chaired by Sonia García Blanco/ Pepijn Pinkse
Cartesian Light Propagation in a 3D Crystal of Photonic-Bandgap Cavities
Sjoerd Hack (MACS & COPS)
Waveguide amplifiers based on potassium rare-earth double tungstates
Yean-Sheng Yong (OS)
Secure Quantum Key Distribution via a Multimode Fibre
Lyuba Amitonova (COPS & BMPI)
Chip-based hybrid lasers with record-high coherence
Youwen Fan (LPNO)
Cartesian Light Propagation in a 3D Crystal of Photonic-Bandgap Cavities, Sjoerd Hack (MACS & COPS)
We study the unconventional propagation of light through a three-dimensional (3D) superstructure of resonant cavities in a 3D photonic band gap crystal. Such a 3D crystal of cavities is the photonic analogue of the Anderson model that describes electronic and spin excitations . Here, we calculate the coupled-cavity modes.
We find that the dispersion is accurately described by a hopping Hamiltonian, wherein the light only hops in the Cartesian x-y-z directions and in the xz diagonal directions, which strongly differs from the usual Bloch wave propagation. The bandwidth of the dispersion depends strongly on direction, which is explained by negative xz-couplings.
Recently, our group has started to fabricate 3D crystals of cavities in inverse woodpile crystals . We predict how these unconventional modes of light can be excited in our optical experiments, by employing wavefront shaping.
 Anderson, Phys. Rev. 109, 1492 (1958)
 Grishina et al., Nanotechnology 26, 505302 (2015)
Waveguide amplifiers based on potassium rare-earth, Yean-Sheng Yong (OS)
The down-scaling of rare-earth-doped amplifier to chip-scale device length is beneficial for signal amplification within short-reach interconnects such as optical backplanes and for realization of novel active/passive integrated optical chips. Our group investigated the use of highly Yb3+-doped or Er3+-doped potassium rare-earth double tungstate, i.e. KRE(WO4)2, epitaxial layers for the fabrication of waveguide amplifiers. These active layers exhibit high transition cross-sections and excellent active-ion solubility which are favourable for achieving high gain per unit length. No sign of lifetime quenching was observed for Yb3+-doped samples with concentrations up to 57 at.% and Er3+-doped samples with concentrations up to 10 at.%. High optical gain of >800 dB/cm for 57 at.% Yb3+-doped layers at 981 nm wavelength and >10 dB/cm for 6 at.% Er3+-doped layers at 1534.75 nm wavelength were demonstrated. The gain performance of Er3+-doped KRE(WO4)2 amplifiers is limited by energy-transfer upconversion and excited-state absorption processes. As a result, higher Er3+ concentration does not lead to better gain. On the other hand, the gain in Yb3+-doped KRE(WO4)2 amplifiers is limited by pump-induced thermal effects, which may be originated from quantum defect as well as parasitic energy-transfer upconversion processes.
Secure Quantum Key Distribution via a Multimode Fibre, Lyuba Amitonova (COPS & BMPI)
Quantum cryptography, in theory, provides the highest possible level of communication security; however, real-life implementations put demanding requirements on the quality of the quantum light sources, detectors and other components limiting the communication distance, speed and security. Key elements of worldwide communication are single-mode optical fibers; however, they allow information to be encoded into the time-frequency domain only. The advent of multimode fibers in combination with wavefront shaping technique and quantum cryptography offer a powerful platform for a new type of quantum key distribution (QKD) protocols.
Here we demonstrate an original high-dimensional QKD protocol by encoding the spatial information carried by photons into guided modes of a multimode fiber. In contrast to other QKD approaches, the proposed method allows to decode the information, which is transferred in a deterministic manner, instantaneously during the transmission and thereby provides an unique platform for quantum secure direct communication. The multimode-fiber-based QKD maintains security in simple practical setup real-life implementations and makes commercial applications conceivable.
Chip-based hybrid lasers with record-high coherence, Youwen Fan (LPNO)
Tunable diode lasers are of great interest for both fundamental and technical applications. Of particular importance is their degree of coherence, which is usually the ultimate determining factor of performance in integrated photonic systems.
Here we report the experimental realization of a chip-based, wavelength-tunable InP-Si3N4 hybrid laser. The laser is formed by longitudinally coupling an InP diode chip and a Si3N4 waveguide chip. Via implementing a widely tunable feedback circuit (a chain of three microring resonators in Vernier configuration), any wavelength within the diode gain bandwidth (1500 nm to 1581 nm) becomes addressable. In measurements we observed that the significant optical roundtrip length of the feedback circuit (50 cm on a chip) imposes an extremely low intrinsic spectral bandwidth of about 300 Hz Hertz (quantum limited). This bandwidth is far below that of any other tunable hybrid laser, and thereby marks a new paradigm for achieving ultra-high coherence.