Silicon Photonic Crystal Microcavities Research Articles

Designs toward synchronization of optical limit cycles with coupled silicon photonic crystal microcavities: erratum.

Correction for the citation of a reference.

Enhanced spontaneous emission from two-photon-pumped quantum dots in a porous silicon microcavity.

Photoluminescence (PL)-based sensing techniques have been significantly developed in practice due to their key advantages in terms of sensitivity and versatility of the approach. Recently, various nanostructured and hybrid materials have been used to improve the PL quantum yield and the spectral resolution. The near-infrared (NIR) fluorescence excitation has attracted much attention because it offers deep tissue penetration and it avoids the autofluorescence of the biological samples. In our study, we have shown both spectral and temporal PL modifications under two-photon excitation of quantum dots (QDs) placed in one-dimensional porous silicon photonic crystal (PhC) microcavities. We have demonstrated an up-to-4.3-fold Purcell enhancement of the radiative relaxation rate under two-photon excitation. The data show that the use of porous silicon PhC microcavities operating in the weak coupling regime permits the enhancement of the PL quantum yield of QDs under two-photon excitation, thus extending the limits of their biosensing applications in the NIR region of the optical spectrum.

Designs toward synchronization of optical limit cycles with coupled silicon photonic crystal microcavities

A driven high-Q Si microcavity is known to exhibit limit cycle oscillation originating from carrier-induced and thermo-optic nonlinearities. We propose a novel nanophotonic device to realize synchronized optical limit cycle oscillations with coupled silicon (Si) photonic crystal (PhC) microcavities. Here, coupled limit cycle oscillators are realized by using coherently coupled Si PhC microcavities. By simulating coupled-mode equations, we theoretically demonstrate mutual synchronization (entrainment) of two limit cycles induced by coherent coupling. Furthermore, we interpret the numerically simulated synchronization in the framework of phase description. Since our proposed design is perfectly compatible with current silicon photonics fabrication processes, the synchronization of optical limit cycle oscillations will be implemented in future silicon photonic circuits.

Photon-correlation measurements of stochastic limit cycles emerging from high- Q nonlinear silicon photonic crystal microcavities

We performed measurements of photon correlation [$g^{(2)}(\tau)$] in driven nonlinear high-$Q$ silicon (Si) photonic crystal (PhC) microcavities. The measured $g^{(2)}(\tau)$ exhibits a damped oscillatory behavior when input pump power exceeds a critical value. From comparison between experiments and simulations, we attribute the measured oscillation of $g^{(2)}(\tau)$ to self-pulsing (a limit cycle) emerging from an interplay between photon, carrier, and thermal dynamics. Namely, the oscillation frequency of $g^{(2)}(\tau)$ corresponds to the oscillation period of the limit cycle, while its finite coherence (damping) time originates from the stochastic nature of the limit cycle. From the standpoint of phase reduction theory, we interpret the measured coherence time of $g^{(2)}(\tau)$ as the coherence (diffusion) time of a generalized phase of the limit cycle. Furthermore, we show that an increase in laser input power enhances the coherence time of $g^{(2)}(\tau)$ up to the order of microseconds, which could be a demonstration of the stabilization of a stochastic limit cycle through pumping.

Effect of anisotropy on the spectral characteristics of one-dimensional porous silicon photonic crystal microcavity for optical sensing applications

We present a numerical study of the effect of anisotropy on the spectral characteristics of one-dimensional porous silicon microcavity (1D-PSMC) with single defect layer. These structures have strong potential applications in optical sensing of chemicals and bioanalytes. Bruggeman’s effective medium approximation (BEMA) and (4 × 4) general transfer matrix method are used for theoretical modeling of spectral response of anisotropic 1D-PSMC. The potential of this structure as a sensing material is illustrated by analyzing wavelength shift in the defect mode induced by infiltration of biochemical analytes of different refractive indices inside the pores. Observed wavelength shift is found to be linearly dependent on the refractive index of analytes. We propose two 1D-PSMC-based sensors with a microcavity wavelength around 800 and 1200 nm. An anisotropic sensor with an operating wavelength around 800 nm shows a maximum sensitivity of 190 while an isotropic sensor with the same design parameters displays a maximum sensitivity of 150. In the case of an anisotropic sensor designed around 1200 nm, the maximum sensitivity is 260; for the isotropic sensor with similar structure, maximum sensitivity of 210 is obtained. Increased sensitivity is observed in anisotropic structures as compared to the isotropic ones. Design parameters play an integral role to obtain desired sensitivity in 1D-PSMC structure for sensor applications. These sensors can be used for high precision optical sensing of chemical-analytes, bioanalytes, gases, and environmental pollutants.

