Optical frequency distribution using laser repeater stations with planar lightwave circuits.

We report a cascaded optical fiber link which connects laboratories in RIKEN, the University of Tokyo, and NTT within a 100-km region using a transfer light at 1397 nm, a subharmonic of the Sr clock frequency. The multiple cascaded link employing several laser repeater stations benefits from a wide feedback bandwidth for fiber noise compensation, which allows constructing optical lattice clock networks based on the master-slave configuration. We developed the laser repeater stations based on planar lightwave circuits to significantly reduce the interferometer noise for improved link stability. We implemented a 240-km-long cascaded link in a UTokyo-NTT-UTokyo loop using light sent from RIKEN via a 30-km-long link. In environments with large fiber noise, the link instability is 3 × 10-16 at an averaging time of 1 s and reaches 1 × 10-18 at 2,600 s.

Diameter-tailored telecom-band luminescence in InP/InAs heterostructure nanowires grown on InP (111)B substrate with continuously-modulated diameter from microscale to nanoscale.

We report diameter-tailored luminescence in telecom band of InP/InAs multi-heterostructure nanowires with continuously-modulated diameter from microscale to nanoscale. By using the self-catalyzed vapor-solid-liquid approach, we tune the indium particle size, and consequently the InP/InAs nanowire diameter, during growth by modulating the flow rate of the indium source material. This technique allows a high degree of continuous tuning in a wide scale from microscale to nanoscale. Hence it offers an original way to bridge the gap between microscale-featured photolithographic and nanoscale-featured nanolithographic processes and to incorporate InAs quantum disks with tunable diameters into a single InP/InAs quantum heterostructure nanowire. We realized site-defined nanowires with nanoscale diameters initiated from site-defined microscale-diameter particles made with a conventional photolithographic process. The luminescence wavelength from InAs quantum disks is directly connected to the nanowire diameter, by which the strain in the InAs quantum disks is tailored. This work provides new opportunities in the fabrication and design of nanowire devices that extends beyond what is achievable with the current technologies and enables the nanowire shape to be engineered thus offering the potential to broaden the application range of nanowire devices.

Drift-Induced Enhancement of Cubic Dresselhaus Spin-Orbit Interaction in a Two-Dimensional Electron Gas.

We investigated the effect of an in-plane electric field on drifting spins in a GaAs quantum well. Kerr rotation images of the drifting spins revealed that the spin precession wavelength increases with increasing drift velocity regardless of the transport direction. A model developed for drifting spins with a heated electron distribution suggests that the in-plane electric field enhances the effective magnetic field component originating from the cubic Dresselhaus spin-orbit interaction.

Controlled 1.1-1.6 µm luminescence in gold-free multi-stacked InAs/InP heterostructure nanowires.

We report controlled 1.1-1.6 µm luminescence in gold-free multi-stacked InAs/InP heterostructure nanowires (NWs). We realized the NWs by using an indium-particle-assisted vapor-liquid-solid synthesis approach. The growth temperature, as low as 320 °C, enables the formation of an atomically abrupt InP/InAs interface by supressing the diffusion and weakening the reservoir effect in the indium droplet. The low growth temperature also enables us to grow multi-stacked InAs/InP NWs in the axial direction without any growth on the NW side face. The high controllability of the growth technology ensures that the luminescence can be tailored by the thickness of InAs segment in InP NWs and cover the 1.3-1.5 µm telecommunication window range. By using the nanoscale-spatial-resolution technology combing cathodoluminescence with scanning electron microscopy, we directly correlated the site of different-thickness InAs segments with its luminescence property in a single NW and demonstrate the InAs-thickness-controlled energy of optical emission in 1.1-1.6 µm.

Carrier-envelope offset locking with a 2f-to-3f self-referencing interferometer using a dual-pitch PPLN ridge waveguide.

We demonstrate that a 2f-to-3f self-referencing interferometer (SRI) becomes a useful tool for stabilizing a carrier-envelope offset frequency of an Er-doped fiber laser. A dual-pitch periodically poled lithium niobate (PPLN) ridge waveguide, consisting of two monolithically integrated segments with different quasi-phase matching pitch sizes, allows us to generate third-harmonic light with high efficiency. By using this device, we obtain a 45-dB signal-to-noise ratio in 100-kHz bandwidth of a heterodyne beat signal and instability of the in-loop f CEO of 8 × 10(-18) at 1 s of averaging time. This result is important for f CEO stabilization of a frequency comb, for which it is difficult to obtain a one-octave supercontinuum spectrum.

