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In a previous work, we demonstrated a coupled cavity system where photons in one storage cavity can be transferred to another storage cavity at an arbitrary time by applying a voltage pulse to a third cavity placed in a p-i-n junction. In this work, we demonstrate methods to improve the transfer efficiency and photon lifetimes of such a coupled system. Firstly, we designed a photonic-crystal structure that achieves a large coupling coefficient without reducing the radiation quality factor compared to the previously proposed structure: The photonic-crystal design was changed to a more symmetric configuration to suppress radiation losses and then optimized using an automatic structure tuning method based on the Covariance Matrix Adaptive Evolutional Strategy (CMAES). Here we added two improvements to achieve an evolution toward the desired direction in the two-dimensional target parameter space (spanned by the coupling coefficient and the inverse radiation loss). Secondly, to improve the experimental cavity quality factors, we developed a fabrication process that reduces the surface contamination associated with the fabrication of the p-i-n junction: We covered the photonic structure with a SiO 2 mask to avoid the contamination and the electrode material was changed from Al to Au/Cr to enable cleaning by a weak acid. Owing to these improvements of the cavity design and the fabrication process, the obtained system provides coupling strengths that are about three times stronger and photon lifetimes that are about two times longer, compared to the previously reported system.

Here, we report on the increase of the quality-factors of photonic crystal nanocavities fabricated by a CMOS-compatible process. We fabricated nanocavities with the same cavity design but used either a binary photomask or a phase-shift photomask in the photolithography step to assess the impact of the photomask-type on the fabrication accuracy of the air holes. We characterized 62 cavities using time-resolved measurements and the best cavity had a quality-factor of 6.65 × 10 6 . All cavities exhibited a quality-factor larger than 2 million and the overall average was 3.25 × 10 6 . While the estimated magnitude of the scattering loss due to the air hole variations in the 33 cavities fabricated with the phase-shift photomask was slightly lower than that in the 29 cavities fabricated with binary photomask, the phase-shift photomask did not provide a significant improvement in the fabrication accuracy. On average, the scattering loss in these samples is more than 3 times larger than that of nanocavities fabricated using electron-beam lithography, which indicates room for further improvement.

It is known that the quality factors (Q) of photonic crystal nanocavities vary from sample to sample due to air-hole fabrication fluctuations. In other words, for the mass production of a cavity with a given design, we need to consider that the Q can vary significantly. So far, we have studied the sample-to-sample variation in Q for symmetric nanocavity designs, that is, nanocavity designs where the positions of the holes maintain mirror symmetry with respect to both symmetry axes of the nanocavity. Here we investigate the variation of Q for a nanocavity design in which the air-hole pattern has no mirror symmetry (a so-called asymmetric cavity design). First, an asymmetric cavity design with a Q of about 250,000 was developed by machine learning using neural networks, and then we fabricated fifty cavities with the same design. We also fabricated fifty symmetric cavities with a design Q of about 250,000 for comparison. The variation of the measured Q values of the asymmetric cavities was 39% smaller than that of the symmetric cavities. This result is consistent with simulations in which the air-hole positions and radii are randomly varied. Asymmetric nanocavity designs may be useful for mass production since the variation in Q is suppressed.

A Raman nanocavity laser can emit light into free space and into a properly designed waveguide adjacent to the cavity by mode coupling. In common device designs, the emission from the edge of this waveguide is relatively weak. However, a Raman silicon nanocavity laser with strong emission from the waveguide edge would be advantageous for certain applications. Here we investigate the increase in the edge emission that can be achieved by adding photonic mirrors to the waveguides adjacent to the nanocavity. We experimentally compare devices with and without photonic mirrors: the edge emission for devices with mirrors is 4.3 times stronger on average. This increase is analyzed using coupled-mode theory. The results indicate that the control of the round-trip phase shift (between the nanocavity and the mirror) and an increase of the quality factors of the nanocavity are important for further enhancement.

