Abstract
We present new kind of microwave phase shifters (MPS) based on dispersion of PbS colloidal quantum dots (QDs) in commercially available photoresist SU8 after a ligand exchange process. Ridge PbS-SU8 waveguides are implemented by integration of the nanocomposite in a silicon platform. When these waveguides are pumped at wavelengths below the band-gap of the PbS QDs, a phase shift in an optically conveyed (at 1550 nm) microwave signal is produced. The strong light confinement produced in the ridge waveguides allows an improvement of the phase shift as compared to the case of planar structures. Moreover, a novel ridge bilayer waveguide composed by a PbS-SU8 nanocomposite and a SU8 passive layer is proposed to decrease the propagation losses of the pump beam and in consequence to improve the microwave phase shift up to 36.5° at 25 GHz. Experimental results are reproduced by a theoretical model based on the slow light effect produced in a semiconductor waveguide due to the coherent population oscillations. The resulting device shows potential benefits respect to the current MPS technologies since it allows a fast tunability of the phase shift and a high level of integration due to its small size.
Highlights
Microwave photonics (MWP) [1] enables the broadband, interference-immune and low loss transport and processing of radiofrequency and millimeter-wave signals conveyed by optical carriers opening the possibility of multiple functionalities which are key in a considerable number of application fields such as fiber-wireless communication and 5G networks, radar, signal filtering, sensing or instrumentation [2]
In this work a novel approach to implement a microwave phase shifters (MPS) based on a PbS-SU8 nanocomposite is reported, which is used to fabricate waveguides integrated into silicon platforms
When PbS quantum dots (QDs) are pumped below their absorption bandgap they are able to modify the group refractive index of an optical carrier at 1550 nm, producing in consequence a shift in the phase of the microwave signal
Summary
Microwave photonics (MWP) [1] enables the broadband, interference-immune and low loss transport and processing of radiofrequency and millimeter-wave signals conveyed by optical carriers opening the possibility of multiple functionalities which are key in a considerable number of application fields such as fiber-wireless communication and 5G networks, radar, signal filtering, sensing or instrumentation [2]. An additional advantage of MWP systems is that they enable to perform RF functionalities, such as fast tunability or reconfigurability, that are either complex or not even possible to achieve with conventional RF architectures [3] These potential practical functionalities include as well arbitrary waveform generation, tunable and reconfigurable filtering, optoelectronic oscillators, analog to digital conversion or beam steering, among others. SBS based phase shifters require both long fiber sections or high pump powers [16], SOA based architectures have distortion and RIN noise limitations while the SOI ring and FBG approximations feature a narrowband behavior To overcome those constraints here we proposed a novel technology approach that is able to integrate photonic MPS with organic waveguides implemented in silicon platforms. A refractive index model of the PbS-SU8 nanocomposite is implemented, where main results are explained by considering the absorption saturation produced in the QDs and developing a model able to fit accurately the experimental data
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