Despite all the eminent advantages of silicon photonics, other materials need to be integrated to fulfill the functions that are difficult to realize with silicon alone. This is because silicon has a low light emission efficiency and a low electro-optic coefficient, limiting the use of silicon as a material for light sources and modulators. A strong two-photon absorption (TPA) at high intensities also limits the use of silicon in applications exploiting nonlinear effects. In addition, signal amplification is needed to compensate the insertion and propagation losses in silicon nanowaveguides. To address these issues we have demonstrated the integration of atomic layer deposited nanolaminates on silicon waveguides.
Firstly we demonstrate slot waveguide ring resonators patterned on a silicon-on-insulator (SOI) wafer coated with an atomic layer deposited organic/inorganic nanolaminate structure, which consists of alternating layers of tantalum pentoxide (Ta2O5) and polyimide (PI) . These materials were selected since the ALD process for depositing Ta2O5/PI nanolaminate films is already available  and both materials exhibit high third order nonlinearities [3-4]. In our nanolaminate ring resonators, the optical power is not only confined in the narrow central air slot but also in several parallel sub-10 nm wide vertical polyimide slots. This indicates that the mode profiles in the silicon slot waveguide can be accurately tuned by the atomic layer deposition (ALD) method. Our results show that ALD of organic and inorganic materials can be combined with conventional silicon waveguide fabrication techniques to create slot waveguide ring resonators with varying mode profiles.
Secondly we demonstrate the integration of atomic layer deposited erbium-doped aluminum oxide (Al2O3) nanolaminates on silicon waveguides. This method provides an efficient way for controlling the concentration and distribution of erbium ions. We have applied this method on silicon strip and slot waveguides and show signal enhancement.
Our results show that atomic layer deposited nanolaminates can potentially open new possibilities for various photonic applications, such as silicon photonic devices for light emission and amplification, optical sensing and all-optical signal processing.
 A. Autere, L. Karvonen, A. Säynätjoki, M. Roussey, E. Färm, M. Kemell, X. Tu, T.Y. Liow, G.Q. Lo, M. Ritala, M. Leskelä, S. Honkanen, H. Lipsanen, and Z. Sun, "Slot waveguide ring resonators coated by an atomic layer deposited organic/inorganic nanolaminate," Opt. Express 23, 26940-26951 (2015)
 L. D. Salmi, E. Puukilainen, M. Vehkamäki, M. Heikkilä, and M. Ritala, “Atomic layer deposition of Ta2O5/polyimide nanolaminates,” Chem. Vap. Deposition 15, 221–226 (2009).
 S. Morino, T. Yamashita, K. Horie, T. Wada, and H. Sasabe, “Third-order nonlinear optical properties of aromatic polyisoimides,” React. Funct. Polym. 44, 183–188 (2000).
 C.-Y. Tai, J. Wilkinson, N. Perney, M. Netti, F. Cattaneo, C. Finlayson, and J. Baumberg, “Determination of nonlinear refractive index in a Ta2O5 rib waveguide using self-phase modulation,” Opt. Express 12, 5110–5116 (2004).
This paper focuses on latest progress in experimental and theoretical studies on silicon-based carrier-depletion PNjunction phase shifters in terms of high modulation efficiency for energy-efficient photonic networks of high transmission capacity. Modulation efficiency of rib-waveguide phase shifters having various PN-junction configuration are characterized with respect to DC figure of merit defined for phase shifters using carrier-plasma dispersion as the physical principle of refractive-index modulation. In addition, RF drive voltage required for 10-Gb/s on-off keying is characterized for rib-waveguide phase shifters including lateral and vertical PN-junction configurations.
Slot waveguide based ring resonators filled with atomic layer deposited (ALD) aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) were fabricated and characterized. Our results demonstrate that ALD can be used to create slot waveguide ring resonators with relatively high Q-factors, which opens new possibilities for various photonic applications, such as optical sensing and all-optical signal processing.
Latest computational and experimental studies on high-speed monolithic silicon-based Mach-Zehnder optical modulators
are studied in the light of photonic integrated circuits for digital coherent communication at a bit rate as fast as 128 Gb/s
per wavelength channel. Lateral PN-junction rib-waveguide phase shifters are elaborated with experimental
characteristics of DC phase shifter response in comparison with computational characteristics. High-speed response in
refractive-index dynamics including electron and hole transport in the PN junction is simulated to study speed limit of
the phase shifters. The performance in quadrature phase-shift keying signal generation is characterized in experimental
and computational constellation diagrams. Silicon waveguides for polarization-division multiplexing are designed in
common design rules with the rib-waveguide phase shifters. Long-haul transmission in polarization-multiplexed
quadrature phase-shift keying in 1000-km single-mode fiber link is confirmed with a monolithic silicon Mach-Zehnder
modulator assembled with modulator drivers in a ceramic-based metal package.
Low-loss high-speed traveling-wave silicon Mach-Zehnder modulator with reduced series resistance is studied in
microwave and optical measurements. Microwave impedance and propagation loss under reverse bias are characterized
by S-parameter measurements. Resonant loss due to series inductance-resistance-capacitance coupling limits microwave
performances of the traveling-wave modulator. High-speed optical performances are characterized, based on eyediagram
measurements in on-off keying at 10-32 Gb/s and constellation and eye-diagram measurements in differential
phase-shift keying at 20 Gb/s. Dispersion tolerance in error-free transmission in 10-Gb/s on-off keying and 20-Gb/s
differential phase-shift keying is obtained as +/-950 ps/nm and +/-220 ps/nm, respectively by path-penalty measurements.
Transmission performance in 10-Gbps on-off keying is comparable with lithium niobate Mach-Zehnder modulator.
We demonstrate low-loss silicon slot waveguides filled with single and dual atomic layer deposited oxide layers.
Propagation losses less than 5 dB/cm and 8 dB/cm are achieved for the waveguides with single (Al<sub>2</sub>O<sub>3</sub>) and double
(Al<sub>2</sub>O<sub>3</sub>-TiO<sub>2</sub>) layers, respectively. The devices are fabricated using low-temperature CMOS compatible processes. The
geometries allow nonlinearities nearly two orders of magnitude smaller than plain silicon waveguides.
Different types of slot waveguide couplers are studied theoretically and experimentally. We present strip-to-slot
waveguide couplers with a length as small as 10 μm and with their feature sizes no less than 150 nm, exhibiting efficient
coupling into the slot mode and negligible scattering. We demonstrate coupling loss of 0.11 dB for a 20-micron long
coupler from a strip waveguide to slot waveguide, fabricated using the 248 nm deep-UV lithography. We also discuss
ring resonator couplers, multimode multislot structures for 2x2 couplers, and prism coupling characterization of multislot
The linear electro-optic (Pockels) effect of wurtzite gallium nitride (GaN) films and six-period GaN/Al<sub>x</sub>Ga<sub>1-x</sub>N
superlattices with different quantum structures were demonstrated by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55μm. The samples were prepared on (0001)
sapphire substrate by low-temperature metalorganic chemical vapor deposition (MOCVD). The measured coefficients of
the GaN/Al<sub>x</sub>Ga<sub>1-x</sub>N superlattices are much larger than those of bulk material. Taking advantage of the strong field
localization due to resonances, GaN/Al<sub>x</sub>Ga<sub>1-x</sub>N SL can be proposed to engineer the nonlinear responses.
Electro-optical modulator with dual capacitors is designed and based on this design basic configuration of
device is realized in laboratory. Exceeding GHz switching speed and high phase modulation efficiency can
be expected with this device.