Free-carrier Plasma Dispersion Effect Research Articles

Terahertz programmable metasurface for phase modulation based on free carrier plasma dispersion effect

Terahertz programmable metasurface for phase modulation based on free carrier plasma dispersion effect

Tunable electro-optic frequency-comb generation around 8 µm wavelength

Electro-optic frequency-comb is an interesting method for comb generation as it offers the possibility to electrically tune the generated frequency-comb by simply tuning the electrical signal applied on the modulator. Integrated modulators operating in a wide spectral range in the mid-IR have been demonstrated recently, relying on free carrier plasma dispersion effect in a Schottky diode embedded in a Ge-rich graded SiGe waveguide. Such integrated mid-infrared modulators have been used to generate electro-optic frequency-combs with more than 200 lines around the 8 µm wavelength optical carrier.

Mid-infrared Integrated Electro-optic Modulator Operating up to 225 MHz between 6.4 and 10.7 μm Wavelength

Mid-infrared spectroscopy is essential for identifying molecular species, while related electro-optic modulators are crucial for signal-to-noise enhancement via synchronous detection. Therefore, the development of integrated modulators is expected to have a major impact in compact and widespread sensing applications. In this work, we experimentally demonstrate a broadband integrated electro-optic modulator, based on a graded-index SiGe photonics platform and free-carrier plasma dispersion effect. Optical modulation is reported from 6.4 to 10.7 μm wavelength, showing an operational frequency up to 225 MHz. These results pave the way for the development of multimolecule on-chip spectroscopic systems, operating at the longest mid-infrared wavelengths.

Fully Reconfigurable Waveguide Bragg Gratings for Programmable Photonic Signal Processing

Fiber Bragg gratings have been invented for over three decades now and have been intensively employed in optical communication and sensor systems. The key limitation of a fiber Bragg grating is that once fabricated, it is hard to be tuned or with very limited tunability via thermal or mechanical tuning. Recently, great efforts have been directed into the study of realizing Bragg gratings on silicon-on-insulator (SOI) platform. The key advantage of Bragg gratings based on SOI is that the gratings can be fast and electrically reconfigured with the use of free-carrier plasma dispersion effect in silicon, which can be employed for fast programmable photonic signal processing. In this article, Bragg gratings based on SOI that are electrically programmable will be reviewed and their use for advanced programmable optical and microwave signal processing will be discussed.

Nonlinear performance and small signal model of junction-less microring modulator

In this paper, we have studied the nonlinear performance and modeled the junction-less microring modulator (JL-MRM) designed on the standard silicon-on-insulator (SOI) platform. The JL-MRM utilizes the electrostatic doping effect to achieve the free carrier plasma dispersion effect in a silicon waveguide. A compact voltage-dependent steady-state model is analytically derived for JL-MRM. Using the compact steady-state model, closed-form expressions for the on–off extinction ratio (ER), optical modulation amplitude (OMA), and the quality of resonance (Q-factor) are also presented. We have also analyzed the nonlinear and chirp performance of JL-MRM. A small signal model for the MRM is also analyzed. The proposed device is simulated using commercially available TCAD and mode solver tools. Simulation results show that for a JL-MRM with 8.75 μm radius, the on–off ER and insertion loss (IL) are 20.5 dB and 1.98 dB, respectively. The estimated Q-value for MRM is approximately 11,630. From the proposed small signal model, the estimated 3-dB bandwidth, chirp parameter, and Q-value are found to be 28.29 GHz, -0.19, and 9979, respectively. From the nonlinear analysis, we also found that the proposed MRM for the input power level of 0 dBm offers approximately 82.15 dB.Hz1/2 and 109.87 dB.Hz2/3 of spurious free dynamic range (SFDR) for second harmonic and third harmonic distortions, respectively.

