On-Chip Monolithically Fabricated Plasmonic-Waveguide Nanolaser

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On-chip monolithically fabricated plasmonic-waveguide nanolaser Ya-Lun Ho, J. Kenji Clark, Kamal Syazwan, and Jean-Jacques Delaunay Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03531 • Publication Date (Web): 13 Nov 2018 Downloaded from http://pubs.acs.org on November 13, 2018

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Nano Letters

On-chip monolithically fabricated plasmonicwaveguide nanolaser Ya-Lun Ho,† J. Kenji Clark,† A. Syazwan A. Kamal, and Jean-Jacques Delaunay* School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan * Address correspondence to [email protected]

ABSTRACT

Plasmonic-waveguide lasers, which exhibit sub-diffraction limit lasing and light propagation, are promising for the next-generation of nanophotonic devices in computation, communication, and biosensing. Plasmonic lasers supporting waveguide modes are often based on nanowires grown with bottom-up techniques that need to be transferred and aligned for use in optical circuits. Here, we demonstrate a monolithically fabricated ZnO/Al plasmonic-waveguide nanolaser compatible with the fabrication requirements of on-chip circuits. The nanolaser is designed with a plasmonic metal layer on the top of the laser cavity only, providing highly efficient energy transfer between photons, excitons and plasmons, and achieving lasing in the ultraviolet region up to 330 K with a low threshold intensity (0.20 mJ/cm2 at room temperature). This work demonstrates the realization of a plasmonic-waveguide nanolaser without the need for transfer and positioning steps, which is the key for on-chip integration of nanophotonic devices.

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KEYWORDS: Nanolaser, plasmonic laser, plasmonic waveguide, ZnO, aluminum.


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TEXT The theoretical prediction of surface plasmon lasers, known as spasers,1 in the early 2000’s has spurred research into nanoscale light sources smaller than the diffraction limit.2−5 The nanoscale size of plasmonic nanolasers makes it possible to integrate them directly on chip with modern electronic devices in integrated optical circuits in order to achieve ultrafast on-chip optical data processing. In a plasmonic nanolaser, surface plasmons confine light to a deep subwavelength area at the metal-gain material interface, enhancing light-matter interactions and supporting lasing modes much smaller than the optical wavelength. The first demonstration of a plasmonic nanolaser was reported using Au/silica/dye core-shell nanoparticles for omnidirectional lasing.6 Further demonstrations of nanolasers without directional emission were made following this.7−10 In order to achieve directional emission, a plasmonic laser based on nanoarrays are demonstrated,5,11-15 showing the out-of-plane directional beam emission. For plasmonic nanolasers to be viable in integrated devices, the lasing of a subwavelength waveguide mode with low loss is desired as the key towards the integrated devices such as integrated optical circuits. A nanowire-based plasmonic laser, which supports a waveguide mode propagating along the nanowire axis, was reported.16 Semiconductor nanowires were first synthesized and then transferred to a metal substrate coated with a dielectric layer. This structure supports a subwavelength hybrid photonic-plasmonic waveguide mode17,18 for nanolasing. Nanowire plasmonic lasers have attracted a high level of interest,19,20 and in recent years nanowire plasmonic lasers operating at a wide variety of wavelengths from the nearinfrared,21,22 the visible,23,24 to the ultraviolet25−28 have been shown. In addition to the large number of nanowire plasmonic lasers, lasing of a waveguide mode from a heterostructure was reported.29 In all of these plasmonic-waveguide nanolasers, the gain material must be fabricated


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on a separate substrate and then transferred to the metallic layer, making monolithic fabrication impossible. Since plasmonic lasing modes are highly confined at the metal-gain material interface, minimizing ohmic loss in the metal is critical. For this, a deposition technique that produces a smooth film must be used, and the metal must be carefully selected according to the lasing wavelength region desired.23,27 By taking these two factors into account, nanowire-based lasing of a pure plasmonic waveguide mode without the need for hybrid waveguide modes was achieved. 27 This additionally allowed for smaller mode sizes, thus enabling nanolaser sizes even smaller than the hybrid mode nanolasers. The practical application of nanolasers in integrated optical circuits requires that they can be integrated on chip with waveguides. For this to be possible, a fabrication strategy that enables nanolasers to be fabricated with a well-controlled shape, position, and alignment must be developed. The fabrication of plasmonic nanowire lasers is based on a bottom-up synthesis that is performed on a separate substrate from the final device. The nanowires must then be transferred to the device substrate and aligned with the other optical components (e.g., waveguides and detectors). These steps are difficult and reproducibility is low, which limits the practical applicability of nanowire lasers. Direct top-down fabrication of a plasmonic-waveguide lasers on a chip without the need for a subsequent transfer and alignment process has not been widely pursued. Metal-coated cavities fabricated using a top-down technique30 forming MIM heterostructures with an InGaAsP layer on an InP substrate were shown to support a plasmonic waveguide mode.31,32 In this design, the cavity was fully covered with metal including the two ends of the cavity, and the total height of the structure was in the micrometer scale. In a different approach, a nanogap structure bounded by metallic gratings with a total size of several


