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![(a) QCSE from a Ge/SiGe MQW 34-µm-long planar waveguide obtained from optical transmission measurements [43], and (b) a stand-alone Ge/SiGe MQW waveguide optical modulator. (c) Scanning electron microscope (SEM) image of the stand-alone optical modulator [46]. (d) Schematic view of SOI-waveguide-integrated Ge/SiGe MQW optical modulator using a butt coupling approach [65]. (e) Schematic and SEM views of a SiGe waveguide-integrated Ge/SiGe MQW optical modulator using a linear taper coupling approach [68] (Reproduced with permission from [43] and [46] © 2011 and 2012 the Optical Society, [65] © 2011 IEEE, and [68] © 2014 Springer Nature).](publication/331490159/figure/fig2/AS:732716156850179@1551704553294/a-QCSE-from-a-Ge-SiGe-MQW-34-m-long-planar-waveguide-obtained-from-optical.jpg)
(a) QCSE from a Ge/SiGe MQW 34-µm-long planar waveguide obtained from optical transmission measurements [43], and (b) a stand-alone Ge/SiGe MQW waveguide optical modulator. (c) Scanning electron microscope (SEM) image of the stand-alone optical modulator [46]. (d) Schematic view of SOI-waveguide-integrated Ge/SiGe MQW optical modulator using a butt coupling approach [65]. (e) Schematic and SEM views of a SiGe waveguide-integrated Ge/SiGe MQW optical modulator using a linear taper coupling approach [68] (Reproduced with permission from [43] and [46] © 2011 and 2012 the Optical Society, [65] © 2011 IEEE, and [68] © 2014 Springer Nature).
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Germanium/Silicon-Germanium (Ge/SiGe) multiple quantum wells receive great attention for the realization of Si-based optical modulators, photodetectors, and light emitters for short distance optical interconnects on Si chips. Ge quantum wells incorporated between SiGe barriers, allowing a strong electro-absorption mechanism of the quantum-confined...
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... the high-speed performance of vertical-incidence modulators, moderate values of 3-dB modulation bandwidth of 3-4 GHz obtained from [39,40,42] was expected to be due to the large diameter of the tested device, and expected to be circumvented by decreasing the device dimensions. In 2011, as in Figure 2a, Chaisakul et al. investigated Ge/SiGe MQW modulators in a planar waveguide structure from 34-µm-long and 64-µm-long planar waveguides [43]. An input fiber was used to directly couple light into the tested device. ...
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... paper showed that a large and sharp optical absorption peak at the LH1-c1 transition could be employed to improve the compactness of the modulators with competitive ER and IL values. In 2012, Chaisakul et al. [46] reported a 3-µm-wide and 90-µm-long Ge/SiGe MQW optical modulator, as shown in Figure 2b,c. The compact waveguide modulator can provide as high as 9-dB ER with a voltage swing of 1 V (~1.5 × 10 4 V/cm) between 3 and 4 V at the optical wavelength of 1435 nm. ...
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... the taper section could be long (~100-300 µm) and increase the device footprint and optical absorption loss, a two-step taper was proposed to shorten the coupling length (~40-80 µm) in [64] and [53]. In 2012, Ren et al. theoretically studied [65] and experimentally demonstrated [66] an SOIwaveguide-integrated Ge/SiGe MQW optical modulator using a butt coupling approach, as shown in Figure 2d for example. The tested device had a very compact footprint of 8 µm 2 and operated with only 1 V swing. ...
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... coupling integration with a large core Si waveguide was experimentally investigated by Claussen et al. [67], which could be adopted for significantly-lower coupling loss. In 2014, Chaisakul et al. [68] demonstrated an evanescent coupling integration between a SiGe waveguide and Ge/SiGe MQWs using a 100-μm-long linear taper, as shown in Figure 2e. An ER of 3-4 dB was simultaneously obtained with 2-3 dB IL over 20 nm spectral ranges from 1430 to 1450 nm using a bias voltage swing between 0 and 3 V. ...
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... Figure 3 shows the results of the photoelectric performance of GeSi/Ge detectors. As shown in Fig. 3(a), when the bias is −1 V and the laser power is set to 1 mW, the photocurrent of the device at 1310 and 1550 nm are 5.08E-4 and 1.72E-4 A, respectively, while the responsivity of the device at these wavelengths corresponds to 0.51 and 0.17 A/W, respectively, which are the highest values of vertical germanium quantum well photodetectors in the optical communication band [47,48]. Furthermore, these detectors demonstrate the corresponding light-dark current ratio of the detector is ∼10 3 . ...