Modeling and Analysis of Optical Modulators Based on Free-Carrier Plasma Dispersion Effect

Silicon photonic networks are revolutionizing computing systems by improving the energy efficiency, bandwidth, and latency of data movements. Optical modulators, such as microresonators (MRs) and Mach–Zehnder interferometers (MZIs), are the basic building blocks of silicon photonic networks. This paper proposes a SPICE-compatible electro-optical co-simulation model, basic optical switch integration model (BOSIM), to systematically study optical modulators using PN, PIN, and metal–insulator–silicon (MIS) capacitor device technologies. BOSIM holistically models both transient and steady state properties, such as switching speed, power, transmission spectrum, area, and carrier distribution. BOSIM is validated by the measured data from eight research groups and companies. Compared to MRs, BOSIM shows MZIs are fast, with a high extinction ratio and large bandwidth but in the sacrifice of loss, energy, and area. Using a PIN diode over a PN diode can save area, but retain the loss and energy, while an MIS capacitor has the shock response of carrier distribution in a narrow range and is marginalized gradually. For instance, an MZI can achieve a $2.5 {\times }$ bit rate, $6.06{\times }$ extinction ratio, $71.04 {\times }\,\,3$ -dB bandwidth, but costs at least $1.93 {\times }$ passing loss, $1.46 {\times }$ energy consumption, and $16.67 {\times }$ area, compared with MR.

High-speed silicon modulators for the 2 μm wavelength band

The 2 μm wavelength band has become a promising candidate to be the next communication window. We demonstrate high-speed modulators based on a 220 nm silicon-on-insulator platform working at a wavelength of 1950 nm, using the free carrier plasma dispersion effect in silicon. A Mach–Zehnder interferometer modulator and a microring modulator have been characterized. At 1950 nm, the carrier-depletion modulator operates at a data rate of 20 Gbit/s with an extinction ratio of 5.8 dB and insertion loss of 13 dB. The modulation efficiency (V π ·L π ) is 2.68 V·cm at 4 V reverse bias. The device operation is broadband, and we also characterize its performance at 1550 nm. At 1550 nm, an open eye is obtained at 30 Gbit/s. The difference in bandwidth is caused by the bandwidth limit of the 2 μm measurement setup. We also show a ring modulator paired with a low power integrated driver working in hybrid carrier depletion and injection mode at a data rate of 3 Gbit/s with power consumption of 2.38 pJ/bit in the 2 μm wavelength range. This work is a proof of principle demonstration and paves a route toward a full silicon-based transceiver in the 2 μm window.

Performance analysis of an electrostatic doping assisted silicon microring modulator

Abstract In this paper, we propose and numerically analyze electrostatic doping (ED) assisted silicon microring modulator (MRM) realizable on SOI platform having ∼ 10 nm free spectral range (FSR). The proposed modulator consists of a vertical metal–insulator–semiconductor (MIS) junction and a lateral metal–semiconductor (MS) junction. It utilizes ED to achieve free carriers inside the rib waveguide that eventually introduces the free carrier plasma dispersion effect. The proposed device does not employ any lateral or vertical PN or PIN junction and offers high operating speed with negligible static power consumption. Numerical simulation using commercially available TCAD tools and mode solvers are used to estimate the performance of the proposed modulators. Palladium (Pd) and aluminum (Al) are used as electrodes to realize ED enabled MIS–MS charge plasma diode. The performance of the proposed modulator is extensively studied both in steady state and transient state. Results predict that a maximum 37.8 dB and 33 dB of extinction ratio (ER) is achievable by the modulator while operating in inversion and accumulation mode, respectively. Predicted insertion losses are 1.97 dB and 1.49 dB when operated in inversion and accumulation mode respectively. The proposed modulator is expected to provide a maximum of 26.3 Gbps and 32.2 Gbps operating speed with a 9.21 GHz and 11.29 GHz of 3-dB optical bandwidth in inversion and accumulation mode of operation, respectively. Simulation result predicts that the dynamic energy per bit for the proposed ED assisted MRM is approximately 31 fJ/bit and 68 fJ/bit in inversion and accumulation mode of operation, respectively.

Design of a carrier-depletion Mach-Zehnder modulator in 250 nm silicon-on-insulator technology

