Free-carrier Effect Research Articles

Free carrier-induced optical nonlinearities in β-Ga2O3 single crystals at 355 nm

Gallium oxide (Ga2O3) is a promising candidate in future photonic integrated circuits devices due to its ultra-wide band gap and high stability. However, the nonlinear optical properties of Ga2O3 still remain elusive. In this work, we investigate the free-carrier absorption and refraction in pristine β-Ga2O3 single crystal via the degenerate pump-probe measurement with phase object technique at 355 nm wavelength. The free-carrier absorption cross-section and free-carrier refraction index of β-Ga2O3 on the sub-nanosecond time scale are determined to be ∼0.6 × 10−22 m2, and ∼−0.62 × 10−28 m3, respectively. And the two-photon absorption coefficient of β-Ga2O3 is characterized as ∼0.53 × 10−11 m W−1. It is found that the free-carrier absorption and free-carrier refraction effect in β-Ga2O3 are significantly smaller than that in other wide bandgap semiconductors such as GaN and ZnO. Our results can serve as guidelines for designing β-Ga2O3 based ultra-low loss waveguides and integrated photonic applications in UV spectral range.

Effects of free carriers on the optical properties of high mobility transition metal doped In2O3 transparent conductors

Transition metal doped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ with high mobility can be used as a transparent conductor with enhanced transparency spectral window. In this work, we carried out a comprehensive study on the electrical and optical properties of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ doped with several transition metal (TM) species (${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{TM}$) including W, Zr, Mo, and Ti. Detailed optical properties obtained by spectroscopic ellipsometry (SE) are correlated with electrical properties obtained by Hall effect measurements. We find that the mobility of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{TM}$ thin films lies in the range of $50\text{--}75\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{\ensuremath{-}}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{\ensuremath{-}}1}$, much higher than the typical mobility of $30\text{--}40\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{\ensuremath{-}}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{\ensuremath{-}}1}$ for conventional ITO. The complex dielectric functions of the thin films reveal remarkable carrier density dependent changes in the optical properties. SE analyses show that the electron effective mass of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{TM}$ at the bottom of the conduction band ${m}_{o}^{*}$ ($0.11\text{--}0.14{m}_{o}$) is much smaller than the reported ${m}_{o}^{*}\ensuremath{\sim}0.18\ensuremath{-}0.30{m}_{o}$ for ITO, which directly results in their higher mobility. This low ${m}_{o}^{*}$ is consistent with recent theoretical studies which proposed that $4d$ donor states of the TMs are resonance in the CB. For films with comparably low resistivity of $1\text{--}2\ifmmode\times\else\texttimes\fi{}{10}^{\text{\ensuremath{-}}4}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$, we find that ${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{TM}$ films have \ensuremath{\sim}4--10 times lower absorption coefficient at $\ensuremath{\lambda}=1300\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ due to free carrier absorption and have their plasma reflection edge extended to \ensuremath{\sim}1.7 \ensuremath{\mu}m compared to \ensuremath{\sim}1.2--1.4 \ensuremath{\mu}m for ITO. Hence, using TM doping we have achieved transparent conductors with conductivity comparable to ITO but with transmission extended to >1600 nm. These materials will be potentially important as transparent conductors for optoelectronic devices utilizing NIR photons.

Impact of nonlinear effects in Si towards integrated microwave-photonic applications.

As one of major integrated microwave photonics (IMWP) platforms, Si photonics exhibits the intensity-dependent Kerr effect and two-photon absorption (TPA) with associated free carrier effects (FCE). At the commonly used 1.55 µm, TPA losses and the associated FCE would eventually limit the dynamic range of Si photonic links. Resonating structures such as ring resonators (RRs) experience enhanced nonlinear effects due to significant intensity buildup. According to the bandgap characteristics of Si, TPA can be eliminated at and beyond 2.2 µm. In this work, a systemic simulation of straight waveguides and RRs is performed at wavelengths from 1.55 to 2.2 µm where the wavelength-dependent TPA loss is investigated. Moreover, the Kerr effect leads to unwanted change of refractive index, which shifts the RR resonant wavelength at both 1.55 and 2.2 µm, thus needing shift compensation. Compensated RRs operating at 2.2µm could open a new venue for Si photonics towards IMWP applications.

