Light Coupling and Modulation in Coupled Nanowire Ring−Fabry

NANO LETTERS

Light Coupling and Modulation in Coupled Nanowire Ring-Fabry-Pe´rot Cavity

2009 Vol. 9, No. 7 2697-2703

Ren-Min Ma,† Xian-Long Wei,‡ Lun Dai,*,† Shi-Feng Liu,§ Ting Chen,† Song Yue,† Zhi Li,† Qing Chen,‡ and G. G. Qin*,† State Key Lab for Mesoscopic Physics and School of Physics, Peking UniVersity, Beijing 100871, China, Key Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics, Peking UniVersity, Beijing 100871, China, and HORIBA Jobin YVon Beijing RepresentatiVe Office, Beijing 100044, China Received April 14, 2009; Revised Manuscript Received May 27, 2009

ABSTRACT CdS nanowire (NW) ring cavities were fabricated and studied for the first time. The rings with radii from 2.1 to 5.9 µm were fabricated by a nanoprobe system installed in a scanning electron microscope. Radius dependent whispering gallery modes (WGMs) were observed. A straight CdS NW with Fabry-Pe´rot (F-P) cavity structure was fabricated and placed by the side of a NW ring cavity to form a coupled ring-F-P cavity. When the NW ring was excited by a focused laser, a bright green light spot was observed at the output end of the straight NW, indicating that the latter had served as an effect waveguide to couple the light out from the ring cavity. The corresponding light spectrum showed that the WGMs had been modulated. We confirmed that the NW F-P cavity had served as a modulator. Such a coupled cavity has potential application in a nanophotonic system.

Introduction. Semiconductor nanowires (NWs) with high refractive indices contrasting to surroundings are attractive for functioning as waveguides for guiding and manipulating light on a subwavelength scale.1,2 With flat end facets serving as reflecting mirrors, straight semiconductor NW can serve as a microscale laser with an axial Fabry-Pe´rot (F-P) cavity.3 Up to now, NW lasing has been observed from straight NWs for various semiconductors, such as ZnO,3-6 GaN,7,8 CdS,9,10 multiquantum-well GaN/InGaN,11 and core-shell GaAs/ GaAsP,12 etc. Microdisk and microring cavities, which operate on whispering gallery modes (WGMs), are another important type of microcavity that has key merits of high quality factor (Q) and compact size.13-17 Besides the application in high Q lasers, the strong confinement of electromagnetic energy in a small microring cavity can result in very functional microphotonic integrated circuits. Alloptical switches and modulators, electro-optic modulators, channel add-drop filters, and optical cross-connect networks have been achieved by coupling microring cavities to straight waveguides in various material systems.18-23 However, only a few studies have been reported on NW ring cavities so far.1,24 CdS NWs have shown wide application in nanoscale devices. In recent years, a variety of crucial nanophotonic devices, such as waveguides, photodetectors, electro-optic * Corresponding author, [email protected]; [email protected]. † State Key Lab for Mesoscopic Physics and School of Physics, Peking University. ‡ Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University. § HORIBA Jobin Yvon Beijing Representative Office. 10.1021/nl901190v CCC: $40.75 Published on Web 06/17/2009

 2009 American Chemical Society

Figure 1. Schematic illustrations of the experiment setups for optical measurements. (a) For the in situ PL measurement, the laser beam is aligned to the center of the microscope view field. (b) For the spatially resolved PL measurement, the laser beam is deviated from the center by the mirror in the custom design module. Only those signals coming from the center can be collected, and other signals are filtered by the confocal pinhole.

modulators, light-emitting diodes, lasers, etc., have been fabricated based on CdS NWs.2,9,10,25-30 In this Letter, we reported the fabrication and study of CdS NW ring cavities for the first time. We also constructed a coupled ring-F-P (R-F-P) cavity by placing a cleaved straight NW by the

Figure 2. (a, b) SEM images of the NW cleaving process. (c) Dark field image of a CdS NW. The scale bar is 2 µm. (d) PL image of the CdS NW depicted in (c) excited by a focused 488 nm laser at the middle part. The scale bar is 2 µm. (e) Normalized in situ (black line) and spatially resolved (red line) PL spectra of a NW with cleaved flat end facets. (f) Mode spacing at 517 nm versus NW length for 22 independent straight NWs. Black squares are experimental points, the red dot is an extrapolated point for infinite length, and the line is the linear fit to the data. (g) Wavelength dependence of the group indices of the CdS NWs.

side of a NW ring cavity with a nanoprobe system installed in a scanning electron microscope (SEM). Herein, the straight NW served not only as a waveguide to couple the light out from the NW ring but also as a modulator to modulate the WGMs of the NW ring cavity. Experiments. The CdS NWs were synthesized via the chemical vapor deposition method described previously.31 The formation mechanism of the CdS NWs is based on the wellknown vapor-liquid-solid process with Au as catalyst.32 CdS NWs typically grow along the [0001] direction and can be easily cleaved to be with flat end facets which can function as two reflecting mirrors.9,31 In our experiments, the NWs were dispersed onto oxidized silicon substrates, each of which had an 800 nm thick SiO2 layer. A nanoprobe system (Kleindiek MM3A) installed in a SEM (FEI XL 30F) was employed to rationally cleave and manipulate the NWs.33 To study mode behavior of the NW cavities, a microzone confocal Raman image system (HORIBA Jobin Yvon LabRam HR 800) was 2698

employed. The 488 nm line of an Ar+ laser was used as the excitation light. Spatially resolved photoluminescence (PL) was realized using a custom design module that can make the excitation laser spot deviate from the collection point (focal point of the microscope). The deviated distance can be changed in a range of several microns. Only the signals that come from the center of the microscope can be collected. The confocal pinhole in signal optic route ensures a high lateral resolution of about 1 µm. Figure 1 shows schematic illustrations of the optical measurement setups. In the PL measurements of the NW ring with radius of 2.1 µm, a 100× objective was used. The corresponding focused laser spot size and excitation power density were about 0.5 µm2 and 15 kW/cm2, respectively. In other PL measurements, a 50× objective was used. The corresponding focused laser spot size was about 2.3 µm2, and the excitation power density was about 16 kW/cm2. All experiments were done at room temperature. Nano Lett., Vol. 9, No. 7, 2009

Figure 3. (a-c) SEM images of the process for a straight CdS NW being bent to a ring by two nanoprobes. Inset of (c): Magnified SEM image of the joint area of the NW ring. (d, e) NW rings with side-by-side overlap coupling geometries, with the overlap length to be about 3.75 and 0.61 µm, respectively. (f) In situ PL spectra of a NW before (black line) and after (red line) being bent into the NW ring depicted in panel c. (g-i) Spatially resolved PL spectra of the NW rings depicted in (c-e), respectively. Nano Lett., Vol. 9, No. 7, 2009

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Figure 4. (a-c) Coupling efficiency simulated by the beam propagation method for two straight NWs (d ) 150 nm) with cleaved end facets versus different coupling geometries: end-to-end coupling geometry with an air gap (the coupling angle is zero) (a), end-to-end coupling geometry with a coupling angle (the nearest air gap is zero) (b), and side-by-side coupling geometry with an overlap length (c). (d) Bending loss Rb simulated by the finite element method for NW rings constructed on NWs with different diameters versus ring radium. (e-g) Electromagnetic power propagation simulation result by the beam propagation method for the three coupling geometries depicted in panels c-e of Figure 3.

Results and Discussion. Parts a and b of Figure 2 show the process of cleaving a CdS NW by a nanoprobe. Figure 2c shows a dark-field image of a 15 µm long NW. Figure 2d shows the PL image of the NW excited at the middle part. Two bright green light spots can be observed at the ends of the NW, indicating that the CdS NW has functioned as an effective waveguide.2,28 Figure 2e shows in situ (exciting and collecting at the same point) and spatially resolved (exciting at the body of the NW, collecting at an end of it) PL spectra of a straight CdS NW with a length of 10 µm and a diameter of 250 nm. The in situ PL has a peak around 511 nm with a full width at half-maximum (fwhm) of about 12 nm, which is the typical characteristic of the near bandedge spontaneous emission of the CdS NW. The spatially resolved PL has a main emission peaked around 517 nm. 2700

The 6 nm red shift compared to the in situ PL emission is due to the reabsorption of the CdS NW.9,28 Obvious periodic intensity variation is observed in the spatially resolved PL. As many as 15 modes are clearly resolved. The mode spacing (∆λ) varies from 1.32 to 2.92 nm with wavelength increasing from 513 to 542 nm. For a F-P type cavity, ∆λ is given by λ2/(2L(ne - λ(dne/dλ))), where λ is the resonant wavelength, L is the length of the NW, ne is the effective refractive index, and dne/dλ is the dispersion relation. Experimental results for more than 20 straight NWs show that for a given λ, ∆λ ∝ 1/L, which confirms that the NWs are acting as axial F-P type cavities. Mode spacing at 517 nm is plotted versus inverse NW length for 22 straight NWs in Figure 2f. According to Figure 2e, the relation of group index (ng ) ne Nano Lett., Vol. 9, No. 7, 2009

- λ(dne/dλ)) of the CdS NW versus λ can be obtained, which is shown in Figure 2g. We use the equation ne2 ) A + B

λ2 λ -C 2

referring to the Sellmerier equation, to approximately describe the dispersion relation of ne. By fitting the curve using the ng and ne expressions, we can obtain ne of the CdS NW. The fitting result (black line) is shown in Figure 2g. The ne obtained from the fitting parameters A (4.15), B (0.045), and C (0.248 µm2) is about 2.23 at 512 nm. This value is smaller than the reported refractive index (n ) 2.8) of CdS films34 for the same wavelength. This is because the electromagnetic power is not totally confined in the CdS NW with small volume. CdS NW rings were fabricated by two nanoprobes. Figure 3a-c show the process of a straight NW being bent to a ring. The diameter (d) of the NW and the radius (R) of the ring are about 187 nm and 5.1 µm, respectively. Two optical coupling geometries, end-to-end and side-by-side overlap, have been adopted. The inset of Figure 3c shows a magnified SEM image of the joint area of an end-to-end coupling geometry. The narrowest air gap between the two end facets is about 20 nm. The coupling angle of the two NWs, which is defined as the deviation angle of the coupling NW from propagation along a straight path of the coupled NW, is 45°. Panels d and e of Figure 3 show the SEM of two NW rings with side-by-side overlap coupling geometry. The radii of the rings are about 5.9 and 2.1 µm, respectively. The diameters of the NWs are about 130 and 150 nm, and the overlap lengths are about 3.75 and 0.61 µm, respectively. The coupling loss at the joint area (Rc) can be estimated by simulating the coupling efficiency ()e-2πRRc) using the beam propagation method. Panels a and b of Figure 4 show the simulated coupling efficiency for two straight NWs (d ) 150 nm) coupled with two kinds of end-to-end geometries, respectively. Note that the cross section shape of NW is set as square in all simulations, the d represents the side length of square. One has an air gap and the coupling angle is zero, the other has a coupling angle and the narrowest air gap between the two end facets is zero. For the end-to-end coupling geometry, the smaller the air gap length and coupling angle are, the higher the coupling efficiency is. Figure 4c shows the simulated coupling efficiency for two straight NWs (d ) 150 nm) coupled with side-by-side geometry. For the side-by-side coupling geometry, the coupling efficiency strongly depends on the overlap length and varies with it periodically. For the 150 nm thick NW, the coupling length (i.e., the interaction length needed to completely transfer the electromagnetic power from one NW to another) is about 1 µm (Figure 4c). The coupling efficiency reaches the maximum value when the overlap length equals an odd integral number of the coupling length. The electromagnetic power propagation simulation result by the beam propagation method for the end-to-end coupling geometry depicted in Figure 3c is shown in Figure 4e. From this figure, we can obtain the coupling efficiency to be about 47.8%. Hence, the Rc is about 230 cm-1. The calculated results of the side-by-side overlap coupling geometries depicted in parts Nano Lett., Vol. 9, No. 7, 2009

d and e of Figure 3 are shown in parts f and g of Figure 4, respectively. The coupling efficiencies are about 60% and 66.8%, respectively. The corresponding Rc values are about 138 and 306 cm-1, respectively. It is worth noting that in our case none of the three ring cavities has reached the maximum coupling efficiency yet. Figure 3f shows the in situ PL spectra of the NW before and after being bent to the NW ring depicted in Figure 3c. The in situ PL of the straight NW has a peak around 510 nm with fwhm of about 13.5 nm, while that of the NW ring has a peak around 513.6 nm with fwhm of about 14.7 nm. We think that the red shift and broadening effect of the PL emission are due to bending strain. We confirm this by bending a NW and measuring the in situ PL spectra at the straight part (without strain) and the curving part (with strain). Corresponding red shift and broadening effect have also been observed (not shown). Parts g-i of Figure 3 show the spatially resolved PL spectra of the rings depicted in parts c-e of Figure 3, respectively. For the spatially resolved PL measurements of the NW ring cavities, the collection spots were at the joint areas, while the excitation spots were 9 µm (for the 50× objective) or 4 µm (for the 100× objective) away from the collection spots on the rings. The joint area of a NW ring can act as a defect and couple the light out. Periodic intensity variations are observed in these figures. For a ring type cavity, ∆λ is given by λ2/(2πR(ne - λ(dne/dλ))). With this equation together with the PL spectra shown in Figure 3g-i, the ng-λ curves for R equal to 5.1, 5.9, and 2.1 µm are depicted in Figure 2g. These ng-λ curves fit well with the fitting line for the straight NW F-P cavity. This confirms that the observed periodic intensity variations originate from the WGMs in the NW ring cavities. Basically, a ring cavity has two key merits: high Q value and compact size. From Figure 3h, the fwhm of the cavity mode at 519.4 nm is about 0.45 nm (background subtracted), corresponding to a Q value to be about 1150. This value is close to the Q value (∼1300 at 516 nm) we obtained from a straight NW F-P cavity with a comparable optical path in a round trip (the length of the NW is about 20 µm). In order to figure out the reason why the NW ring cavity does not show an enhanced Q compared to the straight F-P cavity, we have investigated the factors that affect the Q value as below. The Q of a resonator is given by Q)

λ0 λfwhm

)

πa1/2Lng (1 - a)λ0

where λ0 is the resonance wavelength, a is the inner circulation factor (a ) exp(-RLc)), where R is the total loss in the cavity, Lc is optical path in a round trip, and ng is the group index.20,22 For a NW F-P cavity, R ) Rm + Rs + Re, where Rm is the material loss, Rs is the scattering loss due to adhered particles and defects on the NW, and Re is the end-facets loss (i.e., the mirrors loss) 2701

Figure 5. SEM (a), dark field (b), and PL image (c) of a NW ring coupled to a straight NW. Light spots marked by arrows 1 are due to adhered particles, those marked by arrows 2 are due to defects on the NW. The corresponding positions are also marked in the SEM image in (a). (d) Spatially resolved PL spectrum of the straight NW prior to being contacted to the NW ring. (e) PL spectrum collected at the output end of the straight NW with the exciting point on the NW ring. Red and green lines mark the respective mode positions of the F-P cavity and the ring cavity.

Re )

1 1 ln L Re

L is the length of the NW and Re is the reflection coefficient. In our case, the Rs is very low since there are few adhered particles on the measured straight NW. In contrast to a conventional edge emitting semiconductor laser cavity, in NW F-P cavity, Rm , Re, due to the much smaller cavity length and reflection coefficient.5 Therefore, the main loss of the NW F-P cavity is Re. According to the Fresnel equation, the reflection coefficient for CdS NW is only about 22% (here, the refractive index n of CdS NW is taken to be 2.8).34 Therefore, Re is about 757 cm-1 for a 20 µm long NW. For a NW ring cavity, R ) Rm + Rs + Rc + Rb, where Rc is the coupling loss at the joint area and Rb is radiation loss due to bending. According to our simulation results with the finite element method as shown in Figure 4d, the bending loss Rb increases with the decrease of the radius of ring and the diameter of NW and is very sensitive to the diameter when it is smaller than 130 nm. From these results, we can see that a NW ring constructed on a 130 nm thick NW with a radius down to 2 µm has a Rb of only about 13 cm-1, much smaller than the above-mentioned end-facets loss of NW F-P cavity. Bending loss of the NW ring cavity (R ) 5.9 µm, d ) 130 nm) depicted in Figure 3d can be calculated to be only about 2 cm-1, much lower than the Rc of the ring (∼138 cm-1 obtained from Figure 4f). Thus, we think the coupling loss and scattering loss (referring to the bright scattered light spots due to adhered particles on the NW ring in the PL image in Figure 5c) are the main losses for the NW ring 2702

cavity. We think the Q factor of the as-fabricated NW ring cavity can be increased and exceed that of the NW F-P cavity by reducing the scattering loss via employing NW free from adhered particles and improving the coupling efficiency via optimizing the overlap length. In order to make an effort to couple the light out from the NW ring cavity,35-37 a cleaved straight NW was placed by the side of the NW ring depicted in Figure 3d with a sideby-side overlap coupling geometry. The length and the diameter of the NW are about 13.5 µm and 280 nm, respectively. Panels a and b of Figure 5 show the SEM and dark field images of the coupled R-F-P cavity. When the NW ring was excited by a focused laser, a bright green light spot was observed at the output end of the straight NW (Figure 5c), indicating that the light in the NW ring had been coupled out through the straight NW waveguide efficiently. This suggests a potential application of the NW ring cavity in nanophotonic integrated circuits, such as switches, routers, filters, etc.18-23 Note that in Figure 5c we can see several light spots on the NW ring body. This is due to the scattering light from defects and adhered particles on the NW. The NWs may have natural defects, and the nanoprobe may also cause some damage on the NWs during the manipulation process. All these scattering centers contribute to scattering loss and decrease the quality factor Q of the ring cavity. Selecting NWs free from adhered particles or removing adhered particles from NW rings by the nanoprobe will decrease the scattering loss and then improve the Q factor. Defects on NWs could be reduced by improving the synthesis Nano Lett., Vol. 9, No. 7, 2009

process and the nanoprobe manipulation process. Arrows are added to highlight the emission spot of interest in the PL image in Figure 5c. The corresponding positions are also marked in the SEM image in Figure 5a. Figure 5e shows the PL spectrum collected from the output end of the straight NW with the excitation point on the ring. Significantly, the PL does not just imprint the WGMs of the NW ring cavity as shown in Figure 3h. Some modes are strengthened, while some others are suppressed. The spatially resolved PL spectrum of the straight NW prior to being contacted to the NW ring was plotted in Figure 5d. Periodic intensity variation of the spectrum fits well with the F-P cavity with a length of 13.5 µm. The corresponding mode positions of the F-P cavity (red lines) and ring cavity (green lines) in the coupled R-F-P cavity are marked schematically in Figure 5e for comparison. We can see that those WGMs which coincide spectrally with F-P modes are enforced while others are suppressed. This indicates that the straight NW has served as a modulator in the coupled R-F-P cavity. We note that, the frequencies of the enforced modes are determined by the mode spacings of both the ring and F-P cavities. When the optical path of the ring cavity is fixed, mode modulation can be realized by adjusting the mode spacing of the F-P cavity through varying its cavity length or excitation power.38 Moreover, increasing the excitation laser power can help to realize lasing where the intensity of the cavity modes exceeds the spontaneous emission background by orders of magnitude.5 However, in our case, the light spectrum got even worse (the peak encountered a red shift and its fwhm became wider) when the laser power increased to about 100 kW/cm2. This is because of the heating effect of the laser. We think that using pulsed laser to replace the continuous wave laser and/or performing the experiment in lower temperature may help to achieve lasing in both NW rings and straight NWs.5,10 In that case, the NW ring can act as a laser, while the straight NW can serve as the modulator to further realize single-frequency modulation.38-41 Conclusion. We have fabricated and studied CdS NW ring cavities for the first time. The rings with radii from 2.1 to 5.9 µm were fabricated by a nanoprobe system installed in a SEM. Radius-dependent WGMs were observed and confirmed. Further, a coupled R-F-P cavity was constructed by placing a straight NW by the side of a NW ring. The experimental results indicated that the straight NW served not only as a waveguide to couple the light out but also as a modulator to modulate WGMs of the ring cavity. Such a coupled cavity has potential applications in nanophotonic integrated circuits. Acknowledgment. The authors thank Professors Ruo-Peng Wang and Wei-Xi Chen, and Mr. Jian-Jun Chen for helpful discussions. This work was supported by the National Natural Science Foundation of China (Nos. 60576037, 10774007, 10574008, 50732001) and National Basic Research Program of China (Nos.2006CB921607, 2007CB613402). References (1) Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269. Nano Lett., Vol. 9, No. 7, 2009

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