Materials and devices.for compact optical amplification in Si photonics is reviewed. In particular, as the requirement for
high gain per length together with high refractive index renders traditional oxide-based approach problematic, Er-doping
of silicon-rich silicon nitride and erbium silicate nanocrystals are proposed and shown to be promising alternatives.
Using such new materials, microdisk resonators and slot waveguides that concentrate the light in a compact volume for
high functionality are fabricated and characterized.
We report on fabricating ErxY2-xSiO5 nanocrystals using ErCl3•6H2O and YCl3•6H2O solutions and Si nanowires grown
by VSL method. Use of crystalline host allows incorporation of up to 25 at % Er without clustering and loss of optical
activity, and use of Y enables continuous mixing of Er and Y for controlling cooperative upconversion. We obtain a
cooperative upconversion coefficients of (2.2±1.1)×10-18 cm3/s and (5.4±2.7)×10-18 cm3/s at an Er concentration of
1.2×1021 cm-3 and 2.0×1021 cm-3, respectively. These values are up to 10 times lower at 10 times higher Er concentration
than those reported for Er-doped silica, and shows that up to 69 dB/cm gain could be achieved for ultra-compact optical
amplification. Also, we report on the deposition of ErxY2-xSiO5 thin film on Si substrate using ion beam sputter
deposition. Rapid thermal annealing at 1100°C is enough to form crystal phase the film and activate most of Er3+ ions.
Micro-disk resonators were fabricated using Er doped silicon-rich silicon nitride (SRSN:Er). SRSN:Er thin films are fully CMOS-compatible and show efficient Er3+ luminescence pumped off-resonantly via Si nanoclusters. The high refractive index of SRSN (>2.0 at 1.5 μm) allows freedom in designing compact micro-disk resonators. Micro-disks with two different contents of Er (0.2 at.%, 0.02 at.%) with a diameter of 25 μm were fabricated, and characterized using evanescent coupling using tapered fibers. Whispering gallery modes with Q-factors in excess of 13000 were obtained, and FDTD calculations indicate that much higher values should be possible. Finally, we demonstrate excitation of fundamental whispering gallery mode via off-resonant, top-pumping of the SRSN microdisk.
In this paper, we provide a detailed performance analysis of an nc-Si EDWA for the real application. Optical gain (small signal / saturation regime), noise figure and required pump density has been assessed in terms of the device structure. Results show a high feasibility of achieving 10dBm output power with 0dBm of input signal, using an array of commercially available high-power blue-green LEDs as the top pump. In numerical model section, we suggest simplified coupled rate equation and 2-D propagation equation constructed to investigate amplifier performance. In performance analysis section, firstly, we compared population inversion characteristics for Er with/without nc-Si condition to confirm the widening of high inversion region by introducing nc-Si sensitizer, which means less pump intensity requirement for same input signal power. In addition, to test the feasibility of the NC-EDWA for the metro-network applications, we simulated the amplifier performance for varying the width of amplifier region. With 50x7μm2 active core and bottom mirror, only 15.8 W/cm2 (8.8 W/cm2 for 100μm width) of pump intensity was sufficient to meet the target operating condition. We also compare the inversion distributions of NC-Si EDWA, for the 4 types of EDWA structures under investigation (straight without/with mirror, adiabatic without/with mirror). As another key performance factor, we also calculated noise figures for different NC-Si EDWA structures.
We report the development of a novel low energy optical switch that consists of a silica microsphere optical resonator coated with a layer of silicon nanocrystals. A 150 μm-diameter silica microsphere was coated with a 140 nm thick layer of silicon rich silicon oxide (SRSO) by PECVD. The microsphere/SRSO was annealed in argon at 1100C to facilitate nanocrystal growth. The optical properties of the microsphere were characterized by evanescently coupling 1450 μm tunable laser light through a tapered optical fiber into the whispering gallery mode resonances of the microsphere. A quality factor of 3×105 was measured at this wavelength. Light from an Ar+ laser at 488 nm was introduced into the tapered fiber and was used to excite the nanocrystals near the whispering gallery modes (WGM) of the sphere. WGM resonance wavelengths shifts of 5 pm at an operating wavelength of 1450 nm were observed when the Ar+ light was coupled into the tapered fiber. Powers as low as 3 μW were sufficient to shift the resonance by a half a linewidth and cause full switching of the 1450 nm signal with a fast rise time (which was limited by the time width of the laser pulse). The speed of the switch is limited by the fall time, which has a time constant of 30 ms.
