We report the first observation of supercontinuum (SC) generation in single semiconductor nanoribbons (NRs). By launching a continuous wave (CW) 532-nm pump light along a 200-μm-length CdS NR for waveguiding excitation, SC generation is realized with a threshold down to sub-milliwatt level, which is ~3 orders lower compared with previous CW-pumped SC generated in glass fibers. The low threshold is benefitted from the favorable material properties and waveguide geometries including high Raman gains, strong light confinement, more optical guided modes and phonon modes. Our work paves the way to low-threshold nanoscale SC sources and may find widespread applications ranging from spectroscopic analysis and biological imaging to material research.
© 2012 OSA
Supercontinuum (SC) generation, i.e., spectral broadening of the original pump light by launching intense continuous wave (CW) or pulsed light into nonlinear media, has received a lot of attentions over the past decades [1–4], due to its complex physics and great potentials in various applications ranging from spectroscopy and telecommunication to medicine. So far, SC that covers the visible and near-infrared regions has been successfully observed in various nonlinear media such as liquids , glasses [6–13], and semiconductors [14,15], based on complex processes such as self-phase modulation, four-wave mixing (FWM), stimulated Raman scattering (SRS) and modulation instability. To enhance effective nonlinear interaction of light with mater, high pump power or long interaction lengths are usually required for efficient SC generation. On the other hand, low operation power and compact size are critical to match the rapid growing nanophotonic circuits and devices with ultra-broadband and high density , which spurs substantial effort for realizing low power optical nonlinear effects with miniature feature sizes.
Recently it has been demonstrated that waveguides with subwavelength dimensions, fabricated from a variety of materials such as glass and semiconductor, can provide low-threshold optical nonlinear effects [17–20], miniaturized footprints  and tailorable waveguide dispersion [17,18,22,23], which may open new opportunities to generate SC with extraordinary low power on a lower dimension. Of the well-studied subwavelength waveguides, chemically synthesized [24–26] (e.g., vapor-liquid-solid growth ) free-standing semiconductor nanoribbons (NRs) show favorable material properties and waveguide geometries for SC generation: (1) semiconductors usually have high Raman gains (~10−6−10−7 cm2, about 3−4 orders of magnitude larger than that of silica)  for low-threshold pumping, large refractive indices for strong confinement of guided light, single-crystalline structure quality that allows lattices to vibrate more collectively than that in amorphous glass and photonic-crystal fibers , and material diversity for various spectral ranges [28,29]; (2) compared with nanowires or fibers that have symmetrical cross-sections, the asymmetrical cross-section of NRs can support more optical guided modes for facilitating phase matching , more phonon modes for spectral broadening, and higher strength of the exciton-phonon coupling for increasing Raman scattering [27,31,32]. In this work, for the first time, we demonstrate SC generation in single semiconductor NRs. By launching a CW 532-nm light along a 200-μm-length CdS NR for waveguiding excitation, SC generation with a threshold down to sub-milliwatt level and a conversion efficiency of ~30% is obtained.
Here we chose the CdS as the NR material, which is a typical II-VI semiconductor with favorable and well-studied optoelectronic properties [26,31–33]. Figure 1 shows the room- temperature Raman spectrum (Jobin-Yvon LabRam-1B) of the CdS NRs with 633-nm excitation. Compared to bulk CdS that usually provide two prominent LO and 2LO modes [32,34], CdS NRs used here show 9 optical phonon modes, which is helpful for Raman spectral broadening. The Raman peaks of 4 (301 cm-l) and 9 (600 cm-l) corresponds to the first and second-order longitudinal optical (LO) phonon modes of CdS, respectively. The peaks of 1 (214 cm-l), 5 (326 cm-l), 6 (348 cm-l), 7 (370 cm-l), and 8 (560 cm-l) correspond to multiphonon scattering . The peak of 2 (235 cm-l) is attributed to the A1 transverse optical (TO) mode, E1 TO mode, or their combination, and the peak of 3 (255 cm-l) is attributed to the E2 phonon mode. It is noted that the intensity of the 1st peak (214 cm-l) is larger than those of the LO and TO modes, suggesting the enhanced multiphonon responses in NR structure. Moreover, the ratio of the 2LO mode intensity to the 1LO mode intensity (I2LO/I1LO) is about 0.75, suggesting high strength of the exciton-phonon coupling in CdS NRs [31,32].
