Letter pubs.acs.org/NanoLett
A Nanowire-Based Plasmonic Quantum Dot Laser Jinfa Ho,*,† Jun Tatebayashi,† Sylvain Sergent,† Chee Fai Fong,‡ Yasutomo Ota,† Satoshi Iwamoto,†,‡ and Yasuhiko Arakawa*,†,‡ †
Institute for Nano Quantum Information Electronics and ‡Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan S Supporting Information *
ABSTRACT: Quantum dots enable strong carrier confinement and exhibit a delta-function like density of states, offering significant improvements to laser performance and hightemperature stability when used as a gain medium. However, quantum dot lasers have been limited to photonic cavities that are diffraction-limited and further miniaturization to meet the demands of nanophotonic-electronic integration applications is challenging based on existing designs. Here we introduce the first quantum dot-based plasmonic laser to reduce the crosssectional area of nanowire quantum dot lasers below the cutoff limit of photonic modes while maintaining the length in the order of the lasing wavelength. Metal organic chemical vapor deposition grown GaAs−AlGaAs core−shell nanowires containing InGaAs quantum dot stacks are placed directly on a silver film, and lasing was observed from single nanowires originating from the InGaAs quantum dot emission into the low-loss higher order plasmonic mode. Lasing threshold pump fluences as low as ∼120 μJ/cm2 was observed at 7 K, and lasing was observed up to 125 K. Temperature stability from the quantum dot gain, leading to a high characteristic temperature was demonstrated. These results indicate that high-performance, miniaturized quantum dot lasers can be realized with plasmonics. KEYWORDS: Nanowire, plasmonic laser, quantum dots, near-infrared, temperature stability
Q
nanowires,17−22 metal−insulator−metal waveguides,23 nanopans,24 nanopatches,25 and metallic bowtie arrays,26 incorporating quantum dots as a gain medium in plasmonic lasers has yet to be achieved. This can be attributed to difficulties in fabricating quantum dots with homogeneous sizes to provide sufficient gain to overcome the characteristically large plasmonic losses. We demonstrate here a plasmonic quantum dot laser based on the widely studied nanowire structure. In0.22Ga0.78As quantum dots are embedded in the nanowire core region and are separated by GaAs spacer layers, as shown in Figure 1a,b. The size variation between the dots is suppressed by using a diffusion model to account for the different growth rates of the dots along the nanowire and tuning the growth duration of each quantum dot layer to ensure homogeneity.27 Conventional plasmonic nanowire lasers consist of a semiconductor nanowire separated from a silver substrate by a thin dielectric spacer layer. The hybrid waveguide-plasmonic mode in such structures alleviate losses by reducing mode penetration into the metallic region, but mode overlap with the nanowire is poor. To enable better mode overlap between the quantum dots and the plasmonic mode, we disperse the nanowires directly on a silver thin film.22 While the fundamental
uantum dots are zero-dimensional structures comprising of a low bandgap semiconductor embedded in a larger bandgap material. Electrons and holes are strongly confined spatially in all three directions, resulting in discretized energy levels and a delta function-like density of states. When used as an active medium in lasers, quantum dots offer the potential for high differential gain, low-lasing threshold, and temperature insensitivity1,2 that are expected to surpass other low-dimensional structures such as quantum wells.3,4 Progress in fabrication technologies using molecular beam epitaxy5,6 and metal organic chemical vapor deposition (MOCVD)7,8 has enabled the growth of III−V semiconductor quantum dots with high densities and also the stacking of quantum dot layers for increased gain for laser applications. Such growth technologies have enabled the demonstration of quantum dot lasers with sizes that are rapidly decreasing. However, quantum dot lasers to date are based on photonic cavities such as planar waveguides,9−12 photonic crystals,13−15 and nanowires,16 and further miniaturization based on current designs is thus subject to the diffraction limit. This is insufficient to meet the demands of applications such as high density nanophotonic−electronic integration, where the realization of a compact quantum dot laser source at scales comparable to electrical components remains a major challenge and an area of active research. A promising approach to further miniaturize quantum dot lasers is to use surface plasmons that can be excited in metallic structures. Despite the wide variety of plasmonic nanolasers that have been demonstrated using geometries such as © 2016 American Chemical Society
Received: February 18, 2016 Revised: March 28, 2016 Published: March 30, 2016 2845
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Figure 1. (a) Schematic diagram of the as-grown nanowire sample. The quantum dots have a diameter of ∼40 nm and thicknesses that can be tuned by their growth duration, separated by GaAs spacer layers to give a dot density of ∼1 per 40 nm. The nanowires have an overall diameter of ∼180 nm. (b) Cross-section TEM (left) and close-up STEM (right) images of the nanowire. The location of the InGaAs quantum dots can be inferred from the regions dark/bright contrast indicating the presence of strain fields. Scale bars are 100 and 50 nm for the left and right images, respectively. (c) SEM image of as-grown nanowire samples. Scale bar corresponds to 2 μm. (d) SEM image of a nanowire after dispersion on the silver/silicon (111) substrate. The lengths of individual nanowires varies greatly due to the ultrasonication breaking process. Scale bar corresponds to 500 nm.