... As the wavelength increases, the responsivity gradually decreases and reaches a cutoff near ∼1620 nm. Compared with the cutoff wavelength (∼1410-1540 nm [47][48][49][50][51]) of the conventional Ge MQW photodetectors, our GeSi/Ge MQW photodetectors have an extended optical response covering the infrared region of the telecom O, E, S, C and L bands. This extension of the cutoff wavelength in our detectors is rooted from the induced tensile strain in Ge layers by cyclic annealing treatment. ...
This work demonstrates a high-performance photodetector with a 4-cycle Ge0.86Si0.14/Ge multi-quantum well (MQW) structure grown by reduced pressure chemical vapor deposition techniques on a Ge-buffered Si (100) substrate. At −1 V bias, the dark current density of the fabricated PIN mesa devices is as low as 3 mA/cm², and the optical responsivities are 0.51 and 0.17 A/W at 1310 and 1550 nm, respectively, corresponding to the cutoff wavelength of 1620 nm. At the same time, the device has good high-power performance and continuous repeatable light response. On the other hand, the temperature coefficient of resistance (TCR) of the device is as high as −5.18%/K, surpassing all commercial thermal detectors. These results indicate that the CMOS-compatible and low-cost Ge0.86Si0.14/Ge multilayer structure is promising for short-wave infrared and uncooled infrared imaging.
... However, with their standard cubic-diamond crystal structure silicon, germanium and SiGe-alloys are all indirect band gap semiconductors, impeding the use of silicon-based materials for lasers and optical amplifiers 30 for integrated photonics [1]. Several strategies have been investigated for integrating light emitting materials on silicon, including III-V [2,3], GeSn [4][5][6], strained Ge [7,8], and SiGe quantum wells and dots [9][10][11][12][13][14][15], but remain challenging due to various reasons. When trans- 35 formed into the hexagonal crystal structure, the hex-Si 1−x Ge x alloys [16] are direct bandgap semiconductors with the fundamental bandgap at the Γ-point. ...
Silicon is indisputably the most advanced material for scalable electronics, but it is a poor choice for active photonic applications, due to its indirect band gap. The recently developed hexagonal (hex-)Si1−xGex semiconductor features a direct bandgap at least for x > 0.65, and the realization of quantum heterostructures would unlock new opportunities for advanced optoelectronic devices based on the SiGe system. Here, we demonstrate the synthesis and characterization of direct bandgap quantum wells (QW)s realized in the hex-Si1−xGex system. Photoluminescence experiments on hex-Ge/Si0.2Ge0.8 QWs demonstrate quantum confinement in the hex-Ge segment with type-I band alignment, showing light emission up to room temperature. Moreover, the tuning range of the QW emission energy can be extended using hex-Si1−xGex/Si1−yGey QWs with additional Si in the well. These experimental findings are supported with ab initio bandstructure calculations. A direct bandgap with type-I band alignment is pivotal for the development of novel low-dimensional light emitting devices based on hex-Si1−xGex alloys, which have been out of reach for this material system until now.
... [11] In general, the simple single-layer QW possesses an A-B-A sandwich-type layer structure, in which the conduction band (CB) and the valence band (VB) positions of the semiconductor at the two A layers should straddle those of the semiconductor at the middle B layer (Scheme 1c). [12] From the energy band structure standpoint, the semiconductors at the A and B layers can be referred to as the barrier and well materials, respectively. Although the energy band structure of QWs is basically similar to that of the dual "type I" semiconductor heterojunctions (Scheme S1), the strong quantum size effect Scheme 1. Schematic illustration of a) type II heterojunction, b) S-scheme heterojunction, c) the traditional QWs, d) the SRQWs and e) the enhanced interfacial electric field in SRQWs. ...
