L. Sirleto

Near-Infrared Sub-Bandgap All-Silicon Photodetectors: State of the Art and Perspectives

Due to recent breakthroughs, silicon photonics is now the most active discipline within the field of integrated optics and, at the same time, a present reality with commercial products available on the market. Silicon photodiodes are excellent detectors at visible wavelengths, but the development of high-performance photodetectors on silicon CMOS platforms at wavelengths of interest for telecommunications has remained an imperative but unaccomplished task so far. In recent years, however, a number of near-infrared all-silicon photodetectors have been proposed and demonstrated for optical interconnect and power-monitoring applications. In this paper, a review of the state of the art is presented. Devices based on mid-bandgap absorption, surface-state absorption, internal photoemission absorption and two-photon absorption are reported, their working principles elucidated and their performance discussed and compared.

Advance in thermo-optical switches: principles, materials, design, and device structure

All-optical networking can be the sole approach to provide the huge bandwidth required for future networks. The essential elements in such an optical network are optical switches. A number of options have been proposed in order to implement them efficiently. We focus on thermo-optical switches. First, the physical principles of the thermo-optic effect are briefly introduced. A description of the most common technologies used for the fabrication of thermo-optic switches is provided along with the values of thermo-optic coefficient for a number of materials. The main steps useful in order to design thermo-optical switches are also briefly introduced. Finally, a birds-eye view of the main and recent proposals of switches based on the thermo-optic effect is reported and their performances compared.

Electro-optical switch and continuously tunable filter based on a Bragg grating in a planar waveguide with a liquid crystal overlayer

The possibility of using smectic liquid crystals in active wave- guide devices is explored through the analysis of both an integrated electro-optic switch and a continuously tunable filter. The two devices are based on a Bragg grating in planar waveguide with a liquid crystal over- layer, which enables changing the spectral behavior of the device. The fast and bistable switching of smectic C* in the surface-stabilized liquid crystal structure is used to investigate the possibility of realizing an inte- grated electro-optical switch. The principal advantage of this device is its spectral signature, which enables us to overcome the problems of inten- sity dependent devices. The soft-mode of smectic A* liquid crystals, enabling a continuous modulation of extraordinary refractive index, is used to design an integrated wavelength filter in the wavelength range of interest for optical communications. The principal advantages of such device include fast tuning speed, wide tuning range, low power dissipa- tion, and low cost.

Cu/p-Si Schottky barrier-based near infrared photodetector integrated with a silicon-on-insulator waveguide

In this letter, a near infrared all-silicon (all-Si) photodetector integrated into a silicon-on-insulator waveguide is demonstrated. The device is based on the internal photoemission effect through a metal/Si Schottky junction placed transversally to the optical field confined into the waveguide. The technological steps utilized to fabricate the device allow an efficiently monolithic integration with complementary metal-oxide semiconductor compatible structures. Preliminary results show a responsivity of 0.08 mA/W at 1550 nm with a reverse bias of 1 V and an efficient behavior both in C and L band. Finally, an estimation of bandwidth for GHz range is deduced.

Giant Raman gain in silicon nanocrystals

Nanostructured silicon has generated a lot of interest in the past decades as a key material for silicon-based photonics. The low absorption coefficient makes silicon nanocrystals attractive as an active medium in waveguide structures, and their third-order nonlinear optical properties are crucial for the development of next generation nonlinear photonic devices. Here we report the first observation of stimulated Raman scattering in silicon nanocrystals embedded in a silica matrix under non-resonant excitation at infrared wavelengths (~1.5 μm). Raman gain is directly measured as a function of the silicon content. A giant Raman gain from the silicon nanocrystals is obtained that is up to four orders of magnitude greater than in crystalline silicon. These results demonstrate the first Raman amplifier based on silicon nanocrystals in a silica matrix, thus opening new perspectives for the realization of more efficient Raman lasers with ultra-small sizes, which would increase the synergy between electronic and photonic devices.

Silicon resonant cavity enhanced photodetector based on the internal photoemission effect at 1.55μm: Fabrication and characterization

In this paper, the realization and the characterization of a resonant cavity enhanced (RCE) photodetector, completely silicon compatible and working at 1.55μm, are reported. The detector is a RCE structure incorporating a Schottky diode and its working principle is based on the internal photoemission effect. Taking advantage of a Cu∕Si Schottky diode fed on a high reflectivity Bragg mirror, an improvement in responsivity at 1.55μm is experimentally demonstrated.

Asymmetric MSM sub-bandgap all-silicon photodetector with low dark current

Design, fabrication, and characterization of an asymmetric metal-semiconductor-metal photodetector, based on internal photoemission effect and integrated into a silicon-on-insulator waveguide, are reported. For this photodetector, a responsivity of 4.5 mA/W has been measured at 1550 nm, making it suitable for power monitoring applications. Because the absorbing metal is deposited strictly around the vertical output facet of the waveguide, a very small contact area of about 3 µm2 is obtained and a transit-time-limited bandwidth of about 1 GHz is demonstrated. Taking advantage of this small area and electrode asymmetry, a significant reduction in the dark current (2.2 nA at -21 V) is achieved. Interestingly, applying reverse voltage, the photodetector is able to tune its cut-off wavelength, extending its range of application into the MID infrared regime.

Critically coupled silicon Fabry-Perot photodetectors based on the internal photoemission effect at 1550 nm

In this paper, design, fabrication and characterization of an all-silicon photodetector (PD) at 1550 nm, have been reported. Our device is a surface-illuminated PD constituted by a Fabry-Perot microcavity incorporating a Cu/p-Si Schottky diode. Its absorption mechanism, based on the internal photoemission effect (IPE), has been enhanced by critical coupling condition. Our experimental findings prove a peak responsivity of 0.063 mA/W, which is the highest value obtained in a surface-illuminated IPE-based Si PD around 1550 nm. Finally, device capacitance measurements have been carried out demonstrating a capacitance < 5 pF which has the potential for GHz operation subject to a reduction of the series resistance of the ohmic contact.

Raman gain in niobium-phosphate glasses

In this paper, niobium-phosphate glasses doped with rare earths (Er and Sm) are investigated by Raman scattering. The goal of Raman characterization is twofold: (a) to perform a fine structural characterization of the synthesized glasses and (b) to measure the Raman gain coefficient of the samples and to compare it with fused silica. The results reveal the presence of NbO6 octahedra and Nb–O–P–Nb–O mixed chains. A broadening of bandwidth and a significant enhancement (∼24 times) in gain coefficient G with respect to conventional silica glasses are also demonstrated.

Cavity Enhanced Internal Photoemission Effect in Silicon Photodiode for Sub-Bandgap Detection

In this paper, a new approach for the near infrared sub-bandgap detection in Si-based devices is investigated. In particular, the design, the realization and the characterization of a back illuminated silicon resonant cavity enhanced Schottky photodetectors, working at 1.55 μm, are reported. The photodetectors are constituted by Fabry-Perot microcavity incorporating a Schottky diode. The working principle is based on the internal photoemission effect enhanced by cavity effect. Performances devices in terms of responsivity, free spectral range, finesse and estimated bandwidth are reported.