Hisashi Sumikura

All-silicon sub-Gb/s telecom detector with low dark current and high quantum efficiency on chip

We demonstrate channel selective 0.1-Gb/s photoreceiver operation at telecom wavelength using a silicon high-Q photonic crystal nanocavity with a laterally integrated p-i-n diode. Due to the good crystal property of silicon the measured dark current is only 15 pA. The linear and nonlinear characteristics are investigated in detail, in which we found that the photocurrent is enhanced of more than 105 due to the ultrahigh-Q (Q≃105). With the help of two-photon absorption, which is visible at a surprisingly low input power of 10−8 W, the quantum efficiency of this device reaches ∼10%.

Fast Purcell-enhanced single photon source in 1,550-nm telecom band from a resonant quantum dot-cavity coupling

High-bit-rate nanocavity-based single photon sources in the 1,550-nm telecom band are challenges facing the development of fibre-based long-haul quantum communication networks. Here we report a very fast single photon source in the 1,550-nm telecom band, which is achieved by a large Purcell enhancement that results from the coupling of a single InAs quantum dot and an InP photonic crystal nanocavity. At a resonance, the spontaneous emission rate was enhanced by a factor of 5 resulting a record fast emission lifetime of 0.2 ns at 1,550 nm. We also demonstrate that this emission exhibits an enhanced anti-bunching dip. This is the first realization of nanocavity-enhanced single photon emitters in the 1,550-nm telecom band. This coupled quantum dot cavity system in the telecom band thus provides a bright high-bit-rate non-classical single photon source that offers appealing novel opportunities for the development of a long-haul quantum telecommunication system via optical fibres.

Low-power nanophotonic devices based on photonic crystals towards dense photonic network on chip

The authors review their recent studies on various nanophotonic devices including all-optical switches, optical memories, electro-optic modulators, photo-detectors and lasers, all of which are based on photonic crystal (PhC) nanocavities. The strong light confinement achieved in PhC nanocavities has enabled these devices with ultrasmall footprint and ultralow power/energy consumption. These characteristics are ideally suited for constructing dense photonic network on chip, which will overcome the limitation of future CMOS chips in terms of high-speed operation with less energy consumption and heat generation.

Faraday ellipticity and Faraday rotation of a doped-silicon wafer studied by terahertz time-domain spectroscopy

Free-carrier Faraday ellipticity and Faraday rotation are measured for a moderately doped n-type silicon wafer with the resistivity of 1.1Ωcm under magnetic fields of ±3T using the terahertz time-domain spectroscopy. From the experimental data, we obtain the time evolution of the electric-field vector of the terahertz radiation pulses. When the magnetic field is applied to the sample, the transmitted radiation has an elliptic polarization with its major axis rotated from the polarization direction of the incident radiation (Faraday effect). The Faraday ellipticity and Faraday rotation angle are obtained for the directly transmitted pulse (first terahertz pulse) and the pulse reflected twice at the sample surfaces (second terahertz pulse) separately. They are compared with the calculations using the Drude model. A slight deviation is observed between the experimental and calculated Faraday ellipticities and Faraday rotation angles probably due to the energy dependence of the carrier scattering time.Free-carrier Faraday ellipticity and Faraday rotation are measured for a moderately doped n-type silicon wafer with the resistivity of 1.1Ωcm under magnetic fields of ±3T using the terahertz time-domain spectroscopy. From the experimental data, we obtain the time evolution of the electric-field vector of the terahertz radiation pulses. When the magnetic field is applied to the sample, the transmitted radiation has an elliptic polarization with its major axis rotated from the polarization direction of the incident radiation (Faraday effect). The Faraday ellipticity and Faraday rotation angle are obtained for the directly transmitted pulse (first terahertz pulse) and the pulse reflected twice at the sample surfaces (second terahertz pulse) separately. They are compared with the calculations using the Drude model. A slight deviation is observed between the experimental and calculated Faraday ellipticities and Faraday rotation angles probably due to the energy dependence of the carrier scattering time.

