Polymer and Hybrid Optical Devices Manipulated by the Thermo-Optic Effect

PLC-Based Polymer/Silica Hybrid Inverted Ridge LP11 Mode Rotator

The mode rotator is an important component in a PLC-based mode-division multiplexing (MDM) system, which is used to implement high-order modes with vertical intensity peaks, such as LP11b mode conversions from LP11a in PLC chips. In this paper, an LP11 mode rotator based on a polymer/silica hybrid inverted ridge waveguide is demonstrated. The proposed mode rotator is composed of an asymmetrical waveguide with a trench. According to the simulation results, the broadband conversion efficiency between the LP11a and LP11b modes is greater than 98.5%, covering the C-band after optimization. The highest mode conversion efficiency (MCE) is 99.2% at 1550 nm. The large fabrication tolerance of the proposed rotator enables its wide application in on-chip MDM systems.

Ge-polymer bridge waveguide for mode-locked laser pulse generation

A Ge-polymer hybrid waveguide is sandwiched between an indium phosphide (InP) reflective gain chip and a fiber Bragg grating (FBG) to construct a laser system. The hybrid waveguide serves as a bridge between the gain chip and the fiber with tailored mode-field matching at both facets. The 50-nm amorphous Ge (α-Ge) layer shows a nonlinear absorption effect at 1550 nm. The hybrid waveguide is further verified by a femtosecond laser transmission experiment to show the pulse width compression effect. Such waveguide is then integrated inside the laser cavity as a passive saturable absorber to modulate the longitudinal modes for a pulsed output. This polymer-bridged mode-locked laser adopts an InP gain chip for compact assembly and also a FBG with a flexible length to adjust the pulse repetition rate. The mode-locked laser output around the designed 50 MHz repetition rate is demonstrated. The pulse width is measured as 147 ps, and the signal-to-noise ratio is larger than 50 dB. This work introduces a "ternary" mode-locked laser system, taking advantage of discrete photonic components bridged by a polymer-based waveguide. It also proves the feasibility of applying α-Ge films as practical and low-cost saturable absorbers in photonic devices.

Comparative Investigation of Ultrafast Excited-State Electron Transfer in Both Polyfluorene-Graphene Carboxylate and Polyfluorene-DCB Interfaces

The Photophysical properties, such as fluorescence quenching, and photoexcitation dynamics of bimolecular non-covalent systems consisting of cationic poly[(9,9-di(3,3'-N,N'-trimethyl-ammonium) propyl fluorenyl-2,7-diyl)-alt-co-(9,9-dioctyl-fluorenyl-2,7-diyl)] diiodide salt (PFN) and anionic graphene carboxylate (GC) have been discovered for the first time via steady-state and time-resolved femtosecond transient absorption (TA) spectroscopy with broadband capabilities. The steady-state fluorescence of PFN is quenched with high efficiency by the GC acceptor. Fluorescence lifetime measurements reveal that the quenching mechanism of PFN by GC is static. Here, the quenching mechanisms are well proven via the TA spectra of PFN/GC systems. For PFN/GC systems, the photo electron transfer (PET) and charge recombination (CR) processes are ultrafast (within a few tens of ps) compared to static interactions, whereas for PFN/1,4-dicyanobenzene DCB systems, the PET takes place in a few hundreds of ps (217.50 ps), suggesting a diffusion-controlled PET process. In the latter case, the PFN+•-DCB-• radical ion pairs as the result of the PET from the PFN to DCB are clearly resolved, and they are long-lived. The slow CR process (in 30 ns time scales) suggests that PFN+• and DCB-• may already form separated radical ion pairs through the charge separation (CS) process, which recombine back to the initial state with a characteristic time constant of 30 ns. The advantage of the present positively charged polyfluorene used in this work is the control over the electrostatic interactions and electron transfers in non-covalent polyfluorene/quencher systems in DMSO solution.

Polymer-Embedding Germanium Nanostrip Waveguide of High Polarization Extinction

Germanium (Ge) nanostrip was embedded in a polymer and studied as a waveguide. The measurements reveal that this new type of semiconductor/polymer heterogeneous waveguide exhibits strong absorption for the TE mode from 1500 nm to 2004 nm, while the propagation loss for the TM mode declines from 20.56 dB/cm at 1500 nm to 4.89 dB/cm at 2004 nm. The transmission characteristics serve as an essential tool for verifying the optical parameters (n-κ, refractive index, and extinction coefficient) of the strip, addressing the ambiguity raised by spectroscopic ellipsometry regarding highly absorbing materials. Furthermore, the observed strong absorption for the TE mode at 2004 nm is well beyond the cut-off wavelength of the crystalline bulk Ge (~1850 nm at room temperature). This redshift is modeled to manifest the narrowing of the Tauc-fitted bandgap due to the grain order effect in the amorphous Ge layer. The accurate measurement of the nanometer-scale light-absorbing strips in a waveguide form is a crucial step toward the accurate design of integrated photonic devices that utilize such components.