Plasma Dispersion Effect Research Articles

Challenges and Opportunities of Near and Mid-Infrared Photonics Based on SiGe and Ge

Silicon-germanium (SiGe) and germanium (Ge) have been investigated as high-mobility channel materials for high-performance metal-oxide-semiconductor field-effect transistors (MOSFETs). The intense development of heterogeneous integration of SiGe and Ge on Si has given us opportunities to integrate new functionalities on Si platform. Since SiGe and Ge have narrow bandgap energies which match to photon energies used in optical fiber communication, SiGe/Ge photodetectors monolithically integrated on Si are available, evoking the emergence of Si photonics technology as a standard photonics integrated circuit (PIC) platform. In addition to photodetector (PD) applications, we have investigated the heterogeneous integration of SiGe and Ge on Si to extend the functionalities for Si photonics platform. In this paper, we present challenges and opportunities of SiG and Ge for near and mid-infrared integrated photonics. We have investigated strained SiGe to enhance the modulation efficiency of Si-based optical modulators. Since there are no significant electro-optic effects in Si, free-carrier modulation is a common method to modulate optical properties such as refractive index and absorption coefficient. However, the weak free-carrier effects such as the plasma dispersion effect and free-carrier absorption result in low modulation efficiency. To cover the shortcoming of Si, we have proposed to introduce strained SiGe. We have predicted that biaxial compressive strain enhances the free-carrier effect because of the light hole effective mass in SiGe [1]. Owing to the lattice-mismatch between Ge and Si, biaxial compressive strain is introduced into SiGe grown on Si. According to the Drude model, we have estimated approximately three times greater plasma dispersion effect in SiGe than in Si when a Ge content in SiGe is 0.5. To demonstrate the enhanced free-carrier effects in SiGe, carrier-injection SiGe optical modulators have been explored. First we have examined the free-carrier absorption by measuring the optical attenuation in the SiGe modulator with varied injection current. We have observed approximately two times greater absorption in the SiGe device than in the Si device [2]. We have also fabricated a SiGe Mach-Zehnder interferometer (MZI) optical modulator to evaluate refractive index change precisely. The enhanced plasma dispersion effect in SiGe have bee also confirmed [3], proving the concept of the strain-induced enhancement in the free-carrier effect. We have also investigated SiGe optical modulators based on accumulation [4] and depletion [5]. Thus, strained SiGe is also very promising for optical modulator applications as well as transistor application. We have also investigated Ge-based photonics by using CMOS compatible process technologies. First we have examined the surface passivation impact on the dark current of Ge PDs. Owing to the high-quality Ge oxide/Ge interface, we have achieved extremely low dark current density of 0.032 mA/cm2 in conjunction with the gas phase doped n+/p junction [6]. We have also revealed the impact of fixed charged in the passivation layer on the dark current [7]. Since Ge is transparent in mid-infrared wavelengths, Ge photonics is emerging as mid-infrared integrated photonic platform for bio/medical and environmental sensor applications. We have proposed the Ge CMOS photonics platform by using a Ge-on-insulator (GeOI) wafer [8]. We have fabricated a high-quality GeOI wafer by wafer bonding. Ge waveguide devices including rib waveguides, bend waveguides, MMI couplers, ring resonators, optical modulators have been successfully demonstrated on the GeOI wafer [9, 10]. In conclusion, the heterogeneous integration of SiGe and Ge on Si opens up many possibilities for optoelectronic application for near and mid-infrared wavelengths as well as electronics applications. This work was supported by a Grant-in-Aid for Young Scientists (S) from MEXT, and NEDO “PECST” project.

Challenges and Opportunities of Near and Mid-Infrared Photonics Based on SiGe and Ge

Silicon-germanium (SiGe) and germanium (Ge) have been investigated as high-mobility channel materials for high-performance metal-oxide-semiconductor field-effect transistors (MOSFETs). The intense development of heterogeneous integration of SiGe and Ge on Si has given us opportunities to extend functionalities of photonic integrated circuits based on Si photonics technologies. We have investigated strain engineering by SiGe for enhancing modulation efficiency in Si optical modulators. By using the light hole effective mass in strained SiGe, we have demonstrated the enhanced plasma dispersion effect in SiGe. In addition to low-dark-current Ge photodetectors with GeO2 passivation, we have proposed Ge CMOS photonics for mid-infrared integrated photonics platform. We have successfully demonstrated Ge passive waveguide devices as well as carrier-injection variable optical attenuator operated at a 2-µm wavelength on Ge-on-Insulator wafer.

