices, nanophotonic structures and optical components as well as their interconnecting network of waveguides.  
As such RI_Eng adopts the philosophy of microelectronic VLSI technology and exports it to the optoelectronic arena. 

An example of an electrooptical device fabricated by RI_Eng techniques is the channel waveguide array. It is illustrated in Figure 2 and described in detail in [REF18].

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Figure 2: left: Schematic description of the channel array; right: the view of the front end of the channel waveguide array (3.5 µm  period) illuminated from the back plane by a white light source.

As a generic method for constructing integrated photonic circuits that operate in the visible spectral range, RI_Eng has the potential to provide the physical chassis for lab-on-a chip applications in which a broad range of spectroscopic methods are combined with microfluidic capabilities for processing minute quantities of chemical and biological substances. It can also provide the physical chassis for cost effective implementations of electroholographic wavelength selective switches.