Linear and nonlinear microwave responses of a microwave photonic filter based on a photonic crystal microcavity

We propose and demonstrate an ultracompact bandpass microwave photonic filter (MPF) based on a silicon photonic crystal (PhC) microcavity. Taking the fabricated PhC microcavity as an example, we comprehensively investigate both the linear and nonlinear microwave responses of the MPF based on silicon waveguide devices. Two cases are discussed in the experiment, i.e., the optical carrier wavelength is located on the left or right side of the notch resonant wavelength of the PhC microcavity. The experimental results agree well with the theoretical analyses. For the former case, the central frequency of MPF increases monotonically when fixing the optical carrier wavelength and increasing the optical carrier power. For the latter case, the nonlinear response at a fixed optical carrier wavelength shows a decrease first and then an increase in the central frequency of MPF when increasing the optical carrier power. A jump of the response is observed in the switching process. Moreover, we also observe an interesting bistable microwave response in the experiment under an optical carrier power of around −2.6 dBm in the latter case.

Review of design principles of 2D photonic crystal microcavity biosensors in silicon and their applications

In this paper, we reviewed the design principles of two-dimensional (2D) silicon photonic crystal microcavity (PCM) biosensors coupled to photonic crystal waveguides (PCWs). Microcavity radiation loss is controlled by engineered the cavity mode volume. Coupling loss into the waveguide is controlled by adjusting the position of the microcavity from the waveguide. We also investigated the dependence of analyte overlap integral (also called fill fraction) of the resonant mode as well as the effect of group index of the coupling waveguide at the resonant wavelength of the microcavity. In addition to the cavity properties, absorbance of the sensing medium or analyte together with the affinity constant of the probe and target biomarkers involved in the biochemical reaction also limits the minimum detection limits. We summarized our results in applications in cancer biomarker detection, heavy metal sensing and therapeutic drug monitoring.

Schottky MSM junctions for carrier depletion in silicon photonic crystal microcavities

Collection of free carriers is a key issue in silicon photonics devices. We show that a lateral metal-semiconductor-metal Schottky junction is an efficient and simple way of dealing with that issue in a photonic crystal microcavity. Using a simple electrode design, and taking into account the optical mode profile, the resulting carrier distribution in the structure is calculated. We show that the corresponding effective free carrier lifetime can be reduced by 50 times when the bias is tuned. This allows one to maintain a high cavity quality factor under strong optical injection. In the fabricated structures, carrier depletion is correlated with transmission spectra and directly visualized by Electron Beam Induced Current pictures. These measurements demonstrate the validity of this carrier extraction principle. The design can still be optimized in order to obtain full carrier depletion at a smaller energy cost.

Influence of two-photon absorption on bistable switching in a silicon photonic crystal microcavity

By employing a simplified nonlinear coupled mode theory, we discuss the influence of two-photon absorption (TPA) on the characteristics of bistable switching. It is revealed that the critical value of frequency detuning for bistability rises linearly with increasing TPA coefficient k (when k is less than 30), and eventually access to a saturated value. It is also found that TPA effect will be enhanced for a greater frequency detuning, especially when transmission reaches its peak value. As a result, the peak transmission will decrease monotonously with the increasing frequency detuning. Based on this simplified model, the TPA-induced temperature rise in microcavity is also estimated. The theoretical predictions show good agreement with the simultaneous results, as well as the proposed experimental phenomena.