Nanocavity devices based on silicon that can operate in the 1.2-µm band would be beneficial for several applications. We fabricate fifteen cavities with resonance wavelengths between 1.20 and 1.23 µm. Experimental quality (Q) factors larger than one million are obtained and the average Q values are lower for shorter wavelengths. Furthermore, we observe continuous-wave operation of a Raman silicon laser with an excitation wavelength of 1.20 µm and a Raman laser wavelength of 1.28 µm. The Q values of the nanocavity modes used to confine the excitation light and the Raman scattered light are about half of those for our Raman silicon laser operating in the 1.55-µm band. Nevertheless, this device exhibits an input-output characteristic with a clear laser threshold. Finally, we consider the effect of the higher scattering probability at shorter wavelengths on the Raman laser performance in the 1.2-µm band.

Raman silicon lasers based on photonic crystal nanocavities with a threshold of several hundred microwatts for continuous-wave lasing have been realized. In particular, the threshold depends on the degree of confinement of the excitation light and the Raman scattering light in the two nanocavity modes. Here, we report lower threshold values for Raman silicon nanocavity lasers achieved by increasing the quality (Q) factors of the two cavity modes. By using an optimization method based on machine learning, we first increase the product of the two theoretical Q values by a factor of 17.0 compared to the conventional cavity. The experimental evaluation demonstrates that, on average, the actually achieved product is more than 2.5 times larger than that of the conventional cavity. The input-output characteristic of a Raman laser with a threshold of 90 nW is presented and the lowest threshold obtained in our experiments is 40 nW.

The performance of a Raman silicon laser based on a high quality-factor nanocavity depends on the degree of free-carrier absorption, and this characteristic may be useful for certain applications. Here we demonstrate that laser oscillation in a Raman silicon nanocavity laser stops abruptly after an exposure to a weak flux of negatively ionized air for a few seconds. Spectral measurements reveal that the laser interruption is mainly caused by the transfer of extra electrons from the negatively ionized air molecules to the silicon nanocavity. These electrons affect the efficiency of the Raman laser by free carrier absorption. We find that the laser output gradually recovers as the extra electrons escape from the nanocavity and confirm that such a detection of ionized air is repeatable. These results show that a Raman silicon nanocavity laser can be used for the detection of ionized air with a high spatial resolution.

Near-field thermophotovoltaic (TPV) power generation has been attracting increasing attention as a promising approach for efficient conversion of heat into electricity with high output power density. Here, we numerically investigate near-field TPV devices with surrounding reflectors for efficient recycling of low-energy photons, which do not contribute to the power generation. We reveal that the conversion efficiency of a near-field TPV system can be drastically increased by introducing a pair of reflectors above and below the system, especially when the two mirrors are not in contact with the emitter and absorber. In addition, we investigate the influence of non-perfect photon recycling on the TPV efficiency and reveal that near-field TPV systems are more robust against the decrease of the reflectivity of the reflectors than the far-field TPV systems.

An approach to optimizing the Q factors of two-dimensional photonic crystal (2D-PC) nanocavities based on deep learning is hereby proposed and demonstrated. We prepare a data set consisting of 1000 nanocavities generated by randomly displacing the positions of many air holes in a base nanocavity and calculate their Q factors using a first-principles method. We train a four-layer neural network including a convolutional layer to recognize the relationship between the air holes' displacements and the Q factors using the prepared data set. After the training, the neural network is able to estimate the Q factors from the air holes' displacements with an error of 13% in standard deviation. Crucially, the trained neural network can estimate the gradient of the Q factor with respect to the air holes' displacements very quickly using back-propagation. A nanocavity structure with an extremely high Q factor of 1.58 × 10 9 was successfully obtained by optimizing the positions of 50 holes over ~10 6 iterations, taking advantage of the very fast evaluation of the gradient in high-dimensional parameter spaces. The obtained Q factor is more than one order of magnitude higher than that of the base cavity and more than twice that of the highest Q factors reported so far for cavities with similar modal volumes. This approach can optimize 2D-PC structures over a parameter space of a size unfeasibly large for previous optimization methods that were based solely on direct calculations. We believe that this approach is also useful for improving other optical characteristics.