Semiconductor quantum wells (QWs) exhibit high charge‐utilization efficiency for light‐emitting applications due to their strong charge confinement effect. Inspired by this effect, herein, this work proposes a new idea to significantly improve the photo‐generated charge separation for attaining a highly‐efficient solar‐to‐fuels conversion process through “semi‐reversing” the conventional QWs to confine only the photo‐generated electrons. This electron confinement‐improved charge separation is implemented in the well‐designed model of the CdS/TiO2/CdS semi‐reversed QW (SRQW) structure. The latter is fabricated by selectively assembling CdS quantum dots (QDs) onto the {101} facets (ultra‐thin edge regions) of the TiO2 nanosheets (NSs). Upon light excitation, the photo‐generated electrons of SRQW can be confined on the TiO2‐{101} facets in the vicinity of the CdS/TiO2 hetero‐interface. Thereby, the continuous multi‐electron injection to the adsorbed reactants on the interfacial active‐sites is significantly accelerated. Thus, the CdS/TiO2/CdS SRQW exhibits ≈35.7 and ≈56.0‐fold enhancements on the photocatalytic activities for water and CO2 reduction, respectively, compared to those of pure TiO2. Correspondingly, its CH4‐product selectivity is increased by ≈180%. This work provides a novel charge separation mechanism, which is of great importance for the design of the next‐generation quantum‐sized photocatalysts for solar‐to‐fuels conversion.
... To be comparable to the previous related works [24,31], the absorption changes due to the FKE, which is intrinsically fast [48], between the electric field of ~ 10 kV/cm (α ~ 150 cm -1 , taking into account the Ge indirect-gap absorption) for the on-state operation, and ~ 95 kV/cm (α ~ 650 cm -1 ) for the off-state operation in the 170-nm-thick Ge-rich GeSi layer are employed as indicate in Fig. 4(b-c). This also ensures that the driving voltage would be less than 2 V drivable voltage of CMOS [49][50][51], and the electric field would not be higher than the breakdown field of the bulk Ge material ~ 100 kV/cm [52]. The free carrier absorption data in the p-and n-layers with a doping concentration of ~ 1×10 18 and ~ 5×10 18 cm -3 were calculated using the Drude model [53,54]. ...
Si3N4 photonic integrated circuits have gain significant and rapid interest in different photonic applications thanks to its superior passive performance. Nevertheless, optical integration between Si3N4 and Ge-based optical components remains critically challenging especially for optical modulation. In this paper, via 3D-FDTD calculations we investigate the optical integration between Si3N4 and Ge-based waveguides using vertical coupling configuration employing amorphous Si ( $\alpha $ -Si) as an optical bridge showing efficient and robust coupling efficiency, which can be maintained according to the tolerant analysis with respect to the variations in optical wavelengths and critical parameters of the coupling structure. In addition, with respect to the recent theoretically-optimized SOI waveguide-integrated and also laterally-coupled Si3N4 waveguide-integrated Ge-based optical modulators, we found that the studied coupling structure could be employed to enable a low-voltage Si3N4 waveguide-integrated Ge-based optical modulator with a competitive extinction ratio/insertion loss performance, increasing the prospect of Si3N4-based photonic integrated circuits for low-energy optical interconnects.
... This work focuses specifically on an optical wireless data communication modality intended for low-power wireless implants with ultrasmall footprints. In practice, uplink data (data emitted by the implant) can be transmitted from the inside to the outside of the body by using SiGe optical modulators or light sources (LEDs or lasers) [12][13][14][15][16]. However, SiGe modulators require large reverse bias voltages (1-3V), restricting the circuitry that can be used to drive them. ...
Optical links for medical implants have recently been explored as an attractive option primarily because it provides a route to ultrasmall wireless implant systems. Existing devices for optical communication either are not CMOS compatible, require large bias voltages to operate, or consume substantial amounts of power. Here, we present a high-Q CMOS-compatible electro-optic modulator that enables establishing an optical data uplink to implants. The modulator acts as a pF-scale capacitor, requires no bias voltage, and operates at CMOS voltages of down to 0.5V. We believe this technology would provide a path towards the realization of millimeter (mm)- and sub-mm scale wireless implants for use in bio-sensing applications.
... LTHOUGH being an indirect-gap semiconductor, the direct-gap transition of Ge has been intensively investigated for optical modulation and detection around the telecommunication wavelength regions at room temperature to enable Si-compatible monolithic electronicphotonic integration [1][2][3][4][5][6]. Significantly, quantum-confined Stark effect (QCSE) from Germanium/Silicon-germanium multiple quantum wells (Ge/SiGe MQWs) has been theoretically and experimentally explored toward the realization of compact and low-energy waveguide-integrated electroabsorption (EA) optical modulators that are potentially compatible with CMOS infrastructure and fabrication processes [7][8][9][10][11]. For instance, Ge/SiGe MQW waveguide optical modulators were experimentally reported to provide as high as 9-dB extinction ratio (ER) using a voltage swing of 1 V [12], and the waveguide-integrated devices with promising S. Srikam, W. Traiwattanapong, and P. Chaisakul are with department of Physics, Faculty of Science, Kasetsart University, Bangkok, 10900, Thailand (e-mail: saranison.s@ku.th; ...
