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ABSTRACT: We report on newly-designed H1-type photonic crystal (PhC) nanocavities that simultaneously exhibit high Q factors, small mode volumes, and high external coupling efficiencies (η<sub>⊥</sub>) of light radiated above the PhC membrane. Dipole modes of the H1 PhC nanocavities, which are doubly-degenerate and orthogonally-polarized in theory, are investigated both by numerical calculations and experiments. Through modifying the sizes and positions of the air-holes near to the defect cavity, a Q factor of 62,000 is achieved, accompanied with an improved η<sub>⊥</sub> of 0.38 (assuming an objective lens with a numerical aperture of 0.65). A further increase of η<sub>⊥</sub> to more than 0.60 is observed at the expense of slight degradation of Q factor (down to 50,000). We further experimentally confirm the increase of both Q and η<sub>⊥</sub>, using micro-photoluminescence measurements, and demonstrate high Q factors up to 25,000: the highest value ever reported for dipole modes in H1 PhC nanocavities.
Optics Express 12/2012; 20(27):28292-300. · 3.59 Impact Factor
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ABSTRACT: We experimentally investigated photoluminescence (PL) from silicon photonic crystal nanocavities with different mode volumes at room temperature. The integrated cavity mode intensity, which was estimated from the observed PL signal by considering extraction and collection efficiencies for each cavity mode, increased as the cavity mode volume decreased. This result suggests that smaller cavities have larger mode emission efficiency per volume than that for larger cavities at room temperature.
Applied Physics Letters 05/2011; · 3.84 Impact Factor
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ABSTRACT: We demonstrated the strong coupling between a one-dimensional photonic crystal nanobeam cavity and a single quantum dot (QD). Thanks to a high quality factor ( ∼ 25 000) with small mode volume [0.38×(n/λ)3] of the nanobeam cavity, an anticrossing behavior with a vacuum Rabi splitting of 226 μeV was observed. The ratio of the QD-cavity coupling strength to the cavity decay rate, which is a figure of merit of quantum optical applications, is estimated to 2.1. This is the highest value among any QD-based cavity quantum electrodynamics systems reported so far.
Applied Physics Letters 04/2011; 98(17):173104-173104-3. · 3.84 Impact Factor
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ABSTRACT: Photonic crystals 1,2 have been extensively used in the control and manipulation of photons in engineered electromagnetic environments provided by means of photonic bandgap effects. These effects are key to realizing future optoelectronic devices, including highly efficient lasers. To date, lasers based on photonic crystal cavities have been exclusively demon-strated in two-dimensional photonic crystal geometries 3–6 . However, full confinement of photons and control of their inter-action with materials can only be achieved with the use of three-dimensional photonic crystals with complete photonic bandgaps 7–16 . We demonstrate, for the first time, the realization of lasing oscillation in a three-dimensional photonic crystal nanocavity. The laser is constructed by coupling a cavity mode exhibiting the highest quality factor yet achieved (∼38,500) with quantum dots. This achievement provides means for exploring the physics of light–matter interactions in a nanocavity–single quantum dot coupling system in which both photons and electrons are confined in three dimen-sions, as well as for realizing three-dimensional integrated photonic circuits. Three-dimensional photonic crystals have the facility to inhibit light propagation in any direction by means of their complete photonic bandgaps (PBGs). By introducing an artificial defect into a perfect crystal, three-dimensional photonic-crystal nanocavity modes with high quality (Q) factors and small mode volumes (of the order of a cubic wavelength) can be localized in a complete PBG, a feature that is essential for achieving ultimate control of light–matter interactions. Three-dimensional photonic crystals also offer the possibility of directly funnelling the light output from the cavity 17 and subsequently guiding it through optical circuit paths to other optical devices on the same chip with low loss 8,13 . Furthermore, three-dimensional photonic crystals can also be made to couple with material with gain polarized in any direction as a result of the omnidirectional property of the complete PBG 1,18 . Despite these striking advantages, the realization of three-dimen-sional photonic crystal lasers has been hindered by difficulties in fabricating intricate three-dimensional structures that have a com-plete PBG and contain a high-Q cavity incorporating an efficient light-emitting material. Lasing in three-dimensional photonic crystal structures has previously been attempted using a pseudo-gap 19–22 , but not a complete PBG. It would be difficult to use such structures to gain the abovementioned advantages because they can manipulate photons only in a specific direction. Although several efforts have been devoted to improving the quality of active cavities embedded in complete-PBG crystals 14–16 , lasing oscil-lation has yet to be realized because of relatively low Q factors and/or insufficient material gain. In this Letter, we report the first demonstration of a three-dimensional photonic crystal nanocavity laser under pulsed operation at low temperature, by coupling a high-Q cavity mode, which is localized in a complete PBG and has a record Q factor of 38,500, with high-quality InAs quantum dots 23 . We also demonstrate a systematic change in the laser charac-teristics, including the threshold and spontaneous emission coup-ling factor (b), by controlling the crystal size, which consequently changes the strength of photon confinement in the third dimension. A schematic representation of the fabricated three-dimensional photonic crystal, a so-called woodpile structure, is shown in Fig. 1a. The structure comprises a stack of GaAs-based two-dimen-sional thin layers, each containing a line-and-space pattern with 11 in-plane (x–y plane) rods fabricated using micromanipulation tech-niques 15,16 . An active layer embedding a point-defect structure (dimensions, 1.15 mm × 1.15 mm) and three-layer stacked InAs quantum-dot layers (see Fig. 1b) with a dot density of 4 × 10 10 cm 22 per layer (as cavity and light emitter, respectively) was sandwiched between the upper and lower layers. The quantum dot ground-state emission peak was at 1.26 mm at 7 K. The in-plane periodicity, thickness and width of the rods of each layer, including the active layer, were set at 500 nm, 150 nm and 130 nm, respectively. The number of lower layers of the structure was set at 12, and the number of upper layers increased from an initial value of 2 to 12. Scanning electron microscopy (SEM) images of the fabricated structure are shown in Fig. 1c. The rec-tangular posts that can be observed in the image help to guide the layers to a designated position, allowing high-precision assembly with a stacking error of less than 50 nm. The fabrication time required for assembling one layer was less than 10 min. Details of the fabrication process are provided in the Methods. As further upper layers were stacked onto the structure, photolu-minescence measurements were performed at 7 K. We initially investigated the effect of the number of upper layers on the Q factor of a high-Q cavity mode. Figure 2a shows the photolumines-cence spectrum for the structure with six upper layers, which demonstrates a number of cavity modes in a complete PBG between 1,085 and 1,335 nm, predicted by calculations using a plane-wave expansion (PWE) method. The Q factor of the mode with the most pronounced intensity at 1,200 nm, very close to the centre wavelength of the PBG where the localization of photons was the strongest 16 , is plotted as a function of the number of upper layers in Fig. 2b. We estimated the value of the measured Q from a Lorentzian fit to the experimental data measured at the transparency pump power. The Q factor grew expo-nentially with the number of upper layers as a result of the strength-ening of the PBG effect 24 , reaching a value of 38,500 (Fig. 2c) for the structure with 12 upper layers. This is the highest Q value reported to date for three-dimensional photonic crystal nanocavities. These results agreed well with theoretical calculations. In addition to
Nature Photonics 01/2011; 5:91. · 29.28 Impact Factor
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ABSTRACT: We demonstrate lasing oscillation in a three-dimensional photonic crystal nanocavity. The laser is realized by coupling a cavity mode, which is localized in a complete photonic bandgap and exhibits the highest quality factor of ~38,500, with high-quality semiconductor quantum dots. We show a systematic change in the laser characteristics, including the threshold and the spontaneous emission coupling factor by controlling the crystal size, which consequently changes the strength of photon confinement in the third dimension. This opens up many interesting possibilities for realizing future ultimate light sources and three-dimensional integrated photonic circuits and for more fundamental studies of physics in the field of cavity quantum electrodynamics. Comment: 14 pages, 4 figures
06/2010;
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ABSTRACT: We experimentally investigated the excitation power dependence of a strongly coupled quantum dot (QD)-photonic crystal nanocavity system by photoluminescence (PL) measurements. At a low-excitation power regime, we observed vacuum Rabi doublet emission at 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. Comment: 9 pages, 3 figures
06/2009;
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IEICE Transactions. 01/2009; 92-C:217-223.
