Free-carrier Dispersion Effect Research Articles

Investigation of an absorption plasmonic electro-optical modulator based on the free carrier dispersion effect in the aluminum doped zinc oxide layer

A plasmonic electro-optical modulator, which is based on the free carrier dispersion effect, has been introduced here. The structure of the proposed modulator is a substrate of fused silica/gold/aluminum doped zinc oxide/hafnium dioxide/gold/air. The free carrier dispersion effect occurs in the aluminum doped zinc oxide (AZO) layer. As the electrical permittivity of the AZO layer is near zero (the epsilon near zero effect) at a wavelength of 1.55 μm, the amplitude of an electrical field is high in this layer, therefore, the modulator is highly sensitive to the varying refractive index of this layer. By applying a voltage to two gold layers, the electrical charge density changes in the AZO layer. Therefore, the real and imaginary parts of the refractive index have been changed, which led to a change in the absorption of the modulator. In order to obtain the charge density distribution in the AZO layer, the Poisson equation is solved by using the finite difference method. To investigate modulator absorption, the Nelder-Mead method is implemented in order to solve the dispersion equation numerically. Finally, the magnetic field, the electric field, and the time average of the Poynting vector have been given by using the least squares approximation method.

Highly efficient silicon modulator via a slow-wave Michelson structure.

The weak free carrier dispersion effect significantly hinders the adoption of silicon modulators in low-power applications. While various structures have been demonstrated to reduce the half-wave voltage, it is always challenging to balance the trade-off between modulation efficiency and the bandwidth. Here, we demonstrated a slow-wave Michelson structure with 1-mm-long active length. The modulator was designed at the emerging 2-μm wave band which has a stronger free carrier effect. A record high modulation efficiency of 0.29 V·cm was achieved under a carrier depletion mode. The T-rail traveling wave electrodes were designed to improve the modulation bandwidth to 13.3 GHz. Up to 20 Gb/s intensity modulation was achieved at a wavelength of 1976 nm.

Optomechanical Frequency Comb Based on Multiple Nonlinear Dynamics.

Phonon-based frequency combs that can be generated in the optical and microwave frequency domains have attracted much attention due to the small repetition rates and the simple setup. Here, we experimentally demonstrate a new type of phonon-based frequency comb in a silicon optomechanical crystal cavity including both a breathing mechanical mode (∼GHz) and flexural mechanical modes (tens of MHz). We observe strong mode competition between two approximate flexural mechanical modes, i.e., 77.19 and 90.17MHz, resulting in only one preponderant lasing, while maintaining the lasing of the breathing mechanical mode. These simultaneous observations of two-mode phonon lasing state and significant mode competition are counterintuitive. We have formulated comprehensive theories to elucidate this phenomenon in response to this intriguing outcome. In particular, the self-pulse induced by the free carrier dispersion and thermo-optic effects interacts with two approximate flexural mechanical modes, resulting in the repetition rate of the comb frequency-locked to exact fractions of one of the flexural mechanical modes and the mode hopping between them. This phonon-based frequency comb has at least 260 comblines and a repetition rate as low as a simple fraction of the flexural mechanical frequency. Our demonstration offers an alternative optomechanical frequency comb for sensing, timing, and metrology applications.

Microdisk modulator-assisted optical nonlinear activation functions for photonic neural networks

