Lorenzo Colace

Near-infrared wavemeter based on an array of polycrystalline Ge-on-Si photodetectors

We report on a novel solid state wavelength meter in the near infrared. The device is an array of six photodetectors based on polycrystalline germanium film evaporated on a silicon substrate and each element is a wavelength sensitive detector. We describe the design, the fabrication and the characterization of such device and we demonstrate its capability in the measurement of the wavelength of quasi- monochromatic light beams.

Polycrystalline Ge detectors integrated on SOI waveguide: device modeling and experimental results

Integration of near infrared (NIR) photodetectors on a silicon substrate is a key step for the fabrication of an all silicon based NIR transceiver. To this extent, polycrystalline germanium (poly-Ge) technology is attractive due to the low deposition temperature and cost. Poly-Ge detectors demonstrated broad response, covering the whole NIR spectrum to 1.55 micron, fast, subnanosecond, speed and excellent versatility. In this work we present our recent results on the integration of a poly-Ge photodetector on a SOI silicon waveguide. The use of a waveguide for light in-coupling is appealing for telecom applications where signal is transported on an optical fibre, but, at the same time, it allows to increase detector responsivity. In fact, in this device the light is absorbed into the thin sensitive layer of the poly-Ge/Si heterojunction in a distributed way, during propagation. This releases the strong constraint of the absorption length being smaller than photocarrier collection length typical of normal incidence photodetectors. In the paper, both design issues and experimental results are reported.

Sensitivity of PBS colloidal quantum dot photoconductors: A comparison of different readout methods

We report on colloidal PbS quantum dot near infrared photoconductors operating at low voltage. In order to maximize the device sensitivity, we exploited the advantages of different measurement techniques and methods for dark current cancellation and noise reduction including the DC volt-amperometric measurement with offset cancellation, a Wheatstone-bridge configuration and impedance measurements. We demonstrate that even photodetectors with moderate detectivity (10 9 -10 10 cmHz 1/2 /W), exhibit very large SNR.

Polycrystalline Germanium on Silicon for Near Infrared Detectors

The diffusion of optical communications is still hindered by the high cost of transceivers operating in the near infrared. Such systems are currently realized by hybrid integration of sources, modulators, detectors and control/processing electronics. The development of a Silicon-compatible technology for the fabrication of sources and detectors is expected to dramatically reduce transceivers costs through a more mature technology (compared to III-V semiconductors) and monolithic integration (possibly including amplifying and biasing electronics) on a single optoelectronics integrated circuit (OEIC). Germanium-on-Silicon detectors have been introduced for the wavelengths of interest (1.3 and 1.6 μm), with characteristics comparable with those of commercial III-V (InGaAs, InGaAsP) photodiodes [1]. In spite of the remarkable performance of detectors fabricated by epitaxial Ge [2,3], the required high temperatures and aggressive cleaning processes hinder a seamless integration of these devices with standard Silicon IC. Among the approaches intended to increase such compatibility, we have recently proposed and demonstrated the use of polycrystalline Ge. Poly-Ge can be deposited at a low (250-300°C) substrate temperature (without substrate cleaning) by thermal evaporation. The films exhibit absorption spectra similar to those of monocrystalline Ge but, due to the inferior material quality, mobility and lifetimes are reduced. Nevertheless, polyGe-on-Si heterojunction photodiodes have been reported with a near infrared (NIR) responsivity extending to 1.6 μm and ranging from 16mA/W (at 1.3 μm) to 5mA/W (at 1.55 μm)[4]. Moreover, the full compatibility of the polyGe-on-Si process has been demonstrated by integrating linear-arrays of NIR photodiodes on a custom-prepared Si CMOS integrated circuit [5]. In this Communicaion we discuss advantages and drawbacks of the poly-Ge approach and report on the most recent advances in polyGe based near infrared photodetection. We will present both discrete photodiodes with increased responsivity (up to 50mA/W at 1.3μm) operating at 2.5Gbit/s and twodimensional arrays of detectors integrated with silicon CMOS electronics. Fig.1: Eye diagram obtained at 2.5Gbit/s with a PRBS at 1.54μm, without a transimpedance amplifier.

Spatial optical solitons in Reverse Proton Exchanged PPLN waveguides

Low-threshold spatial optical solitons are observed for the first time in buried planar waveguides obtained by reverse-proton-exchanged periodically poled LiNbO3.

Novel spatial solitons in nonlinearly coupled waveguides in unpoled lithium niobate

Based on cascading between coupled-modes in planar guides, we undertook a study of spatial optical solitons in this novel geometry encompassing two waveguides and orthogonal eigenmodes.

The /spl gamma//sub 0/ chip: a new front end for the gamma camera based on the ISPA tube

In this paper we present an IC chip, the /spl gamma//sub 0/, specifically developed for gamma imaging in medical applications. The chip is designed to be used inside an ISPA (integrated silicon pixel array) tube, in simple description, a one stage opto-electronic camera consisting of sealed vacuum tube in which a photocathode, evaporated on a scintillating crystal window, is comforted to a pixelated electronic imaging device. The /spl gamma//sub 0/, that has a total area of 0.5 /spl times/ 0.5 cm/sup 2/, is composed by a matrix of 32 by 32 square 135/spl mu/ pixels. Each silicon pixel is designed to be bonded by a Pb-Sn bump of 50/spl mu/ diameter to a PIN detector. The pixel has a front-end channel, a shaper, an individual 3-bit threshold adjustment and a discriminator. Every cell has a ten bit event buffer. The /spl gamma//sub 0/ is equipped with a novel on chip energy discrimination system that will allow the ISPA camera to self trigger on the gamma event with no extra electronics. The chip is self biased by a set of configurable internal DAC devices. For maximum simplicity in the design of the readout electronic system, every pixel and DAC device is memory mapped.

X-ray and optical characterization of multilayer semiconductor waveguides

Nowadays refractive-index engineering has become a challenging area for experimentalists in semiconductor integrated optics, whereas design constraints are often more strict than both standard technology tolerances and model accuracies. In fact, it is crucial to non-destructively evaluate thicknesses and refractive indices of a multilayer waveguide independently, and to this aim we resorted to X-ray reflectometry and effective index measurements on MBE-grown AlGaAs waveguides, respectively. With the first technique interference effects (Kiessig fringes) arise, which are related to layer thicknesses. By standard data processing, thickness accuracies of +/- 0.05 nm are readily achieved. Effective index measurements were performed at several wavelengths on both slab and rib waveguides, through grating-assisted distributed coupling with both photoresist and etched gratings. Effective indices were determined with an absolute precision as good as 1/2000, adequate for phase matching in parametric devices. Merging thickness and effective index evaluations, the refractive indices of the constituent layers were determined with unprecedented accuracies, in substantial agreement with existing models.