±RS 目 477.9nm 4523nm 4196 383.1nm Pump power density (KW cm-3 360380400420440460480500 Wavelength (nm) Figure 3 MQW nanowire photoluminescence. a, Photoluminescence images(false colour) recorded from GaN/no. s Gao. gsN (left) and GaNino.x Gao. N (right MQw nanowire structures. Scale bars are 5 um. b, Normalized spectra collected from three representative 26MQW nanowire structures with increasing In composition; excitation power density 150kW cm-2. c, Photoluminescence spectra of a 26MQw nanowire recorded at excitation power densities of 250 and 1, 300 kW cm-2, respectively Spectra are offset for darity Inset: LogH-og plot of output power versus pump power density. Red open circles are experimental data points and the dashed line is the fit calculated with the rate equations. d, Normalized photoluminescence spectra collected from four representative 26MQw nanowire structures with increasing In composition pumped at -250 kWcm-2and-700 kW cm-2, respectively. Spectra are offset for clarity. In addition, we have investigated power-dependent excitation In GaN-based material. Overall, these data demonstrated that and resulting photoluminescence from MQW nanowire structures our concept and realization of MQW nanowire heterostructures as a function of In% in the Ingan quantum well. Significantly, yield unprecedented tunability for nanowire-based lasers at normalized photoluminescence spectra acquired from four room temperature. representative MQW nanowire structures with increasing In%, Given the unique geometry of our MQW nanowire lasers low and above thresholds( Fig. 3d), show lasing behaviour (Fig 4a), where the MQW gain medium is separated from the with peak maxima at 383, 420, 452 and 478 nm, spanning a nanowire core in contrast to previous homogeneous nar substantial portion of the ultraviolet to visible region of the systems'-, we have investigated how the number of quantum wells electromagnetic spectrum. The threshold power densities for in the gain medium affects the laser threshold For conventional these four MQW nanowire lasers are all below 700 kWcm-. rate equation analysis, the threshold can be approximated as Finally, preliminary studies of a higher In composition 26MQW nanowire structure( Supplementary Information, Fig. S4)yielded LN.,Nw V o/or N n go er-dependent photoluminescence spectra suggestive of lasing ni at 494 nm. Although further studies will be needed to characterize where Lth is the threshold, Nw is the number tum wells fully this MQW nanowire composition regime, this is to the is the active volume per well, B is the bin best of our knowledge the longest length lasing for an coefficient, go is the differential gain, Nr is nsparent carrier naturematerialsIVol7iSeptEmbeR2008Iwww.nature.com/naturematerials 2008 Macmillan Publishers Limited. All rights reservedLETTERS 383.1 nm 419.6 nm 452.3 nm 477.9 nm Normalized intensity (a.u.) 360 380 400 420 440 460 480 500 Wavelength (nm) Normalized intensity (a.u.) 300 400 500 600 700 Wavelength (nm) Intensity (a.u.) 430 440 450 460 Wavelength (nm) 2 3 4 5 1,000 Peak output intensity (a.u.) Pump power density (kW cm–2) 80 60 40 20 0 100 80 60 40 20 0 120 101 102 103 104 6 2 3 4 56 a b c d Figure 3 MQW nanowire photoluminescence. a, Photoluminescence images (false colour) recorded from GaN/In0.05Ga0.95N (left) and GaN/In0.23Ga0.77N (right) MQW nanowire structures. Scale bars are 5 µm. b, Normalized spectra collected from three representative 26MQW nanowire structures with increasing In composition; excitation power density ∼150 kW cm−2 . c, Photoluminescence spectra of a 26MQW nanowire recorded at excitation power densities of 250 and 1,300 kW cm−2 , respectively. Spectra are offset for clarity. Inset: Log–log plot of output power versus pump power density. Red open circles are experimental data points and the dashed line is the fit calculated with the rate equations. d, Normalized photoluminescence spectra collected from four representative 26MQW nanowire structures with increasing In composition pumped at ∼250 kW cm−2 and ∼700 kW cm−2 , respectively. Spectra are offset for clarity. In addition, we have investigated power-dependent excitation and resulting photoluminescence from MQW nanowire structures as a function of In% in the InGaN quantum well. Significantly, normalized photoluminescence spectra acquired from four representative MQW nanowire structures with increasing In%, below and above thresholds (Fig. 3d), show lasing behaviour with peak maxima at 383, 420, 452 and 478 nm, spanning a substantial portion of the ultraviolet to visible region of the electromagnetic spectrum. The threshold power densities for these four MQW nanowire lasers are all below 700 kW cm−2 . Finally, preliminary studies of a higher In composition 26MQW nanowire structure (Supplementary Information, Fig. S4) yielded power-dependent photoluminescence spectra suggestive of lasing at 494 nm. Although further studies will be needed to characterize fully this MQW nanowire composition regime, this is to the best of our knowledge the longest wavelength lasing for an InGaN-based material30. Overall, these data demonstrated that our concept and realization of MQW nanowire heterostructures yield unprecedented tunability for nanowire-based lasers at room temperature. Given the unique geometry of our MQW nanowire lasers (Fig. 4a), where the MQW gain medium is separated from the nanowire core in contrast to previous homogeneous nanowire systems7–13, we have investigated how the number of quantum wells in the gain medium affects the laser threshold. For conventional rate equation analysis, the threshold can be approximated as17: L (Nw) th ∼ qNwV1BN2 tr ηi e 2ω/Qvg NwΓ1 g0 , where Lth is the threshold, Nw is the number of quantum wells, V1 is the active volume per well, B is the bimolecular recombination coefficient, g0 is the differential gain, Ntr is the transparent carrier 704 nature materials VOL 7 SEPTEMBER 2008 www.nature.com/naturematerials © 2008 Macmillan Publishers Limited. All rights reserved