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Applied Physics Letters. 01/2009; 94:033102--3.
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Applied Physics Letters. 11/2008; 93:183114--3.
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ABSTRACT: We report on effects of cavity mode volume V on the enhancement of PL from crystalline silicon, demonstrating that nanocavities with smaller mode volume are useful to obtain a larger improvement of the internal quantum efficiency of crystalline silicon.
Nano-Optoelectronics Workshop, 2008. i-NOW 2008. International; 09/2008
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ABSTRACT: The authors demonstrate highly efficient selective excitation of a single InGaAs self-assembled quantum dot (QD) embedded in a photonic crystal defect nanocavity. A single QD, whose first excited state is resonant with the cavity mode resonance, shows strong light emission from the ground state under cavity resonant excitation. The light emission is nearly ten times stronger than that of a single QD located at an unpatterned area. This result is attributed to the local enhancement of the effective absorption by the cavity resonant effect. This highly efficient excitation technique can be useful for single photon sources.
Applied Physics Letters 12/2006; 89(24):241124-241124-3. · 3.84 Impact Factor
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ABSTRACT: Si photonic wire waveguides are attractive for constructing various optical devices that are extremely small because the waveguides can be bent with extremely small curvatures of less than a few micrometers of bending radius. We have fabricated optical directional couplers with the waveguides and demonstrated their fundamental characteristics. Their coupling length was extremely short, several micrometers, because of strong optical coupling between the waveguide cores. We have also demonstrated wavelength-demultiplexing functions for these devices with a long coupled waveguide. Optical outputs from a device with a 100-mum-long coupled waveguide changed reciprocally with a 20-nm wavelength spacing between the parallel and cross ports. We also demonstrated the operation of ultrasmall optical add-drop multiplexers (OADMs) with Bragg grating reflectors made up of the waveguides. The dropping wavelength bandwidth of the OADMs was less than 0.7 nm, and these dropping wavelengths could be precisely designed by adjusting the grating period. Using the Si photonic wire waveguide, we have also demonstrated thermo-optic switches. Metal thin-film heaters were evaporated onto the branch of a Mach-Zehnder interferometer that incorporated the waveguide to achieve switching operations by thermo-optic effects. In these switching operations, we observed more than 30 dB of extinction ratio, less than 90 mW of switching power, and less than 100 mus of switching speed
IEEE Journal of Selected Topics in Quantum Electronics 12/2006; 12(6):1371-1379. · 3.78 Impact Factor
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ABSTRACT: The authors demonstrate highly efficient optical pumping of photonic crystal (PhC) nanocavity lasers with InGaAs single quantum wells (QWs) using cavity resonant excitation at 10 K. The laser threshold power is largely reduced when the central wavelength of the excitation pulse is resonant with a higher-order cavity mode. The localized excitation by the cavity resonant effect increases the effective absorption of the QW region in the nanocavity. The direct photocarrier generation in the QW also results in the highly efficient optical pumping. This cavity resonant excitation technique can lower the laser threshold of PhC nanocavity lasers and avoid detrimental device heating.
Applied Physics Letters 10/2006; 89(16):161111-161111-3. · 3.84 Impact Factor
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ABSTRACT: We have investigated cavity resonant excitation effects on InGaAs
quantum dots (QDs) embedded in high-quality-factor photonic crystal
nanocavities. The light emission of the lowest-order cavity mode at 1.06
μm is enhanced by more than a factor of ten, as compared with
nonresonant excitation, when the excitation wavelength is resonant with
higher order cavity modes. This result can be attributed to an
enhancement in the effective absorption coefficient due to a local
enhancement of the excitation light, which couples with the cavity mode.