Direct measurement of ferroelectric polarization in a tunable semimetal

Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges. Recent measurements, however, demonstrated signatures of ferroelectric switching in the electrical conductance of bilayers and trilayers of WTe2, a semimetallic transition metal dichalcogenide with broken inversion symmetry. An especially promising aspect of this system is that the density of electrons and holes can be continuously tuned by an external gate voltage. This degree of freedom enables measurement of the spontaneous polarization as free carriers are added to the system. Here we employ capacitive sensing in dual-gated mesoscopic devices of bilayer WTe2 to directly measure the spontaneous polarization in the metallic state and quantify the effect of free carriers on the polarization in the conduction and valence bands, separately. We compare our results to a low-energy model for the electronic bands and identify the layer-polarized states that contribute to transport and polarization simultaneously. Bilayer WTe2 is thus shown to be a fully tunable ferroelectric metal and an ideal platform for exploring polar ordering, ferroelectric transitions, and applications in the presence of free carriers.

Review on ZnO-based piezotronics and piezoelectric nanogenerators: aspects of piezopotential and screening effect

Among various piezoelectric materials, ZnO has attracted a great deal of attention due to facile preparations and exceptional semiconductor characteristics compared to other conventional piezoceramics or organic piezoelectric materials. One of the issues hindering ZnO from progressing into applications is the screening effect, where the intrinsic piezopotential generated upon mechanical deformations is screened and becomes waned or even diminished by the presence of intrinsic free carriers in ZnO. Consequently, ZnO-based piezoelectric devices often suffer from low output voltages, resulting in low total output power generation even though the output current could be larger than those made of insulating piezoelectric materials, such as PZT, polyvinylidene fluoride, and barium titanate. It is therefore vital to fully understand the impact of the screening effect and produce strategies to handle this issue in the context of piezotronics and piezoelectric nanogenerators (PENG). Therefore, this article presents a comprehensive review of growth methodologies for various ZnO nanostructures, structure modifications, effects of free carriers on the screening effect and strategies for device applications, including strain-gated transistors, PENG and piezotronic sensors for gas, humidity and bio-molecules etc.

Design of a high extinction ratio silicon optical modulator at 2 µm using the cascaded compensation method

The effect of the free carrier effect is more significant in the mid-infrared band when compared with the 1310 band and 1550 band. In this paper, we propose a cascaded Mach-Zehnder interference (MZI) structure to improve the extinction ratio (ER) of the modulator in the mid-infrared band. The cascaded compensation method is to add the next-stage equal-arm MZI device to the two phase shifters of the major MZI. The output light intensity of the two phase shifters can be maintained at the same level by adjusting the output loss of both the equal-arm MZI. With the cascaded compensation method, the simulated ER of the optical modulator is increased from 36 dB to 55 dB under −4 Vbias while the device still maintains a low insertion loss (IL) of 12.5 dB. Through the cascaded compensation method, the modulation depth of the modulator at −2 V, −4 V, −6 V, and −8 V are 58 dB, 53 dB, 57 dB, and 59 dB, respectively. Meanwhile, the dynamic ER is 9.2 dB at a data rate of 40 Gbps, which is 4.5 dB higher than that of the original one.

Practical security of a chip-based continuous-variable quantum-key-distribution system

A chip-based continous-variable quantum-key-distribution (CVQKD) system with a high practical confidentiality performance is crucial for constructing quantum metropolitan communication networks, but imperfections in the chip-based modulation will threaten the practical security of the chip-based CVQKD system. In this paper, we combine the plasma dispersion effect of free carriers to model the carrier fluctuations and reveal the essential mechanism of carrier fluctuations' influence on the system. The simulations show that the chip-based CVQKD system may face potential loophole threats or its performance will dramatically decrease under different carrier fluctuations. Moreover, two preliminary defense strategies are proposed to completely solve the practical security problems commonly induced by modulators in general chip-based CVQKD systems. This work proposes a set of modeling and analysis methods for general chip-based CVQKD systems' modulators, which provides constructive methods to build the chip-based CVQKD system with more rigorous practical security.