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micrometers was fabricated using a top-down technique33,34 and later used for plasmonic lasing.35 Despite these significant advances, the realization of plasmonic-waveguide nanolasers with a subwavelength device size fabricated directly on chip without any transfer process remains a challenge. To develop an appropriate nanolaser design that can be fabricated by topdown techniques is the key for the plasmonic-waveguide nanolaser to be integrated on-chip together with optical circuits. Here, we report the experimental demonstration of a ZnO/Al plasmonic-waveguide nanolaser monolithically fabricated on chip without the need for any transfer or manipulation steps, thus realizing a design suitable for integrated optical circuits. The plasmonic-waveguide nanolaser design, as shown in Figure 1a, is composed of a gain material cavity coated on top by a plasmonically active metal layer, a design which differs significantly from reported nanowire and nanocavity lasers. The design with the metal on the top of the gain medium offers four key advantages: 1. the absorption of the pump light is highly concentrated in the gain medium and has good overlap with the plasmonic-waveguide mode; 2. the scattering loss at the metal/gain material interface is reduced and the effective area of plasmonic-waveguide mode in the gain material is maximized; 3. the metal layer acts as a mask for the top-down nanofabrication process, preventing damage to the metal/gain medium interface during fabrication; and 4. the open-ended cavity design allows the laser light to be coupled from the ends to a waveguide fabricated using top-down techniques on the same chip. The nanolaser design enables efficient energy transfer both between the photons of the incident pump light and excitons in the gain medium, and between excitons in the gain medium and plasmons of the plasmonic-waveguide mode, making efficient plasmonic-waveguide lasing possible. The ZnO/Al nanolaser shows single-mode lasing in the ultraviolet region from 147 to 330 K with a low threshold intensity


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(0.20 mJ/cm2 at room temperature) and a lasing mode cross-sectional size in the sub-wavelength regime. Results Design of monolithically fabricated plasmonic-waveguide nanolasers A schematic diagram of the monolithically fabricated plasmonic-waveguide nanolasers is shown in Figure 1a. The structure consists of ZnO cavities with Al top layers deposited on a sapphire substrate. In contrast to nanolaser devices based on semiconductor nanowires, which must be transferred from their synthesis environment to the device substrate, the proposed ZnO cavities are fabricated directly on the device substrate. The use of electron-beam lithography and Arbased inductively coupled plasma (ICP) ion etching in the top-down nanofabrication process are optimized with the design of top plasmonic layer (Al), which also functions as an appropriate mask for the ICP ion etching with a high etching selectivity between gain material and metal (see Methods and Supporting Information). The design of top metal layer and the fabrication strategy allows for accurate control of the dimensions of a single nanolaser cavity, and its alignment on the substrate. The entire process is compatible with large-scale integrated optical circuit fabrication and is free of any transfer process. The scanning electron microscopy image of Figure 1b shows the nanolaser cavity, and the insets show two cavities with different widths. The optical microscopy image of Figure 1c shows an array of the nanolasers illustrating our ability to fabricate cavities with specific lengths, widths, and positions over a large area. Microscopy images illustrating the lasing obtained with cavity lengths of 30 and 20 μm are given in Figure 1d. Because of the need for a high-quality gain medium, the metal (Al) layer is deposited on top of a predeposited gain medium (ZnO) thin-film. The metal layer acts as both a mask for the


