Newton C. Frateschi

Reconfigurable silicon thermo-optical ring resonator switch based on Vernier effect control

A proof-of-concept for a new and entirely CMOS compatible thermo-optic reconfigurable switch based on a coupled ring resonator structure is experimentally demonstrated in this paper. Preliminary results show that a single optical device is capable of combining several functionalities, such as tunable filtering, non-blocking switching and reconfigurability, in a single device with compact footprint (~50 μm x 30 μm).

Purcell effect in sub-wavelength semiconductor lasers.

We present a formal treatment of the modification of spontaneous emission rate by a cavity (Purcell effect) in sub-wavelength semiconductor lasers. To explicitly express the assumptions upon which our formalism builds, we summarize the results of non-relativistic quantum electrodynamics (QED) and the emitter-field-reservoir model in the quantum theory of damping. Within this model, the emitter-field interaction is modified to the extent that the field mode is modified by its environment. We show that the Purcell factor expressions frequently encountered in the literature are recovered only in the hypothetical condition when the gain medium is replaced by a transparent medium. Further, we argue that to accurately evaluate the Purcell effect, both the passive cavity boundary and the collective effect of all emitters must be included as part of the mode environment.

Embedded coupled microrings with high-finesse and close-spaced resonances for optical signal processing

Single microring resonators have been used in applications such as wavelength multicasting and microwave photonics, but the dependence of the free spectral range with ring radius imposes a trade-off between the required GHz optical channel spacing, footprint and power consumption. We demonstrate four-channel all-optical wavelength multicasting using only 1 mW of control power, with converted channel spacing of 40-60 GHz. Our device is based on a compact embedded microring design fabricated on a scalable SOI platform. The coexistence of close resonance spacing and high finesse (205) in a compact footprint is possible due to enhanced quality factors (30,000) resulting from the embedded configuration and the coupling-strength dependence of resonance spacing, instead of ring size. In addition, we discuss the possibility of achieving continuously mode splitting from a single-notch resonance up to 40 GHz.

Amorphous Al 2 O 3 Shield for Thermal Management in Electrically Pumped Metallo-Dielectric Nanolasers

We analyze amorphous Al2O3 (α-Al2O3) for use as a thick thermally conductive shield in metallo-dielectric semiconductor nanolasers, and show that the use of α-Al2O3 allows a laser to efficiently dissipate heat through its shield. This new mechanism for thermal management leads to a significantly lower operating temperature within the laser, compared with lasers with less thermally conductive shields, such as SiO2. We implement the shield in a continuous wave electrically pumped cavity, and analyze its experimental performance by jointly investigating its optical, electrical, thermal, and material gain properties. Our analysis shows that the primary obstacle to room temperature lasing was the devices high threshold gain. At the high pump levels required to achieve the gain threshold, particularly at room temperature, the gain spectrum broadened and shifted, leading to detrimental mode competition. Further simulations predict that an increase in the pedestal undercut depth should enable room temperature lasing in a device with the same footprint and gain volume. Through the integrated treatment of various physical effects, this analysis shows the promise of α-Al2O3 for nanolaser thermal management, and enables better understanding of nanolaser behavior, as well as more informed design of reliable nanolasers.

Silicon technology compatible photonic molecules for compact optical signal processing

Photonic molecules (PMs) based on multiple inner coupled microring resonators allow to surpass the fundamental constraint between the total quality factor (QT), free spectral range (FSR), and resonator size. In this work, we use a PM that presents doublets and triplets resonance splitting, all with high QT. We demonstrate the use of the doublet splitting for 34.2 GHz signal extraction by filtering the sidebands of a modulated optical signal. We also demonstrate that very compact optical modulators operating 2.75 times beyond its resonator linewidth limit may be obtained using the PM triplet splitting, with separation of ∼55 GHz.

