Masayuki Shirane

Absolute scale of second-order nonlinear-optical coefficients

The absolute scale of the second-order nonlinear-optical coefficients of several important nonlinear-optical materials has been obtained with improved accuracy. Second-harmonic generation, parametric fluorescence, and difference-frequency generation measurements have been made at several wavelengths in the near-infrared region. The second-harmonic generation measurement was performed at the fundamental wavelengths of 1.548, 1.533, 1.313, 1.064, and 0.852 µm. The materials measured included congruent LiNbO3,1%MgO:LiNbO3,5%MgO:LiNbO3,LiTaO3,KNbO3,KTiOPO4,KH2PO4, quartz, GaAs, GaP, α-ZnS, CdS, ZnSe, and CdTe. We made the parametric fluorescence measurement to determine the nonlinear-optical coefficients of congruent LiNbO3 and 5%MgO:LiNbO3 at pump wavelengths of 0.532 and 0.488 µm. We made the difference-frequency generation measurement for congruent LiNbO3 at a pump wavelength of 0.532 µm. The second-harmonic generation, parametric fluorescence, and difference-frequency generation measurements yielded consistent data on the nonlinear-optical coefficients of the materials. We found that many of the currently accepted standard values are overestimated because of neglect of the multiple-reflection effect in (nearly) plane-parallel-plate samples. The dispersion of the nonlinear-optical coefficients showed that Miller’s Δ is barely constant over the wavelength range measured and thus that Miller’s rule is not so good as other methods for wavelength scaling of the nonlinear-optical coefficients.

Nonlinear-Optic Silicon-Nanowire Waveguides

Using a 4-mm-long compact silicon-nanowire waveguide, we demonstrated nonlinear-optic effects such as the spectral broadening of optical short pulses due to self-phase modulation and nonlinear transmittance due to two-photon absorption. At a 12 W input power level, we observed a 1.5-π nonlinear phase shift and a strong saturation of optical output power in a sample. We also estimated the third-order nonlinear coefficient n2 and the two-photon absorption coefficient β, and compared them with those previously reported.

Systematic design of antireflection coating for semi-infinite one-dimensional photonic crystals using Bloch wave expansion

We present a systematic method for designing a perfect antireflection coating (ARC) for a semi-infinite one-dimensional (1D) photonic crystal (PC) with an arbitrary unit cell. We use Bloch wave expansion and time reversal symmetry, which leads exactly to analytic formulas of structural parameters for the ARC and renormalized Fresnel coefficients of the PC. Surface immittance (admittance and impedance) matching plays an essential role in designing the ARCs of 1D PCs, which is shown together with a practical example.

A compact optical sampling measurement system using mode-locked laser-diode modules

A compact optical sampling measurement system with a temporal resolution of 2 ps has been developed. External-cavity mode-locked laser-diode modules, which directly generate coherent 2-ps optical pulses, were used as the optical sampling pulse sources. Real-time measurement of the recovery dynamics in semiconductor saturable absorber devices and 160-Gbit/s pseudorandom return-to-zero optical signals with this system has been demonstrated.

Mode identification of high-quality-factor single-defect nanocavities in quantum dot-embedded photonic crystals

We investigate the quality (Q) factor and the mode dispersion of single-defect nanocavities based on a triangular-lattice GaAs photonic-crystal (PC) membrane, which contain InAs quantum dots (QDs) as a broadband emitter. To obtain a high Q factor for the dipole mode, we modulate the radii and positions of the air holes surrounding the nanocavity while keeping sixfold symmetry. A maximum Q of 17 000 is experimentally demonstrated with a mode volume of V=0.39(λ∕n)3. We obtain a Q∕V of 44 000(n∕λ)3, one of the highest values ever reported with QD-embedded PC nanocavities. We also observe ten cavity modes within the first photonic band gap for the modulated structure. Their dispersion and polarization properties agree well with the numerical results.

Vacuum Rabi splitting with a single quantum dot embedded in a H1 photonic crystal nanocavity

We report here the first observation of vacuum Rabi splitting in a single quantum dot (QD) embedded in a H1 photonic crystal nanocavity by photoluminescence measurement. The QD emission was tuned into a cavity mode by controlling the temperature. At the resonance condition, clear anticrossing with a Rabi splitting of ∼124 μeV was observed, where the cavity mode possesses the smallest mode volume V∼0.43(λ/n)3 among strongly coupled QD-cavity systems reported to date.

Immittance matching for multidimensional open-system photonic crystals

An electromagnetic (EM) Bloch wave propagating in a photonic crystal (PC) is characterized by the immittance (impedance and admittance) of the wave. The immittance is used to investigate transmission and reflection at a surface or an interface of the PC. In particular, the general properties of immittance are useful for clarifying the wave propagation characteristics. We give a general proof that the immittance of EM Bloch waves on a plane in infinite one- and two-dimensional (2D) PCs is real when the plane is a reflection plane of the PC and the Bloch wave vector is perpendicular to the plane. We also show that the pure-real feature of immittance on a reflection plane for an infinite three-dimensional PC is good approximation based on the numerical calculations. The analytical proof indicates that the method used for immittance matching is extremely simplified since only the real part of the immittance function is needed for analysis without numerical verification. As an application of the proof, we describe a method based on immittance matching for qualitatively evaluating the reflection at the surface of a semi-infinite 2D PC, at the interface between a semi-infinite slab waveguide (WG) and a semi-infinite 2D PC line-defect WG, and at the interface between a semi-infinite channel WG and a semi-infinite 2D PC slab line-defect WG.

Investigation of the Spectral Triplet in Strongly Coupled Quantum Dot–Nanocavity System

We experimentally investigated the excitation power dependence of a strongly coupled quantum dot (QD)–photonic crystal nanocavity system by photoluminescence measurements. At a low excitation power regime, we observed a vacuum Rabi doublet emission at the QD–cavity resonance condition. With increasing excitation power, in addition to the doublet, a third emission peak appeared. This observed spectral change is unexpected from conventional atomic cavity quantum electrodynamics. The observations can be attributed to featured pumping processes in the semiconductor QD–cavity system.

Optical sampling measurement with all-optical clock recovery using mode-locked diode lasers

Optical sampling measurement of 160-Gbps signals incorporating all-optical clock recovery has been demonstrated. External-cavity mode-locked semiconductor laser modules, which directly generate coherent 2-ps optical pulses, were used for optical clock recovery and as an optical sampling pulse source. We accomplished the real-time observation of waveform distortion in 160-Gbps optical data signals due to nonlinear optical effect in the transmission fiber.

Highly reliable mode-locked semiconductor lasers and their applications

Monolithic and external-cavity mode-locked semiconductor lasers are highly stabilized by means of electrical or optical signal injection incorporating a compact device-module packaging. Potential applications of these MLLDs are described for high-bit-rate optical communication systems at over 40 Gbps rate, and for ultrahigh-speed optical measurement systems.