In0.22Ga0.78As quantum dots with thicknesses of ∼3 nm (see Supporting Information). Because of carrier capture in the quantum dots, emission from bulk GaAs is barely observed in the samples. Two Fabry−Pérot mode peaks are visible in the spectra at ∼845 and 870 nm. As the pump power increases, the spontaneous emission background broadens due to band filling. At a pump fluence of ∼120 μJ/cm2 per pulse, the peak at 845 nm increases sharply in intensity to several orders of magnitude larger than the spontaneous emission background. Figure 2c shows the integrated output power of the peak at 845 nm (blue dots) and the spontaneous emission background (red dots) as a function of the pump power, obtained by fitting the spectra with Gaussian curves (see Supporting Information). While the peak at 870 nm shows a linear power dependence, an S-shaped power dependence characteristic of lasing behavior is obtained for the peak at 845 nm, which is close to the center of the emission bandwidth. The concurrent onset of the fwhm line width narrowing plateau and superlinear kink at ∼120 μJ/cm2 per pulse, together with spontaneous emission clamping, are indicators of the lasing threshold. Compared with similar nanowire plasmonic lasers with bulk GaAs as the gain medium that lase at ∼800 nm,22 the lasing wavelength in this sample is longer due to the use of InGaAs quantum dots as the gain medium. The insets in Figure 2c show microscope images of the nanowire emission in the spontaneous emission, stimulated emission, and lasing regimes. In the spontaneous emission regime, emission is only observed from the nanowire end facets, which is in contrast to reports on photonic nanowires that show strong emission from the entire nanowire bulk.16,30,31 This provides evidence that the observed cavity modes are plasmonic in origin, as unlike photonic modes plasmonic modes are tightly confined to the metal−dielectric interface until they are
plasmonic mode becomes too lossy for lasing to be achieved, the higher order plasmonic mode has lower losses that can be compensated by gain from the quantum dots embedded in the nanowire core (see Supporting Information). After the growth of the nanowire core, GaAs is overgrown in the lateral direction and coated with an Al0.1Ga0.9As shell layer to prevent nonradiative recombination from the surface states. The entire nanowire is finally coated with a ∼ 10 nm GaAs layer to prevent oxidation of the aluminum content in the AlGaAs shell layer. The silver film is deposited on a silicon (111) substrate by electron beam evaporation, followed by an annealing process to give an ordered silver (111) film with a surface roughness of ∼0.4 nm. Detailed fabrication procedures of the nanowire and silver film are reported in refs 16 and 22, respectively. The as-grown nanowires (Figure 1c) are immersed in an isopropyl alcohol (IPA) solution and broken by ultrasonication and are dispersed onto the preheated (100 °C) silver/silicon substrate via drop casting. Figure 1d shows the SEM image of a dispersed nanowire, showing good contact between the nanowire and the silver film. The nanowires are well dispersed, allowing for the photoluminescence of each individual nanowire to be measured separately. The nanowire samples were optically excited by a pulsed laser focused to a 2 μm diameter spot at 785 nm with 50 ps pulses at a repetition rate of 5 MHz (duty cycle ∼2.5 × 10−4) (Figure 2a). The low-temperature (7 K) power-dependent spectra of a lasing sample with a diameter of ∼180 nm and length of ∼2.3 μm are shown in Figure 2b. At low pump fluences, a broad Gaussian-like spontaneous emission background centered at ∼850 nm is observed. This is longer in wavelength than the expected GaAs bandgap at low temperatures of 819 nm28,29 and originates from the emission from 2846
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Figure 2. (a) Schematic diagram of an optically pumped nanowire lying on a silver/silicon (111) substrate. A 785 nm pulsed laser source (red arrow) was used as the pump. (b) Power-dependent spectra (plotted on a logarithmic scale) of a lasing sample (d ∼ 180 nm, L ∼ 2.3 μm) at 10 K, showing the transition from spontaneous emission (74 μJ/cm2) to stimulated emission (155 μJ/cm2) and lasing (214 μJ/cm2). Broad Gaussian-like spontaneous emission is observed at low pump fluences, while a peak at ∼845 nm appears and its intensity increases rapidly to several orders of magnitude larger than the background above the lasing threshold. The sharp peak at ∼870 nm is a Fabry−Pérot cavity mode and is nonlasing. (c) Integrated output power of the lasing peak at ∼845 nm (blue dots) and the broad background Gaussian emission extending from ∼820 to 880 