Phase-noise characteristics of a 25-GHz-spaced optical frequency comb based on a phase- and intensity-modulated laser.

We investigated phase-noise characteristics of both a phase/intensity-modulated laser with 25-GHz mode spacing and a mode-locked fiber laser with carrier-envelope-offset (CEO) locking. As the separation from the frequency of the continuous wave (CW) laser diode (LD) for a seed light source increases, the integrated phase noise of each comb mode of both the phase/intensity-modulated laser and supercontinuum light originating from it increases with the same slope as a function of mode number. The dependence of the integrated phase noise on mode number with the phase/intensity-modulated laser is much larger than with the mode-locked fiber laser of the CEO locking. However, the phase noise of the phase/intensity-modulated laser is extremely lower than that of the mode-locked fiber laser with CEO locking in the frequency region around the CW LD. The phase noise of the phase/intensity-modulated laser with 25-GHz mode spacing and that of the mode-locked fiber laser with the CEO locking could be estimated and were found to be almost the same at the wavelengths required in an f-to-2f self-referencing interferometer. Our experimental results indicate the possibility of achieving an offset-frequency-locked frequency comb with the phase/intensity-modulated laser.

VLS growth of alternating InAsP/InP heterostructure nanowires for multiple-quantum-dot structures.

We investigated the Au-assisted growth of alternating InAsP/InP heterostructures in wurtzite InP nanowires on InP(111)B substrates for constructing multiple-quantum-dot structures. Vertical InP nanowires without stacking faults were obtained at a high PH(3)/TMIn mole flow ratio of 300-1000. We found that the growth rate changed largely when approximately 40 min passed. Ten InAsP layers were inserted in the InP nanowire, and it was found that both the InP growth rate and the background As level increased after the As supply. We also grew the same structure using TBAs/TBP and could reduce the As level in the InP segments. A simulation using a finite-difference time-domain method suggests that the nanowire growth was dominated by the diffusion of the reaction species with long residence time on the surface. For TBAs/TBP, when the source gases were changed, the formed surface species showed a short diffusion length so as to reduce the As background after the InAsP growth.

Generation of 120-fs laser pulses at 1-GHz repetition rate derived from continuous wave laser diode.

We report the first demonstration of continuous-wave laser diode based 100-fs-class pulse lasers operating at a gigahertz repetition rate without a mode-locking technique. We describe the performance of a 1-W, 120-fs optical pulse train at 1 GHz and a 1-W, 80-fs optical pulse train at 250 MHz by using a simple configuration. Sub-100-fs pulse durations are achieved by using a progressive expansion of the spectrum in the self-phase modulation process in an erbium-doped fibre amplifier. Our scheme can achieve continuously tunable repetition rate in the range of ± 20%, and develop powerful tools for use in nanomechanical systems and nanobiotechnology.

Vibration amplification, damping, and self-oscillations in micromechanical resonators induced by optomechanical coupling through carrier excitation.

Carrier-induced dynamic backaction in micromechanical resonators is demonstrated. Thermal vibration of an n-GaAs/i-GaAs bilayer cantilever is amplified by optical band-gap excitation, and for the excitation power above a critical value, self-oscillations are induced. These phenomena are found in the [1[over ¯]10]-oriented cantilever, whereas the damping (deamplification) is observed in the [1[over ¯]10] orientation. This optomechanical coupling does not require any optical cavities but is instead based on the piezoelectric effect that is generated by photoinduced carriers.

Optical nonlinearity enhancement with graphene-decorated silicon waveguides.

Broadband on-chip optical frequency combs (OFCs) are important for expanding the functionality of photonic integrated circuits. Here, we demonstrate a huge local optical nonlinearity enhancement using graphene. A waveguide is decorated with graphene by precisely manipulating graphene's area and position. Our approach simultaneously achieves both an extremely efficient supercontinuum and ultra-short pulse generation. With our graphene-decorated silicon waveguide (G-SWG), we have achieved enhanced spectral broadening of femtosecond pump pulses, along with an eightfold increase in the output optical intensity at a wavelength approximately 200 nm shorter than that of the pump pulses. We also found that this huge nonlinearity works as a compressor that effectively compresses pulse width from 80 to 15.7 fs. Our results clearly show the potential for our G-SWG to greatly boost the speed and capacity of future communications with lower power consumption, and our method will further decrease the required pump laser power because it can be applied to decorate various kinds of waveguides with various two-dimensional materials.

Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures.

The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure-cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron-hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays.