Angelica Simbula

Integrated Source of Spectrally Filtered Correlated Photons for Large-Scale Quantum Photonic Systems

We demonstrate the generation of quantum-correlated photon pairs combined with the spectral filtering of the pump field by more than 95 dB on a single silicon chip using electrically tunable ring resonators and passive Bragg reflectors. Moreover, we perform the demultiplexing and routing of signal and idler photons after transferring them via an optical fiber to a second identical chip. Nonclassical two-photon temporal correlations with a coincidence-to-accidental ratio of 50 are measured without further off-chip filtering. Our system, fabricated with high yield and reproducibility in a CMOS-compatible process, paves the way toward large-scale quantum photonic circuits by allowing sources and detectors of single photons to be integrated on the same chip.

Energy correlations of photon pairs generated by a silicon microring resonator probed by Stimulated Four Wave Mixing

Compact silicon integrated devices, such as micro-ring resonators, have recently been demonstrated as efficient sources of quantum correlated photon pairs. The mass production of integrated devices demands the implementation of fast and reliable techniques to monitor the device performances. In the case of time-energy correlations, this is particularly challenging, as it requires high spectral resolution that is not currently achievable in coincidence measurements. Here we reconstruct the joint spectral density of photons pairs generated by spontaneous four-wave mixing in a silicon ring resonator by studying the corresponding stimulated process, namely stimulated four wave mixing. We show that this approach, featuring high spectral resolution and short measurement times, allows one to discriminate between nearly-uncorrelated and highly-correlated photon pairs.

Efficient continuous-wave nonlinear frequency conversion in high-Q gallium nitride photonic crystal cavities on silicon

We report on nonlinear frequency conversion from the telecom range via second harmonic generation (SHG) and third harmonic generation (THG) in suspended gallium nitride slab photonic crystal (PhC) cavities on silicon, under continuous-wave resonant excitation. Optimized two-dimensional PhC cavities with augmented far-field coupling have been characterized with quality factors as high as 4.4 × 104, approaching the computed theoretical values. The strong enhancement in light confinement has enabled efficient SHG, achieving a normalized conversion efficiency of 2.4 × 10−3 W−1, as well as simultaneous THG. SHG emission power of up to 0.74 nW has been detected without saturation. The results herein validate the suitability of gallium nitride for integrated nonlinear optical processing.

Enhanced Telecom Emission from Single Group-IV Quantum Dots by Precise CMOS-Compatible Positioning in Photonic Crystal Cavities

Efficient coupling to integrated high-quality-factor cavities is crucial for the employment of germanium quantum dot (QD) emitters in future monolithic silicon-based optoelectronic platforms. We report on strongly enhanced emission from single Ge QDs into L3 photonic crystal resonator (PCR) modes based on precise positioning of these dots at the maximum of the respective mode field energy density. Perfect site control of Ge QDs grown on prepatterned silicon-on-insulator substrates was exploited to fabricate in one processing run almost 300 PCRs containing single QDs in systematically varying positions within the cavities. Extensive photoluminescence studies on this cavity chip enable a direct evaluation of the position-dependent coupling efficiency between single dots and selected cavity modes. The experimental results demonstrate the great potential of the approach allowing CMOS-compatible parallel fabrication of arrays of spatially matched dot/cavity systems for group-IV-based data transfer or quantum optical systems in the telecom regime.

Low-power four-wave mixing in porous silicon microring resonators

We report the measurement of low-power continuous-wave four-wave mixing in porous silicon microring resonators operating in the 1550 nm telecom band. Resonantly enhanced stimulated four-wave mixing has been measured in rings with 25 μm radius and quality factor around 5000 for pump powers as low as a few hundreds of microwatts. A waveguide nonlinear parameter γ = 20 W–1 m−1 has been determined. These results suggest further research on porous silicon for low-power nonlinear optics, possibly taking advantage of its tunable porosity.

Demonstration of continuous-wave second and third harmonic generation in high-Q gallium nitride photonic crystal cavities

Wide bandgap semiconductors such as gallium nitride (GaN) are essential constituents of future optical circuits, as their optical response can accommodate a broad wavelength range while suppressing two-photon absorption and free-carrier absorption effects that are encountered with silicon (Si) structures. Their direct wide bandgap is also favourable for the incorporation of active elements. Furthermore, nonlinear optical processes can be harnessed by exploiting the higher-order susceptibility tensors of the crystal structure to achieve advanced light control modalities, enabling all-optical processing and the generation of entangled photon states. We will report on nonlinear frequency conversion from the telecom range via second harmonic generation (SHG) and third harmonic generation (THG) in suspended gallium nitride slab photonic crystal (PhC) cavities on silicon, under continuous-wave resonant excitation. Genetic optimization is applied to sweep parameter space for the highest cavity quality factors, and simultaneously accounting for power in-coupling. While there is a clear trade-off theoretically between coupling efficiency and Q-factor for a given cavity design, the upper limit on the Q-factor that is imposed by loss channels, given the disorder figure of current fabrication technology, makes room for introducing improved far-field coupling to enhance nonlinear processes without sacrificing the experimentally achievable light confinement. Far-field coupling is addressed through various PhC cavity designs, which enable the excitation of the fundamental mode with a Gaussian beam. Optimized two-dimensional PhC cavities with increased far-field coupling have been characterized with quality factors as high as 44′000, approaching the computed theoretical values. The strong enhancement in light confinement has enabled second harmonic generation (SHG) under continuous-wave excitation, with up-conversion from both 1300 nm and 1550 nm wavelength bands, confirmed by spectral and power dependence measurements. At 1550 nm, normalized SHG conversion efficiency as large as 2.4×10−3 W−1 are measured as well as simultaneous THG. SHG emission power of up to 0.74 nW has been detected without saturation

High-Q/V photonic crystal cavities realized by an effective Aubry-André-Harper bichromatic potential

We report on the realization of high-Q/V silicon photonic crystal cavities with resonance wavelengths in the telecom window around 1.55 µm. The cavity designs are based on an effective Aubry-Andrè-Harper bichromatic potential, defined by the superposition of two one-dimensional lattices with an incommensurate ratio between their periodicity constants. This peculiar confinement mechanism allows to achieve an ultra-high-Q factor and diffraction-limited mode volume. Several photonic crystal nanocavities in a silicon membrane geometry have been realized with measured Q-factors in the one million range, as determined by resonant scattering experiments. The generality of the proposed designs and their easy implementation and scalability make these results particularly interesting for realizing highly performing photonic nanocavities on different material platforms and operational wavelengths.

Efficient harmonic generation in high-Q gallium nitride photonic crystal cavities on silicon

Frequency conversion from the telecom range into the visible is demonstrated in suspended GaN slab photonic crystal (PhC) cavities through second harmonic generation (SHG) and third harmonic generation (THG), under continuous-wave resonant excitation. PhC cavities based on L3 and H0 designs are implemented with embedded injectors, the latter shown in Fig. 1a, enabling improved coupling to the fundamental cavity mode using far-field excitation. Q-factors as high as 2.2×105 near λ=1550 nm were obtained through automated optimization runs for the localized cavity modes [1], based on guided-mode expansion computations. To confirm scalability across the telecom range, two design wavelengths at 1300 nm and 1550 nm were targeted for the devices.

Measurement of energy correlations of photon pairs generated in silicon ring resonators

With unprecedented resolution we measure the joint spectral density of photon pairs that would be generated by spontaneous four-wave-mixing in a silicon ring resonator, and show how the quantum correlations can be tailored.