nm (red dots), shown on a double logarithmic scale. While the background emission is clamped beyond the lasing threshold of ∼120 μJ/cm2, the lasing peak shows an S-shaped power dependence with a superlinear increase in output power in the stimulated emission regime highlighted in purple. The corresponding line width narrowing behavior (open blue circles) is a signature of lasing. Microscope images of the nanowire emission in the spontaneous emission regime (top), stimulated emission regime (middle), and lasing regime (bottom) are shown, and interference fringes can be observed above threshold, which is an indication of coherence in the nanowire emission. Above threshold, the charge-coupled device camera is saturated by the nanowire bulk emission and the interference pattern along the nanowire body cannot be resolved despite the length of the nanowire being above the diffraction limit. All microscope images were obtained after filtering out the excitation laser light with a 800 nm long pass filter.
scattered at the end facets of the nanowires.22 In the stimulated emission and lasing regimes, interference fringes can be observed, which is indicative of coherent radiation generated in the nanowire cavity.31 The lasing peak output power can be fitted to a rate equation model (see Supporting Information), and a surprisingly small spontaneous emission coupling β factor of 0.005 was obtained. This is 2 orders of magnitude smaller than that reported in previous demonstrations of plasmonic nanowire lasers with β factors typically in the order of ∼0.1.17−22 A combination of factors may explain this difference. First, the quantum dots are located away from the high electric field intensity of the higher order plasmonic mode (near the silver/nanowire interface) and are unlikely to benefit from high Purcell factors. Second, finite-difference time-domain (FDTD) simulations reveal that only ∼50% of the quantum dots are located in the antinodes of the higher order plasmonic mode in the longitudinal direction (see Supporting Information). The quantum dots also act as carrier traps along the nanowire length, preventing carrier diffusion from regions of slow carrier recombination near the longitudinal nodes of the lasing mode
where the Purcell factor is small. Finally, the fabrication process of the quantum dot-in-nanowire structure may introduce additional nonradiative recombination sites at the interface between the InGaAs quantum dots and the GaAs spacer layers, resulting in a steeper increase of the laser output power in the stimulated emission regime. As our rate equation model does not account for such nonradiative recombinations, it tends to underestimate the β factor. To confirm the nature of the lasing mode, microphotoluminescence experiments were performed to eliminate the possibility of photonic mode lasing and to differentiate between the fundamental and higher order plasmonic modes. The potential for leaky photonic modes that may contribute to lasing below photonic mode cutoff32 can be eliminated based on polarization and mode spacing measurements (see Supporting Information). The typical power-dependent spectra of nanowires dispersed on a SiO2 substrate, excited under the same conditions as the lasing sample, are shown in Figure 3a. Such samples do not support plasmonic modes and the diameters are below the photonic mode cutoff of ∼200 nm.33,34 2847
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Figure 3. (a) Power-dependent spectra of a single nanowire (d ∼ 180 nm) dispersed on SiO2 and Gaussian fits to the experimental data. No mode structure is observed as the diameter is below the photonic mode cutoff. (b) Spectra of nanowires with diameters of 150 nm (blue, magnified 10 times for clarity) and 180 nm (red) dispersed on silver/silicon (111) substrates at a pump power of 150 μJ/cm2. Corresponding microscope images of each nanowire and their polarization dependence are shown as insets. The fundamental plasmonic mode exhibits broad Fabry−Pérot peaks that are polarized parallel to the nanowire axis, while the higher order plasmonic mode exhibits sharp peaks and is polarized perpendicular to the nanowire axis. (c) Experimentally observed mode spacings of the fundamental (blue triangles) and the higher order plasmonic mode (red circles), and the theoretically calculated mode spacings (solid lines). The modes are identified from their polarization dependence. The group index of the fundamental plasmonic mode (blue line) is ∼7.3, while that of the higher order plasmonic mode (red line) is ∼5.6. Insets display the cross-sectional electric-field distributions of the higher order (top left) and fundamental (bottom right) plasmonic modes.
