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We demonstrate heterogeneous integration of active semiconductor materials into the conventional passive metal-insulator-metal (MIM) waveguides to provide compact on-chip light generation and detection capabilities for chip-scale active nanophotonic platforms. Depending on its bias conditions, a metal-semiconductor-metal section can function as either a light emitting diode or a photodetector directly connected to the MIM waveguides. We experimentally verify the independent and combined operations of electrically-driven on-chip light sources and photodetectors.

Recently, research on the use of additives in photoactive layers to improve the efficiency of solar cells has continued to be conducted. Among them, carbon-based additives serve to increase a device's efficiency by simply and effectively improving the carrier movement. In this work, inverted polymer solar cells (iPSCs) are fabricated based on a blend of PTB7-Th:PC71BM and PTB7:PC71BM bulk heterojunction as photoactive materials using mixed additives of 4Cl-carbon-based contorted hexabenzocoronene (4Cl-cHBC). The 4Cl-cHBC additives, a kind of contorted polycyclic aromatic hydrocarbon, added to bulk heterojunction in the iPSC devices, provide a cocrystal modifier and effective charge separation to enhance device performance. The reason for this efficiency improvement is that cocrystal is formed due to a combination between 4Cl-cHBC and the PTB7-Th:PC71BM bulk heterojunction, which reduces bimolecular recombination in the device and significantly increases short-circuit current and fill factor. The RS and RSh values of iPSCs based on PTB7-Th:PC71BM with and without the 4Cl-cHBC additives decreased from 7.0 Ω cm2 to 4.0 Ω cm2, and increased from 813 Ω cm2 to 1226 Ω cm2, respectively. The calculated average carrier lifetimes at 500 nm for PTB7-Th:PC71BM and PTB7-Th:PC71BM with 4Cl-cHBC additives are 4.46 ps and 3.06 ps, respectively. The carrier lifetimes show that the charge separation of PTB7-Th:PC71BM with 4Cl-cHBC additives is more than 45% faster. As a result, the iPSCs based on the blend of PTB7-Th:PC71BM and PTB7:PC71BM with 4Cl-cHBC additives showed maximum power conversion efficiencies of 9.46 and 8.62%, which were 20.6 and 19.95% higher than those of the reference, respectively.

In this work, we present a model for spectral transmittance of extrinsic fiber Fabry-Perot cavities using paraxial Gaussian approximation and ray transfer matrices. The finesse is obtained through a Lorentzian fitting of the spectral transmittance derived from the proposed model. The relationship between the finesse and the radius of curvature on both sides of the cavity was analyzed to obtain high finesse. The predictions of the model are found to be in good agreement with the experimental results for the fabricated cavities.

Negative photoconductivity (NPC), a reduction in photoconductivity under light illumination, could provide low power consumption and high-speed frequency response. The NPC has been generally observed in low-dimensional materials, which can be easily affected by the trapping of photocarriers. However, a gradual transition between NPC and positive photoconductivity (PPC) by controlling the light intensity has not been reported. In this study, a gradual and reversible switching between NPC and PPC is achieved in a van der Waals heterostructure of graphene and MoTe2. The initially observed NPC state becomes a PPC state with the increase in light intensity. The switching between NPC and PPC is considered to originate from the hole trapping in MoTe2. The hole trapping can induce a shift in the Fermi level of MoTe2 and thus change the junction characteristics between the graphene and MoTe2, which determine the photoresponse type (NPC or PPC). Notably, the switching from one state to the other can also be reversed, depending on the gate bias. The stable and reversible effect upon light illumination and application of a gate voltage could be used in optoelectronic devices and optical communications.