Nanocrystal Si (nc-Si) sensitization of Er in a silica matrix to obtain high optical activity in a Si-compatible material is investigated. Er-doped silicon-rich silicon oxide (SRSO) films, which consist of nc-Si embedded inside an SiO2 matrix, were deposited by electron-cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) using SiH4 and O2 with concurrent sputtering of Er followed by a high temperature anneal. For comparison, Er-free SRSO films were also deposited. Detailed investigation of processing conditions indicates that an annealing process consisting of 30 min anneal at 950°C without hydrogenation to be optimum for activation of Er. Investigation of MOS diode structure with Er-doped and Er-free SRSO films indicates that a mesa-type structure with n+ poly-silicon top contact, p-type substrate, and SRSO Si content of less than 40% gives the best diode performance. Er-free SRSO diodes fabricated using the optimum conditions show electroluminescence under forward bias. Er-doped SRSO diodes show photoresponse at 1.54 μm due to nanocrystal -- Er interactions, showing the promise of developing integrated, Si-based 1.54 μm light detectors for integrated microphotonic devices.
The role of the size of amorphous silicon quantum dots in the Er luminescence at 1.54 µm was investigated. As the dot size was increased, the more Er ions were located near one dot due to its large surface area and more Er ions interacted with other Er ions. This Er-Er interaction caused a weak photoluminescence intensity despite the increase in the effective excitation cross section. The critical dot size, needed to take advantage of the positive effect on Er luminescence, is considered to be about 2.0 nm, below which a small dot is very effective in the efficient luminescence of Er. However, the hydrogenation is considered to suppress this Er-Er interaction.
The Er3+ luminescent properties of Er-doped Si/SiO2 superlattices are investigated. The superlattices were deposited either by electron cyclotron resonance plasma enhanced chemical vapor deposition or by ultra-high vacuum ion beam sputter deposition method and subsequently annealed at 950 degrees C. The thickness of the layers was varied 0.6 to 4.8 nm, and location of Er controlled within sub-nm. The structure and the composition of the films were confirmed using transmissions electron microscopy and medium energy ion spectroscopy. By carefully controlling the Si and SiO2 layer thickness and the locations of Er, we demonstrate several orders of magnitude enhancement of Er3+ luminescence and suppression of de-excitation mechanisms. We also demonstrate fabrication of waveguides using Er-doped Si/SiO2 superlattices, and discuss implications for possible applications.
Excitation and de-excitation mechanisms of rare earth doped nanocrystalline silicon and its implications for waveguide amplifier applications are investigated. Er, Nd, and Pr doped silicon rich silicon oxide (SRSO) thin films were prepared by electron cyclotron resonance enhanced chemical vapor deposition with co-sputtering of target and subsequent anneal at 950 degrees C. Temperature and pump-power dependence of Er3+ photoluminescence shows that carrier-mediated non-radiative de-excitation are strongly suppressed indicating feasibility of population inversion. Detailed investigations of dependence of Er3+ luminescence intensity and lifetime on pump width indicate that exciton-erbium coupling is dominant over carrier- exciton coupling, and that the luminescent Er ions are not inside the Si nanoclusters but in the SiO2 matrix near the clusters. Luminescence properties of Nd-doped SRSO is similar to that of Er-doped SRSO, but the temperature dependence of Nd3+ luminescence intensity is different from that of Er3+ luminescence, an effect which we ascribe to its higher transition energy. In contrast, no luminescence could be observed from Pr-doped SRSO. Erbium-doped SRSO waveguides are fabricated using the standard Si processing techniques, and guiding of 1.55 micrometers light with strong Er luminescence is observed. These results indicate that for rare erath-doped SRSO waveguides to become practical, formation of high density of small Si nanoclusters must be induced.
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