Figure 4. (a) Integrated output power for the lasing peak at 7 and 125 K (solid circles) and rate equation fitting (solid lines) for a nanowire with length of 1.2 μm and diameter of 180 nm. The respective line widths are shown by the open circles. (b) The threshold pulse fluence as a function of the temperature for the quantum dot nanowire laser (black circles) and the bulk GaAs nanowire laser (red circles). The quantum dot nanowire laser has a lower temperature sensitivity and a T0 of ∼300 ± 50 K, while the bulk GaAs nanowire laser has a T0 of ∼200 ± 20 K. Inset: Normalized temperature-dependent PL spectra of the quantum dot nanowire laser at a pump power of 500 μJ/cm2. The lasing peak redshifts from 882 nm at 7 K to 885 nm at 125 K, and originates from the higher order plasmonic mode based on its polarization. A weaker peak originating from the fundamental plasmonic mode is also observed at ∼877 nm.
emission of each individual quantum dot with the large number of quantum dots in the nanowire core. In order to confirm that the emission originates from quantum dots rather than quantum well or bulk structures, photoluminescence experiments were performed on nanowires with a single quantum dot embedded in the core, where sharp delta function-like spectra
Broad Gaussian-like emission is observed for all pump powers, which confirms the absence of cavity modes. The fwhm of the nanowire emission is ∼28 nm, which is much larger than bulk GaAs emission fwhm of ∼10 nm. This is attributed to variations in the quantum dot size within each individual nanowire, resulting in spectral broadening.16 It is difficult to resolve the 2848
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basis of rate equation fitting, the β factors are in the order of 0.01. The larger β factor compared to the previous sample could be due to sample-to-sample fluctuations in the quantum dot-to-silver substrate distance. Specifically, the β factor decreases from ∼0.035 at 7 K to ∼0.02 at 125 K, which can be accounted for by the increase in nonradiative recombination at higher temperatures (see Supporting Information). Finally, a key feature of quantum dot lasers is temperature stability that manifests as a high characteristic temperature (T0). The threshold pump pulse fluence increases by only a factor of ∼1.5 times as the temperature is increased from 7 to 125 K as shown in Figure 4b and we obtain a high T0 of ∼300 ± 50 K for this quantum dot laser, significantly higher compared to a T0 of ∼200 ± 20 K for a bulk GaAs plasmonic nanowire laser that operates in the same temperature range.22 It should be noted that this was based on lasing up to 125 K and may not be directly comparable to the T0 measured for lasers that function up to room temperature as the lasing threshold could increase significantly at higher temperatures. In summary, we have demonstrated a plasmonic laser based on quantum dots as the gain medium below the photonic mode cutoff diameter, lasing in the near-infrared region of ∼840−890 nm. A remarkably low lasing threshold pump fluence of ∼120 μJ/cm2 was achieved at 7 K due to significant mode overlap with the quantum dot region and relatively low losses of the higher order plasmonic mode. Despite gain limitations due to the finite number of quantum dots that can be stacked in the nanowire core, lasing was observed up to 125 K. Further improvements can be expected from better control over the quantum dot size, which can provide more gain in the wavelengths of interest and also the use of sidewall quantum dots that can be placed in the regions of high electric field intensity and benefit from enhanced Purcell and spontaneous emission coupling factors. This is the first demonstration of a quantum dot-based plasmonic laser and will be useful for the future development of plasmonic-based quantum dot devices.