In this paper, we numerically investigate a method to obtain narrow-bandwidth near-field thermal radiation spectra by using two-dimensional (2D) photonic crystal (PC) slabs. Our examination reveals that near-field thermal radiation spectra can be artificially controlled via the photonic band engineering of 2D-PC slabs, where the radiation is enhanced in a range of frequencies of the flat bands and suppressed inside the photonic bandgap. By designing a thermal emitter with a 2D-PC slab of appropriate thickness, and by adjusting the gap between the emitter and the absorber, we can implement narrowband near-field thermal radiation that overcomes the far-field blackbody limit in the near-infrared range. Further, its linewidth is as small as Δλ = 0.14 µm.

Lithium niobate-on-insulator (LNOI) waveguides fabricated on a silicon wafer using a room-temperature bonding method have potential application as Si-based high-density photonic integrated circuits. A surface-activated bonding method using a Si nanoadhesive layer was found to produce a strong bond between LN and SiO 2 /Si at room temperature, which is sufficient to withstand both the wafer-thinning (LN thickness <5 μm) and surface micromachining processes used to form the strongly confined waveguides. In addition, the bond quality and optical propagation characteristics of the resulting LNOI waveguides were investigated, and the applicability of this bonding method to low-loss LNOI waveguide fabrication is discussed. The propagation loss for the ridged waveguide was approximately 2 dB/cm at a wavelength of 1550 nm, which was sufficiently low for the device application. The results of the present study will be of significant use in the development of fabrication techniques for waveguides with any bonded materials using this room-temperature bonding method, and not only LN core/SiO 2 cladding waveguides.

We propose a scheme for near-field thermophotovoltaic (TPV) energy conversion, where thermal emission from an emitter is extracted by an intermediate transparent substrate attached to the top of a photovoltaic (PV) cell. The addition of an intermediate transparent substrate suppresses the unwanted heat transfer from the emitter to the PV cell due to the surface modes on the PV cell while maintaining the enhancement in the interband absorption. We confirm that our scheme is applicable for near-field TPV systems using a silicon (Si) or tungsten (W) emitter. As a specific example, we designed a near-field TPV system composed of a one-dimensional Si photonic crystal thermal emitter, an InGaAs PV cell, and an intermediate Si substrate, and displayed that our scheme could realize both high power density (>5 × 10 4 W/m 2 ) and high power conversion efficiency (>40%) at a 50-nm gap between the emitter and the intermediate substrate.

We fabricated photonic crystal high-quality factor (Q) nanocavities on a 300-mm-wide silicon-on-insulator wafer by using argon fluoride immersion photolithography. The heterostructure nanocavities showed an average experimental Q value of 1.5 million for 12 measured samples. The highest Q value was 2.3 million, which represents a record for a nanocavity fabricated by complementary metal-oxide-semiconductor (CMOS)-compatible machinery. We also demonstrated an eight-channel drop filter with 4 nm spacing consisting of arrayed nanocavities with three missing air holes. The standard deviation in the drop wavelength was less than 1 nm. These results will accelerate ultrahigh-Q nanocavity research in various areas.

Photonic crystal nanocavities that simultaneously possess small modal volumes and high quality (Q) factors have opened up novel research areas in photonics during this decade. Here, we present an important key for the increase of Q factors to ranges beyond ten million. A systematic investigation on photon lifetimes of air-bridge-type heterostructure nanocavities fabricated from silicon on insulator (SOI) substrates indicated the importance of cleaning the bottom side (buried oxide side) of the nanaocavites. Repeated thermal oxidation and an oxide removal process applied after the removal of the buried oxide layer underneath the nanocavities realized an experimental Q factor greater than eleven million, which is the highest experimental Q ever recorded. The results provide important information not only for Si PC nanocavities but also for general Si nanophotonic devices and photonic electronic convergence systems.