On-chip implementation of optical nonlinear activation functions (NAFs) is essential for realizing large-scale photonic neural chips. To improve the reconfigurability and enrich the functions of photonic neural network (PNN), an NAF unit with a unique capability of performing multiple types of NAFs is highly preferred. In this work, we propose and experimentally demonstrate a microdisk modulator (MDM)-assisted NAF unit, in which multiple types of electro-optic NAFs with a high response speed and multiple types of all-optical NAFs with a low latency and reduced power consumption are performed in a single device. The fabricated MDM has an add-drop configuration, in which a lateral PN junction is incorporated to achieve high-speed resonance wavelength tuning. With the use of the high-speed nonlinear electro-optic effect, three different kinds of electro-optic NAFs including sigmoid function, radial basis function (RBF), and negative rectified linear unit (ReLU) function are realized by exploiting the free-carrier dispersion effect on silicon. Moreover, thanks to the strong optical confinement in the disk cavity, all-optical NAFs can be performed by exploiting the thermo-optic effect on silicon. In the experiment, four different kinds of all-optical NAFs including softplus function, RBF, clamped ReLU function, and leaky ReLU function are demonstrated. With the use of the realized clamped ReLU function, a convolutional neural network is simulated to perform a handwritten digit classification benchmark task, and an accuracy as high as 98 % is demonstrated. Thanks to its strong electro-optic and thermo-optic effects, the proposed MDM provides a unique capability of performing multiple types of electro-optic and all-optical NAFs, which is potential to be served as a flexible nonlinear unit in large-scale PNN chips.

Harnessing self-heating effect for ultralow-crosstalk electro-optic Mach–Zehnder switches

This paper presents a novel approach to counterbalance free-carrier-absorption (FCA) in electro-optic (E-O) Mach–Zehnder interferometer (MZI) cells by harnessing the self-heating effect. We show insights on crosstalk limitations in MZIs with direct carrier-injection and provide a detailed design methodology on a differential phase shifter pair. Leveraging both free-carrier dispersion (FCD) and self-heating effects, our design enables arbitrary phase tuning with balanced FCA loss in the pair of arms, eliminating the need for additional phase corrections and creating ultralow crosstalk MZI elements. This neat design disengages from the commonly used nested structure, thus providing an opportunity of embedding tunable couplers for correcting imperfect splitting ratios given that only two are needed. We show that with the use of tunable directional couplers, a standard ±10 nm process variation is tolerated, while achieving a crosstalk ratio below −40 dB. By direct carrier injection in both arms, the proposed device operates at nanosecond scales and can bring about a breakthrough in the scalability of E-O switch fabrics, as well as other silicon integrated circuits that have stringent requirements for crosstalk leakage.

An investigation and analysis of plasmonic modulators: a review

Abstract Plasmonics is an emerging and very advantageous technology which provides high speed and tiny size devices for fulfilling the demand of today’s high-speed world. SPPs are the information carrying elements in plasmonics, which are capable of breaking the diffraction limit. Plasmonics technology has shown its application in uncountable nanophotonic applications like switching, filtering, light modulation, sensing and in many more fields. Modulators are the key components of integrated photonic system. Various modulators which work on different effects are discussed in this study for providing a universal idea of modulators to researchers. Some useful plasmonic active materials are also discussed which are used in most of plasmonic modulators and other active devices. Previously, many researchers have worked on many kinds of modulators and switches, which operate on different kind of operating principles. For providing an overview about plasmonic modulators, their classification and their operation, we have discussed the state of art of some previously introduced modulators and switches which operates on electro-refractive effects and include electro-optic effect, Pockels effect, free charge carrier dispersion effect, phase change effect, elasto-optic effect, magneto-optic effect, and thermo-optic effect. Instead of different effects used in plasmonic switches and modulators different active materials like liquid crystals, graphene, vanadium di-oxide, chalcogenides, polymers, indium tin oxide, bismuth ferrite, barium titanate, and lithium niobate are also explained with their properties. Additionally, we also compared modulators based on different effects in terms of their design characteristics and performances.

Electromagnetically induced transparency from first-order dynamical systems

We show how a strongly driven single-mode oscillator coupled to a first-order dynamical system gives rise to induced absorption or gain of a weak probe beam, and associated fast or slow light depending on the detuning conditions. We derive the analytic solutions to the dynamic equations of motion, showing that the electromagnetically induced transparency (EIT) like response is a general phenomenology, potentially occurring in any nonlinear oscillator coupled to first-order dynamical systems. The resulting group delay (or advance) of the probe is fundamentally determined by the system damping rate. To illustrate the practical impact of this general theoretical framework, we quantitatively assess the observable consequences of either thermo-optic or free-carrier dispersion effects in conventional semiconductor microcavities in control/probe experiments, highlighting the generality of this physical mechanism and its potential for the realization of EIT-like phenomena in integrated and cost-effective photonic devices.