Comparison of Plasmonic Arrays of Holes Recorded by Interference Lithography and Focused Ion Beam

In this paper, we compare the geometric characteristics and the optical properties of plasmonic hole arrays recorded in gold (Au) films using two different techniques, namely, focused ion beam (FIB) and interference lithography (IL). The morphology of the samples was analyzed using a scanning electron microscope (SEM), and the plasmonic peaks were measured from the transmission spectrum of the samples. The diameters of the holes recorded by IL present approximately the same statistical deviation as those fabricated by FIB but in a much larger area. Although the transmittance measurements of both types of samples exhibit the characteristic plasmonic peaks, the intrinsic fabrication errors of each technique affect differently the optical spectra.

Thermal considerations in electrically-pumped metallo-dielectric nanolasers

Metal nanocavity-based lasers show promise for dense integration in nanophotonic devices, thanks to their compact size and lack of crosstalk. Thermal considerations in these devices have been largely overlooked in design, despite the importance of self-heating and heat dissipation to device performance. We discuss the sources of self-heating in electrically-pumped wavelength-scale nanolasers, and the incorporation of these heat sources into a heat dissipation model to calculate laser operating temperature. We apply this thermal model to an example electrically-pumped nanolaser operating at room temperature.

Spectral Engineering With CMOS Compatible SOI Photonic Molecules

Photonic systems based on microring resonators have a fundamental constraint given by the strict relationship among free spectral range, total quality factor QT , and resonator size, intrinsically making filter spacing, photonic lifetime, and footprint interdependent. Here, we break this paradigm employing CMOS-compatible silicon-on-insulator photonic molecules based on coupled multiple inner ring resonators. The resonance wavelengths and their respective linewidths are controlled by the hybridization of the quasi-orthogonal photonic states. We demonstrate photonic molecules with doublet and triplet resonances with spectral splitting only achievable with single-ring orders of magnitude larger in footprint. In addition, this splitting is potentially controllable based on the coupling (bonds) between resonators. Finally, the spatial distribution of the hybrid states allows up to sevenfold QT enhancement.

Modeling quasi-dark states with temporal coupled-mode theory.

Coupled resonators are commonly used to achieve tailored spectral responses and allow novel functionalities in a broad range of applications. The Temporal Coupled-Mode Theory (TCMT) provides a simple and general tool that is widely used to model these devices. Relying on TCMT to model coupled resonators might however be misleading in some circumstances due to the lumped-element nature of the model. In this article, we report an important limitation of TCMT related to the prediction of dark states. Studying a coupled system composed of three microring resonators, we demonstrate that TCMT predicts the existence of a dark state that is in disagreement with experimental observations and with the more general results obtained with the Transfer Matrix Method (TMM) and the Finite-Difference Time-Domain (FDTD) simulations. We identify the limitation in the TCMT model to be related to the mechanism of excitation/decay of the supermodes and we propose a correction that effectively reconciles the model with expected results. Our discussion based on coupled microring resonators can be useful for other electromagnetic resonant systems due to the generality and far-reach of the TCMT formalism.

Carrier saturation in multiple quantum well metallo-dielectric semiconductor nanolaser: is bulk material a better choice for gain media?

Although multi quantum well (MQW) structure is frequently suggested as the appropriate medium for providing optical gain in nanolasers with low threshold current, we demonstrate that in general bulk gain medium can be a better choice. We show that the high threshold gain required for nanolasers demands high threshold carrier concentrations and therefore a highly degenerate condition in which the barriers between the quantum wells are heavily pumped. As a result, there occurs spontaneous emission from the barrier in very dissipative low Q modes or undesired confined higher Q modes with resonance wavelengths close to the barrier bandgap. This results in a competition between wells and barriers that suppresses lasing. A complete model involving the optical properties of the resonant cavity combined with the carrier injection in the multilayer structure is presented to support our argument. With this theoretical model we show that while lasing is achieved in the nanolaser with bulk gain media, the nanolaser with MQW gain structure exhibits well emission saturation due to the onset of barrier emission.