We experimentally study photonic crystal L3 nanocavities whose design Q factors (Q design ) have been improved with the visualization of leaky components design method. The experimental Q values (Q exp ) are monotonically increased from 6,000 to 2,100,000 by iteratively modifying the positions of some of the air holes, as determined by the referred design method. We investigate the Q exp tolerance to imperfections in the fabricated samples, which reveals that the cavities improved by the visualization method tend to lose some tolerance to structural differences between the fabricated samples and the design values.

We demonstrate a single-mode high-Q (Q>100) mid-infrared thermal emitter operating with high power-utilization efficiency. The emitter consists of a rod-type photonic crystal (PC) slab interacting with GaAs/AlGaAs multiple quantum wells (MQWs), a GaAs substrate frame supporting the PC slab, and electric wires for Joule heating of the device. We carefully design the structure of the PC slab and the supporting frame/wires to minimize unwanted thermal losses and realize narrowband thermal emission having a peak intensity, under a given electrical input power, that is an order of magnitude higher than that of a reference blackbody emitter due to the efficient increase of the device temperature.

We investigate the radiation angle of an oblique waveguide in a stripe-stacked three-dimensional photonic crystal. We show that the output-light is radiated in a different direction from the oblique waveguide direction. Moreover, the radiation polar angle varies from 30° to 50° depending on the frequency. To inhibit the frequency dependence and obtain vertical radiation, we introduced a symmetric structure at the end of the waveguide. As a result of cancellation of the in-plane asymmetric wavenumber, the radiation polar angle is less than 6° from the surface-normal direction and does not depend on frequency.

A method that simply improves the quality (Q) factors of two-dimensional photonic crystal nanocavities using a three-dimensional finite-difference time domain calculation is described. The leaky area for a high-Q nanocavity mode is visualized in a real cavity structure by extracting the leaky components within a light cone in momentum space and by transferring them back into real space using an inverse Fourier transformation. The Q factor is remarkably improved by appropriately shifting the positions of air holes at the leaky area. We design three-missing-air-hole and zero-cell-defect nanocavities with Q factors of 5,000,000 and 1,700,000, respectively, for demonstration.

We carry out the structural design of photonic crystals (PCs) using sensitivity analysis for enhancing optical absorption of thin film microcrystalline silicon (μc-Si) solar cells. In this paper, we employ a model which includes absorption of not only the thin film μc-Si, but also the transparent conductive oxide and metal back reflector for design accuracy. We carry out structural design for this model using sensitivity analysis which maximizes optical absorption in μc-Si layer. As a result, we succeed in obtaining the maximum short circuit current density of 25.2 mA/cm2 for thin film (600-nm thick) μc-Si solar cells (1.4-fold increase compared to the case without a PC).

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.

We have precisely measured the Raman shift of photonic crystal silicon heterostructure nanocavities for Raman laser applications. We utilized a near-infrared excitation laser of wavelength 1.42 μm in order to avoid local sample heating and exploited two high-Q nanocavity modes to calibrate the Raman frequency. The measured Raman shift was 15.606 THz (520.71 cm(-1)) with a small uncertainty of 1.0 × 10(-3) THz. In addition, we investigated the compressive stress generated in a photonic crystal slab in which a ~5.1 × 10(-3) THz blue shift of the Raman peak and a slight warpage of the slab were observed. We also demonstrated that the stress could be eliminated by using a cantilever structure.

