Compact symmetric polarization rotator-splitter on InP.

Symmetric polarization rotator-splitter (PRS) is proposed and experimentally demonstrated on InP for the first time. Instead of integrating a mode-selective splitter, we employ a symmetric multimode-interference (MMI) splitter at the output of an adiabatic taper section to extract the linear superpositions of the transverse-electric (TE) and the transverse-magnetic (TM) components of the input signal. As a result, the entire device functions as a PRS with its basis on the S2-S3 plane of the Poincaré sphere, whereas we can fully eliminate complicated asymmetric structures that are challenging to fabricate on InP. Moreover, the adiabatic taper, which operates as a mode-evolution-based polarization converter, is designed judiciously to minimize the overall length. The designed InP PRS with a total length of 750 µm is fabricated by a simple single-etching process. A polarization extinction ratio of more than 16.3 dB and a polarization-dependent loss of 0.67 dB are demonstrated experimentally at a 1550-nm wavelength.

Comparative analysis of speckle-based single-pixel imaging using uniform and non-redundant optical phased arrays.

An optical phased array (OPA) is a compact high-speed wavefront modulation device that is promising for next-generation optical sensing systems. In particular, speckle-based single-pixel imaging (SSPI) using OPA is an attractive scheme since precise tuning of optical phases is unnecessary. In this work, we present a comprehensive analysis of SSPI using an OPA with N phase shifters by comparing two classes of OPAs: uniformly spaced OPA (UOPA) and non-redundant OPA (NROPA). Through singular value decomposition analysis of the illumination patterns generated from the OPA, we clarify the theoretical limit of the imaging resolution for each case. As a result, the number of resolvable points can be as large as N 2-N+1 for the case of NROPA. This is in clear contrast to the case of UOPA, where the number of resolvable points can only be as large as 2N-1. Finally, imaging results of a test target are compared to study the impact of the array layout in OPA-based SSPI. Our work provides theoretical understanding of OPA-based SSPI and reveals the effectiveness of SSPI using NROPA.

Strong Proton-Electron Coupling in π-Planar Metal Complex with Redox-Active Ligands.

Proton-coupled electron transfer (PCET) of metal complexes has been widely studied, especially in biochemistry and catalytic chemistry. Although metal complexes bearing redox-active ligands play a part in these research areas, those with π-planar structure remain entirely unexplored, which are vital for future development of iono-electronics. Here, proton-electron coupling of a π-planar nickel complex bearing redox-active N,S-ligands, Ni(itsq)2 , was investigated by combining experimental and theoretical approaches. Strong proton-electron coupling was manifested in a large potential shift, which is twice greater than that of a typical PCET-type π-planar metal complex with redox-inactive ligands, [Ni(dcpdt)2 ]2- . Theoretical calculations affirmed that the stabilization of frontier orbitals by protonation is greater in Ni(itsq)2 than that in [Ni(dcpdt)2 ]2- . These results indicate that π-planar metal complexes with redox-active ligands are promising for developing novel PCET-type materials.

Integrated dual-polarization coherent receiver without a polarization splitter-rotator.

We propose and demonstrate a simple integrated dual-polarization (DP) coherent receiver that does not require a polarization splitter-rotator (PSR). Based on a novel concept, a DP coherent signal is mixed with the local-oscillator (LO) waves inside a single interferometer and detected by five single-ended photodetectors. The signal-signal and LO-LO beat noises are eliminated through differential detection. We design and fabricate a proof-of-concept device on InP and experimentally demonstrate complete retrieval of DP quadrature phase-shift keyed signals. Requiring minimal number of optical components without a PSR, the demonstrated scheme would be attractive particularly for the InP and thick-silicon photonic platforms due to its significantly reduced footprint and ease of fabrication.

Efficient InGaAsP MQW-based polarization controller without active-passive integration.