and single photon emission were observed. Furthermore, the peak emission wavelength of the samples can be tuned by varying the quantum dot heights and are consistent with the calculated emission wavelengths (see Supporting Information). In contrast, nanowires dispersed on the silver/silicon (111) substrate exhibit clear, regularly spaced Fabry−Pérot modes as shown in Figure 3b. Depending on the diameter of the nanowire, two distinct types of Fabry−Pérot modes arising from surface plasmons can be observed. For nanowires with diameters larger than 150 nm, sharp peaks polarized perpendicular to the nanowire axis originating from the higher order plasmonic mode are observed. On the other hand, nanowires with diameters below 150 nm display broad peaks that are polarized parallel to the nanowire long axis. These samples support only the fundamental plasmonic mode that is strongly confined to the nanowire/silver interface. As the nanowires are in direct contact with silver, the fundamental mode exhibits large propagation losses in our samples, and only broad peaks that are indicative of a low cavity quality factor are observed. The observed polarization dependences of both the fundamental and higher order modes match expected polarization from near-field FDTD simulations (see Supporting Information). The mode spacings also exhibit a 1/L dependence (see Figure 3c) and show a good agreement with the theoretical mode spacing calculated by −1 ⎛ λ 2 ⎞⎛ dn ⎞ Δλ = ⎜ ⎟⎜neff − λ eff ⎟ dλ ⎠ ⎝ 2L ⎠⎝
(1)
where neff is the effective mode index of a longitudinal guided mode in a NW with length L, and (dneff/dλ) is the dispersion relation of the mode. On the basis of the polarization properties and mode spacing of the lasing device, we conclude that the lasing mode is the higher order plasmonic mode. Although the fundamental mode is supported for all nanowire diameters, it is not observed in samples with larger diameters. One possible reason for this could be the presence of other higher order mode peaks in the nanowire, leading to mode competition that results in the suppression of the lossy fundamental mode. To demonstrate the tunability of the quantum dot emission by changing its thickness, we increased the quantum dot thickness to 5 nm and the lasing wavelength was red shifted to ∼882 nm at 7 K. The higher-temperature lasing characteristics of this sample was investigated. The inset of Figure 4b shows the temperature-dependent emission spectra of the sample at 500 μJ/cm2, and the lasing wavelength was seen to further redshift to ∼885 nm at 125 K. On the basis of polarization measurements, the lasing peak originates from the higher order plasmonic mode, while the weak peak at ∼877 nm originates from the fundamental plasmonic mode. The short nanowire length (l ∼ 1.2 μm, almost half the length of the previous sample) results in large mode spacings for both the fundamental and higher order mode, so only one peak arising from each mode can be observed in the spectra (Figure 4b, inset). The reduced mode competition from the absence of other higher order mode peaks in the emission spectrum allows for the observation of the fundamental mode despite the large nanowire diameter. The integrated output power of the lasing peak is plotted in Figure 4a for 7 and 125 K. The lasing threshold for this sample increases from ∼290 μJ/cm2 at 7 K to ∼430 μJ/cm2 at 125 K. The higher threshold at 7 K compared to the previous nanowire sample can be attributed to reduced differential gain due to the shorter nanowire length. On the
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00706. Comparison between the fundamental plasmonic, higher order plasmonic and plasmon-waveguide hybrid modes, absence of lasing leaky photonic modes, FDTD simulations of near-field radiation patterns, longitudinal mode distribution, rate equation fitting, and evidence for quantum dot emission. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the KAKENHI Grant-in-Aid for specially promoted Research (15H05700) and the Project for Developing Innovation Systems of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. 2849
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