This paper demonstrates the application of ultra-precision cutting to the fabrication of ridged LiNbO₃ waveguides for use in low-loss photonic integrated circuits. Ridged waveguides with sidewall verticality of 88° and ultra-smooth sidewalls were obtained in LiNbO₃ crystals using this technique. In addition, the possibility of fabricating bent ridged waveguides via this mechanical micromachining method was examined. The root mean square surface roughness of the machined sidewall was 4.5 nm over an area of 2.5 × 10 µm, which is sufficiently low so as to minimize scattering losses of guided light. The propagation loss of the ridged waveguide produced during this work was well below 1 dB/cm at a wavelength of 1550 nm. The present technique should have significant applicability to the micromachining of ferroelectric materials and the fabrication of highly confined optical waveguides such as ridged waveguides and photonic wires.

We develop a gallium arsenide (GaAs) photonic crystal nanocavity device capable of capturing and releasing a pulse of light by dynamic control of the Q factor through free carrier photoexcitation. Unlike silicon-based devices where the performance of this dynamic optical control is limited by absorption from free carriers with nanosecond-order lifetimes, the short carrier lifetime (∼ 7 ps) of our equivalent GaAs devices enables dynamic control with negligible absorption losses. We capture a 4 ps optical pulse by briefly cycling the Q factor from 40,000 to 7900 and back just as the light couples to the nanocavity and confirm that the captured energy can be subsequently released on demand by a second injection of free carriers. Demonstrating dynamic control with negligible loss in a GaAs nanophotonic device also opens the door to dynamic control of cavity quantum electrodynamics with potential application towards quantum information processing.

We demonstrated 32-channel drop filters with 100 GHz spacing consisting of arrayed nanocavities and a waveguide in a photonic crystal silicon slab. Changing the lattice constant of the nanocavities on the subnanometer scale successfully controlled the drop wavelengths at 100 GHz spacing in the wavelength range between 1510 and 1550 nm. The device size was as small as 15 μm × 270 μm, and the variation in drop wavelengths was less than 0.3 nm in terms of standard deviation. We also present a movie showing the operation of the drop filter, demonstrating that the arrayed nanocavities have the potential for developing ultracompact 100 GHz spaced filters in a dense wavelength division multiplexing system.

We have investigated absorption losses due to surface water adsorbed on the surface of silicon heterostructure nanocavities with quality (Q) factors of several million. Measurements performed while changing the ambient humidity that the nanocavity is exposed to show that the Q value depends linearly on humidity. We also reveal that chemical treatment to change the degree of hydrophilicity of the surface results in a drastic increase of Q; we have obtained an experimental value of 9 million, which represents a new record for a heterostructure nanocavity. We analytically determine the absolute value of absorption loss by exploiting the degree of fluctuation of Q values between different samples.

In this paper we investigate the optical forces induced by localized optical modes propagating along three parallel waveguides, of which only the central one is free to move. In this configuration, when all three waveguides are identical, the components of the optical-force acting on the free beam are decoupled along the axis of symmetry. As a result, two dimensional optomechanical control of the central waveguide, like single-mode optical trapping, can be achieved. We also study non symmetric configurations, that can be used, for example, to tailor the position of the optical trap. Unlike other techniques that rely on buckling, multi-mode excitation or radiation-pressure, single-mode optomechanical-operation should help the realization of faster and simpler on-chip positioning of a single nanobeam since most of the parameters involved can be controlled with great precision.

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.

We propose a set of principles to tailor and enhance optical forces between parallel, periodic dielectric waveguides by molding the eigen-mode field distribution via the combined effects of highly symmetric slow light modes and waveguide morphology. The geometries here considered are amenable to standard lithographic techniques and possess strong repulsive and attractive optical forces that can be enhanced via slow-light band edge modes. This new methodology should enable the fabrication of optomechanical devices for applications in sensing, switching and nano-optomechanical systems.

