Efficient generation of ultra-broadband parametric fluorescence using chirped quasi-phase-matched waveguide devices

Tailored second harmonic generation in Ti-diffused PPLN waveguides using micro-heaters

Frequency conversion based on χ(2) nonlinear optical interactions can be made very efficient in waveguide structures. Fabrication imperfections remain very often a limiting factor. They can induce strong distortions in the spectral shape and lower the efficiency. To overcome these imperfections a post-trimming method based on a cascade of ten micro-heaters on top of the waveguide along the interaction length is demonstrated. As an example, the second harmonic generation in a Ti-indiffused waveguide in periodically poled LiNbO3 is investigated. A tailoring of the spectral characteristics of the phase-matching curves could be successfully demonstrated.

Non-collinear generation of ultra-broadband parametric fluorescence photon pairs using chirped quasi-phase matching slab waveguides

Many optical quantum applications rely on broadband frequency correlated photon pair sources. We previously reported a scheme for collinear emission of high-efficiency and ultra-broadband photon pairs using chirped quasi-phase matching (QPM) periodically poled stoichiometric lithium tantalate (PPSLT) ridge waveguides. However, collinearly emitted photon pairs cannot be directly adopted for applications that are based on two-photon interference, such as quantum optical coherence tomography (QOCT). In this work, we developed a chirped QPM device with a slab waveguide structure. This device was designed to produce spatially separable (photon pair non-collinear emission) parametric fluorescence photon pairs with an ultra-broadband bandwidth in an extremely efficient manner. Using a non-chirped QPM slab waveguide, we observed a photon pair spectrum with a full-width-at-half-maximum (FWHM) bandwidth of 26 nm. When using a 3% chirped QPM slab waveguide, the FWHM bandwidth of the spectrum increased to 190 nm, and the base-to-base width is 308 nm. We also confirmed a generation efficiency of 2.4×106 pairs/(μW·s) using the non-chirped device, and a efficiency of 8×105 pairs/(μW·s) using the 3% chirped device under non-collinear emission conditions after single-mode fiber coupling. This is, to the best of our knowledge, the first report of frequency correlated photon pairs generation using slab waveguide device as a source. In addition, using slab waveguides as photon pair sources, we performed two-photon interference experiments with the non-chirped device and obtained a Hong-Ou-Mandel (HOM) dip with a FWHM of 7.7 μm and visibility of 98%. When using the 3% chirped device as photon pair source, the HOM measurement gave a 2 μm FWHM dip and 74% visibility.

High-depth-resolution imaging of dispersive samples using quantum optical coherence tomography

Quantum optical coherence tomography (QOCT) is a promising approach to overcome the degradation of the resolution in optical coherence tomography (OCT) due to dispersion. Here, we report on an experimental demonstration of QOCT imaging in the high-resolution regime. We achieved a depth resolution of 2.5 μm, which is the highest value for QOCT imaging, to the best of our knowledge. We show that the QOCT image of a dispersive material remains clear whereas the OCT image is drastically degraded.

Femtosecond laser writing of lithium niobate ferroelectric nanodomains

Lithium niobate (LiNbO₃) is viewed as a promising material for optical communications and quantum photonic chips^1,2. Recent breakthroughs in LiNbO₃ nanophotonics have considerably boosted the development of high-speed electro-optic modulators^3-5, frequency combs^6,7 and broadband spectrometers⁸. However, the traditional method of electrical poling for ferroelectric domain engineering in optic^9-13, acoustic^14-17 and electronic applications^18,19 is limited to two-dimensional space and micrometre-scale resolution. Here we demonstrate a non-reciprocal near-infrared laser-writing technique for reconfigurable three-dimensional ferroelectric domain engineering in LiNbO₃ with nanoscale resolution. The proposed method is based on a laser-induced electric field that can either write or erase domain structures in the crystal, depending on the laser-writing direction. This approach offers a pathway for controllable nanoscale domain engineering in LiNbO₃ and other transparent ferroelectric crystals, which has potential applications in high-efficiency frequency mixing^20,21, high-frequency acoustic resonators^14-17 and high-capacity non-volatile ferroelectric memory^19,22.