Design of high extinction ratio, low loss, and compact optical modulators and switches based on GST phase change material

In this paper, an electrically driven optical modulator and switch based on micro-ring resonators using the phase change material (PCM) of G e 2 S b 2 T e 5 (GST) is designed and their optical properties are investigated. The change in refractive index between two states in PCMs is orders of magnitude larger than conventional electro-optic and free-carrier dispersion effects. These extraordinary variations make PCMs excellent candidates for optical active devices such as modulators and switches. In our design, GST is used as the active material because it is far more stable and results in lower insertion loss compared to other PCMs such as vanadium dioxide ( V O 2 ) (at λ = 1.55 µ m , the imaginary part of the refractive index is 1.49 for GST and 2.53 for V O 2 is their metallic phases). By optimizing the GST dimensions, the device figure of merit is maximized. Three-dimensional finite-difference time-domain simulations show a very high extinction ratio of 37.3 dB with a low insertion loss of 0.427 dB for the proposed modulator and high extinction ratios of 19 dB and 10 dB in off and on states and insertion losses of 1.52 dB and 2.02 dB in off and on states, respectively, of the proposed 1 × 2 switch.

4-Bit All-Optical Serial-to-Parallel Converter With Sub-dB/cm Delay Lines Based on Rib Waveguides

This paper extends our previous work on the microring resonator-type serial-to-parallel conversion scheme aimed for all-optical label processing on the silicon photonic platform. In the previously proposed scheme, the architecture of the delay lines was silicon wires. This resulted in considerable propagation loss, confining multibit operation of serial-to-parallel conversion to a label packet of 2 bits. However, during a practical application, propagation loss at the delay lines has to be improved to support label packets comprising of a greater number of bits. The purpose of this article is twofold. First, we confirm that propagation loss can be lowered to sub-dB/cm values by introducing rib waveguides via simulations and experiment. Second, we demonstrate multibit serial-to-parallel conversion on a 4-bit label “1101” by utilizing a cascaded microring resonator device of which the straight sections of the delay lines are of rib waveguide architecture. In order to focus on free-carrier dispersion effect of silicon during operation, we left out a temporal delay between the pump pulses and probe bits to be extracted. As a result, serial-to-parallel conversion was viable even at peak pump power levels as low as ∼100 mW. We hereby verify that the proposed serial-to-parallel conversion scheme incorporated with rib waveguide delay lines is a realistic solution in realizing a photonics-based solution to all-optical label processing.

Electrically tuned fractional-order temporal differentiator in silicon photonics

Here, full-wave analysis and design of an electrically tunable optical fractional-order temporal differentiator (DIFF) based on an ultra-compact microring resonator (MRR) has been presented. Electrical tunability has been realized through free carrier dispersion (FCD) effect in the resonator with P-I-N and P-N junctions. The first-order optical DIFF is realized based on the critically-coupled resonator without applying any bias voltages in the junctions. Also, fractional-order DIFF with differentiation order α > 1 is realized by over-coupled resonator with the forward-biased P-I-N and P-N junctions. While, fractional-order DIFF with α < 1 is realized by under-coupled resonator with reverse-biased P-N junction. The high performance of the designed tunable optical DIFFs and their high accuracy have been confirmed considering an ultra-fast Gaussian input pulse with short full-width at half-maximum (FWHM) of 6.3 ps.

Free carrier induced dark pulse generation in microresonators.

We theoretically demonstrate a novel, to the best of our knowledge, mechanism for dark pulse excitation in normal dispersion microresonators exploiting free carrier dispersion and free carrier absorption effects due to multi-photon absorption. Dark pulses can be generated in the three- and four-photon absorption regimes in the presence or absence of external reverse bias to control the lifetime of free carriers, respectively. Direct generation of dark pulses is proven to be feasible in both regimes with a frequency fixed laser. The dynamics of their temporal and spectral evolution have also been investigated. Our findings establish a reliable path for dark pulse and Kerr microcomb generation in related platforms with simplified controlling and tuning techniques.