The effect of the nanosecond laser annealing on the photoluminescence (PL) property of phosphorus ions (P(+)) implanted ZnOnanorods (NRs) has been investigated. The nanosecond laser annealing was performed with the third harmonic of a Q-switched Nd:YAG laser (355nm, 10ns/pulse) at a fluence of 100mJ/cm(2). It turned out that nanosecond laser annealing is more effective in the recovery of the PL property compared with the thermal annealing using an electric furnace. As the results, the I-V characteristics of the p-n homojunctions along ZnO NRs showed rectifying property with a threshold voltage of approximately 6V.

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.

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.

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.

We have measured the quality (Q) factors and resonant wavelengths for 80 photonic crystal nanocavities with the same heterostructure. In this statistical evaluation, the Q factors varied according to a normal distribution centered at 3 million and ranging between 2.3 million and 3.9 million. The resonant wavelengths also fluctuated but with a standard deviation of only 0.33 nm. Such a high average Q factor and highly controlled resonant wavelength will be important for the development of advanced applications of photonic crystal nanocavities. Comparing the experimental values with calculated values suggests that factors other than structural variations of air holes, which decrease the Q factor, are indeed present in the fabricated nanocavities.

We propose to use a combination of intersubband transitions in semiconductor quantum wells with a two dimensional photonic crystal cavity to obtain narrow, strong thermal radiation spectra. Single peak thermal radiation is obtained due to the Lorentzian shape absorption spectrum of the intersubband transition and the single mode cavity embedded within the photonic band gap. We present an analysis based on the quantum Langevin theory. It is shown that local radiance of the narrow emission peak can be maximized to approximately 80% of the radiation from the blackbody devices when the photon dissipation rates of the cavity mode due to the intersubband absorption and that due to the radiation to the free space modes are equal. Guidelines for concrete device design are introduced, and an example device structure is shown.

We have investigated the localized design of heterostructure photonic crystal nanocavities in order to make them more suitable for integration. While retaining theoretical quality factors of more than ten million, the total length of the heterostructure nanocavity can be reduced to ~5 mum and the shifted air holes comprising the heterostructure can be restricted to the two rows nearest the nanocavity on each side. Though the area for the heterostructure nanocavity investigated thus far was larger than 10 x 10 microm(2) in the photonic crystal slab, heterostructure nanocavities of this new design have sizes of approximately 3 x 5 microm(2), thus allowing dense integration.

We introduce recent advances in dynamic control over the Q factor of a photonic crystal nanocavity system. By carefully timing a rapid increase of the Q factor from 3800 to 22,000, we succeed in capturing a 4ps signal pulse within the nanocavity with a photon lifetime of 18ps. By performing an additional transition of the Q factor within the photon lifetime, the held light is once again ejected from of the system on demand.

Solid-state cavity quantum-electrodynamics (QED) has great potential owing to advances such as coupled systems combining a nanocavity and a quantum dot (QD). These systems involve two photon-emission mechanisms: the Purcell effect in the weak coupling regime and vacuum Rabi-splitting in the strong coupling regime. In this paper, we describe a third emission mechanism based on the quantum anti-Zeno effect (AZE) induced by the pure-dephasing in a QD. This is significantly enhanced by the inherent characteristics of the nanocavity. This mechanism explains the origin of strong photon emission at a cavity mode largely detuned from a QD, previously considered a counterintuitive, prima facie non-energy-conserving, light-emission phenomenon. These findings could help in controlling the decay and emission characteristics of solid-state cavity QED, and developing solid-state quantum devices.

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.

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.

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.

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.

We report a theoretical and experimental study of a channel drop filter with two cascaded point-defects between two line-defects in a two-dimensional photonic-crystal slab. Using coupled-mode analysis and a three-dimensional finite-difference time-domain method, we design a filter to engineer the line shape of the drop spectrum. A flat-top and sharp roll-off response is theoretically and experimentally achieved by the designed and fabricated filters. Furthermore, we theoretically demonstrate that drop efficiency is increased dramatically, up to 93%, by introducing hetero-photonic-crystals. We also describe a method to modify the bandwidth of the spectrum.

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.