Movable high Q nanoresonators realized by semiconductor nanowires on a Si photonic crystal platform
M. D. Birowosuto, A. Yokoo, G. Zhang, K. Tateno, E. Kuramochi, H. Taniyama, M. Takiguchi, M. Notomi
aa r X i v : . [ phy s i c s . op ti c s ] M a r Movable High- Q Nanoresonators Realized by Sub-wavelength III/V SemiconductorNanowires on a Si Photonic Crystal Platform
Muhammad Danang Birowosuto,
1, 2
Atsushi Yokoo,
1, 2
Guoqiang Zhang, KoutaTateno, Eiichi Kuramochi,
1, 2
Hideaki Taniyama,
1, 2
Masato Takiguchi,
1, 2 and MasayaNotomi
1, 2 NTT Basic Research Laboratories, NTT Corporation,3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198,Japan NTT Nanophotonics Center, NTT Corporation, 3-1 Morinosato Wakamiya,Atsugi, Kanagawa 243-0198, Japan (Dated: March 19, 2014)
Sub-wavelength semiconductor nanowires have been attracting strong interest re-cently for photonic applications because they possess various unique optical propertiesand offer great potential for miniaturizing devices. However, with these nanowires,it is not easy to realize tight light confinement or efficient coupling with photoniccircuits. Here we show that a high Q nanocavity can be created by placing a sin-gle III/V semiconductor nanowire with the diameter less than 100 nm in a groovedwaveguide in a Si photonic crystal, and employing nanoprobe manipulation. We haveobserved very fast spontaneous emission (91 ps) from nanowires accelerated by thestrong Purcell enhancement in nanocavities, which proves that unprecedented stronglight confinement can be achieved in nanowires. Furthermore, this unique system en-ables us to move the nanocavity anywhere along the waveguide. This configurationprovides us tremendous flexibility in integrated photonics because we can add anddisplace various functionalities of III/V nanocavity devices in Si photonic circuits.1t has been well established that various semiconductor nanowires (NWs) with diameterssmaller than 100 nm can be grown by epitaxial methods, such as the vapor-liquid-solid (VLS)method . A variety of structures, including core-shell , multi-layer heterostructures , andp-i-n junctions , have been implemented specifically for III/V semiconductor NWs by ap-propriately arranging the growth sequence. Several interesting NW photonic devices havebeen reported, such as lasers , light-emitters , single photon source , photo-detectors ,and wavelength converters . The dimensions of the NWs naturally suggest that NW-basedphotonic devices are promising for ultrasmall and power-efficient device operation . How-ever, their performance has been limited because no effective light confinement has beenachieved for NWs when the NW dimensions are in the sub-wavelength regime. NWs cou-pled with plasmonic waveguide modes have been reported , but the quality factor ( Q )is rather low ( <
10) and the mode size is smaller than (thus, not compatible with) the NWdimension. Besides the plasmonics, some studies have reported NWs coupled with relatively-small resonators or photonic crystals , but the light confinement volume is much larger thanthe wavelength of light and the NWs are too small to achieve a sufficient overlap with theconfined mode. This means that we cannot enjoy the various enhancements which wouldarise from strong light confinement in a tiny active material.In this report, we propose and demonstrate the design shown in Fig. 1a in which a sub-wavelength III/V NW is placed in a grooved waveguide in a Si photonic crystal. It has beenpreviously shown that a small local modulation of a line defect waveguide produces high- Q nanocavity modes resulting from the spatial modulation of the mode gap frequency in theline defect. This is a modulated mode-gap cavity, which exhibits a Q of millions and aneffective mode volume ( V ) of ∼ ( λ/n ) for well-optimized structures, which λ and n are thecavity wavelength and the medium index, respectively . It is noteworthy that a surprisinglysmall modulation can lead to ultra-strong light confinement in this type of structure . InFig. 