Reconfigurable all-optical nonlinear activation functions for neuromorphic photonics.

We experimentally demonstrate all-optical reconfigurable nonlinear activation functions in a cavity-loaded Mach-Zehnder interferometer device on a silicon photonics platform, via the free-carrier dispersion effect. Our device is programmable to generate various nonlinear activation functions, including sigmoid, radial-basis, clamped rectified linear unit, and softplus, with tunable thresholds. We simulate benchmark tasks such as XOR and MNIST handwritten digit classifications with experimentally measured activation functions and obtain accuracies of 100% and 94%, respectively. Our device can serve as nonlinear units in photonic neural networks, while its nonlinear transfer function can be flexibly programmed to optimize the performance of different neuromorphic tasks.

Numerical study of a plasmonic absorption electro-optical modulator based on n-doped silicon and barium titanate

In this paper, a numerical solution for a plasmonic absorption modulator with a six-layer structure consisting of an air superstrate, gold, Barium Titanate, n-type Silicon, gold layers and thin-shell nanolattice aluminum oxide substrate is presented. Regarding the suggested structure, parameters related to the absorption modulation are investigated at different thicknesses. Here, the Pockels effect and the free carrier dispersion effect are considered simultaneously. The dispersion relation of this structure is analytically obtained and numerically solved by the Nelder-Mead method. According to our results, it is understood that this modulator has a high ability to be utilized in optical communication systems.

MZI-based all-optical serial-to-parallel conversion circuit by free-carrier dispersion effect

Device applications of nonlinear optical phenomena on the silicon-on-insulator platform are attracting significant attention in silicon photonics. We report a novel on-chip Mach–Zehnder interferometer-type serial-to-parallel converter aimed at all-optical label processing of optical packets. We focus on the free-carrier dispersion effect during phase modulation of the serial probe signal, and a pump signal is used to generate sufficient free-carriers within the waveguide structure. By leaving out a delay between the pump and probe pulses, we avoid the effects of pump-induced cross-phase modulation and two-photon absorption, and perform serial-to-parallel conversion at a lower pump power. In the theoretical analysis, serial-to-parallel conversion of a 10-Gbps probe signal is considered. By numerical simulations, the shortest arm length of the Mach–Zehnder interferometric structure required for operation is calculated. When doing so, the pump pulse width and delay between the pulses are varied while the pulse energy of the pump is held constant at 25 pJ, and the combination of values that corresponds to the shortest arm length is taken as optimum values. Our study reveals that the optimum values for delay and pump pulse width are 30 ps and 25 ps, respectively. Finally, we also demonstrate single-bit serial-to-parallel conversion experimentally on a Mach–Zehnder interferometric device. Phase modulation due to free-carrier dispersion was observed at peak pump power levels of 1.0 W at the waveguide input.

Single photon detection by cavity-assisted all-optical gain

We consider the free carrier dispersion effect in a semiconductor nanocavity in the limit of discrete photoexcited electron-hole pairs. This analysis reveals the possibility of ultrafast, incoherent transduction and gain from a single photon signal to a strong coherent probe field. Homodyne detection of the displaced probe field enables a new method for room temperature, photon-number-resolving single photon detection. In particular, we estimate that a single photon absorbed within a silicon nanocavity can, within tens of picoseconds, be detected with $\sim 99\%$ efficiency and a dark count rate on the order of kHz assuming a mode volume $V_\text{eff}\sim 10^{-2}(\lambda/n_\text{Si})^3$ for a 4.5 micron probe wavelength and a loaded quality factor $Q$ on the order of $10^4$.