Free-carrier-assisted mid-infrared microcavity soliton generation

Multi-photon absorption (MPA) and free-carrier (FC) effects, usually considered to be detrimental to microcomb generation by introducing strong nonlinear loss, also offer opportunities to overwhelm the thermal-optic effect by modifying the refractive index. Here, we derive the theoretical expression of solitons expanded with MPA and FC effects, accompanied by numerical simulations to reveal the dynamics and mechanism for capturing steady mid-infrared solitons in both Si and Ge microcavities. It is found that with increased FC lifetime, the intensity-dependent MPA underlies non-monotonous variation of nonlinear detuning which enables soliton generation. More interestingly, proper control on the FC lifetime admits bidirectional switching of soliton states (i.e., both decreasing and increasing the number of solitons) as well as self-starting solitons, yielding different technique routes toward microcavity solitons. This research could contribute to a better understanding of nonlinear behaviors influenced by FC effects and find practical applications by releasing the demand on precise control of laser sources, which is especially meaningful for the mid-infrared region.

Dynamic all-optical control in ultrashort double-pulse laser ablation

Double-pulse femtosecond laser ablation of thin aluminum films and bulk aluminum counterintuitively demonstrated a strong (60–70%) raise of the thickness-dependent thresholds for inter-pulse delays of 20–200 ps, preventing material removal at above-threshold fluencies. Time-resolved optical pump-probe reflection and double-pump transmission studies were performed and confirmed the variation of the ablation threshold depending on the interpulse delay. To support the experimental measurements, the process of double-pulse laser ablation was modelled with the combined atomistic-continuum model. The applied model can describe the laser-induced non-equilibrium phase transition processes at atomic precision, whereas the effect of free carriers, playing a determinant role for the case of ultrashort laser pulses, is accounted for in the continuum. The simulations revealed the underlying pre-ablative laser-induced stress dynamics in the hot, acoustically relaxed Al melt, crucially sensitive to the second pump-pulse compressive pressurization. The results of theoretical and experimental study enable efficient dynamic all-optical control of ultrafast laser ablation.

Carrier-induced optical bistability in the silicon micro-ring resonators under continuous wave pumping

The results of a study of the nonlinear frequency response of a silicon micro-ring resonator (MRR) manufactured using silicon-on-insulator technology are presented. Appearance of optical bistability caused by the frequency shift of the MRR resonant spectrum under continuous wave pumping is experimentally demonstrated. The experimental results are explained in the framework of an original theory, which accounts for the linear loss, Kerr nonlinearity, two-photon absorption, free-carrier-induced absorption and dispersion, and the thermo-optic effect. It is shown that the bistability of the frequency response originates from competition between frequency shifts of opposite signs that appear due to the presence of the thermo-optic and free-carrier effects, with the latter one being dominant.

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.

All-optical nanophotonic resonant element for switching and routing applications exploiting graphene saturable absorption

A silicon disk resonator overlaid with a uniform graphene layer in an add-drop configuration is proposed as an all-optical routing element. Operation is based on the saturable absorption effect provided by the graphene layer. The element is thoroughly analyzed as a two-channel device in the context of an appropriate nonlinear framework combining perturbation theory and temporal coupled-mode theory. Taking into consideration the primary nonlinear effect, which is graphene saturable absorption, a design path is carefully developed that eventually leads to a traveling-wave resonant element with low-power requirements, low insertion loss, high extinction ratio, and sufficient bandwidth. In a subsequent step, other important nonlinear effects originating from graphene and the silicon disk, including the Kerr effect and free-carrier effects, are considered and means for counterbalancing their action are demonstrated. A low control power of 9mW together with a bandwidth of 20GHz is shown possible, with the insertion loss of almost 3dB and an extinction ratio over 10dB in both ports (add and drop).

Determination of Electrodynamic Parameters of In2O3 thin Films by Terahertz and Infrared Spectroscopy

Indium oxide is a transparent semiconductor widely used in optical-electronic systems of the visible and near infrared range, but insufficiently studied in the terahertz frequency range. In this study, the dielectric response function of indium oxide thin films on glass substrates was investigated by terahertz and infrared spectroscopy techniques to determine the effect of free carriers on the optical characteristics of films. The experimental results were analyzed applying multiparametric dispersion model of Drude–Lorentz. The basic electrodynamic parameters of In2O3 films are determined. Model transmission and conductivity spectra for films of different thickness are calculated. The possibility of using thin films of indium oxide as a transparent conductive layer in terahertz optoelectronics devices is shown.

