Highly efficient second-harmonic generation of a reflective waveguide-coupled photonic nanocavity.

In order to enhance second-harmonic generation (SHG) efficiency in a waveguide-coupled photonic nanocavity, we introduce a reflector at the edge of the waveguide and investigate its influence on input power and SHG efficiency. SHG efficiency and critical input power of the reflective waveguide-coupled cavity are controlled by the reflection amplitude and phase delay of the reflector. At input powers considerably lower than the critical power, SHG efficiency increases by up to three orders of magnitude higher than the case without reflector. Moreover, SHG efficiency of 100% at critical input power, which is twice that of the previous result, can be achieved over a wide phase range. These results prove the feasibility and controllability of highly efficient nonlinear optical devices.

Design of thin-film photonic crystals with complete photonic bandgap.

We theoretically investigate the optical characteristics of a thin-film photonic crystal structure with a complete photonic bandgap for both polarization of the transverse electric and transverse magnetic modes for any in-plane direction. The structure consists of three-layer stacked two-dimensional photonic crystal slabs, and the thickness of the structure is less than a few wavelengths. We show that a wide complete photonic bandgap can be obtained in the asymmetrically stacked photonic crystal structure. In addition, we designed a waveguide with a broad bandwidth of 100 nm and a nanocavity with a quality factor of 3.7 × 107 in the structures.

Measurement of optical loss in nanophotonic waveguides using integrated cavities.

Measurement of optical loss in nanophotonic waveguides is necessary for monitoring the properties of integrated photonic devices. We propose a simple method of measuring the optical loss using integrated nanocavities. It is shown theoretically that weak coupling between the waveguide and cavities leads to a direct estimation of the optical loss by measuring light radiated from the cavities. In addition, we experimentally demonstrate the optical loss in a fabricated photonic crystal waveguide. Our method gives not only a degree of freedom in real-time monitoring of the optical properties of nanophotonic structures, but it also can be used for various waveguide-based applications.

Investigation of Electrical and Optical Properties of Highly Transparent TCO/Ag/TCO Multilayer.

Transparent conductive oxides (TCOs) have been widely used as transparent electrodes for opto-electronic devices, such as solar cells, flat-panel displays, and light-emitting diodes, because of their unique characteristics of high optical transmittance and low electrical resistivity. Among various TCO materials, zinc oxide based films have recently received much attention because they have advantages over commonly used indium and tin-based oxide films. Most TCO films, however, exhibit valleys of transmittance in the wavelength range of 550-700 nm, lowering the average transmittance in the visible region and decreasing short-circuit current (Isc) of solar cells. A TCO/Ag/TCO multi-layer structure has emerged as an attractive alternative because it provides optical characteristics without the valley of transmittance compared with a 100-nm-thick single-layer TCO. In this article, we report the electrical, optical and surface properties of TCO/Ag/TCO. These multi-layers were deposited at room temperature with various Ag film thicknesses from 5 to 15 nm while the thickness of TCO thin film was fixed at 40 nm. The TCO/Ag/TCO multi-layer with a 10-nm-thick Ag film showed optimum transmittance in the visible (400-800 nm) wavelength region. These multi-layer structures have advantages over TCO layers of the same thickness.

Multiple-channel wavelength conversions in a photonic crystal cavity.

We demonstrate multiple-channel wavelength conversions of second harmonic and sum frequency generations in a silicon carbide photonic crystal cavity. The cavity is designed to have multiple modes including a nanocavity mode and Fabry-Pérot modes. Multiple-channel wavelength conversions in the nanocavity and Fabry-Pérot modes are shown experimentally. Furthermore, we investigate the polarization characteristics of wavelength-converted light. The experimental results of the polarization are in good agreement with calculation.

Investigation of electric/magnetic local interaction between Si photonic-crystal nanocavities and Au meta-atoms.

We experimentally investigate nanoscale local interaction in a composite system consisting of dielectric photonic-crystal nanocavity and metallic meta-atoms. The Q factor of the composite system changes by a maximum of about 10 dB based on the relative position and the type of meta-atoms. The emission by meta-atoms dominates the nanocavity emission when they are in the electric or magnetic antinodes of the cavity field. Circularly polarized emission is achieved by tuning the polarization, the position, and the coupling phase of each meta-atom with respect to the nanocavity. Utilization of the local interaction with meta-atoms is shown to be useful as a new degree of freedom in tailoring the characteristics of nanocavities.

