Gallium phosphide (GaP) is an indirect bandgap semiconductor used widely in solid-state lighting. Despite numerous intriguing optical properties---including large χ(2) and χ(3) coefficients, a high refractive index (>3), and transparency from visible to long-infrared wavelengths (0.55−11μm)---its application as an integrated photonics material has been little studied. Here we explore GaP-on-insulator as a platform for nonlinear photonics, exploiting a direct wafer bonding approach to realize integrated waveguides with 1.2 dB/cm loss in the telecommunications C-band (on par with Si-on-insulator). High quality (Q>105), grating-coupled ring resonators are fabricated and studied. We directly observe the Kerr effect in response measurements, and obtain the first experimental estimate of the nonlinear index of GaP at telecommunication wavelengths: n2=1.2(5)×10−17m2/W. We also observe Kerr frequency comb generation in resonators with engineered dispersion. Parametric threshold powers as low as 3 mW are realized, followed by broadband (>100 nm) frequency combs with sub-THz spacing, frequency-doubled combs and, in a separate device, efficient Raman lasing. These results signal the emergence of GaP-on-insulator as a novel platform for integrated nonlinear photonics.
I H Baek1,6, J M Hamm2, K J Ahn1, B J Kang1, S S Oh2, S Bae3, S Y Choi1, B H Hong4, D-I Yeom1, B Min5,O Hess2, Y U Jeong6 and F Rotermund1,7
http://iopscience.iop.org/article/10.1088/2053-1583/aa5c64/meta
The conical band structure is the cornerstone of graphene's ultra-broadband optical conductivity. For practical use of graphene in nonlinear photonics, however, substantial increases of the light–matter interaction strength will be required while preserving the promising features of monolayers, as the interaction of light with a single atomic layer is limited due to the extremely short interaction length and low density of state, particularly for the long-wavelength region. Here, we report that this demand can be fulfilled by random stacking of high-quality large-area monolayer graphene up to a requested number of layers, which leads to the electronic interaction between layers being effectively switched off due to turbostratic disorder. The nonlinear characteristics of randomly stacked multilayer graphene (RSMG), which originates from a thermo-modulational feedback mechanism through ultrafast free-carrier heating and temperature-dependent carrier-phonon collisions, show clear improvements in the terahertz (THz) regime with increasing layer numbers, whereas as-grown multilayer graphene (AGMG) exhibits limited behaviors due to strong interlayer coupling. This controllable nonlinearity enhancement provides an ideal prerequisite for developing efficient graphene-based THz photonic devices.