Carrier-injection-based efficient polarization controller with a strained InGaAsP multiple-quantum-well (MQW) layer is demonstrated on a regrowth-free InP platform. We employ a straight-line device configuration by cascading an asymmetric polarization rotator (PR) to provide a fixed polarization conversion and a polarization-dependent phase shifter (PD-PS) to enable tunable polarization rotation. Based on a novel design concept, both the PR and PD-PS sections are integrated monolithically without active-passive integration. Using the fabricated device, we experimentally demonstrate efficient polarization conversion over the entire Poincaré sphere with a total current of less than 40 mA. With the capability of monolithically integrating other InP-based active components, the demonstrated scheme should be attractive for various applications, such-as low-cost coherent communication, microwave photonics, and quantum key distribution.

Monolithic InP optical unitary converter based on multi-plane light conversion.

Integrated reconfigurable optical unitary converters (OUCs) are crucial in realizing all-optical spatial mode demultiplexing for mode-division-multiplexed transmission systems and programmable photonic processing for optical neural networks. In this work, we present the first experimental demonstration of 4×4 OUC monolithically integrated on InP. To avoid the difficulty of integrating a large number of Mach-Zehnder interferometer couplers on the InP platform, we apply the concept of multi-plane light conversion and use cascaded stages of 4-port multimode interference couplers, which are more scalable and easier to fabricate on InP. By optimizing the phase shifters, we demonstrate reconfigurable 4-mode sorting as well as error-free switching of 40-Gbit/s signal.

Complete retrieval of multi-level Stokes vector signal by an InP-based photonic integrated circuit.

An integrated Stokes vector receiver (SVR) that can retrieve state of polarization of light in the three-dimensional (3D) Stokes space has widespread applications, such as short-reach communication links, polarization-sensitive imaging, and sensing. While various approaches have been demonstrated to date, monolithic integration of polarization components on InP has been a challenging issue. In this paper, we develop a novel 4-port SVR circuit integrated on a compact InP chip to retrieve complete Stokes parameters of incoming light with various intensity and degree-of-polarization. By judiciously designing the lengths and positions of asymmetric waveguide sections, we demonstrate that the SV of signal can be projected onto four vertices of a regular tetrahedron inscribed in the Poincaré sphere. Additionally, we employ this device in decoding 10-Gbaud 4-ary and 8-ary Stokes-vector-modulated signals in the 3D Stokes space.

Ghost imaging using a large-scale silicon photonic phased array chip.

We experimentally demonstrate the use of a large-scale silicon-photonic optical phased array (OPA) chip as a compact, low-cost, and potentially high-speed light illuminating device for ghost imaging (GI) applications. By driving 128 phase shifters of a newly developed silicon OPA chip using rapidly changing random electrical signals, we successfully retrieve a slit pattern with over 90 resolvable points in one dimension. We then demonstrate 2D imaging capability by sweeping the wavelength. With the potential of integrating high-speed phase modulators, tunable lasers, grating couplers, and CMOS driver circuit on the same silicon platform, this work paves the way towards realizing ultrahigh-speed and low-cost single-chip GI devices.

Mapping molecular motions leading to charge delocalization with ultrabright electrons.

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

First-principles modeling of GaN(0001)/water interface: Effect of surface charging.

The accumulation properties of photogenerated carriers at the semiconductor surface determine the performance of photoelectrodes. However, to the best of our knowledge, there are no computational studies that methodically examine the effect of "surface charging" on photocatalytic activities. In this work, the effect of excess carriers at the semiconductor surface on the geometric and electronic structures of the semiconductor/electrolyte interface is studied systematically with the aid of first-principles calculations. We found that the number of water molecules that can be dissociated follows the "extended" electron counting rule; the dissociation limit is smaller than that predicted by the standard electron counting rule (0.375 ML) by the number of excess holes at the interface. When the geometric structure of the GaN/water interface obeys the extended electron counting rule, the Ga-originated surface states are removed from the bandgap due to the excess holes and adsorbates, and correspondingly, the Fermi level becomes free from pinning. Clearly, the excess charge has a great impact on the interface structure and most likely on the chemical reactions. This study serves as a basis for further studies on the semiconductor/electrolyte interface under working conditions.

Reconfigurable all-optical on-chip MIMO three-mode demultiplexing based on multi-plane light conversion.

