Single-Nanowire Single-Mode Laser

LETTER pubs.acs.org/NanoLett

Single-Nanowire Single-Mode Laser Yao Xiao,† Chao Meng,† Pan Wang,† Yu Ye,‡ Huakang Yu,† Shanshan Wang,† Fuxing Gu,† Lun Dai,‡ and Limin Tong*,† †

State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China ‡ State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, China

bS Supporting Information ABSTRACT: We demonstrate single-mode laser emission in single nanowires. By folding a 200 nm diameter CdSe nanowire to form loop mirrors, single-mode laser emission around 738 nm wavelength is obtained with line width of 0.12 nm and low threshold. The mode selection is realized by the vernier effect of coupled cavities in the folded nanowire. In addition, the loop structure makes it possible to tune the nanowire cavity, opening an opportunity to realize a tunable single-mode nanowire laser. KEYWORDS: Nanowire, laser, single mode, Vernier effect, coupled cavity, loop mirror

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wning to their excellent properties as gain media, resonance cavities and passive waveguides, one-dimensional semiconductor nanowires (NWs) have been attracting growing interests in constructing nanoscale photonic and optoelectronic devices and circuits.1,2 Among the nanowire-based functional elements, such as photodetectors,3,4 lasers,5-15 and light-emitting devices,16-19 nanowire lasers have gained considerable attentions due to their promises as integrable nanoscale coherent light sources for wide applications including optical communications, sensing, and signal processing.1,2,20-22 Generally, in a semiconductor NW when the round-trip gain, sustained by a certain feedback (e.g., end-face reflection), can compensate round-trip losses, the lasing occurs. Typically, the lasing cavity of this kind of nanolaser is formed by endface reflection8,12,19,23 or ring resonance,7 and the lasing emission of the nanowire is usually multimode due to the lacking of mode selection capability. One possible route to obtain single-mode nanowire lasers, proposed by numerical simulation,24 is to fabricate distributed-Bragg-reflector (DBR) mirrors on a nanowire, but experimentally it is very difficult to fabricate high-quality DBR structures on a single nanowire. Another possible scheme indicated by the previous studies, is expanding the free space range (FSR) of the multimodes by significantly shortening the optical path of the lasing cavity, until only one mode is left.12 However, shortening the cavity path will inevitably reduce the round-trip gain, resulting in high threshold for lasing action. In this Letter, we report a single-mode nanowire laser with low threshold. By simply folding one or two ends of a nanowire into micro loops to form loop mirrors (LMs) and coupled resonant cavities, mode selection can be readily realized by means of Vernier effect,25 which is a commonly used mode-selection technique for macroscopic lasing cavity, 26,27 but have not been applied in nanowire lasers. Since the reflectivity of a LM is much higher than r 2011 American Chemical Society

that of an endface of the nanowire,28 the nanowire laser operates with low threshold. Additionally, the loop structure makes it possible to tune the cavity by micromanipulation, which opens an opportunity to realize a tunable single-mode nanowire laser. Here the CdSe NWs are grown on the Si wafers using a chemical vapor transport process.29 As-synthesized NWs are directly deposited onto a MgF2 substrate and then cut to desired length using a bend-to-fracture process,30 leaving a perfect fracture endface along (001) face. A typical scanning electron microscope (SEM) image of a 50 μm length 200 nm- diameter CdSe nanowire is shown in Figure 1a in which excellent uniformity and flat endface (inset of Figure 1a) of the NW are clearly seen. The folding of the nanowire is carried out under an optical microscope equipped with superlong-working-distance objective. By using fiber probes for micromanipulation31,32 (see also Supporting Information), a single nanowire can be readily folded into micro loops at both ends, as shown in Figure 1b. The closed loops may act as LMs with high reflectivity31 via evanescent coupling between NWs,33,34 leading to a high-quality lasing cavity in a single nanowire. Optical characterization of the NWs (or folded NWs) is carried out under an optical microscope, as schematically illustrated in Figure 2a. To excite the CdSe NW (supported on MgF2 substrate), 532 nm laser pulses (5 kHz repetition rate, 15 ns pulse width) from a frequency doubled Nd:YAG laser was focused to a spot size ∼100 μm through a 20
objective (NA = 0.45), offering a pulsed energy density of 0-1000 μJ/cm2 in the excitation region. The photoluminescence (PL) signals are collected by the same objective and directed through a dichroic Received: November 18, 2010 Revised: January 24, 2011 Published: February 15, 2011 1122

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Nano Letters mirror and a 532 nm notch filter, and then split by a dichroic beam splitter to a spectrograph and a CCD camera for spectral measurement and imaging, respectively. All measurements are performed at room temperature. Figure 2b shows a PL microscope image of an excited 60 μm length 200 nm diameter CdSe NW. Two bright lighting spots are clearly observed at both ends of the NW, indicating that the PL emission can be confined and waveguided along the length of the wire. Spectral measurement shows that the PL is centered around 714 nm with a full width at half-maximum (fwhm) of about 34 nm (Figure 2c). Owing to the self-absorption, PL intensity decreases exponentially along the length of the wire (i.e., obeys the Lambert-Beer law,35,36 see Figure 2d), with estimated absorption coefficient R of about 224 cm-1 (see Supporting Information),37,38 which is on the same level with those measured in CdS NWs.39,40 To investigate the lasing action in a single NW, we folded a 200 nm diameter CdSe NW with length of about 75 μm into

Figure 1. (a) Scanning electron microscopy (SEM) image of a 200 nm diameter 50 μm length CdSe NW. Inset, a close view of the right end of the NW with a flat endface. Scale bar, 200 nm. (b) SEM image of a CdSe single NW folded into micro loops at both ends.