1a, the sub-wavelength NW produces a certain refractive-index modulation in the linedefect, and we confirmed numerically that this modulation is sufficient to form a high- Q nanocavity mode exactly at the NW position . As we show later, the cavity mode is mostly2ocalized in the NW. Thus, it can solve the aforementioned problem for NW-based photonicdevices. But this is not the only merit of this design. It also enables us to implement variousIII/V-semiconductor-based nanocavity devices in a Si waveguide platform. Si photonics isnow being extensively studied to provide a future photonic platform for integrated circuits,but it suffers severely from the poor optical functionality of Si . With our proposeddesign, we can add various III/V functional nanocavity devices at arbitrary positions inSi waveguides. From another viewpoint, NW photonic devices generally suffer from poorinput/output coupling , but with our present configuration the NW cavity mode can beeasily coupled to Si photonics waveguides as already demonstrated for a simple waveguide .Moreover, this NW cavity is movable, which is exceptional as regards high- Q nanocav-ities. Since strong light confinement with a small loss is only achieved by using a specialarrangement of photonic crystal cavities or strongly disordered media , wavelength-sized high- Q nanocavities are prefixed to the surrounding arrangement and are immovable.In our present design, the cavity is created by the perturbation from the NW and is notfixed to the surrounding photonic crystal lattice. Although reconfigurable cavities have beenproposed and reported in various forms in photonic crystals , but none of them demon-strated the position manipulation of the cavity as a result of moving the single nano-objectscatterer (NW). Also with our method, an active material can be embedded in the scattererso that a nanocavity with active function becomes movable. This feature adds more flexibil-ity and tunability to the application for Si photonics, and more importantly it may open upan interesting application when combined with a microfluidic circuit . If we fill the groovewith a certain liquid, a NW can flow along the groove. Consequently, we may be able todeliver an active NW nanocavity to another location in a microfluidic Si photonic circuit.In this study, we demonstrate the formation and movability of a cavity by manipulatingInAsP/InP NWs on Si photonic crystals with a scanning probe, in other words, by atomicforce microscopy (AFM) manipulation. Here we also report the clear observation of a largePurcell enhancement of spontaneous emission from NWs.First of all, we describe the procedure for making the proposed structure (shown in Fig.3a) by AFM manipulation . After we measured the emission intensities of a number ofNWs, we chose some of the NWs for our experiment. Then, we transferred these NWs fromthe outside of the photonic crystal using an AFM tip with a scanning velocity of 50 nm/s,shown as process (1) in Fig. 1a. The AFM manipulation technique has been used to slightlyadjust the position of nanoparticles on the photonic crystal cavity , but now we use AFMfor manipulating much larger semiconductor NWs over a much larger distance. In fact, thelonger dimension of NW is advantageous for long-distance transfer because the NWs willnot fall down into photonic crystal holes. By manipulating the AFM tip, we put the NWinto the square-grooved waveguide of the photonic crystal, shown as process (2) in Fig. 1a.Process (3) corresponds to the movement of the NW along the groove that we describe later.Note that the existence of the groove is a great help as regards the controllability of AFMmanipulation, and also enlarges the light confinement as shown below.The electric field intensity profile for the resonant mode calculated by three-dimensionalfinite difference time domain (3D FDTD) is shown in Fig. 1a, where we assumed a photoniccrystal with a lattice constant a = 352.5 nm and a square groove along the waveguide witha depth d groove of 75 nm and a width w groove of 150 nm. The radius of the hole r , theslab thickness h , and the width of the waveguide W are 100 nm, 200 nm, and 0.98 √ a ,respectively. We modelled a square-cross-section NW with a length L NW = 2 µ m and aside length d NW = 90 nm (see also Supplementary Fig. S1). These parameters are closeto those used in our experiment. Fig. 1b shows simulated results for the NW on a normalwaveguide without a groove. It is apparent that tight light confinement is achieved aroundthe NW after the NW placement. When we place the NW in the groove, the NW has betteroverlap with the confined field and the maximum intensity point is moved to the inside ofthe NW (see Supplementary Fig. S2). Using the fraction of the area of the confined fieldinside the NW in comparison with the total field, ∼
20 - 30 % of photons are coupled tothe NW for the NW inside the groove while < ∼