Abstract. We present the design of a single-drive Mach-Zehnder modulator for amplitude modulation in silicon-on-insulator technology with 250 nm active layer thickness. The applied RF signal modulates the carrier density in a reverse biased lateral pn-junction. The free carrier plasma dispersion effect in silicon leads to a change in the refractive index. The modulation efficiency and the optical loss due to free carriers are analyzed for different doping configurations. The intrinsic electrical parameters of the pn-junction of the phase shifter like resistance and capacitance and the corresponding RC-limit are studied. A first prototype in this technology fabricated at the IMS CHIPS Stuttgart is successfully measured. The structure has a modulation efficiency of VπL = 3.1 V ⋅ cm at 2 V reverse bias. The on-chip insertion loss is 4.2 dB. The structure exhibits an extinction ratio of around 32 dB. The length of the phase shifter is 0.5 mm. The cutoff frequency of the entire modulator is 30 GHz at 2 V. Finally, an optimization of the doping structure is presented to reduce the optical loss and to improve the modulation efficiency. The optimized silicon optical modulator shows a theoretical modulation efficiency of VπL = 1.8 V ⋅ cm at 6 V bias and a maximum optical loss due to the free carrier absorption of around 3.1 dB cm−1. An ultra-low fiber-to-fiber loss of approximately 4.8 dB is expected using the state of the art optical components in the used technology.

Simplified modeling and optimization of silicon modulators based on free-carrier plasma dispersion effect.

In this paper, a simplified model of silicon phase modulators is presented that enables favorable accuracy together with a substantial reduction in computational effort and without the requirement of semiconductor TCAD device simulation software. This permits fast optimization of the different parameters of a modulator. The model was successfully implemented in Phoenix Optodesigner optical software allowing the optimization of silicon phase shifters for different applications. Moreover, this model presents a great potential for the simulation of modulators based on PN interdigitated junctions, which normally require complex and time consuming 3D simulations. Simulation time was reduced by a factor of 6 for the lateral PN junction based modulator, and two orders of magnitude reduction was obtained for interdigitated PN junctions based modulators.

All-optical Mach–Zehnder interferometer switching based on the phase-shift multiplication effect of an analog on the electromagnetically induced transparency effect

We theoretically and numerically investigate all-optical Mach–Zehnder interferometer switching based on the phase-shift multiplication effect of an all-optical analog on the electromagnetically induced transparency effect. The free-carrier plasma dispersion effect modulation method is applied to improve the tuning rate with a response time of picoseconds. All observed schemes are analyzed rigorously through finite-difference time-domain simulations and coupled-mode formalism. Compared with no phase-shift multiplication effect, the average pump power of all-optical switching required to yield the π -phase shift difference decreases by 55.1%, and the size of the modulation region is reduced by 50.1% when the average pump power reaches 60.8 mW. This work provides a new direction for low-power consumption and miniaturization of microstructure integration light-controlled switching devices in optical communication and quantum information processing.

Characterizing the effects of free carriers in fully etched, dielectric-clad silicon waveguides

We theoretically characterize the free-carrier plasma dispersion effect in fully etched silicon waveguides, with various dielectric material claddings, due to fixed interface charges and trap states at the silicon-dielectric interfaces. The values used for these charges are obtained from the measured capacitance-voltage characteristics of SiO2, SiNx, and Al2O3 thin films deposited on silicon substrates. The effect of the charges on the properties of silicon waveguides is then calculated using the semiconductor physics tool Silvaco in combination with the finite-difference time-domain method solver Lumerical. Our results show that, in addition to being a critical factor in the analysis of such active devices as capacitively driven silicon modulators, this effect should also be taken into account when considering the propagation losses of passive silicon waveguides.

Designs of Silicon MIS Phase Modulator With a Deposited AlN Film as the Gate Dielectric

We propose and analyze a silicon metal-insulator- semiconductor phase modulator based on a poly-Si/AlN/Si horizontal slot waveguide. The AlN gate dielectric exhibits an inherent Pockels effect, which can provide additional phase modulation besides that provided by the free-carrier plasma dispersion effect of Si. The proposed modulator with an optimized geometry offers a high modulation efficiency of 0.95 V · cm for the 1.55-μm transverse magnetic light even the electrooptical coefficient of AlN (r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> ) is only 1 pm/V, which is ~40% better than the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> counterpart with the same equivalent oxide thickness. The modulation efficiency increases quickly with r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> increasing, reaching 0.2 V · cm when r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> is 10 pm/V.

GeSi modulator based on two-mode interference

A GeSi modulator based on two-mode interference is designed in this study. A GeSi layer with a height of 0.22 μm is introduced to decrease the optical power overlap of the two modes. A doping region in which the free carrier plasma dispersion effect exists to change the oscillation period for on- and off-state switching is identified. A doping concentration of 1×10¹⁸ cm⁻³ for both n and p type is selected. The single modulation arm for 3 V operation is 1416.3 μm long. The extinction ratio and insertion loss are 15 and 5 dB, respectively. The traveling electrode design shows 3 dB bandwidth as large as 50 GHz.