... fscipac@ku.ac.th) performance were also shown via simulation [13][14] and measurement [15]. Interestingly, for optical interconnect applications, in which both optical modulation and detection have to be implemented, it is of great interest to have both functionalities realized from the same material platforms on silicon chip [8,[16][17][18][19][20]. As a result, optical detection characteristics from Ge/SiGe MQWs have also been widely studied with a view to obtaining high-performance waveguide photodetectors [16,21,22], required to convert light into an electrical signal at the end of an optical link. ...
... As a result, optical detection characteristics from Ge/SiGe MQWs have also been widely studied with a view to obtaining high-performance waveguide photodetectors [16,21,22], required to convert light into an electrical signal at the end of an optical link. Nevertheless, as the structures of Ge/SiGe MQWs also contain SiGe barriers, compared to bulk Ge waveguide photodetectors, Ge/SiGe MQWs waveguide photodetectors will generally have less Ge absorbing layers, which contributes to the longer device length required to obtain competitive detection performance [8]. Moreover, contrary to the case of Ge on Si in which high quality Ge can be grown directly on Si thanks to annealing techniques [23][24][25], a Ge-rich SiGe relaxed buffer is usually required to grow Ge/SiGe MQWs on Si [7,26,27], and the presence of additional buffer layers further reduces the optical overlap between the guided optical mode and the absorbing Ge/SiGe MQWs region [12,21,22]. ...
We report on 3-D FDTD simulation of waveguide-integrated plasmonic Ge/SiGe MQWs photodetectors. Despite several significant works on Ge/SiGe MQW optical modulators, Ge/SiGe MQW photodetectors still show improvable performance, limiting the potential to employ them for both optical modulation and detection. In this paper, we investigate the use of plasmonic concept to enhance electromagnetic field confinement inside the Ge/SiGe MQWs absorbing region with a view to having a compact and low-voltage waveguide-integrated Ge/SiGe MQWs photodetectors. Optical responsivities in term of applied electric fields, device dimensions of width and length, optical bandwidth, fabrication misalignment, and number of QWs periods are taken into account. The investigation shows that using plasmonic enhancement, a waveguide-integrated Ge/SiGe MQWs photodetector that is as short as 5 µm could be obtained with comparable responsivity and bias voltage values to some of the state-of-the-art bulk Ge-based devices, increasing the potential usage of Ge/SiGe MQWs for both optical modulation and detection in low-energy optical interconnects.
... 6 Given the strong refractive index contrast in the O and C telecommunication bands, SOI passive devices have been efficiently implemented with modulators and detectors. 7 Nevertheless, the demands for lower power consumption and large bandwidth, has driven interests in demonstrating efficient integration of O-band modulators based on the quantum-confined Stark effect (QCSE) 8,9 on silicon. QCSE type modulators are formed by barrier and well layers realised through material engineering which compose the active region for operation at specific wavelengths. ...
The integration of fast and power efficient electro-absorption modulators on silicon is of utmost importance for a wide range of applications. To date, Franz Keldysh modulators formed of bulk Ge or GeSi have been widely adopted due to the simplicity of integration required by the modulation scheme. Nevertheless, to obtain operation for a wider range of wavelengths (O to C band) a thick stack of Ge/GeSi layers forming quantum wells is required, leading to a dramatic increase in the complexity linked to sub-micron waveguide coupling. In this work, we present a proof-of-concept integration between micro-metric waveguides, through the butt-coupling of a 1.25 μm thick N-rich silicon nitride (SiN) waveguide with a 1.25 μm thick silicon waveguide for O-band operation. A numerical analysis is conducted for the design of the waveguide-to-waveguide interface, with the aim to minimize the power coupling loss and back-reflection levels. The theoretical results are compared to the measured data, demonstrating a coupling loss level of 0.5 dB for TE and TM polarisation. Based on the SiN-SOI interconnection, a theoretical analysis of a quantum-confined Stark effect (QCSE) stack waveguide coupled to an SiN waveguide is then proposed.