We present, to the best of our knowledge, the first experimental demonstration of reconfigurable all-optical on-chip multi-input-multi-output three-mode demultiplexing based on multi-plane light conversion. The demultiplexer consists of cascaded phase shifter arrays and multimode interference couplers integrated on a compact silicon chip. By optimizing the phase shifters, reconfigurable three-mode demultiplexing is experimentally realized with wavelength-dependent loss of less than 3 dB and modal crosstalk of less than -10 dB over a 23 nm optical bandwidth. Error-free mode demultiplexing of 40 Gbps non-return-to-zero signal is also demonstrated.

Material challenges for solar cells in the twenty-first century: directions in emerging technologies.

Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan-French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.

Polarization-analyzing circuit on InP for integrated Stokes vector receiver.

Stokes vector modulation and direct detection (SVM/DD) has immense potentiality to reduce the cost burden for the next-generation short-reach optical communication networks. In this paper, we propose and demonstrate an InGaAsP/InP waveguide-based polarization-analyzing circuit for an integrated Stokes vector (SV) receiver. By transforming the input state-of-polarization (SOP) and projecting its SV onto three different vectors on the Poincare sphere, we show that the actual SOP can be retrieved by simple calculation. We also reveal that this projection matrix has a flexibility and its deviation due to device imperfectness can be calibrated to a certain degree, so that the proposed device would be fundamentally robust against fabrication errors. A proof-of-concept photonic integrated circuit (PIC) is fabricated on InP by using half-ridge waveguides to successfully demonstrate detection of different SOPs scattered on the Poincare sphere.

Active metasurface modulator with electro-optic polymer using bimodal plasmonic resonance.

Electrically tunable metasurfaces have gained special interest as they can realize ultrathin surface-normal modulators in planar geometries. In this paper, we demonstrate a novel metasurface modulator based on electro-optic (EO) polymer that utilizes bimodal resonance inside a metallic subwavelength grating to increase the modulation efficiency. When two metal-insulator-metal (MIM) resonant modes are excited simultaneously inside the grating, they couple strongly to generate a sharp dip in the reflected spectrum. As a result, efficient intensity modulation with 15-dB extinction ratio can be obtained at the resonant wavelength under a small refractive index change of 8.5 × 10-3, corresponding to modulation voltage of less than 10 V. Due to the low parasitic capacitance of EO polymer and high conductivity of metallic gratings which is also used as the electrodes, the RC bandwidth of the device should easily exceed 100 GHz, potentially applicable to high-speed surface-normal modulators.

Magnetic-Nonmagnetic Phase Transition with Interlayer Charge Disproportionation of Nb3 Trimers in the Cluster Compound Nb3Cl8.

We grew large single crystals of the cluster magnet Nb3Cl8 with a magnetic triangular lattice and investigated its magnetic properties and crystal structure. In Nb3Cl8, the [Nb3]8+ cluster has a single unpaired spin, making it an S = 1/2 triangular lattice anti-ferromagnet. At low temperatures, Nb3Cl8 exhibits a magnetic-nonmagnetic phase transition driven by a charge disproportionation, in which the paramagnetic [Nb3]8+ clusters transform into alternating layers of nonmagnetic [Nb3]7+ and [Nb3]9+ clusters. The observed exotic phenomenon with the strong correlation between the magnetism and structure are based on the nature of the cluster magnetism.

The Salts of Copper Octafluoro- and Hexadecafluorophthalocyanines Containing [CuII(F8Pc)4-]2- Dianions and [CuF16Pc]- Monoanions.