The on-resonant excitation technique selectively and efficiently excites
only the QDs in the cavity. On-resonant excitation at energies below the
wetting layer band gap energy can achieve stronger light emission from
the cavity mode than excitation at energies greater than the wetting
layer band gap energy with much less undesirable background emission. It
will be shown that this is primarily due to the enhancement in the
effective absorption by the cavity resonant effect and the direct
carrier generation in QDs.
Japanese Journal of Applied Physics 07/2006; 45:6091. · 1.06 Impact Factor
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ABSTRACT: We demonstrate room temperature continuous-wave laser operation at 1.3 mum in a photonic crystal nanocavity with InAs/GaAs self-assembled quantum dots by optical pumping. By analyzing a coupled rate equation and the experimental light-light characteristic plot, we evaluate the spontaneous emission coupling factor of the laser to be ~ 0.22. Three-dimensional carrier confinement and a low transparent carrier density due to volume effect in a quantum dot system play important roles in the cw laser operation at room temperature as well as a high quality factor photonic crystal nanocavity.
Optics Express 07/2006; 14(13):6308-15. · 3.59 Impact Factor
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ABSTRACT: We have studied excitation wavelength dependence of light emission from InGaAs quantum dots (QDs) embedded in high-quality-factor photonic crystal nanocavities. The light emission of the cavity mode around 1 μm was very weak with below-band-gap excitation in the InGaAs wetting layer. However, the emission of the lowest-order cavity mode was strongly enhanced when the excitation wavelength was resonant with higher-order cavity modes. This phenomenon can be attributed to the local intensity enhancement of the excitation light which couples with the cavity mode. The on-resonant excitation technique selectively and efficiently excites only the QDs in the cavity without undesirable background emission.
Applied Physics Letters 04/2006; 88(14):141108-141108-3. · 3.84 Impact Factor
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ABSTRACT: We fabricated a photonic crystal (PC) line-defect waveguide integrated with a microelectromechanical actuator and demonstrated the optical switching operation. The device consisted of a PC line-defect waveguide fabricated in a silicon-on-insulator substrate and a polycrystalline-Si dielectric plate located above the PC waveguide. An applied voltage moved the dielectric plate towards the PC surface due to the electrostatic force. This motion increased out-of-plane scattering of the guided light through the evanescent interaction with the dielectric plate, and modulated the transmittance of the PC waveguide. With only a 5 μm interaction length, an extinction ratio of ∼ 10 dB was obtained at a wavelength of 1568 nm under an applied voltage of 60 V. The response time of the switching operation was approximately 1 ms.
Applied Physics Letters 01/2006; 88(1):011104-011104-3. · 3.84 Impact Factor
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ABSTRACT: Using silicon photonic wire waveguides, we constructed compact 1 x 1, 1 x 2, and 1 x 4 Mach-Zehnder interferometer type optical switches on a silicon-on-insulator substrate and demonstrated their switching operations through the thermo-optic effect. These switches were smaller than 140 x 65, 85 x 30, and 190 x 75 mum, respectively. At a 1550-nm wavelength, we obtained an extinction ratio larger than 30 dB, a switching power as low as 90 mW, and a switching response time of less than 100 mus. Furthermore, switching operations were successfully demonstrated for the 1 x 4 switch.
Optics Express 01/2006; 13(25):10109-14. · 3.59 Impact Factor
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ABSTRACT: Ultrasmall optical add-drop multiplexers (OADMs) with Si-wire waveguides were demonstrated. Bragg grating reflectors based on Si-wire waveguides were developed and used as the wavelength-selective mechanism in the OADMs. The dropping wavelength bandwidth of the OADMs was less than 0.7 nm, and the dropping wavelengths could be controlled precisely by adjusting the grating period.
Applied Physics Letters 05/2005; 86(19):191107-191107-3. · 3.84 Impact Factor
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Applied Physics Letters. 05/2000; 76:3212--3214.