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.

Silicon-Based On-Chip Electrically-Tunable Spectral Shaper for Continuously Tunable Linearly Chirped Microwave Waveform Generation

A silicon-based on-chip electrically-tunable spectral shaper for the generation of a tunable linearly chirped microwave waveform (LCMW) based on spectral shaping and wavelength-to-time (SS-WTT) mapping is designed, fabricated, and demonstrated. The on-chip spectral shaper has a Michelson interferometer structure with two linearly chirped waveguide Bragg gratings (LC-WBGs) incorporated in its two arms. Due to the wavelength-dependent length difference between the two arms of the interferometer, the spectral response of the spectral shaper exhibits a wavelength-dependent free spectral range, which is required for the generation of an LCMW based on SS-WTT mapping. To enable electrical tuning of the spectral response, a lateral PN junction is introduced to each of the waveguides where the LC-WBGs are inscribed. Thanks to the plasma dispersion effect, the spectral response of the spectral shaper can be tuned by changing the bias voltages applied to the PN junctions, which would lead to the tuning of the generated LCMW. A theoretical analysis on the LCMW generation is performed, which is verified by an experiment in which an electrically-tunable spectral shaper is fabricated using a CMOS-compatible process with 248-nm deep ultraviolet lithography. By independently controlling the bias voltages to the PN junctions, a continuous tuning of the generated LCMW is demonstrated.

First demonstration of SiGe-based carrier-injection Mach-Zehnder modulator with enhanced plasma dispersion effect.

We demonstrate a strained Si0.91Ge0.09-based carrier-injection Mach-Zehnder (MZ) optical modulator using the enhanced plasma dispersion effect in strained SiGe through mass modulation for the first time. The SiGe modulator has an injection current of 1.47 mA for a phase shift of π which is lower than that for a Si modulator. Also, it is expected that the injection current can be further reduced by increasing the strain and Ge fraction, enabling operation at an injection current of less than 1 mA. As an example of the dynamic characteristics, 10 Gbps modulation with clear eye opening was obtained by the pre-emphasis method.

Electro-plasmonic 2 × 2 channel-routing switch arranged on a thin-Si-doped metal/insulator/semiconductor/metal structure.

Plasmonics as a new field of chip-scale technology is the interesting substrate of this study to propose and numerically investigate a metal/insulator/semiconductor/metal (MISM)-structure 2×2 plasmonic routing switch. As a planar subwavelength arrangement, the presented design has two npn-doped side-coupled dual waveguides whose duty is to route the propagating surface plasmon polaritons through the device. Relying on the MISM structure, which has a MOS-like thin-film arrangement of typically 45 nm doped silicon covered by a layer of 8 nm thick HfO(2) gate insulator, the routing configuration is electrically addressed based on the carrier-induced plasma dispersion effects as an external electro-plasmonic switching control. Finite-element-method-conducted electromagnetic simulations are employed to evaluate the switch optical response at telecom wavelength of λ=1550 nm, due to which the balanced operation measure of extinction ratios larger than 10 dB and insertion losses of around -1.8 dB are obtained for both channels of CROSS and STRAIGHT. Compared with other photonic and plasmonic switching counterparts, this configuration, besides its potential for CMOS compatibility, can be utilized as a high-speed compact building block to sustain higher-speed, more miniaturized, and less consuming electro-optic routing/switching protocols toward complicated optical integrated circuits and systems.

A fast SOI-based variable optical attenuator with a p-i-n structure with low polarization dependent loss

According to the plasma dispersion effect of silicon (Si), a silicon-on-insulator (SOI) based variable optical attenuator (VOA) with p-i-n lateral diode structure is demonstrated in this paper. A wire rib waveguide with sub-micrometer cross section is adopted. The device is only about 2 mm long. The power consumption of the VOA is 76.3 mW (0.67 V, 113.9 mA), and due to the carrier absorption, the polarization dependent loss (PDL) is 0.1 dB at 20 dB attenuation. The raise time of the VOA is 34.5 ns, the fall time is 37 ns, and the response time is 71.5 ns.

Decoupling and tuning the light absorption and scattering resonances in metallic composite nanostructures.