Scanning electron microscopy (SEM) image in Fig. 2(a) shows a typical CdS NR (200 nm in thickness, 1.2 μm in width) with smooth surface and uniform rectangular cross section, which are favorable for low-loss optical waveguiding . To adapt the CdS NR (refractive index ~2.6)  for low-loss waveguiding, we place the CdS NR on a low-index MgF2 crystal substrate (refractive index ~1.39). The NR used for SC generation is 300 nm in thickness, 1.5 μm in width, and 200 μm in length, as shown in Figs. 2(c)−(i). We use a 532-nm CW monochromatic laser as the pump source, of which the wavelength is near to the band edge of CdS NR (~2.48 eV). The pump light is launched into the NRs using a butt-coupling technique . As schematically illustrated in Fig. 2(b), a fiber taper is placed in parallel and contact with one end of a CdS NR so that the pump light can be efficiently transferred from the fiber to the NR. For reference, Fig. 2(c) shows an optical micrograph of coupling the 532-nm laser from a silica fiber taper  into the CdS NR. The input power (Pin) launched into the fiber is 10 nW, calibrated by a spectrometer (Ando AQ-6315A). The output emission of the NR is collected using an objective lens and directed to a CCD and spectrometers . A notch filter (Semrock NF01-532U-25) is used to block the excitation laser when taking CCD images (Figs. 2(d)–(i)) and spectra. The output spectra shown in Figs. 3(a)–(c) are obtained using a Maya 2000-Pro spectrometer (Ocean Optics) with the notch filter, and the SC spectrum in Figs. 3(d) is obtained using the AQ-6315A spectrometer without the notch filter. The coupling efficiency between the fiber taper and the CdS NR is measured as ~20% (the detail of pump power calibration and coupling coefficient measurement can be found in Ref. 37).
3. Results and discussions
At relatively low pump intensity (Pin < 1.7 mW), the two-photon absorption induced photoluminescence (TPA-PL) and spontaneous Raman scattering are the dominant processes in the CdS NR. Figure 2(d) and 2(e) show two typical microscope images of the CdS NR with Pin of 0.07 and 0.63 mW (taken with the 532-nm notch filter), respectively, in which cyan light emitted along the whole length of the NR. The peak wavelength of the cyan emission at 505 nm (see spectra in Fig. 3(a)), corresponding to the two-photon absorption (TPA)-induced spontaneous inter-band transition [26,32]. The spontaneous Raman scattering with a broadened first-order Stokes wave (ωs1) at 540 nm and a narrow second-order Stokes wave (ωs2) at 549 nm are also observed with the magnified spectrum provided in the inset of Fig. 3(a), which is in consistent with the Raman spectrum in Fig. 1 (ωs1 corresponds to the 4th peak and ωs2 corresponds to the 9th peak). No evident anti-Stokes scattering peaks are observed. Under this pumping level, the majority of the pump light is guided to the other end of the NR. Only a small fraction of the pump light is consumed and transferred through the nonlinear interaction, in which most of the energy is converted to the TPA-PL. As shown in Fig. 4 , the intensity of the TPA-PL increases quadratically with Pin, while both the intensities of the Raman Stokes wave and the pump from the output end of NR increase near linearly with Pin.
When Pin is increased over ~1.7 mW, the green light spot at the output end of the CdS NR become stronger and stronger, as shown in Fig. 2(f) (Pin = 1.67 mW) and (g) (Pin = 2.33 mW). These coincide with the spectra shown in Fig. 3(b), in which the intensities of 540-nm Stokes wave and the anti-Stokes wave (ωas1) at 520 nm grow up quickly, while the TPA-PL intensity becomes saturated. Under this pumping level, the anti-Stokes photon is mainly parametrically generated via the four-wave mixing (FWM) process, which involves two pump photons and one Stokes Raman photon [1–3]. The saturation of TPA-PL is due to the band filling effect, in which the TPA induced high-density carriers block inter-band transition and in turn induce the saturation of the above-bandgap absorption [38,39]. Figure 4 shows that the intensity of the Stokes waves increases from linearly to exponentially, indicating the turning on of the SRS in CdS NR , with a threshold about 1.7 mW. Once the SRS threshold is reached, the intensity of the transmitted pump light at the output end of the NR shows clear deviation from a linear relation with Pin, which indicates the efficient energy conversion of SRS. It should be also noticed that as Pin increases, the spectral profiles of Raman waves become broadened, and the peaks of the second-order Stokes wave are gradually smoothed out. It can be attributed to the increased multiphonon scattering due to the laser-heating effect in CdS NR [31,40,41].
When Pin is further increased above 2.3 mW, a slightly increase in Pin results in a dramatic increase in both the Stokes and the anti-Stokes intensities. Figure 2(h) and 2(i) show the microscope images of the NR with Pin of 2.56 and 2.73 mW, respectively, in which extremely bright emissions with a white spot in the center, a cyan ring in the middle and a green divergent radiation outside generates at the output end of the CdS NR. The white spot in the center is due to the exposure saturation of CCD camera. Figure 3(c) and Fig. 4 show the sharp increase of the Stocks and anti-Stocks waves. From Fig. 4 we can find that the intensity of the pump output experiences a sharp decrease to almost zero, because the majority of the guided pump energy is consumed and transferred to the Stokes and anti-Stokes waves. All these results indicate the onset of Raman oscillation , with a very low CW excitation threshold about 2.4 mW. As Pin further increases, the Stokes and the anti-Stokes waves show saturation in intensity due to the depletion of the pump energy. Figure 3(d) shows the output spectrum of the NR at Pin = 2.73 mW without the 532-nm notch filter. It clearly demonstrates SC generation with a 20-dB bandwidth of nearly 80 nm. Considering the coupling efficiency between the fiber taper and the CdS NR (20%), the effective pump power received by the CdS NR is about 0.5 mW, indicating a sub-milliwatt-level threshold, and the energy conversion efficiency from the pump to SC is calculated about 30%.
