We demonstrate a compact hybrid structure red-green-ultraviolet three-color laser consisting of three distinct semiconductor nanowires (CdSe, CdS and ZnO) attached to a silica microfiber, which is pumped by 355 nm wavelength laser pulses. The exciting of the nanowires and the collection of the photoluminescence (PL) are implemented by means of evanescent coupling through the same silica microfiber. When pump energy higher than 1.3 μJ, three spatially and spectrally distinct lasing groups can be measured at the same output port simultaneously. The approach can be extended to other materials to produce hybrid lasers that cover ultraviolet to near infrared spectral regions.
© 2009 Optical Society of America
Tremendous attention has currently been paid to integrated photonics, because they have the potential to overcome the problems faced in electronics, such as limitations of speed and power dissipation. The development of materials and structures has been the central to progress in this area [1,2]. Semiconductor nanowires are emerging as ideal materials which can be assembled into integrated optical systems because of their unique properties such as nanoscale size, high gain, and low energy consumption. Rapid progresses have been made in realizing ultraviolet [3,4], green , and infrared  semiconductor nanowire lasers, where the lasing wavelength in these studies corresponding to the fundamental bandgap energies of the respective nanowire materials. Furthermore, wavelength-controlled lasers over a broad range of wavelengths have been obtained in ternary alloy nanoribbons  or multi-quantum-well nanowire structures . It is of scientific and practical importance to generate the multiwavelength laser in a single compact laser system. The compact multicolor laser will lead to many advanced applications in future optoelectronic technology such as full-color laser display, high-resolution laser printing, medicine and biology.
Recently, both theoretical and experimental works have shown the potential to combine different functionalities by using different photonic building blocks in hybrid structures [9–11]. In this letter, we demonstrate a single compact multicolor laser system which generates red-green-ultraviolet three-color laser collected from the same end of a commercial fiber. The laser system consists of three distinct semiconductor nanowires (CdSe, CdS and ZnO) and a silica microfiber. To our knowledge, it is also the first report of CdSe nanowire laser. The pump energy is coupled into nanowire by the evanescent field existing outside microfiber, and the PL will be coupled back into the microfiber by end emission and evanescent field of the nanowire. Comparing with conventional nanowire lasers located on a substrate, this microfiber/nanowire structure will not only reduce the effect of substrate but also show high compatibility with fiber systems that offer variable solutions for the integration of semiconductor nanowire lasers into photonic circuits. Furthermore, the hybrid structure offers good stability and has the potential to obtain high coupling efficiency to both pumping and collection of the semiconductor nanowire lasers. In addition, the simple approach can be extended to other materials, such as GaN, InP, ZnS, CdS1-xSex, and other complex nanostructures to produce hybrid lasers that not only cover ultraviolet to near infrared spectral regions but also can be integrated as multicolor laser source arrays in micro-/nanophotonic devices.
The three-color laser consists of single CdSe, CdS and ZnO nanowire attached to a silica biconical microfiber [Figs. 1(a)-(c) ], as illustrated schematically in Fig. 1(d). Silica biconical microfiber is fabricated by a flame-heated taper drawing technique using two linear stepper motors. Both sides of the biconical fiber are connected to the commercial fibers. The CdSe, CdS and ZnO nanowires are grown with wurtzite crystal structure and the optical axis of the crystal coincides with the geometrical axis of the nanowires [12–14]. To fabricate the three-color laser, we adopt the following approach. A single semiconductor nanowire is taken up from the substrate and moved toward the microfiber by home made probes. When the semiconductor nanowire touches the microfiber, they attract each other by van der Waals force or electrostatic force. By careful manipulation, the nanowire can be attached straight along the microfiber. Using this method, single CdSe, CdS and ZnO nanowires can all be attached to the same microfiber.
The pump source is a frequency-tripled neodymium doped yttrium aluminum garnet (Nd:YAG) laser (355 nm, 6 ns, and 10 Hz). The pumping light is first sent into the input port by lens-focused launching, and then squeezed into the microfiber. Due to the small diameter of the microfiber, the evanescent field existing outside will excite the nanowires attached on the microfiber. The light emission from the nanowires will be coupled back to the same microfiber. The pump energy is measured at the untapered input port of the silica microfiber.