Electrically tunable silicon plasmonic phase modulators with nano-scale optical confinement

Electrically tunable silicon (Si) plasmonic phase modulators with nano-scale optical confinement are presented and analyzed in this study. The modulation is realized based on two mechanisms: free carrier plasma dispersion effect in Si and high electro-optic effect in polymer. The phase modulators can be found potential applications in optical telecommunication and interconnect.

Analysis of Electrooptic Modulator With 1-D Slotted Photonic Crystal Nanobeam Cavity

A new nanocavity-based electrooptic modulator on silicon-on-insulator is proposed. This nanocavity is formed by a one-dimensional photonic crystal nanobeam cavity with a slot embedded. The slot lies in both the cavity and the distributed Bragg reflectors region. This results in an ultrasmall modal volume of 0.047 (λ/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and a high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor of 8.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> . Another advantage of the structure is that polymers can be infiltrated into the slot. Accordingly, we rely on the fast and strong electrooptic effect of polymers, rather than on the free-carrier plasma dispersion effect of silicon. Analysis shows a modulator with a bandwidth of 86 GHz, a switching voltage of 0.2 V, and a length of only 14 μm can be obtained.

Nonlinear Distortion in a Silicon Microring-Based Electro-Optic Modulator for Analog Optical Links

We analyze silicon microring modulators for analog signal generation, in terms of harmonic and intermodulation distortions. Free carrier plasma dispersion effect in silicon, Lorentzian-shaped resonance profile, and cavity photon lifetime are identified as the sources of modulation nonlinearity. Simulations show that the silicon microring modulators exhibit good modulation index and carrier-to-distortion ratio (CDR). For the generation of 10-GHz suboctave analog signals, a high optical-modulation-index of up to 46%, a CDR of 57 dB, and a spurious free dynamic range of 96 dB can be obtained. In fiber transmission, the microring-generated signal CDR can be improved by 10 dB due to the interaction between frequency chirp and chromatic dispersion.

Deposited silicon high-speed integrated electro-optic modulator

We demonstrate a micrometer-scale electro-optic modulator operating at 2.5 Gbps and 10 dB extinction ratio that is fabricated entirely from deposited silicon. The polycrystalline silicon material exhibits properties that simultaneously enable high quality factor optical resonators and sub-nanosecond electrical carrier injection. We use an embedded p(+)n(-)n(+) diode to achieve optical modulation using the free carrier plasma dispersion effect. Active optical devices in a deposited microelectronic material can break the dependence on the traditional single layer silicon-on-insulator platform and help lead to monolithic large-scale integration of photonic networks on a microprocessor chip.

Design of photonic crystal directional couplers for electro-optical wavelength switching in Si technology

Electro-optical wavelength switch in Si technology operating around = 1.55 µm and based on a photonic crystal directional coupler configuration was studied using Plane Wave Method and Finite Difference Time Domain simulations. Owing to the exploitation of slow wave supermodes, ultra-compact active devices (~ 20 µm) exploiting free carrier plasma dispersion effect in Si can be designed. Injection of input/output strip access waveguides with the photonic core area is optimized using tapering stages to gradually reduce light group velocity. With carrier injection around 1018 cm-3, the coupler configuration can be used as an optical switch of individual wavelengths or a multiplexer/demultiplexer device with active routing of wavelengths.

Directional-coupler-based Mach-Zehnder interferometer in silicon-on-insulator technology for optical intensity modulation

Based on silicon-on-insulator (SOI) technology, a Mach-Zehnder interferometer (MZI) is fabricated, in which two directional couplers serve as power splitter and combiner. The free carrier plasma dispersion effect of Si is adopted to achieve the phase modulation and the consequent intensity modulation of optical fields. The device presents an insertion loss of 2.61 dB and an extinction ratio of 19.6 dB. The rise time and fall time are 676 ns and 552 ns, respectively. Detailed analysis and explanation of the performance behaviors are also presented. (c) 2007 Society of Photo-Optical Instrumentation Engineers.