Generation and Wavelength Control of Resonant Luminescence from Silicon Photonic Crystal Microcavities with Ge Dots

Ge self-assembled quantum dots were embedded into two-dimensional photonic crystal microcavities fabricated on silicon-on-insulator substrates by gas-source molecular beam epitaxy (GS-MBE), electron beam lithography, and reactive ion etching (RIE). Light-emission characteristics of Ge quantum dots in the microcavities were characterized by room-temperature microphotoluminescence. Multiple sharp resonant luminescent peaks with quality factors of up to 600, corresponding to the resonant modes supported by the cavity, were observed in the emission range of Ge quantum dots. Control of the wavelengths of the photoluminescence peaks over a wide range from 1.3 to 1.6 µm was demonstrated by adjusting the lattice constant of the photonic crystals. The results show that embedding Ge dots into optical microcavities is a possible candidate for silicon-based light emitting devices.

Local nanofluidic light sources in silicon photonic crystal microcavities

We report on the realization of a rewritable and local source inside a Si-based photonic crystal microcavity by infiltrating a solution of colloidal PbS quantum dots inside a single pore of the structure. We show that the resulting spontaneous emission from the source is both spatially and spectrally redistributed due to the mode structure of the photonic crystal cavity. The coupling of the quantum dot emission to the cavity mode is analyzed by mapping the luminescence signal of the infiltrated solution with a scanning near-field optical microscope at room temperature. Spectral characterization and the mode profile are in good agreement with a three-dimensional numerical calculation of the system.

Room-temperature light-emission from Ge quantum dots in photonic crystals

Multiple layers of Ge self-assembled quantum dots were embedded into two-dimensional silicon photonic crystal microcavities fabricated on silicon-on-insulator substrates. Microphotoluminescence was used to study the light-emission characteristic of the Ge quantum dots in the microcavities. Strong resonant room-temperature light-emission was observed in the telecommunication wavelength region. Significant enhancement of the luminescence from Ge dots was obtained due to the resonance in the cavities. Multiple sharp resonant peaks dominated the spectrum, showing strong optical resonance inside the cavity. By changing the lattice constant of photonic crystal structure, the wavelengths of the resonant peaks are tuned in the wide wavelength range from 1.2 to 1.6 μm.

Nonlinear Fabry-Perot resonator with a silicon photonic crystal waveguide

We derive an equation that describes the nonlinear operation of a Fabry-Perot resonator with a large group index waveguide. Specifically, a silicon photonic crystal microcavity with two-photon-excited free carrier nonlinearity and Kerr nonlinearity is assumed. The equation clearly explains the bistability of the device and the reduction of the required pump energy for a specific nonlinear phase shift at an appropriate phase detuning from the resonance. We present a simple procedure to predict the required optical pump energy for the modulation and the resulting modulation depth by use of the equation and the device parameters.

Porous Silicon One-Dimensional Photonic Crystals for Optical Signal Modulation

Porous silicon one-dimensional (1-D) photonic crystal microcavities are fabricated as building blocks for optical modulators. The microcavities consist of porous silicon layers with alternating low and high porosity. Since the pore size is much smaller than the incident wavelength of light, each porous silicon layer is treated as an effective medium with a specific effective refractive index that corresponds to the given porosity. Active tuning of the microcavity resonance is accomplished through electrical actuation of liquid crystals infiltrated into the pores. As the refractive index of the liquid crystals change in response to the external stimulus, the effective refractive index of the porous silicon layers changes and the resonance wavelength shifts accordingly. The resonance wavelength shift creates an optical signal intensity modulation at a particular wavelength. Extinction ratios greater than 10 dB are achievable for these devices. The performance characteristics of the porous silicon photonic crystal microcavities can be tailored to specific applications based on the choice of infiltrated optically active material and the quality factor of the device

Characterization of buried photonic crystal waveguides and microcavities fabricated by deep ultraviolet lithography

We present results of the optical characterization of silicon photonic crystal waveguides and microcavities that are completely buried in a silicon dioxide cladding and are fabricated by deep ultraviolet (UV) lithography. The advantages of buried waveguides and deep UV lithography are discussed. Transmission spectra and loss factors for photonic crystal waveguides, as well as quality factors for resonant microcavities, are obtained. The observed characteristics are in good agreement with three-dimensional simulations.