Optical modulation in Ge-rich SiGe waveguides in the mid-infrared wavelength range up to 11 \xb5m

Waveguide integrated optical modulators in the mid-infrared wavelength range are of significant interest for molecular spectroscopy. This is because on-chip synchronous detection can improve the performance of detection systems and can also be used for free-space communications where optical modulators working in atmospheric transparency windows are needed. Here we report optical modulation in a mid-infrared photonic circuit, reaching wavelengths larger than 8 µm. Optical modulation in the wavelength range from 5.5 to 11 µm is shown, relying on a broadband Ge-rich graded-SiGe platform. This demonstration experimentally confirms the free-carrier absorption effect modeling. These results pave the way towards efficient high-performance electrically-driven integrated optical modulators in the mid-infrared wavelength range.

Impact of third-order dispersion and three-photon absorption on mid-infrared time magnification via four-wave mixing in Si0.8Ge0.2 waveguides.

We investigate the influence of third-order dispersion of dispersive elements, three-photon absorption and free-carrier effects on mid-infrared time magnification via four-wave mixing (FWM) in ${{\rm Si}_{0.8}}{{\rm Ge}_{0.2}}$Si0.8Ge0.2 waveguides. It is found that the magnified waveform is seriously distorted by these factors, and conversion efficiency is decreased, mainly because of nonlinear absorption. A time lens based on FWM in ${{\rm Si}_{0.8}}{{\rm Ge}_{0.2}}$Si0.8Ge0.2 waveguides is proposed for time magnification of mid-infrared ultrashort pulses, in which the low-distortion, high-magnification in the time domain could be obtained by optimizing system parameters. These results make it possible to analyze the transient dynamic process through oscilloscopes and detectors with gigahertz bandwidth and have important applications in ultrafast process analysis, optical pulse sampling, and optical communications.

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.

Enhanced efficiency thermo-optic phase-shifter by using multi-mode-interference device

We demonstrate a method for power reduction in thermo-optic based Mach-Zehnder interferometer (MZI) by using the multimode region of a 2x2 Multi-Mode-Interferometer (MMI) as the modulation region. The light is circulated through the same multimode region twice and therefore utilizes the already present change in temperature leading to additional phase change, and an increase in efficiency. Power saving of 29.6% compared to a conventional thermo-optic phase shifter using single mode waveguides has been experimentally demonstrated. The reported devices show minimal insertion loss penalty compared to generic devices and do not add any additional fabrication complexity. Such an approach could also be applied to higher speed devices, for example those employing free carrier effects.

Numerical Investigation of Ultrashort Laser-Ablative Synthesis of Metal Nanoparticles in Liquids Using the Atomistic-Continuum Model.

We present a framework based on the atomistic continuum model, combining the Molecular Dynamics (MD) and Two Temperature Model (TTM) approaches, to characterize the growth of metal nanoparticles (NPs) under ultrashort laser ablation from a solid target in water ambient. The model is capable of addressing the kinetics of fast non-equilibrium laser-induced phase transition processes at atomic resolution, while in continuum it accounts for the effect of free carriers, playing a determinant role during short laser pulse interaction processes with metals. The results of our simulations clarify possible mechanisms, which can be responsible for the observed experimental data, including the presence of two populations of NPs, having a small (5–15 nm) and larger (tens of nm) mean size. The formed NPs are of importance for a variety of applications in energy, catalysis and healthcare.

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.

Enhancing Pockels effect in strained silicon waveguides.

The magnitude and origin of the electro-optic measurements in strained silicon devices has been lately the object of a great controversy. Furthermore, recent works underline the importance of the masking effect of free carriers in strained waveguides and the low interaction between the mode and the highly strained areas. In the present work, the use of a p-i-n junction and an asymmetric cladding is proposed to eliminate the unwanted carrier influence and improve the electro-optical modulation response. The proposed configuration enhances the effective refractive index due to the strain-induced Pockels effect in more than two orders of magnitude with respect to the usual configuration.