Crystalline anionic salts with copper octafluoro- and hexadecafluorophthalocyanines, (Bu4N+)2[CuII(F8Pc)4-]2-·2C6H4Cl2 (1) and (PPN+)3[CuF16Pc]33-·2C6H5CN (2), where PPN+ is bis(triphenylphosphoranylidene)ammonium and Pc is phthalocyanine, have been obtained. The absence of noticeable absorption in the NIR range and DFT calculations for 1 indicate that both negative charges are mainly localized on the Pc ligand, and that the [CuII(F8Pc)4-]2- dianions are formed without reduction of CuII. The magnetic moment of 1.60 µB corresponds to the contribution of one S = 1/2 spin per dianion. The spin is localized on the CuII atom, which shows an EPR signal characteristic of CuII. Dianions are isolated in 1, providing only weak magnetic coupling of spins with a Weiss temperature of -4 K. Salt 2 contains closely packed π-π stacks built of [CuF16Pc]- anions of types I and II, and the interplanar distances are 3.187 and 3.275 Å. According to the DFT calculations, the [CuF16Pc]- anions of types I and II can have different charge distributions, with localization of an extra electron on the copper atoms to form diamagnetic [CuI(F16Pc)2-]- monoanions or delocalization of an extra electron on the F16Pc ligand to form [CuII(F16Pc)•3-]•- having an S = 1/2 (CuII) + 1/2 (F16Pc•3-) spin state. In fact, at 300 K, the magnetic moment of 2 of 3.25 µB per formula unit is rather close to the contribution from two [CuII(F16Pc)•3-]•- (calculated µeff is 3.46 µB). The Weiss temperature of -21.5 K indicates antiferromagnetic coupling of spins, which can be modeled by stronger intermolecular coupling between (F16Pc)•3- with J1/kB = -23.5 K and weaker intramolecular coupling between CuII and (F16Pc)•3- with J2/kB = -8.1 K. This interaction is realized in the {[CuII(F16Pc)•3-]•-}2 dimers separated by diamagnetic [CuI(F16Pc)2-]- species. In spite of the stacking arrangement of phthalocyanine macrocycles in 2, the inhomogeneous charge distribution and nonuniform distances between the macrocycles should suppress electrical conductivity.

Conducting π Columns of Highly Symmetric Coronene, The Smallest Fragment of Graphene.

Coronene, which is the smallest D6h -symmetric polycyclic aromatic hydrocarbon, attracts particular attention as a basic component of electronic materials because it is the smallest fragment of graphene. However, carrier generation by physical methods, such as photo- or electric field-effect, has barely been studied, primarily because of the poor π-conduction pathway in pristine coronene solid. In this work we have developed unprecedented π-stacking columns of cationic coronene molecules by electrochemical hole-doping with polyoxometallate dianions. The face-to-face π-π interactions as well as the partially charged state lead to electrical conductivity at room temperature of up to 3 S cm(-1) , which is more than 10 orders of magnitude higher than that of pristine coronene solid. Additionally, the robust π-π interactions strongly suppress the in-plane rotation of the coronene molecules, which has allowed the first direct observation of the static Jahn-Teller distortion of cationic coronene molecules.

Coordination Complexes of Transition Metals (M = Mo, Fe, Rh, and Ru) with Tin(II) Phthalocyanine in Neutral, Monoanionic, and Dianionic States.

The ability of tin atoms to form stable Sn-M bonds with transition metals was used to prepare transition metal complexes with tin(II) phthalocyanine in neutral, monoanionic, and dianionic states. These complexes were obtained via the interactions of [Sn(IV)Cl2Pc(3-)](•-) or [Sn(II)Pc(3-)](•-) radical anions with {Cp*Mo(CO)2}2, {CpFe(CO)2}2, {CpMo(CO)3}2, Fe3(CO)12, {Cp*RhCl2}2, or Ph5CpRu(CO)2Cl. The neutral coordination complexes of Cp*MoBr(CO)2[Sn(II)Pc(2-)]·0.5C6H4Cl2 (1) and CpFe(CO)2[Sn(II)Pc(2-)]·2C6H4Cl2 (2) were obtained from [Sn(IV)Cl2Pc(3-)](•-). On the other hand, the coordination of transition metals to [Sn(II)Pc(3-)](•-) yielded anionic coordination complexes preserving the spin on [Sn(II)Pc(3-)](•-). However, in the case of {cryptand[2,2,2](Na(+))}{CpFe(II)(CO)2[Sn(II)Pc(4-)]}(-)·C6H4Cl2 (4), charge transfer from CpFe(I)(CO)2 to [Sn(II)Pc(3-)](•-) took place to form the diamagnetic [Sn(II)Pc(4-)](2-) dianion and {CpFe(II)(CO)2}(+). The complexes {cryptand[2,2,2](Na(+))}{Fe(CO)4[Sn(II)Pc(3-)](•-)} (5), {cryptand[2,2,2](Na(+))}{CpMo(CO)2[Sn(II)Pc(2-)Sn(II)Pc(3-)(•-)]} (6), and {cryptand[2,2,2](Na(+))}{Cp*RhCl2[Sn(II)Pc(3-)](•-)} (7) have magnetic moments of 1.75, 2.41, and 1.75 µ(B), respectively, owing to the presence of S = 1/2 spins on [Sn(II)Pc(3-)](•-) and CpMo(I)(CO)2 (for 6). In addition, the strong antiferromagnetic coupling of spins with Weiss temperatures of -35.5 -28.6 K was realized between the CpMo(I)(CO)2 and the [Sn(II)Pc(3-)](•-) units in 6 and the π-stacking {Fe(CO)4[Sn(II)Pc(3-)](•-)}2 dimers of 5, respectively. The [Sn(II)Pc(3-)](•-) radical anions substituted the chloride anions in Ph5CpRu(CO)2Cl to form the formally neutral compound {Ph5CpRu(II)(CO)2[Sn(II)Pc(3-)]} (8) in which the negative charge and spin are preserved on [Sn(II)Pc(3-)](•-). The strong antiferromagnetic coupling of spins with a magnetic exchange interaction J/k(B) = -183 K in 8 is explained by the close packing of [Sn(II)Pc(3-)](•-) in the π-stacked {Ph5CpRu(II)(CO)2[Sn(II)Pc(3-)](•-)}2 dimers.