1% of photons for the NW placed on thewaveguide. The calculated Q and V values (normalized by ( λ/n NW ) where n NW is the NWindex) for the cavity formed by the NW inside the groove are 33,200 and 0.95, respectively4see Methods). When we compare NWs of the same size on the top and at the center of thephotonic crystal waveguide, the Q and V of the former are three-fold larger and four-foldsmaller, respectively (see Supplementary Fig. S1). Our previous simulation showed that d NW , w groove , and d groove should be optimized for increasing Q . In addition, d NW should beapproximately equal to d groove .Generally speaking, this type of cavities (modulated modegap cavities) are not so sensitiveto the detailed shape of the modulation. The overall effective index modulation determinesthe cavity confinement . In fact, it was shown that random disorder in mode-gap waveguidescan also create an ultrahigh- Q localized modes , which indicates that an irregular shapeof NW does not necessarily lead to low Q . From our simulation, the NW diameter influences Q greatly, especially when it becomes closer to the groove width (see Supplementary Fig.S3 and Ref. ). We regard that this is more critical issue for creating higher Q .For our experiments, we fabricated a series of grooved line defect waveguides in Si photoniccrystals. We used InAsP/InP heterostructure NWs with ten InAsP quantum disk (QD)layers that worked as emitters as illustrated in Fig. 1c. A scanning electron micrograph ofthese NWs in Fig. 1d shows that L NW and d NW are homogeneous and L NW for this sampleis about 2 µ m. The InAsP QDs inside a single InP NW without a cap are shown in thetransmission electron micrograph in Fig. 1e. The details of the fabrication and structureare described in Methods and Supplementary Information.In this manuscript, we use sample A (sample B) to refer to the NW with L NW = 1,760nm (940 nm) and d NW = 83 nm (95 nm) in a photonic crystal with a = 352.5 nm (350 nm).Fig. 2a shows an AFM image of the NW after we manipulated the NW inside the groovefor sample A (see Supplementary Fig. S4 for the microscope images). This result showsthat the NW was successfully placed in the middle of the grooved waveguide. The top ofthe NW is slightly above the top surface of the photonic crystal and slightly inclined (seeSupplementary Fig. S5).We can locate the NW position directly from the bright photoluminescence (PL) imageof InAsP QDs inside the NW at room temperature (RT) using a highly sensitive infrared5nGaAs camera (see Fig. 2b). To confirm the NW cavity formation, we investigated the PLspectra of the same NW on sample A before and after we placed the NW inside the groove(Fig. 2c). The PL spectrum (black lines) for a single NW of sample A outside the photoniccrystal on a bare Si on insulator (SOI) exhibits peaks between 1,200-1,500 nm.In the PL spectrum (red lines) of the NW inside the groove in Fig. 2c, we observed adistinctive peak at 1,286 nm. Fig. 2d shows the magnified PL spectrum recorded with higherresolution, which exhibits a strong-intensity peak at 1,286 and another weak-intensity peakat 1,281 nm. From a comparison with the FDTD calculation, we confirm that the strong-intensity peak with Q fexp = 7,100 is related to the fundamental mode of the cavity shown inFig. 1a while the latter with Q hexp = 7,500 is related to the second-order cavity mode. Next,using the emission intensity filtered at 1286 nm (at the resonance peak), we performed aspatially resolved PL scan over the entire area of photonic crystals (as shown in Fig. 2e)(see also Methods). This result confirmed that the intense emission at 1,286 nm is highlylocalized at exactly the position of the manipulated NW (see Supplementary Fig. S4 forthe microscope image as a position comparison). All of these results proved that the NWplacement created a confined resonance mode at the NW position.Other information regarding the origin of the peak emission can be obtained with apolarization measurement. In Fig. 2f, we analyze the polarization properties of the sampleA emission. When the NW is on a bare SOI, the polarization is parallel to the NW axis( θ = 0). This polarization is expected for a zincblende InAsP QD inside a single InP NW but this is also related with the different dielectric, the thickness, and the quantum effectof the QD (see Supplementary Information). However, the polarization of the NW insidethe grooved waveguide is almost perpendicular ( θ = 70 . ± . ◦ ) to the NW. Its degree ofpolarization ρ is 66 % and is defined as ρ = ( I max − I min ) / ( I max + I min ) , where I max and I min are the intensities at the maximum and minimum of the polar plot. The observedpolarization matches the expected polarization for the cavity mode calculated in Fig. 1a,which strongly supports the previous assignment of the observed resonant mode. Note thatthe observed polarization degree is not perfect as simply expected from the cavity mode.6e regard that this imperfect polarization may be due to photons directly radiated intothe free space and polarization variation of intrinsic NWs. In fact, the polarization degreeslightly varies for different samples as shown in Fig. S6.Here, we demonstrate the movability of our NW-induced cavity, which is one of the mostimportant features. To demonstrate the movable cavity, we move the NW inside the groovealong the waveguide direction and observe the narrow characteristic peak of the cavity atthe new position. In this experiment, we used the AFM manipulation technique in sampleB three times: first to locate the NW in the groove, and then to displace it twice. After eachdisplacement, we recorded the AFM images, PL images, PL spectra, and spatially-resolvedPL scan images as shown in Fig. 3. We overlay three AFM images that we obtained afterthe first, second, and third manipulations in Fig. 3a. The displacement by the second andthird manipulations was 3.0, and 6.0 µ m, respectively. We confirmed that the positions ofthe NW in the AFM images are the same as the bright spots of the NW emission in thePL images of Fig. 3b. In all three cases, we successfully observed a narrow cavity peak inthe PL spectra at a wavelength of ∼ Q exp (5,200 - 2,900 -3,200) (see blue curves in Fig. 3c and Supplementary Fig. S3 for the FDTD simulation).At each step, we measured the PL spectrum at a reference NW outside the groove, whichshowed no difference (green curves in Fig. 3c). Next, we performed a spatially-resolved PLscan with a filter centered (2-nm window) at the peak wavelength in Fig. 3c resulting inthe PL maps shown in Fig. 3d. In each image, a single bright peak (in a blue circle) is seenexactly at the NW location shown in Fig. 3a. The weak spot in a green circle correspondsto the unmanipulated NW we used for the reference. These results clearly show that thesharp resonance moved with the NW displacement. This constitutes the first observation ofa movable sub-wavelength-sized high Q cavity.Our target is to create a nanocavity mode concentrated in NW. To demonstrate thisfeature, we next investigated the emission lifetime since the strong light confinement in asingle NW should lead to a large Purcell enhancement. Here we measured the emissiondecay curves at 4 K using an 800-nm pulse excitation laser and a bandpass filter (BPF) at7he cavity wavelength (see Methods). Since at 4 K the nonradiative recombination rates aremuch smaller than those at RT , we should be able to see the Purcell effect more clearly.For our sample, the cavity wavelength and Q exp at 4 K do not differ significantly from thevalues at RT (see Supplementary Fig. S7). In Fig. 4a, the emission decay curve of the NWon a bare SOI is shown by black dots. The data can be well fitted with a single exponentialcurve resulting in an emission lifetime (1 / Γ NW ) of 770 ps. Other NWs measured on thebare SOI also show similar values around 800 ps (see Supplementary Fig. S7). In contrast,the NW inside the groove for sample A (red dots in Fig. 4a) shows distinctive shortening ofthe PL decay yielding a lifetime (1 / Γ cav ) of 187 ps. From a comparison with the emissionlifetime of a single NW on a bare SOI, we obtain a 4-fold reduction in the lifetime.The above result was obtained with a PL decay measurement for different NWs, but wecan directly investigate the Purcell effect by measuring the same NW while varying the cavity Q exp by AFM manipulation. In fact, the result in Fig. 3 shows that the cavity Q exp changesat each displacement step (5,200 - 2,900 - 3,200), although the resonant wavelength is mostlythe same. We consider that this was because the vertical position of the NW in the groovewas changed a little, in other words, part of the NW was lifted up slightly. We confirmedthis speculation by pushing the NW down into the groove with the probe after the thirddisplacement, and observed that Q exp recovered from 3,200 to 4,200 (see SupplementaryFig. S8). Our calculation shows that such misplacement leads to a slight reduction in Q ,but has negligible influence on the cavity volume (see Supplementary Fig. S9). Thus, thismanipulation method is ideally suited for Purcell effect characterization. We performedtime-resolved emission measurements for sample B at each step of the manipulation shownin Fig. 3. For the NW on the bare SOI of sample B, the lifetime (1 / Γ NW ) was 730 ps (blackdots in Fig. 4b). The PL decay curves of the NW in the grooved waveguide at each stepare shown in Fig. 4b with the corresponding Q exp values. The graph clearly shows that theemission becomes faster as the Q exp increases. At the highest Q exp (5,200), the lifetime wasas short as 91 ps. This lifetime is the shortest lifetime reported for the III-V NW. Sincethis measurement was performed for the same NW in the cavity with the same volume, it8epresents a clean demonstration of Purcell enhancement, which was realized by the uniquefeature of our NW-induced nanocavity. For a better comparison, we collect Γ cav / Γ NW in Fig.4c from samples A and B for different Q exp /V values. Note that the V values for samples Aand B are 0.95 and 0.54, respectively. We also added other data points from different NWsof C and D (see Supplementary Figure S10). Sample C (three samples of D) refers to theNW with L NW = 2,620 nm (1,700, 1,800, and 1,900 nm) and d NW = 85 nm (102, 129, and90 nm) in a photonic crystal with a = 416 nm (382.5, 360, and 360 nm). NW in sample Chas a similar structure as A and B while NWs in sample D are InAsP NWs capped withInP layer. The result clearly reveals a linear relationship between Q exp /V and Γ cav / Γ NW ,which is an evidence of the Purcell enhancement.Although the inhibited spontaneous emission in NWs has been reported by manygroups , there has been very little work on Purcell enhancement observation for NWs.Recently, Oulton et al. observed a six-fold shortening of the PL lifetime in a single CdSNW coupled to plasmonic waveguides (no cavity effect) . Since these plasmonic structuresrealize an extremely small mode volume, it can lead to large Purcell enhancement even withvery low Q resonance. Our present result gives the clear Purcell enhancement of a singleNW for the first time in a high- Q dielectric cavity. The achieved enhancement is slightlylarger than the result in the NW-plasmonic system. Although there are pros and cons forboth methods, we regard that our NW system is advantageous for integrated photonicsapplications because of its low loss and ease of integration with the waveguide. Althoughthe confinement factor (15 - 20 %) is comparable to that for our cavities ( ∼
20 - 30 %),the plasmonic mode is much smaller than the NW dimension, which generally degrades theenergy efficiency for device operations. Furthermore, extreme plasmonic modes may sufferfrom nonradiative energy transfer to metals , which makes the observation of large Purcellenhancement difficult. In contrast, our system may still have a room for achieving largerenhancement if we boost up the cavity Q as we discuss below.The theoretical Purcell factor F P for our sample with the fastest emission rate is about732. As is well known, the emission rate will be reduced for several reasons, such as a spec-9ral, spatial, or polarization mismatch . Note that in our experiments, the polarizationmismatch is large (as seen in Fig. 2f). A quantitative investigation of these reduction factorsis beyond the scope of this study. Although we could not determine the exact position ofthe emitter that we measured in this experiment, our AFM manipulation technique is ableto precisely align the emitter position at the field maximum of the cavity if we know theprecise position of the emitter in the NW. This would constitute an important advantage ofour system over other solid-state cavity QED systems .In conclusion, we demonstrated that a high- Q cavity can be created simply by putting asingle NW into a groove in a photonic crystal waveguide. We used the spontaneous emissionof the QDs inside the single NW to probe the cavity formation by comparing the PL spectraand the emission polarization of the NW on a bare SOI and inside the groove. We moved thecavity spatially while manipulating the position of the NW. Furthermore, we also observedemission rate enhancements for the NW inside the groove showing unprecedented stronglight confinement in a single NW with a record of eight-fold emission rate enhancement. Webelieve that our observation will stimulate the discovery of new phenomena in fundamen-tal research on cavities in photonic crystals and also the localized modes in disorderedphotonic systems . As regards the realization of novel nanophotonic devices, our new ap-proach for creating cavities is useful for movable devices such as nanolasers or all-opticalmemories with the capability of integrating them on a Si platform. Especially, a laser isone of the most interesting applications of our system. To achieve lasing, we may need alarger gain volume and higher Q , which remains for a future study. As for the improvementof Q , smaller-diameter NWs in narrower grooves are possible options. Finally, the integra-tion technique of NW is not limited to III-V NWs on Si photonic crystals, e.g. diamondnanowires with suitable refractive index photonic crystals. Another potential method formoving cavities, namely nanofluidic technology remains tantalizing.10 ETHODS
Finite difference time domain and Purcell factor
We performed a 3D FDTD calculation to investigate the field distribution. Some parameters, such asthe calculation area, the perfectly matched layer boundaries, and the grid, have already been describedelsewhere . The strength of the light confinement or Q was calculated from a single-exponential fitting ofthe energy decay curve tail with the narrow band excitation of the cavity. From the fitting, we determinedthe cavity photon lifetime τ ph and we estimated Q in this cavity as Q = τ ph · ω c .The common figure of merit of the spontaneous emission rate enhancement in the cavity compared withthat in the bulk at resonance is the Purcell factor F p , which can be written as follows: F p = 3 Q π V where mode volume V denotes the dimensionless mode volume with a cubic wavelength λ of the emissioninside a single NW with a refractive index n NW . Note that E ( r max ) is located inside the NW. V = R ǫ ( r ) | E ( r ) | d rǫ ( r max ) | E ( r max ) | (cid:16) n NW λ (cid:17) Photonic crystal fabrication
We fabricated a series of Si photonic crystals with a groove in the middle of the waveguide by electron-beam lithography and inductively coupled plasma etching. Holes with a diameter of 95 ± layer were then removed by selective wet etching using HF solution. Thistechnology guarantees a resolution of <
5% in diameter and <
1% in distance between the holes and all theseprocesses resulted a photonic crystal slab with a thickness of 200 nm. Finally, the groove was fabricatedafter a second-mask process. We obtained this desired groove depth after determining a suitable etchingtime through the linear dependence of the groove depth on variations in the etching time.
NW growth and transfer
The NW growth was carried out in a low-pressure horizontal metalorganic vapor phase epitaxy (MOVPE)reactor. Trimethylindium (TMIn) was the group III sources while phosphine (PH ), tertiary-butyl phosphine(TBP), arsine (AsH ), and tertiary-butyl arsine (TBAs) were the group V source. The substrate was nP(111)B and the catalysts were Au particles (20 nm in diameter) obtained from Au colloids. Ten InAsPlayers were grown alternately with InP layers in a single NW capped with a 30-nm diameter-thick InP layer.The InAsP layer thickness was 10 nm and the growth time for each InAsP and InP layer was 10 s. Thecapping growth process was performed at 470 ◦ C for 35 minutes. After the growth process, we obtained avertically-grown NW with alternating InAsP/InP layers located in the middle and in the core-shell region.The NWs were dispersed from the grown substrate to the photonic crystal substrates by pushing gently bothsubstrates to each other resulting a minimum number of broken NWs (mechanical dispersion technique) . Photoluminescence measurements
We used a standard µ PL setup. The sample was optically excited either using the CW diode laserat 640 nm or picosecond pulses from a 80 MHz mode-locked titanium sapphire laser at 800 nm with anaverage excitation power of 100 µ W. A free-space excitation technique was applied through a 0.42- and0.50-numerical-aperture near-infrared microscope objective, by which the beam diameter was estimated tobe ∼ µ m. The fraction of the spontaneously emitted photons that coupled to the radiation modes wascollected through the same microscope objective and filtered with a dichroic beam splitter and a long-wave-pass filter (LWPF). For direct observation of the emission from a single NW, we split the emissionfrom the NW and the visible illumination light using a dichroic mirror (cut off wavelength 1,064 nm). Thedetection parts contained a visible-near infrared CCD camera and a highly sensitive short wave infraredInGaAs camera with an LWPF (cut off wavelength 1,200 nm) for the microscope image and the PL image,respectively. To measure the PL spectrum, we coupled the emission into the multimode fiber and directed itto a grating spectrometer with a cooled InGaAs array. It had resolution limits of 0.24 and 0.05 nm at 1,550nm for 300 grooves/mm and 1,000 grooves/mm, respectively. To obtain the spatial origin of the emission,we also performed a spatially resolved PL scan. We selected the target cavity wavelength and chose thebest resolution limit. Finally, we collected PL spectra from a 7.5 µ m x 18.5 µ m area of photonic crystalswith a step of 500 nm and the initial point of the photonic crystal edge (see Supplementary Fig. S2). Forthe time-resolved PL measurements, we coupled the emission filtered by a band-pass filter of 1,300 ±
30 nmto a single mode fiber and directed it to one-channel NbN superconducting single photon detector (SSPD).Finally, we connected the SSPD and the excitation pulse signals to one input and a synchronization input hannel of a time-correlated single photon counting (TCSPC) board, respectively. ACKNOWLEDGMENTS
We acknowledge S. Fujiura and M. Ono for assistance with the NW manipulation.
AUTHOR CONTRIBUTIONS
M. D. B., A. Y., G. Z., and M. N. conceived the idea and designed the experiments. M.D. B. performed the simulation, conducted the experiments and analysed the data. M. D.B. and M. N. wrote the manuscript. A. Y. manipulated the nanowires. K. T. conducted theNW growth. G. Z. and E. K. involved in the fabrications. M. T. supported the experiments.H. T. supported the simulation. M. N. guided the project.
ADDITIONAL INFORMATION
The authors declare that they have no competing financial interests. Reprints and per-mission information is available online at http://npg.nature.com/reprintsandpermissions/.Correspondence and requests for materials should be addressed toM. D. B. ([email protected]) and M. N. ([email protected])
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The scale bars in (d) and (e) indicate 2 µ m and 50 nm, respectively.Figure 2. Manipulation of single NW and photoluminescence (PL) characterizations.a,b , False-color AFM (a) and PL images (b) of the single NW inside the square-grooved waveguideof the photonic crystal. c , PL spectra of a single NW on a bare Si on insulator (SOI) (black lines)and inside the square-grooved waveguide of the photonic crystal (red lines). d , Details of the cavityspectrum and the green lines show the Lorentzian fits. e , Spatially-resolved PL scan of the cavityat 1286 nm wavelength. The orange and purple lines show the photonic crystal structures, and theAFM image area, respectively. The scale bars in all images indicate 2 µ m. f , Integrated intensityemission of the NW inside the square-grooved waveguide of the photonic crystal (red spheres) andthe NW on a bare SOI (blue spheres) as a function of the linear polarization angle at RT and1286 nm. The green arrow and the solid lines represent the NW orientation and the sinusoidal fits,respectively. All measurements were performed for sample A using continuous-wave (CW) laserexcitation at 640 nm and at room temperature (RT). igure 3. Observation of a movable cavity at three different positions. a,b , False-colorimage of an overlay from three AFM images (a) and PL images (b) of a single NW at three differentpositions inside a groove in a photonic crystal. The black rectangles represent the cropped areaof the AFM images. c , PL spectra of the cavity and the NW background emission shown by blueand green lines, respectively. d , Spatially-resolved PL scan of the cavity intensity integrated overthe 2-nm cavity peak. The blue and green circles correspond to the PL spectra in (c). The orangeand purple lines show the photonic crystal structures and the AFM image size, respectively. Allmeasurements were performed for sample B using CW laser excitation at 640 nm and at RT. Thescale bar represents 2 µ m.Figure 4. Emission rate enhancement for different Q exp /V values. a , Time-resolved emissionof a QD in sample A. b , Time-resolved emission of a QD in sample B with a different position insidethe groove in the waveguides. Decay curves for a single NW on a bare SOI outside the photoniccrystal are shown as black spheres and those for an NW inside the grooves of samples A and Bare shown as red and purple spheres, respectively. The intensity at zero delay of each curve in (a)and (b) is shifted for clarity. The white lines and the green spheres represent a single exponentialfit and an instrumental response function of 45 ps. c , Summary of spontaneous emission rateenhancement for different Q exp /V values. Four samples with different NWs, C and D, were added(see also Supplementary Fig. S10). The blue dashed line is a linear fit. All measurements wereperformed using a picosecond-pulse laser emitting at 800 nm and at 4 K.values. Four samples with different NWs, C and D, were added(see also Supplementary Fig. S10). The blue dashed line is a linear fit. All measurements wereperformed using a picosecond-pulse laser emitting at 800 nm and at 4 K.