The Franz-Keldysh effect and free carrier dispersion effect in germanium

We calculated the changes of refractive index and absorption coefficient in Ge induced by the Franz-Keldysh (FK) effect and free carrier dispersion (FCD) effect, taking advantage of reported absorption spectra.The FK effect is weak near the indirect absorption edge of Ge. FCD effect can induce a much larger change of refractive index. In the wavelength range of 2–10 μm, theoretical formulas about dependences of the changes of absorption coefficient and refractive index on the change of carrier concentrations are presented. In addition, FCD effects in N-type and P-type Ge are compared.

The influence of thermal and free carrier dispersion effects on all-optical wavelength conversion in a silicon racetrack-shaped microring resonator

We experimentally demonstrate ultra-low pump power wavelength conversion based on four-wave mixing in a silicon racetrack-shaped microring resonator. When the pump and signal are located at the resonance wavelengths, wavelength conversion with a pump power of only 1 mW can be realized in this microring resonator because of the resonant enhancement of the device. However, saturation of the conversion efficiency occurs because of the shift of the resonance peak, which is caused by the change of the effective refractive index induced by a combination of thermal and free carrier dispersion effects, and it is demonstrated that the thermal effect is the leading-order factor for the change of the refractive index. The maximum conversion efficiency of −21 dB is obtained when the pump power is less than 12 mW. This ultra-low-power on-chip wavelength convertor based on a silicon microring resonator can find important potential applications in highly integrated optical circuits for all-optical signal processing.

Noise-hidden signal recovery via stochastic resonance in the SOI waveguide resonator

We propose a method to recover weak pulse signals buried in noise via stochastic resonance (SR) based on optical bistability induced by the free-carrier dispersion effect in the silicon-on-insulator (SOI) waveguide resonator. The bistable system threshold is determined by the resonator parameters including the waveguide length, mirror reflectivity, and the free-carrier lifetime. A signal with different power levels can be detected by changing the free-carrier lifetime using a reverse-biased p-i-n junction embedded in the SOI waveguide. The influence of the system parameters on the SR is quantitatively analyzed by calculating the cross-correlation coefficient between the input and output signals. A cross-correlation gain of 5.6 is obtained by optimizing the system parameters of the SOI waveguide resonator. The results show the potential of using this structure to reconstruct and extract weak signals in all-optical integrated systems.

Coupled-ring reflector in an external-cavity tunable laser

Miniaturized tunable lasers are important as light sources in optical networks and sensing systems. However, challenges remain in realizing a low-cost on-chip tunable laser with high tuning accuracy and wide tuning range. Here, an external-cavity tunable laser integrated by using a pair of coupled-ring resonators as a reflector onto a single silicon chip is designed and demonstrated. The silicon coupled-ring reflector provides simultaneous tuning of the central reflective wavelength and the free spectral range of the entire cavity to guarantee phase matching. The effective optical length of the coupled-ring reflector is controlled by the complementary thermo-optic and free-carrier dispersion effects, thus enabling high-resolution wavelength fine-tuning while maintaining wide wavelength tuning range. Experimentally, the tunable laser has high external-cavity efficiency (22%) and high wavelength tuning accuracy (2 pm) throughout the 41.6 nm tuning range. Potential applications of the external-cavity tunable laser include optical networks, optical sensing, and on-chip optoelectronic systems.

Electro-optic directed XOR logic circuits based on parallel-cascaded micro-ring resonators.

We report an electro-optic photonic integrated circuit which can perform the exclusive (XOR) logic operation based on two silicon parallel-cascaded microring resonators (MRRs) fabricated on the silicon-on-insulator (SOI) platform. PIN diodes embedded around MRRs are employed to achieve the carrier injection modulation. Two electrical pulse sequences regarded as two operands of operations are applied to PIN diodes to modulate two MRRs through the free carrier dispersion effect. The final operation result of two operands is output at the Output port in the form of light. The scattering matrix method is employed to establish numerical model of the device, and numerical simulator SG-framework is used to simulate the electrical characteristics of the PIN diodes. XOR operation with the speed of 100Mbps is demonstrated successfully.