Synthesis, structures, and properties of crystalline salts with radical anions of metal-containing and metal-free phthalocyanines.

Radical anion salts of metal-containing and metal-free phthalocyanines [MPc(3-)](·-), where M = Cu(II), Ni(II), H2, Sn(II), Pb(II), Ti(IV)O, and V(IV)O (1-10) with tetraalkylammonium cations have been obtained as single crystals by phthalocyanine reduction with sodium fluorenone ketyl. Their formation is accompanied by the Pc ligand reduction and affects the molecular structure of metal phthalocyanine radical anions as well as their optical and magnetic properties. Radical anions are characterized by the alternation of short and long C-Nimine bonds in the Pc ligand owing to the disruption of its aromaticity. Salts 1-10 show new bands at 833-1041 nm in the NIR range, whereas the Q- and Soret bands are blue-shifted by 0.13-0.25 eV (38-92 nm) and 0.04-0.07 eV (4-13 nm), respectively. Radical anions with Ni(II), Sn(II), Pb(II), and Ti(IV)O have S = 1/2 spin state, whereas [Cu(II)Pc(3-)](·-) and [V(IV)OPc(3-)](·-) containing paramagnetic Cu(II) and V(IV)O have two S = 1/2 spins per radical anion. Central metal atoms strongly affect EPR spectra of phthalocyanine radical anions. Instead of narrow EPR signals characteristic of metal-free phthalocyanine radical anions [H2Pc(3-)](·-) (linewidth of 0.08-0.24 mT), broad EPR signals are manifested (linewidth of 2-70 mT) with g-factors and linewidths that are strongly temperature-dependent. Salt 11 containing the [Na(I)Pc(2-)](-) anions as well as previously studied [Fe(I)Pc(2-)](-) and [Co(I)Pc(2-)](-) anions that are formed without reduction of the Pc ligand do not show changes in molecular structure or optical and magnetic properties characteristic of [MPc(3-)](·-) in 1-10.

Formation of {Co(dppe)}2{µ2-η(2):η(2)-η(2):η(2)-[(C60)2]} Dimers Bonded by Single C-C Bonds and Bridging η(2)-Coordinated Cobalt Atoms.

Coordination of two bridging cobalt atoms to fullerenes by the η(2) type in {Co(dppe)}2{µ2-η(2):η(2)-η(2):η(2)-[(C60)2]}·3C6H4Cl2 [1; dppe = 1,2-bis(diphenylphosphino)ethane] triggers fullerene dimerization with the formation of two intercage C-C bonds of 1.571(4) Å length. Coordination-induced fullerene dimerization opens a path to the design of fullerene structures bonded by both covalent C-C bonds and η(2)-coordination-bridged metal atoms.