Utilizing the localized surface plasmon resonance (LSPR) effect of metallic nanoparticles enables their usage as contrast agents in a variety of applications for medical diagnostics and treatment. Those applications can use both the very strong absorption and scattering properties of the metallic nanoparticle due to their LSPR effects. There are certain applications where domination of the scattering over absorption or vice versa would be an advantage. However, the scattering and absorption resonance peaks have practically the same spectral location for solid noble metal nanoparticles at a certain domination of one over the other. In this paper we present gold nanoparticles coated with silicon that switches the order between the scattering and the absorption magnitude at the resonance peak by up to 34% in scattering-absorption ratio and tune the plasmon resonance over the spectrum by up to 56nm. This is obtained by modifying the refractive index of the silicon coating of the nanoparticle by illuminating it with a pumping light due to the plasma dispersion effect in silicon.

Total internal reflection optical switch using the reverse breakdown of a pn junction in silicon.

We demonstrate a new type of silicon total-internal-reflection optical switch with a simple pn junction functioning both as a reflector and a heater. The reflector is placed between asymmetrically y-branched multimode waveguides with an inclination angle corresponding to half of the branch angle. When the reflector is at rest, incident light is reflected in accordance to the refractive index difference due to the plasma dispersion effect of the pre-doped carriers. Switching to the transmission state is attained under a reverse breakdown of the pn junction by the thermo-optic effect which smears the refractive index difference. From this switching scheme, we confirmed the switching operation with a shallow total-internal-reflection region of 1 μm width. At a 6° branch angle, an extinction ratio of 12 dB and an insertion loss of -4.2 dB are achieved along with a thermal heating power of 151.5 mW.

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.

Highly linear ring modulator from hybrid silicon and lithium niobate.

We present a highly linear ring modulator from the bonding of ion-sliced x-cut lithium niobate onto a silicon ring resonator. The third order intermodulation distortion spurious free dynamic range is measured to be 98.1 dB Hz(2/3) and 87.6 dB Hz(2/3) at 1 GHz and 10 GHz, respectively. The linearity is comparable to a reference lithium niobate Mach-Zehnder interferometer modulator operating at quadrature and over an order of magnitude greater than silicon ring modulators based on plasma dispersion effect. Compact modulators for analog optical links that exploit the second order susceptibility of lithium niobate on the silicon platform are envisioned.

Numerical Analysis of Carrier-Depletion Strained SiGe Optical Modulators With Vertical p-n Junction

The modulation characteristics of carrier-depletion strained SiGe optical modulators with a vertical p-n junction are numerically analyzed by technology computer-aided design simulation and finite-difference optical mode analysis. In addition to the strong optical confinement in the vertical direction for the fundamental transverse electric field mode, the vertical p-n junction effectively depletes the strained SiGe layer in which the plasma dispersion effect is enhanced owing to the mass modulation of holes by strain. We predict that a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> optical modulator exhibits V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> L at 1.55 μm of as small as 0.31 V-cm at a bias voltage of -2 V, which is ~1.8 times smaller than that of a Si optical modulator. The product of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> L and the phase-shifter loss (αV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> L) is also expected to be as low as 18.3 V-dB at -2 V, enabling optical modulation with a 5-dB extinction ratio in a symmetric Mach-Zehnder modulator.

An electro-optic directed decoder based on two cascaded microring resonators

We demonstrate a directed optical decoder device consisting of two cascaded microring resonators, which are both modulated through the plasma dispersion effect. The inherent resonance wavelength mismatch between two microring resonators caused by fabrication errors is compensated for by using microheaters that are fabricated on top of the microring resonators. Two electrical signals generated by pulse pattern generators are used to modulate the PIN diodes that are embedded in the device, and the results are presented by optical signals detected at the four output ports of the device. The working wavelength and driving voltages of two MRRs are measured and analyzed by the static response spectra of the device. Dynamic experimental results show that the decoding operation is achieved at a speed of 100 Mbps.

Hybrid silicon and lithium niobate electro-optical ring modulator

Of all oxides, lithium niobate (LiNbO3) is the gold standard electro-optical material in fiber-optic transmission systems. Modulators based on diffused waveguides in bulk LiNbO3 substrates are, however, relatively large. In contrast, ring modulators based on silicon-on-insulator are of interest for chip-scale electro-optical modulation, but unstrained crystalline silicon does not exhibit a linear electro-optic effect, so modulation is based on alternative mechanisms such as the plasma dispersion effect. Here, we present a hybrid silicon and LiNbO3 electro-optical ring modulator operating at gigahertz frequencies. The modulator consists of a 15 μm radius silicon microring and an ion-sliced LiNbO3 thin film bonded together via benzocyclobutene. Fabricated devices operating in the TE optical mode exhibit an optical loaded quality factor of 14,000 and a resonance tuning of 3.3 pm/V. The small-signal electrical-to-optical 3 dB bandwidth is measured to be 5 GHz. Digital modulation with an extinction ratio greater than 3 dB is demonstrated up to 9 Gb/s. High-speed and low-tuning-power chip-scale modulators that exploit the high-index contrast of silicon with the second-order susceptibility of LiNbO3 are envisioned.