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three types of cavity structures. As shown in Figure 3, we study the NW without LM (Figure 3a), with one LM (Figure 3c), and with double LMs (Figure 3e) successively. Since the spatial distribution of the pumping light may be inhomogeneous, the three types of structures are assembled with similar geometry and size to minimize the difference in pumping conditions. Figure 3a shows a PL microscope image of the NW cavity solely relying on endface reflection (Figure 3b). In Figure 3c, left side of the NW is folded into a LM with minimum bending radius of about 3.1 μm. With the additional LM reflection from the left side, the NW operates with two coupled cavities relying on endface-endface reflection and endface-LM reflection, respectively (Figure 3d). In Figure 3e, both sides of the NW are folded into LMs, forming four coupled cavities in a single NW (Figure 3f). To excite the NW, we focus 532 nm laser pulses (5 kHz repetition rate, 15 ns pulse width) on the whole NW structure using the same experimental setup shown in Figure 2a. Under the pumping intensity well above the lasing threshold, Figure 3a shows two lasing spots (at both ends of the NW) with similar brightness, indicating that the two endfaces have almost equivalent reflectivities. For comparison, in single-LM NW shown in Figure 3c, the free-standing end of the NW gives a much stronger lasing output than the loop end, indicating that the LM offers a much higher reflectivity than the free end. With double LMs, the two ends of the NW give strong lasing output with similar intensity (Figure 3e). Noticeably, compared with the NW in Figure 3a,c, obvious scattering spots (caused by adherent nanoparticles or other kind of surface contamination introduced in the micromanipulation process) is observed along the NW in Figure 3e, indicating much stronger intensity of optical fields confined and recirculated inside the NW due to the much higher quality of the double-LM cavity. The coupled cavities make it possible to realize mode selection of lasing action by means of Vernier effect.25 Generally, in a single nanowire with only one F-P cavity, the lasing action is multimode. As shown in Figure 4, when the 75 μm length NW

Figure 2. (a) Schematic diagram of experimental setup for micromanipulation and optical characterization. (b) PL microscope image of a 60 μm length 200 nm diameter CdSe NW excited by a 532 nm wavelength continuous wave laser, taken with a 532 nm notch filter. (c) PL spectrum of the NW in (b). (d) Propagation-distance-dependent PL output intensities (dots) of a 200 nm diameter long CdSe NW measured by locally exciting the NW at different positions along the length of the NW using a tightly focused 532 nm laser. Gray curve is the fitted curve (first-order exponential decay fit) of the experimental data. 1123

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Nano Letters (Figure 4a, the same NW used in Figure 3) is pumped near (∼117.5 μJ/cm2, gray line) and well above (∼236.6 μJ/cm2, red line) the lasing threshold, multimode lasing spectra are observed with a measured FSR of about 0.75 nm (Figure 4b), corresponding to a group index of ng ≈ 4.8 for the CdSe NW (obtained by FSR ≈ λ2/2Lng, where λ is the wavelength, L = 75 μm is the

Figure 3. PL microscope images and schematic diagrams of lasing cavities of single-NW structures (a,b) without LM, (c,d) with one LM, and (e,f) with double LMs. The NWs used in (a,c,e) are the same CdSe NW with diameter of 200 nm. In (b,d,f), the thin arrow represents endface reflection with low reflectivity, while the thick arrow represents LM reflection with high reflectivity.

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length of the NW). Although, as the pumping intensity increases, the lasing energy tends to concentrate on fewer modes, it is difficult to realize single-mode operation by solely increasing the pumping intensity in the NW without LMs (see Supporting Information). When the left side of the NW is folded into a LM, the measured effective path length L0 (obtained by L0 ≈ L (LLoop/2), where LLoop is the perimeter of the loop) of the second cavity based on endface-LM reflection is about 34 μm, corresponding an FSR0 (FSR ≈ λ2/2L0 ng) of about 0.98 nm. The coupled two cavities (based on endface-endface reflection and endface-LM reflection, respectively) can be used for mode selection via Vernier effect: take the fwhm of the lasing peak (∼0.145 nm) into account, the coupling of the two cavities offers the lowest common multiple of the FSRs (0.75 and 0.98 nm) of about 2.96 nm, which suppresses the lasing modes of the two individual cavities and selects only one dominant mode within the lasing range. The calculated lowest common multiple of the FSRs (2.96 nm) agrees well with the spectral range (about 2.96 nm) measured between the dominant mode (the central peak) and the negligibly side mode (the left weak peak) in Figure 4d. It can also be seen in Figure 4d, lasing with pumping well above the threshold (red line) offers much better suppression of side modes compared with that near the threshold (gray line, in which several weak peaks are clearly seen). For example, as the pumping intensity increases from 69.7 to 177 μJ/cm2, side-mode suppression ratio (SMSR) increases from 2.6 to 8.6, corresponding to an increase of mode suppression factor (MSF) from 4.15 to 9.34 dB.

