Light is one of the most powerful tools in our hands to study the world around us. Lasers have indeed revolutionized our daily life. Since they only work at a few fixed wavelengths, it is crucial to convert their wavelength into the required one. Nonlinear optics provides essential tools to this end. For instance, second harmonic generation is a nonlinear process in which an intense input field at frequency ω generates a new field at the second harmonic frequency 2ω. The maximum efficiency of the nonlinear process is achieved by minimising the wave vector mismatch ∆k, i.e. the difference of the input wave vector at ω and the wave vector of the second harmonic at 2ω, achieving the so-called phase-matching condition (∆k=0). Birefringent phase-matching (BPM) can be obtained in anisotropic nonlinear crystals with moderate nonlinearities (few pm/V). Alternatively, quasi-phase-matching (QPM) makes use of periodically-poled nonlinear crystals with larger nonlinearities (10-20 pm/V). Typical crystals for BPM and QPM have a large efficiency but macroscopic thickness, i.e. millimeter/centimeters, and do not easily lend themselves to on-chip integration. PIONEER aims at revolutionising nonlinear optics at the nanoscale, achieving birefringent and quasi phase-matching in engineered vertical stacks of layered semiconductors, like non-centrosymmetric transition metal dichalcogenides (3R-TMDs), which possess huge nonlinearities (100-1000 pm/V) and promise to achieve the same efficiencies of bulk nonlinear crystals within micron-thicknesses. The goal of PIONEER is the development of ultra-compact on-chip integrable nonlinear devices, like entangled photon sources, nano- lasers and waveguides based on 3R-TMDs, exceeding the performances of state-of-the-art photonic resonators and nonlinear waveguides. PIONEER will initiate the field of phase-matched nonlinear optics with layered semiconductors and will trigger the next revolution of integrated nonlinear optical devices.
The Action has three main objectives, reflecting the logical development steps of the project.
• Objective 1: To measure the second harmonic coherence length Lc of semiconducting 3R-TMDs. I will systematically fabricate by mechanical exfoliation 3R-TMDs with variable thickness, from monolayer (0.65 nm) to bulk (~ 1 μm), and measure the thickness dependent SHG in order to determine Lc (Fig. 3.1) the key parameter to
engineer highly efficient stacks of 3R-TMDs.
• Objective 2: To realize periodically poled TMDs and demonstrate quasi phase-matching. I will realize the
first PP-TMD by exfoliating and stacking flakes of one representative TMD, e.g. MoS2, with thickness comparable to Lc and opposite dipole orientations (Fig 3.2). I expect to achieve a proof of concept of QPM measuring the quadratic enhancement of the SHG with the thickness of the PP-TMD.
• Objective 3: To realize novel applications of the extraordinary nonlinearity of 3R-TMDs. I will exploit 3R- TMD stacks to demonstrate novel nonlinear optical devices: (1) Spontaneous Parametric Down-Conversion (SPDC) for on-chip entangled photon pair generation7; (2) frequency tunable laser action; (3) BPM in waveguides (Fig. 3.3).