Ultrafast spontaneous emission of copper-doped silicon enhanced by an optical nanocavity

Dopants in silicon (Si) have attracted attention in the fields of photonics and quantum optics. However, the optical characteristics are limited by the small spontaneous emission rate of dopants in Si. This study demonstrates a large increase in the spontaneous emission rate of copper isoelectronic centres (Cu-IECs) doped into Si photonic crystal nanocavities. In a cavity with a quality factor (Q) of ~16,000, the photoluminescence (PL) lifetime of the Cu-IECs is 1.1 ns, which is 30 times shorter than the lifetime of a sample without a cavity. The PL decay rate is increased in proportion to Q/Vc (Vc is the cavity mode volume), which indicates the Purcell effect. This is the first demonstration of a cavity-enhanced ultrafast spontaneous emission from dopants in Si, and it may lead to the development of fast and efficient Si light emitters and Si quantum optical devices based on dopants with efficient optical access.

Magnetic-field-induced fourfold azimuthal angle dependence in the terahertz radiation power of (100) InAs

The azimuthal angle dependence in the terahertz radiation power of (100) InAs under 1T magnetic field is presented. Results show that although the dominant radiation mechanism is surge current, azimuthal-angle-dependent radiation due to the nonlinear effect is also observed. The twofold symmetry of the p-polarized terahertz radiation power was modified to a fourfold symmetry with the transverse magnetic field. Moreover, results exhibited fourfold symmetry for the s-polarized terahertz power even with no applied field. The anisotropic intervalley scattering of photocarriers is tentatively proposed as the origin of quadrupole response and the fourfold emission symmetry.

Systematic study of thresholdless oscillation in high-β buried multiple-quantum-well photonic crystal nanocavity lasers.

Buried multiple-quantum-well (MQW) 2D photonic crystal cavities (PhC) achieve low non-radiative recombination and high carrier confinement thus making them highly efficient emitters. In this study, we have investigated the lasing characteristics of high-β(spontaneous emission coupling factor) buried MQW photonic crystal nanocavity lasers to clarify the theoretically-predicted thresholdless operation in high-β nanolasers. The strong light and carrier confinement and low non-radiative recombination in our nanolasers have enabled us to clearly demonstrate very smooth lasing transition in terms of the light-in vs light-out curve and cavity linewidth. To clarify the thresholdless lasing behavior, we carried out a lifetime measurement and a photon correlation measurement, which also confirmed the predicted behavior. In addition, we systematically investigated the dependence of β on the detuning frequency, which was in good agreement with a numerical simulation based on the finite-difference time-domain method. This is the first convincing systematic study of nanolasers based on an MQW close to the thresholdless regime.

Cavity-enhanced Raman scattering of single-walled carbon nanotubes

We have demonstrated the cavity-enhanced Raman scattering of semiconducting single-walled carbon nanotubes (CNTs) deposited in a silicon photonic crystal (Si PhC) nanocavity. In a resonant nanocavity, the detected Raman intensity of the CNTs is 100 times larger than that of the CNTs on a flat Si film. This enhancement results from the large local density of photon states and the large light extraction efficiency of the nanocavity. The cavity-enhanced Raman scattering of the CNTs suggests a way to develop a low-threshold CNT-based Raman laser.

Development of a Cryogen-Free Terahertz Time-Domain Magnetooptical Measurement System

We have developed a terahertz time-domain magnetooptical measurement system for investigating the carrier transport of semiconductors at low temperatures and high magnetic fields. To cool a superconducting magnet and a sample, Gifford–McMahon-type refrigerators were employed to avoid the use of liquid cryogen. However, the vibration of the refrigerator is a critical problem because it induces oscillation noise in measured waveforms of terahertz pulses. In order to overcome this problem, a synchronous data sampling method has been developed utilizing the sound of the refrigerator as a trigger for the waveform acquisition. This sampling method succeeded in eliminating the oscillation noise completely. Our measurement system enables us to obtain the magnetooptical spectra in the frequency range from 0.2 to 1.5 THz at temperatures between 5 and 300 K in magnetic fields of up to ±10 T. As an example, the measurement of the cyclotron resonances of a two-dimensional electron gas in a GaAs/AlGaAs heterostructure is demonstrated.

Time-Domain Coherent Anti-Stokes Raman Scattering Signal Detection for Terahertz Vibrational Spectroscopy Using Chirped Femtosecond Pulses

A new scheme for low-frequency coherent anti-Stokes Raman scattering spectroscopy (CARS) using chirped femtosecond pulses is proposed and demonstrated. Two chirped broadband optical pulses created terahertz (THz) polarization in the sample and generated CARS signal. The chirped CARS signal was then compressed by a pulse compressor and detected by a time gating technique. A resonant Raman band of GaSe at around 0.6 THz was successfully observed with the time-domain CARS technique. With the present system, CARS spectra from 0.2 to 5 THz are obtainable.