Figure 4. SEM images and lasing spectra of single-NW structures (a,b) without LM, (c,d) with one LM, and (e,f) with double LMs. The red lines represent lasing spectra obtained at pump fluence well above the threshold, while the gray lines represent the spectra obtained near the threshold. 1124

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been verified in other looped CdSe nanowires with different diameters and lengths (see Supporting Information). The lasing thresholds of the NW structures are also investigated. As shown in Figure 5a, the measured threshold for the NW without LM is about 97 μJ/cm2 (blue line) that reduces to about 59.7 μJ/cm2 (red line) when one end of the NW is folded into a LM and finally goes down to 34.4 μJ/cm2 (green line) with double LMs. The significant reduction of lasing threshold by folding NW into LM, which is benefitted from the greatly improved cavity quality, suggests a feasible approach to lowthreshold NW lasers. Additionally, the flexible loop structure makes it possible to change the optical path of the lasing cavity by changing the geometry of the loop, and therein tune the wavelength of the lasing mode. To show this, we use a single-mode NW laser assembled with a 240 nm diameter 84 μm length CdSe NW. As shown in Figure 5b, when the size of the right loop is decreased by micromanipulation (right end of the nanowire is shifted slightly to the left), the lasing peak shifts from 733.7 to 726.9 nm due to the reduction in optical path of the lasing cavity, making it a potential way to realize a tunable single-mode NW laser without breaking the nanowire. Compared to laser emission from a nanowire that relies on one resonant cavity, folding a nanowire into LMs does not only offer multiple cavities for mode selection but also improves the cavity quality for reducing the lasing threshold. As mode quality and lasing threshold are two of the most concerned parameters of lasers, the single-mode low-threshold lasing action demonstrated in a single nanowire may open opportunities for practical applications of nanowire lasers, as well as provide a new design for realizing low-threshold single-mode lasers using other types of nanostructures. Figure 5. (a) Emission power versus pump fluence of the excited NW without LM (triangle), with one LM (square), and with double LMs (circle). The intensity is the peak intensity of the dominant mode for single-mode laser (with single/double loops) and the mode with lowest threshold (with lasing peak centered at 735.2 nm in Figure 4b) for the multimode laser (without loop). The integration time is 1 s. (b) Spectral shift of the lasing peak from 733.7 to 726.9 nm by changing the geometry of the loop in a 240 nm diameter 84 μm length CdSe NW. Inset, SEM images of the original cavity with lasing peak at 733.7 nm (bottom right) and the changed cavity with lasing peak at 726.9 nm (up left).

’ ASSOCIATED CONTENT

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Supporting Information. Micromanipulation process of folding one end of a NW into a micro loop. Estimation of PL absorption coefficient R in Figure 2d. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. With both ends of the NW folded into LMs (Figure 4e), the NW laser offers a higher SMSR than in the single-LM structure (13.5 in Figure 4f versus 8.6 in Figure 4d, or 11.3 dB versus 9.34 dB in MSF), which is benefitted from the coupling of more cavities for side-mode suppression. It is also noticed that, the line width of the lasing mode near the threshold decreases from about 0.15 nm in NW without LM (Figure 4b) to 0.12 nm in NW with one LM (Figure 4d) and 0.10 nm in NW with double LMs (Figure 4f), indicating the increase in lasing cavity quality by introducing the LMs. In addition, compared with the single-LM NW laser, the lasing peak shows an obvious red shift of about 2.72 nm (from 735.36 nm in Figure 4d to 738.08 nm in Figure 4f). This may be caused by band gap renormalization7 due to stronger power recirculation inside the double-LM cavity (see Figure 3e), and higher coupling efficiency for light of longer wavelength in the side by side coupling geometry, as previously observed in other types of nanowire lasers.7 By the way, the reproducibility of single-mode operation in single nanowires has

’ ACKNOWLEDGMENT The authors thank Yaoguang Ma, Wei Li, and Xiyuan Li for helpful discussions. This work is supported by the National Basic Research Program of China (Nos. 2007CB307003, 2007CB613402) and by the National Natural Science Foundation of China (Nos. 10974178, 61036012, and 10774007). ’ REFERENCES (1) Li, Y.; Qian, F.; Xiang, J.; Lieber, C. M. Mater. Today 2006, 9, 18–27. (2) Yan, R. X.; Gargas, D.; Yang, P. D. Nat. Photonics 2009, 3, 569–576. (3) Wang, J. F.; Gudiksen, M. S.; Duan, X. F.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455–1457. (4) Kind, H.; Yan, H. Q.; Messer, B.; Law, M.; Yang, P. D. Adv. Mater. 2002, 14, 158. 1125

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