We propose to implement a 2-channel optical-time-division-multiplexed (OTDM) system for short-reach optical interconnects by using a sinusoidally modulated light source instead of a complicated mode-locked laser as an input pulse source. In this system, the OTDM signal is obtained by bit-interleaving two optical return-to-zero (RZ) signals generated by using the sinusoidally modulated light. We operate these RZ signals in the orthogonal in-phase and quadrature domains to avoid the unwanted beat components. After the transmission, the OTDM signal is detected by using single photodetector, and then processed by a 2 × 2 multiple-input multiple-output equalizer. For a demonstration, we generate 150-Gb/s OTDM signal operating in the 8-level pulse-amplitude modulated (PAM-8) format by using commercial LiNbO3 Mach-Zehnder modulators and transmit this OTDM signal over 1.9 km of the standard single-mode fiber (SSMF). In addition, we fabricate the proposed OTDM transmitter in an integrated silicon-photonics chip and use it to demonstrate the transmission of the 64-Gb/s OTDM PAM-4 signal over 2.2 km of SSMF.

Coherent terahertz (THz) wireless communication using silicon photonics technology provides critical solutions for achieving high-capacity wireless transmission beyond 5G and 6G networks and seamless connectivity with fiber-based backbone networks. However, high-quality THz signal generation and noise-robust signal detection remain challenging owing to the presence of inter-channel crosstalk and additive noise in THz wireless environments. Here, we report coherent THz wireless communication using a silicon photonic integrated circuit that includes a dual-parallel Mach-Zehnder modulator (MZM) and advanced digital signal processing (DSP). The structure and fabrication of the dual-parallel MZM-based silicon photonic integrated circuit are systematically optimized using the figure of merit (FOM) method to improve the modulation efficiency while reducing the overall optical loss. The advanced DSP compensates for in-phase and quadrature (IQ) imbalance as well as phase noise by orthogonally decoupling the IQ components in the frequency domain after adaptive signal equalization and carrier phase estimation. The experimental results show a reduction in phase noise that induces degradation of transmission performance, successfully demonstrating error-free 1-m THz wireless transmission with bit-error rates of 10−6 or less at a data rate of 50 Gbps.

We have experimentally demonstrated feedforward and recirculating FPPGAs with MEMS-tunable elements. The static power of the elements is less than 50 nW, and energy required to change the state of the actuator is 8 pJ.

We demonstrate an inch-scale fiber ring resonator with its core partially exposed by droplet-based wet etching for efficient coupling. A quality factor of (3.14±0.3)×107 with a free spectral range of ~2.2 GHz was measured.

We propose and demonstrate an ultra-low-power 3×3 unitary matrix multiplier with MEMS-based tunable couplers and phase shifters. The static power consumption and tuning energy per tuning element are less than 0.13 µW and 115 pJ, respectively.

We present a $1\times 16$ silicon optical phased array using electro-optic phase shifters to attain high-speed operation with low power consumption. The phase shifters are constructed using a $p$ - $i$ - $n$ junction structure with the $p$ - and $n$ -doped regions formed on both sides of the $i$ -region. The $i$ -region width of the phase shifter is optimized considering the power consumption for phase-tuning and the propagation loss in the phase shifter. The fabricated $p$ - $i$ - $n$ phase shifter exhibits a fast operating speed of 20 MHz and a low phase-tuning power of 1.7 mW/ $\pi $ . From a 1-D optical phased array integrated with 2- $\mu \text{m}$ -pitch grating radiators, a wide beam-steering range of 45° is attained along the transversal direction at a $1.55~\mu \text{m}$ wavelength. Average power consumption for the beam-forming operation of the $1\times 16$ OPA is measured to be 39.6 mW and the average transition time to steer the beam is 24 ns.

In this study, we propose the fabrication of a photodetector based on the heterostructure of p-type Si and n-type MoS2. Mechanically exfoliated MoS2 flakes are transferred onto a Si layer; the resulting Si–MoS2 p–n photodiode shows excellent performance with a responsivity (R) and detectivity (D*) of 76.1 A/W and 1012 Jones, respectively. In addition, the effect of the thickness of the depletion layer of the Si–MoS2 heterojunction on performance is investigated using the depletion layer model; based on the obtained results, we optimize the photoresponse of the device by varying the MoS2 thickness. Furthermore, low-frequency noise measurement is performed for the fabricated devices. The optimized device shows a low noise equivalent power (NEP) of 7.82 × 10–15 W Hz–1/2. Therefore, our proposed device could be utilized for various optoelectronic devices for low-light detection.

A high performance polarization beam splitter (PBS) based on cascaded directional couplers (DCs) assisted by effectively anisotropic structures is proposed. The sub-wavelength grating (SWG) structures between the two coupled waveguides act as anisotropic metamaterial cladding to block/enhance light coupling for transverse electric (TE)/magnetic (TM) polarization. Cascading the DCs designed at different central wavelengths can improve the extinction ratio (ER) within the desired bandwidth or broaden the working wavelength bandwidth for TM polarization. Three design examples show the excellent performance of PBS with ER of >35 dB, 20 dB or 14 dB and loss of ER>20dB of the PBS based on the cascaded DCs increases by ~11 times when compared with a single DC.

Molybdenum disulfide (MoS2) has been extensively used as a hole transport layer in many inverted polymer solar cell devices. However, its potential as an electron transport layer in a solar cell device has rarely been reported. In this paper, we demonstrate an iPSC with enhanced photovoltaic performance by the introduction of a MoS2 nanosheet (NS) interlayer. Herein, MoS2 NS thin films were uniformly deposited on a polyethylenimine ethoxylated polymer by an electrostatic interaction. The MoS2 NS interlayer in the iPSC device plays important roles as a sub-photo sensitizer and an electron transport layer and provides effective charge separation for the enhancement of device performance. iPSCs with MoS2 NSs based on bulk heterojunction active materials such as P3HT:PC60BM, PTB7:PC71BM and PTB7-Th:PC71BM showed maximum power conversion efficiencies of 3.54%, 8.12%, and 9.08%, respectively, which were 27%, 11%, and 15% higher than those of the reference, respectively. Furthermore, the enhanced photovoltaic performance and electron transport mechanism of the iPSCs with the MoS2 NSs were confirmed experimentally using finite-difference-time-domain modeling and time-correlated single photon counting (TCSPC) measurements. The computational simulation results showed that the light absorption of the iPSCs with the MoS2 NS interlayer was slightly better than that of the iPSCs without the MoS2 NS interlayer. The electron decay time of PTB7:PC71BM with the MoS2 NS interlayer at a wavelength of 700 nm was reduced to 60 ps from 100 ps of PTB7:PC71BM without the MoS2 NS interlayer. The efficient light absorption and electron transfer characteristics of MoS2 NSs significantly improved the photovoltaic conversion performance of the iPSC devices.

Programmable photonic integrated circuits have mainly been developed based on the single wavelength channel operation of fundamental building blocks consisting of Mach-Zehnder interferometers (MZIs) with tunable phase shifters. We propose and study optical circuit models consisting of cascaded optical resonators that enable the independent operation of multiple wavelength channels in a more compact footprint than the conventional MZIs. By adopting experimental values reported for silicon micro-ring resonators, the fidelities of different types of 2×2 unitary transformations and higher-dimensional unitary transformations are examined by employing the Reck algorithm and the Clements algorithm.

We report on a 32 × 32 silicon photonic micro-electro-mechanical-system (MEMS) switch with gap-adjustable directional couplers. The switch is fabricated on 200-mm silicon-on-insulator wafers in a commercial complementary metal-oxide-semiconductor (CMOS) foundry. The fabricated device has a maximum on-chip loss of 7.7 dB and an extinction ratio of 50.8 dB. The switching voltage is 9.45 V and the 20-dB bandwidth is 28.7 nm. Our work shows a promising path for mass production of silicon photonic MEMS switches in commercial CMOS foundries.

We report on a 16-core recirculating programmable photonic array based on MEMS-tunable directional couplers. The photonic array has a compact footprint (0.04mm2/cell) and negligible static power consumption. Waveguide-coupled single-ring resonators, CROWs, and add-drop filters are demonstrated.

Coupled mode theory and matrix analysis of our proposal

We report on an etching-based fabrication method for conical core-maintained fiber tapers with adiabatic tapering angles, and discuss its adiabatic mode evolution compared to the conventional pulled fibers over the O-/C-bands. The far-field pattern from the etched fiber shows that the fundamental mode is maintained throughout the tapers.

Negative photoconductivity (NPC), a reduction in photoconductivity under light illumination, could provide low power consumption and high-speed frequency response. The NPC has been generally observed in low-dimensional materials, which can be easily affected by the trapping of photocarriers. However, a gradual transition between NPC and positive photoconductivity (PPC) by controlling the light intensity has not been reported. In this study, a gradual and reversible switching between NPC and PPC is achieved in a van der Waals heterostructure of graphene and MoTe

We experimentally demonstrate the use of silicon photonics circuit (SPC) in the simple and cost-effective photonics-aided Terahertz (THz) wireless transmission system. We perform theoretical investigation (with experimental confirmation) to understand that the system performance is more sensitive to the free space path loss (FSPL) at the THz wireless link than the SPC’s insertion loss. The SPC, we design and fabricate, combines two incident optical carriers at different wavelengths and modulates one of two optical carriers with data to transfer, consequently reducing the system footprint that is indeed one of the key challenges that must be tackled for better practicability of the THz communication system. We perform experimental verification to show the feasibility of 40 Gb/s non-return-to-zero (NRZ) on-off-keying (OOK) signal transmission over 1.4 m wireless link for possibly its application in short-reach indoor wireless communication systems utilizing (sub-)THz frequency band such as, e.g., indoor WiFi, distributed antenna/radio systems, rack-to-rack data delivery, etc. The SPC could be further integrated with various photonic elements such as semiconductor optical amplifiers, laser diodes, and photo-mixers, which will enable the path towards all-photonic THz-wave synthesizers.

We demonstrate a lateral, planar fiber-to-waveguide coupling strategy for photonic integrated circuits with diffraction grating couplers using angle-polished silica waveguide blocks fabricated with well-established planar lightwave circuit technologies. Compared to the conventional lateral coupling scheme with angle-polished fibers, the demonstrated scheme can significantly decrease the diverging distance between the reflective angle-polished facet and the grating couplers, and thereby maintains the overall coupling efficiency and alignment tolerances of the vertical coupling approach. The proposed method shows a small penalty in coupling efficiency (< 0.1 dB), and in-plane (out-of-plane) alignment tolerance for 1 dB excess loss is approximately 5 µm (9 µm).

Although hexagonal boron nitride (BN) nanostructures have recently received significant attention due to their unique physical and chemical properties, their applications have been limited by a lack of processability and poor film quality. In this study, a versatile method to transfer-print high-quality BN films composed of densely stacked BN nanosheets based on a desolvation-induced adhesion switching (DIAS) mechanism is developed. It is shown that edge functionalization of BN sheets and rational selection of membrane surface energy combined with systematic control of solvation and desolvation status enable extensive tunability of interfacial interactions at BN-BN, BN-membrane, and BN-substrate boundaries. Therefore, without incorporating any additives in the BN film and applying any surface treatment on target substrates, DIAS achieves a near 100% transfer yield of pure BN films on diverse substrates, including substrates containing significant surface irregularities. The printed BNs demonstrate high optical transparency (>90%) and excellent thermal conductivity (>167 W m-1 K-1) for few-micrometer-thick films due to their dense and well-ordered microstructures. In addition to outstanding heat dissipation capability, substantial optical enhancement effects are confirmed for light-emitting, photoluminescent, and photovoltaic devices, demonstrating their remarkable promise for next-generation optoelectronic device platforms.

We demonstrate Schottky-junction-based near-infrared sub-bandgap organic photodetectors operating up to 1300 nm. Coherent perfect absorption mechanism improves broadband optical absorption over wide incident angles with a responsivity of 0.88 pA/W at 975 nm.

In universal linear optics, programmable photonic circuits perform an arbitrary linear transformation by the sequential multiplication of two-level achromatic unitary operators. Based on cascaded optical resonator-based building blocks, we construct an arbitrary multi-wavelength unitary operator.