As described above, the evolution of the SC generation in CdS NRs experiences four distinct processes: (1) spontaneous Raman scattering, (2) stimulated Raman scattering, (3) Raman oscillation, and (4) saturation, as denoted in Fig. 4. Such behavior is similar to the stimulated Raman emission previous reported in liquids . Although the CW-pumped SC generation has also been demonstrated in glass fibers, however much higher thresholds (watt level) and longer interaction lengths (kilometer level) are needed [3,4,11].
We believe the realization of such compact low-threshold SC source here mainly benefits from the unique CdS NR structure. Firstly, CdS NRs have relatively high refractive index and subwavelength thickness, which results in strong confinement of the guided pump light . Thus the interaction of pump light with NR is dramatically enhanced [43,44], comparing to bulk semiconductor materials. And utilizing the butt-coupling technique is also very helpful to efficiently couple the pump light into NRs. In our work, the 0.5 mW pump energy is capable to generate a power density of ~1 GW/m2 in the CdS NR (300 nm thick by 1.5 μm wide) we tested, which is comparable with the pump intensities of CW or pulsed light used in other works [3,4,6–14]. Thus, a sub-mW level pump power can generate a sufficiently strong optical field for manifesting nonlinear phenomena in a CdS NR.
Secondly, the rectangle cross-section shapes of NRs are essential in the Raman scattering process. Compared with nanowires, NRs with large width-to-thickness ratio usually have more phonon modes [27,31], which is helpful for spectrum broadening in SC generation. Moreover, NRs with relatively large width can support more optical modes than NWs. Thus the group velocity distribution at certain wavelength range is wider in NRs than in NWs. This means the phase matching condition of the anti-Stokes FWM process is more likely to be satisfied for certain high order optical modes, even though the fundamental mode may not satisfy such condition automatically. Indeed, as we tested with similar excitation condition, we did not observe SC generation from CdS NWs, with diameters ranging from 100 nm to several micrometers and lengths ranging from tens to hundreds of micrometers, even at a maximum pump power of 50 mW.
Finally, though the TPA-PL consume certain amount of pump energy, such a process may be critical for the low threshold behavior demonstrated here. The TPA induced carriers can readily emit cascade phonons by relaxation to the bottom (top) of conduction (valence) band , which is helpful for enhancing the Raman scattering. Together with the exciton-phonon interaction enhanced by the NR structure (indicated by the high I2LO/I1LO ratio) [27,31,32], the SRS threshold is greatly reduced. Moreover, the TPA effect may also be helpful for the FWM process to generate anti-Stokes waves. As we studied numerically, the group-velocity dispersion (GVD) of the guided fundamental mode at the pump wavelength is not close to zero in the NR, which means the phase matching condition of FWM is not likely to be satisfied for this mode. When the pump power is above 1.7 mW, the saturation of TPA-PL indicates very high carrier density existing in the NR. The high carrier density will cause the change of optical properties of NR (e.g., refractive index and dispersion relation) [39,45], which in turn may help the realization of the zero GVD near the pump wavelength for fundamental or other low order guided modes. This might be the reason why we see strong anti-Stokes Raman scattering when the pump power is above the saturation of TPA-PL.
In conclusion, we have demonstrated SC generation in a 200-μm-length CdS single NR via CW waveguiding excitation with a sub-milliwatt threshold. Compared to earlier CW-pumped SC generated in glass fibers [3,4,11], here the thresholds are ~3 orders lower and the interaction lengths are ~6 orders less, respectively. The NR structure plays an important role in the low-threshold SC generation, which is also valid for other semiconductor materials. Though the width of our SC spectra is not very broad, it would be more desirable to convert pump efficiently into a specific spectral range than an ultra-broadband one in certain circumstances such as biological and biomedical imaging [3,4,46]. For example, benefitted from material variety/diversity of semiconductors, by using ZnO or alloy ZnSSe NRs , it may allow the realization of blue and ultraviolet SC sources. Our work paves the way to low-threshold nanoscale SC sources and may find widespread applications ranging from biological imaging and spectroscopic analysis to material research.
The authors thank Prof. Yuen-Ron Shen (UC Berkeley) and Bing Guo (Zhejiang Univ.) for helpful discussions. This work was supported by the National Basic Research Programs of China (No. 2007CB307003), the National Natural Science Foundation of China (No. 60907036, 11104245), and Fundamental Research Funds for the Central Universities.
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