3. Results and discussion
CdSe, CdS and ZnO nanowires used in this study are direct bandgap II-VI materials with the bulk bandgaps enabling light emission from ultraviolet to visible region as confirmed for individual nanowires using PL measurements . As shown in Fig. 1(e), red-green-ultraviolet PL from CdSe, CdS, and ZnO nanowires are observed obviously along the microfiber. The end facets of the nanowire will serve as the two mirrors of Fabry-Perot cavity because of the large refractive index contrast between the nanowire (the refractive indices: ZnO 2.45 at 391 nm, CdS 2.6 at 519 nm, CdSe 2.78 at 743 nm) and the surrounding air [4,16,17]. The laser emission can be observed when the round trip gain is larger than the round trip losses. Figure 2 shows the optical spectra for the hybrid structure laser [the same structure shown in Fig. 1(e)] as a function of pump intensity. Single-color (CdSe), dual-color (CdSe and CdS) and three-color (CdSe, CdS and ZnO) laser are obtained in sequence with the increasing of pump intensity. According to Fig. 2, when pump energy higher than 1.3 μJ, three spatially and spectrally distinct lasing groups (centered at 391 nm, 519 nm and 743 nm, respectively) can all be measured at the same output port simultaneously, which is consistent with lasing emission from ZnO, CdS, and CdSe, respectively. The intensity of CdSe laser in Fig. 2 decreases as the intensity of pumping laser increases with pumping energy above 1.21 μJ, which may be induced by the saturation of the CdSe nanowire laser . By changing diameter and length of the nanowires, we can generate the laser which has different sequential lasing from that observed in Fig. 2.
The close-up view spectra for the three distinct nanowire lasing groups of the hybrid laser [the same structure shown in Fig. 1(e)] are shown in Fig. 3(a) , in which multimode can be observed. The mode spacings measured from the lasing groups originated from ZnO nanowire, CdS nanowire and CdSe nanowire are 0.17 nm, 0.8 nm, and 0.85 nm, in good agreement with the length of three nanowires . The linewidths of the three lasing groups are 0.7 nm, 0.6 nm, 0.57 nm, corresponding to the Q factors (quality factor: Q = λ/Δλ, where λ is the center of the wavelength and Δλ is the full width at half maximum of the cavity mode) of 558, 865, and 1303, respectively. Because of the small cavity size and the low reflection of semiconductor nanowires, the Q factor is usually not very high [11, 17]. The Q factor can be calculated by:19, 20]. Low reflection coefficient will reduce the Q factor. For example, the reflection coefficient of ZnO is 0.197 by FDTD simulation, and the corresponding Q factor is 880, which is close to the measured value. In such a low Q factor optical cavity, laser can also be observed due to the large confinement factor of the high refractive index of nanowire materials .
In order to use the multicolor laser as a practical light source, the threshold of the different lasing groups is crucial. Figure 3(b) show the peak intensity vs pump energy of the three lasing groups originating from three distinct nanowires. The measured thresholds of the three lasing groups from CdSe, CdS, ZnO nanowires are about 0.6 μJ, 1.1 μJ, and 1.3 μJ, respectively. Considering the factors that will affect the threshold such as coupling efficiency, taper loss and scattering loss etc, the actual pump energy is lower than the threshold given in Figure 3(b). In general, the threshold is proportional to exp(1/QΓ), where Q is the quality factor of the laser cavity and Γ is the confinement factor of a lasing mode, which is almost determined by the fraction guided power in the fundamental mode η [4, 21, 22]Eq. (1). It is noted that the threshold of the distinct nanowires could be modified by controlling the diameter and the length of the nanowire. Besides, coupling strength will also affect the lasing threshold in this experiment. Generally, the smaller diameter of the microfiber and the nanowire, the higher coupling efficiency between the microfiber and nanowire, which means small diameter is good to reduce the lasing threshold. However, the smaller diameter of the nanowire, the lower end facets reflection, which means small diameter will reduce the Q factor of the nanowire cavity and increase the lasing threshold. In this experiment, by selecting the proper length and diameter, we make these thresholds closely. Otherwise, the nanowire which has the low threshold will be over pumped. Thus, the features of the nanowire cavity will be changed, and the emission laser will be unstable or even vanish.
In addition, the multicolor laser is very sensitive to the position of the three nanowires (from input to output: CdSe, CdS, ZnO). If the small bandgap material is behind the large bandgap material, the laser emission of the large bandgap material will significantly decrease. The reason may be that the emission of the large bandgap nanowire will be absorbed by the small bandgap nanowire followed behind.
In conclusion, we have fabricated a compact multicolor laser based on the hybrid structure of three semiconductor nanowires and a microfiber. The red-green-ultraviolet three-color laser can be obtained at the same output port of a commercial fiber. Optical properties of the nanowire cavity, such as Q factor and threshold have been investigated. The simple approach, which relies upon bottom-up assembly of the semiconductor nanowire laser cavities and a microfiber in a compact system, has the potential to get compact white light laser from the hybrid structure by modifying the intensity of the three nanowire lasers or adopting new materials as laser gain mediums.
This work was supported by the National Natural Science Foundation of China (Grant No. 60706020) and the National Basic Research Program (973) of China (Grant No. 2007CB307003). The authors would like to thank Wenhai Zou, Guanzhong Wang, Huakang Yu, Zongyin Yang, Wei Li, and Guozhang Dai for their help in experiments and Wei Fang for helpful discussions.
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