Supplemental Document

Grating couplers are one of the most significant elements for the coupling of light between optical fibers and photonic integrated circuits. In this paper, we present the design, fabrication and experimental characterization of Silicon on Insulator (SOI) based grating couplers. The mode matching of diffracted optical field from the grating coupler with a single mode fiber at different angles is investigated here with normal surface physical contact (PC) and angled physical contact (APC) fibers. According to our simulations, the 1°tilt in the fiber angle approximately changes the position of the optimum mode coupling for the grating coupler with the single mode fiber by ∼ 300 nm distance. The variation of input wavelength at different grating periods gives the different optimal coupling angles for the PC fibers. The experimental results also demonstrate the coupling efficiency as −3.6 dB at 1550 nm with a simulated 3-dB bandwidth of 48 nm near the telecommunication C-band.

Efficient light energy transfer between optical waveguides has been a critical issue in various areas of photonics and optoelectronics. Especially, the light coupling between optical fibers and integrated waveguide structures provides essential input-output interfaces for photonic integrated circuits (PICs) and plays a crucial role in reliable optical signal transport for a number of applications, such as optical interconnects, optical switching, and integrated quantum optics. Significant efforts have been made to improve light coupling properties, including coupling efficiency, bandwidth, polarization dependence, alignment tolerance, as well as packing density. In this review article, we survey three major light coupling methods between optical fibers and integrated waveguides: end-fire coupling, diffraction grating-based coupling, and adiabatic coupling. Although these waveguide coupling methods are different in terms of their operating principles and physical implementations, they have gradually adopted various nanophotonic structures and techniques to improve the light coupling properties as our understanding to the behavior of light and nano-fabrication technology advances. We compare the pros and cons of each light coupling method and provide an overview of the recent developments in waveguide coupling between optical fibers and integrated photonic circuits.

We report on 50 × 50 silicon photonic micro-electromechanical-system (MEMS) switches with reduced polarization dependence. The switches make use of two-level waveguide crossbars and MEMS-actuated adiabatic couplers. Simulations indicate that by eliminating all polarization-dependent elements (e.g., waveguide crossings), low polarization-dependent losses (

Light-induced manipulation techniques have been utilized to transport, trap, or levitate microscopic objects for a wide range of applications in biology, electronics, and photonics. Without making direct physical contact, they can provide simple yet powerful means for high-precision assembly of microscale functional blocks and components within the integrated circuit platforms, thereby offering a viable alternative to the conventional heterogeneous integration techniques, such as wafer/die bonding and transfer printing. Using a microbubble-based optofluidic pick-and-place assembly process, heterogeneous integration of compact III-V semiconductor photodetectors on a silicon-based photonic integrated circuit chip, enabling direct high-speed vertical electrical contacts for significantly improved photogenerated carrier transit distance/time, is experimentally demonstrated. The microdisk-shaped InGaAs p-i-n photodetector integrated on the silicon waveguide has a 3 dB bandwidth exceeding 50 GHz under the applied bias voltage of -1 V for near-infrared wavelengths around 1.55 mu m. The light-induced optofluidic assembly will provide a promising route for seamless heterogeneous integration of various optoelectronic components with high-speed and low-noise electrical interconnection on the fully processed silicon photonic/electronic integrated circuit platforms.

This paper presents a fabrication method for 2 × 2 twisted fiber-optic directional couplers based on biconical low-loss tapered optical fibers using a two-step wet-etching technique with hydrofluoric acid-based droplets and surface tension-driven flows. The droplet-based wet etching of optical fibers allows us simple and cost-effective fabrication of tapered fibers and multiports directional couplers. The simulation results agree well with the experimental observations on the propagation losses, which can be monitored in real-time by measuring the transmitted optical powers during the chemical etching process. The fabricated 2 × 2 fiber-optic directional coupler shows wavelength-insensitive operation within the telecommunication C-band with an excess insertion loss of less than 0.5 dB.