All-optical tuning of a magnetic-fluid-filled optofluidic ring resonator.

An all-optical tunable optofluidic ring resonator (OFRR) is proposed and experimentally demonstrated. The all-optical control of a silica microresonator is highly attractive, but it is difficult to realize because of the relatively weak Kerr effect and the absence of a plasma dispersion effect of silica. Here, we infuse a silica microcapillary-based optofluidic ring resonator with a magnetic fluid, into which pump light is injected by a fiber taper. Iron oxide nanoparticles dispersed in the magnetic fluid produce a strong pump light absorption, and this leads to a resonance shift of the silica microresonator due to the photothermal effect. To the best of our knowledge, this is the first scheme for all-optical tuning of an OFRR. A tuning sensitivity of up to 0.15 nm mW(-1) and a tuning range of 3.3 nm are achieved. With such excellent performance, the magnetic-fluid-filled OFRR has great potential in filtering, sensing, and signal processing applications.

Optical detection and modulation at 2µm-2.5µm in silicon.

Recently the 2μm wavelength region has emerged as an exciting prospect for the next generation of telecommunications. In this paper we experimentally characterise silicon based plasma dispersion effect optical modulation and defect based photodetection in the 2-2.5μm wavelength range. It is shown that the effectiveness of the plasma dispersion effect is dramatically increased in this wavelength window as compared to the traditional telecommunications wavelengths of 1.3μm and 1.55μm. Experimental results from the defect based photodetectors show that detection is achieved in the 2-2.5μm wavelength range, however the responsivity is reduced as the wavelength is increased away from 1.55μm.

Strain-induced enhancement of plasma dispersion effect and free-carrier absorption in SiGe optical modulators.

The plasma dispersion effect and free-carrier absorption are widely used to change refractive index and absorption coefficient in Si-based optical modulators. However, the weak free-carrier effects in Si cause low modulation efficiency, resulting in large device footprint and power consumption. Here, we theoretically and experimentally investigate the enhancement of the free-carrier effects by strain-induced mass modulation in silicon-germanium (SiGe). The application of compressive strain to SiGe reduces the conductivity effective mass of holes, resulting in the enhanced free-carrier effects. Thus, the strained SiGe-based optical modulator exhibits more than twice modulation efficiency as large as that of the Si modulator. To the best of our knowledge, this is the first demonstration of the enhanced free-carrier effects in strained SiGe at the near-infrared telecommunication wavelength. The strain-induced enhancement technology for the free-carrier effects is expected to boost modulation efficiency of the most Si-based optical modulators thanks to high complementary metal-oxide-semiconductor (CMOS) compatibility.

Progress towards a high-performing a-Si:H-based electro-optic modulator

Hydrogenated amorphous silicon (a-Si:H) has recently emerged as a promising material to provide microchips with passive and active photonic functions through a back-end and CMOS-compatible technological process. In this paper, we discuss the performance achieved with different configurations of a-Si:H-based electro-optical amplitude modulators integrated into passive waveguides. All of the analysed devices are based on the plasma dispersion effect, a phenomenon that allows us to reach useful performance at the communication wavelength of λ ∼ 1.55 μm. The behaviour of the various proposed modulation approaches has been tested by ad hoc interferometric structures, such as Fabry–Perot integrated resonators or integrated Mach–Zehnder interferometer, as well as by multistack devices to enhance the static modulation efficiency. The performance of each modulator has been analysed through several figures of merit.

Demonstration of electro-optic half-adder using silicon photonic integrated circuits

We report a silicon photonic integrated circuit which can perform the operation of half-adder based on two cascaded microring resonators (MRRs). PIN diodes embedded around MRRs are employed to achieve the carrier injection modulation. Two electrical pulse sequences representing the two operands of the half-add operation are applied to PIN diodes to modulate two MRRs through the plasma dispersion effect. The final operation results of bitwise Sum and Carry operation are output at two different output ports of the device. Microheaters fabricated on the top of MRRs are employed to compensate two MRRs resonance mismatch caused by the fabrication error through the thermo-optic effect. Addition operation of two bits with the operation speed of 100 Mbps is demonstrated.