Second-harmonic generation in a silicon-carbide-based photonic crystal nanocavity.

We demonstrate second-harmonic generation (SHG) in a silicon-carbide (SiC)-based heterostructure photonic crystal nanocavity by using a pulsed laser. We observe SHG light radiated from the SiC nanocavity and estimate the conversion efficiency in the cavity to be 2.59×10(-5) (=0.15 W(-1)) at an average input power of 0.17 mW. The near-field patterns and polarization characteristics of the SHG light are investigated experimentally and theoretically, and the results are in qualitatively good agreement.

The capture, hold and forward release of an optical pulse from a dynamic photonic crystal nanocavity.

We develop a silicon photonic crystal nanocavity device capable of performing targeted optical pulse capture and release via distinct ports on demand, based on dynamic Q factor control. The capture of 4 ps pulses and their release up to 332 ps later is directly observed by time-resolved measurements of the energy behaviour in both the nanocavity and emitted from the release port. We also discuss how the behaviour of excited free carriers dictates the performance of such dynamic devices.

Suppression of multiple photon absorption in a SiC photonic crystal nanocavity operating at 1.55 µm.

We show that a SiC photonic crystal cannot only inhibit two photon absorption completely, but also suppress higher-order multiple photon absorption significantly at telecommunication wavelengths, compared to conventional Si-based photonic crystal nanocavities. Resonant spectra of a SiC nanocavity maintain a Lorentzian profile even at input energies 100 times higher than what can be applied to a Si nanocavity without causing nonlinear effects. Theoretical fitting of the results indicates that the four photon absorption coefficient in the SiC nanocavity is less than 2.0 × 10(-5) cm(5)/GW(3). These results will contribute to the development of high-power applications of SiC nanocavities such as harmonic generation, parametric down conversion, and Raman amplification.

Time-resolved catch and release of an optical pulse from a dynamic photonic crystal nanocavity.

We perform time-domain measurements of the interaction between light and silicon photonic crystal nanocavities under dynamic Q factor control. Time-resolved evidence of optical pulse capture and release on demand is demonstrated and compared for samples with dynamic Q ranges from ~3,000 to 26,000 and from 18,500 to 48,000. Observing the energy behaviour in response to dynamic control provides insight not available with time-integrated measurements into factors influencing device performance such as carrier absorption and pulse capture efficiency.

Experimental investigation of thermo-optic effects in SiC and Si photonic crystal nanocavities.

We experimentally investigate and compare the thermo-optic effects of silicon carbide (SiC) and silicon (Si) photonic crystal nanocavities on their resonant wavelengths over a temperature range of 25 °C to nearly 200 °C by using a laser source with a wavelength close to the telecommunication wavelength range of 1550 nm. The measured results clearly show that the shift in the resonant wavelength of the SiC cavity is significantly (by a factor of 3) less than that of the Si cavity for the same ambient temperature changes. In addition, the measured results provide direct estimates of the thermo-optic coefficients (dn/dT) for thin SiC and Si as 3.87×10(-5)/°C and 1.60×10(-4)/°C, respectively, for this temperature range.

Demonstration of two-dimensional photonic crystals based on silicon carbide.

We demonstrate two-dimensional photonic crystals of silicon carbide (SiC)-a wide bandgap semiconductor and one of the hardest materials-at near-infrared wavelengths. Although the refractive index of SiC is lower than that of a conventional semiconductor such as GaAs or Si, we show theoretically that a wide photonic bandgap, a broadband waveguide, and a high-quality nanocavity comparable to those of previous photonic crystals can be obtained in SiC photonic crystals. We also develop a process for fabricating SiC-based photonic crystals that experimentally show a photonic bandgap of 200 nm, a waveguide with a 40-nm bandwidth, and a nanocavity with a high quality factor of 4,500. This demonstration should stimulate further development of resilient and stable photonics at high power and high temperature analogous to SiC power electronics.

Symmetrically glass-clad photonic crystal nanocavities with ultrahigh quality factors.

We design and fabricate ultra-high-quality (Q) photonic nanocavities in a symmetrically glass-clad silicon (Si) two-dimensional (2D) photonic crystal (PhC) structure. We theoretically investigate the dependence of the refractive index of the glass on the Q factors for asymmetric and symmetric structures. We show that the index-symmetric distribution of the glass is a critical factor to realize ultrahigh Q factors for glass-clad 2D PhC structures. We fabricate symmetrically glass-clad Si PhC nanocavities and achieve a record Q factor of 1×10(6), comparable with the highest Q factors of nanocavities in air-bridge structures.