... The unit of variables in this table is pJ/symb or mW/GHz b It's an estimated value assuming that a 10 0 mW semiconductor light emitter is employed in each lane and F clc = 1 GHz c Using the silicon modulators based on carrier effect as reference [160][161][162][163][164][165][166] d Using novel modulators based on surface plasmon polariton or hybrid plasmonic mechanism [160] e The voltage feedback TIA's power is usually restricted by the gain-bandwidth product. E TIA symb is the estimated value based on the data from ref. [167,168], assuming the gain~10 4 and the bandwidth~10 0 GHz f E C symb (C indicates AD or DA) is usually proportional to 2 n (n means n-bit conversion resolution). ...
In recent years, the explosive development of artificial intelligence implementing by artificial neural networks (ANNs) creates inconceivable demands for computing hardware. However, conventional computing hardware based on electronic transistor and von Neumann architecture cannot satisfy such an inconceivable demand due to the unsustainability of Moore’s Law and the failure of Dennard’s scaling rules. Fortunately, analog optical computing offers an alternative way to release unprecedented computational capability to accelerate varies computing drained tasks. In this article, the challenges of the modern computing technologies and potential solutions are briefly explained in Chapter 1. In Chapter 2, the latest research progresses of analog optical computing are separated into three directions: vector/matrix manipulation, reservoir computing and photonic Ising machine. Each direction has been explicitly summarized and discussed. The last chapter explains the prospects and the new challenges of analog optical computing.
... Moreover, they are incompatible with silicon technology. On the other hand, Si-based PDs have been extensively used in the near-infrared (NIR) spectral range due to their matured technology and low cost 13,14 . However, its relatively large bandgap energy of 1.12 eV limits the absorption cutoff ( ) of 1. 1 . ...
This work presents the high-performance Si/SiO 2 distributed Bragg reflector (DBR)-integrated lateral germanium (Ge) p-i-n photodetectors (PDs) for atmospheric gas sensing and fiber-optic telecommunication networks in the short-wave infrared (SWIR) regime. In addition, this study also proposes an opto-electronic compact small-signal noise equivalent circuit model (SSNECM) of the designed device to compute the signal-to-noise ratio (SNR) at the output of the detector. Various figure-of merits including current under dark and illumination, responsivity, detectivity, 3dB bandwidth, and the SNR of the proposed device are estimated at the room-temperature (RT) for an incident optical power of 0.5 μW. Furthermore, the impact of scaling of rib waveguide (WG) width and height on dark current, responsivity, and 3dB bandwidth are investigated to optimize the proposed device. The validation of the proposed model is done by comparing various parameters including dark current, responsivity, and detectivity of the designed device with other Ge PDs. The estimated results show the reduced trade-off between responsivity and 3dB bandwidth of the designed device. At λ=1550 nm, the proposed device achieves the high detectivity and SNR of >2X10 ¹¹ Jones and 120 dB (up to 3 THz), respectively, with the bias voltage of -2V. These encouraging results pave the path for the future development of low noise and high-speed detectors for SWIR applications.
... Indeed, compensation via an increase in spatial mesh points, i.e., Fourier-Bessel coefficients, might be a viable counter-strategy but will increase the computational time. Similar problems will certainly arise for thin but pronounced refractive index wells and dips for example in Bragg fibers [29] (or other Bragg-type waveguides [30]) or quantum well waveguides [31]. With respect to such designs, another disadvantage of the DHT-based solving approach is the inflexible meshing because the sampling is determined by the zeros of the corresponding Bessel function and cannot be chosen freely. ...
It is crucial to be time and resource-efficient when enabling and optimizing novel applications and functionalities of optical fibers, as well as accurate computation of the vectorial field components and the corresponding propagation constants of the guided modes in optical fibers. To address these needs, a novel full-vectorial fiber mode solver based on a discrete Hankel transform is introduced and validated here for the first time for rotationally symmetric fiber designs. It is shown that the effective refractive indices of the guided modes are computed with an absolute error of less than 10−4 with respect to analytical solutions of step-index and graded-index fiber designs. Computational speeds in the order of a few seconds allow to efficiently compute the relevant parameters, e.g., propagation constants and corresponding dispersion profiles, and to optimize fiber designs.