Glass-embedded two-dimensional silicon photonic crystal devices with a broad bandwidth waveguide and a high quality nanocavity.

To enhance the mechanical stability of a two-dimensional photonic crystal slab structure and maintain its excellent performance, we designed a glass-embedded silicon photonic crystal device consisting of a broad bandwidth waveguide and a nanocavity with a high quality (Q) factor, and then fabricated the structure using spin-on glass (SOG). Furthermore, we showed that the refractive index of the SOG could be tuned from 1.37 to 1.57 by varying the curing temperature of the SOG. Finally, we demonstrated a glass-embedded heterostructured cavity with an ultrahigh Q factor of 160,000 by adjusting the refractive index of the SOG.

Resonant-wavelength tuning of a nanocavity by subnanometer control of a two-dimensional silicon-based photonic crystal slab structure.

We demonstrate fine tuning of the resonant wavelength of a nanocavity in a two-dimensional silicon-based photonic crystal slab structure by subnanometer control of the airhole diameter and slab thickness. Theoretical investigation shows that the resonant wavelength depends linearly on the latter two parameters. To experimentally demonstrate the fine tuning of the resonant wavelength, we control these parameters through chemical processes. The resonant-wavelength shift is tuned to 3.25 and 0.36 nm by use of two oxidizing processes. The latter shift, which corresponds to a 0.14 nm thick silicon layer, is considerably smaller than shifts achieved in previous studies.

High-Q nanocavity with a 2-ns photon lifetime.

We have succeeded in fabricating a photonic crystal nanocavity with a photon lifetime of 2.1 ns, which corresponds to a quality factor of 2.5 x 10(6). This lifetime is the longest recorded thus far in photonic crystal cavities, and was brought about by improvements in the fabrication process. Comparing our experimental quality factor with the results of calculations shows that we have suppressed variations in the radii and positions of the air holes composing a nanocavity such that their standard deviations are less than 1 nm.

Investigation of optical nonlinearities in an ultra-high-Q Si nanocavity in a two-dimensional photonic crystal slab.

We investigated the characteristics of an ultra-high-Q photonic nanocavity (Q = ~230,000 and modal volume = ~1.2 cubic wavelengths) at various input light powers. The cavity characteristics were red-shifted as the input power increased. This nonlinearity could be explained by coupled-mode theory, taking into account two-photon absorption, the associated free-carrier absorption, consequent free-carrier absorption, plasma effect, thermo-optic effect, and a Kerr effect. Nonlinear cavity characteristics were observed at an extremely low input light power of 10 muW. We confirmed that these low-power nonlinear optical effects could be attributed to the ultra-high Q factor of the nanocavity.

Analysis of the experimental Q factors (~ 1 million) of photonic crystal nanocavities.

In this letter, we show that the Q factors of the latest high-Q cavities in two dimensional photonic crystals, measured experimentally to be ~1000000, are determined by losses due to imperfections in the fabricated structures, and not by the cavity design. Quantitative analysis shows that the dominant sources of loss include the tilt of air-holes within the cavity, the roughness of the inner walls of the air-holes, variation in the radii of the air-holes, and optical absorption by adsorbed material. We believe that cavities with experimental Q factors of the order of several millions will be obtained in the future by reducing the losses due to imperfections through improved fabrication techniques.

Highly efficient multi-channel drop filter in a two-dimensional hetero photonic crystal.

An in-plane, multi-channel drop filter with high efficiency, in a two-dimensional photonic crystal (PC) slab, is experimentally demonstrated. Based on the concept of heterostructure photonic crystals proposed previously, the device consists of multiple simply connected, PC-based filter units, in which each unit has a structure proportional to an optimized basic unit and operates at a different wavelength. Four-channel drop operation was successfully obtained, with high efficiencies of almost 100%, and equal quality factors, across all channels.

Fine-tuned high-Q photonic-crystal nanocavity.

A photonic nanocavity with a high Q factor of 100,000 and a modal volume V of 0.71 cubic wavelengths, is demonstrated. According to the cavity design rule that we discovered recently, we further improve a point-defect cavity in a two-dimensional (2D) photonic crystal (PC) slab, where the arrangement of six air holes near the cavity edges is fine-tuned. We demonstrate that the measured Q factor for the designed cavity increases by a factor of 20 relative to that for a cavity without displaced air holes, while the calculated modal volume remains almost constant.