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etching process that patterns the cavities and acts as the plasmonic metal layer in the final plasmonic structure. This design differs from existing plasmonic lasers where the metal layer lies under the laser cavity, as in the majority of nanowire-based nanolasers, or covers the whole cavity, as in the nanocavity-based and MIM nanolasers. In this design, the cavity ends are open and the surface of the substrate remains free of any layer. This allows additional optical components, such as waveguides, to be integrated on the surface. Al is selected as the plasmonic metal material due to its low ohmic loss in the region of lasing.27,36 The direct deposition of the metal layer on the gain medium means that a high-quality interface between the metal layer and the gain medium is readily achieved, thus reducing the scattering loss at the interface in comparison to devices relying on nanowires transferred onto a metal surface.25 Note that the plasmonic interface did not damage during the nanofabrication process due to the top metal layer design. The dark-field microscopy image (Inset of Figure 1c) shows light scattering from the Al top layer that remains after the etching process. The electric field distribution for the guided mode of the ZnO/Al nanolaser cavity at the lasing wavelength λ = 372 nm is shown in Figure 1e (left). The cavity width w is 80 nm, ZnO height h is 120 nm, and the Al layer thickness t is 30 nm. The electric field distribution reveals that the plasmonic-waveguide mode is supported (Figure 1e, left). The electric field distribution of the fundamental photonic mode (HE11) is shown for w = 140 nm in the Supporting Information. This is in agreement with the predicted cutoff wavelength of the HE11 mode occurring at w = 125 nm (see Discussion section). The plasmonic mode is confined within the gain medium near the ZnO/Al interface and has a deep-subwavelength mode area of λ2/86. The distribution of the local Purcell factor shows a maximum value of 29 (Supporting Information), suggesting the plasmonic lasing mode and the gain medium are well coupled. Furthermore, the


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monolithically fabricated nanolaser has a planar geometry with a large contact area between the metal and the gain medium, resulting in a large overlap between the plasmonic mode and the gain medium.25 Comparing the electric field distributions of the mode of the monolithically fabricated nanolaser structure (Figure 1e, left) and the mode of a plasmonic nanolaser of a ZnO nanowire, having the same cross-sectional area, on an Al surface (Figure 1e, right) reveals that the plasmonic mode only partially overlaps with the gain material in the case of the nanowire structure. The simulated absorption distributions of 343-nm pump light applied from the top and bottom (through the sapphire substrate) of the cavity are shown in Figure 1f (left). When the light pumps the cavity from the top, it is strongly absorbed in the Al layer and on the sides of the ZnO cavity. However, when the light pumps the cavity from the bottom, the maximum of the absorption lies close to the ZnO/Al interface in the ZnO, such that the absorption in the Al layer is low and hence thermal loss is low. Note that the absorption distribution in the cavity when the light pumps the nanolaser from the bottom has good overlap with the electric field distribution of the plasmonic-waveguide mode of Figure 1e (left), which is mainly confined in the gain medium. This means that exciton generation will occur primarily in an area where the plasmonic mode has a high intensity, allowing for the efficient transfer of energy from excitons to plasmons by stimulated plasmon emission. High-efficiency stimulated emission of plasmons into the plasmonic-waveguide mode will result in low-threshold plasmonic lasing.25,29 Compared to the monolithically fabricated nanolaser structure, the nanowire-on-metal structure has less overlap between the electric-field distribution of the plasmonic mode (Figure 1e, right), which has strong enhancement at the bottom of the cavity, and the absorption distribution of the pump light (Figure 1f, right), which is significant in the center of the cavity. Additionally, the efficient


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pumping from the backside of the substrate offers protection against excessive absorption in and consecutive heating of the metal that can damage the nanolaser structure. Photonic and plasmonic lasing characterization of the monolithically fabricated nanolaser The plasmonic-waveguide nanolaser is pumped by a femtosecond laser at 343 nm (Methods and Supporting Information). The emission measurements, shown in Figure 2, are obtained for cavity widths of 140 nm (black) and 100 nm (violet) at an operating temperature of 147 K. In Figure 2a, the emission spectrum of the cavity with a width of w = 140 nm shows spontaneous emission for a pump energy density below the lasing threshold, and then single-mode lasing above the lasing threshold. The variation in the emission intensity (Eout) with pump energy density (Pin) is shown as a log−log plot in Figure 2b. When the pump energy density reaches 0.04 mJ/cm2, the emission intensity increases nonlinearly. This non-linear region ranges from 0.04 to 0.06 mJ/cm2 and represents the threshold region between spontaneous emission and lasing from the nanolaser. The bandwidth of the emission peak for different pump energy densities is shown in the same figure, and a large variation in the bandwidth is seen over the threshold region. The polarization of the lasing spectrum is examined in Figure 2c, where the emission intensity for light polarized in the direction parallel (z-direction) and perpendicular (x-direction) to the cavity axis is shown. The polarization of the lasing emission is in agreement with the simulated electric field distributions (Figure. 2d) of the fundamental photonic mode (HE11), which shows a characteristic main component in the x-direction, indicative of photonic lasing. The spontaneous emission and single-mode lasing of the cavity with a width of w = 100 nm are shown in Figure 2e. The nonlinear threshold region of the emission intensity starts from 0.09 mJ/cm2, and extends to 0.12 mJ/cm2 as can be seen from the Eout−Pin plot and the corresponding bandwidth variation (Figure 2f). The polarization of the lasing is provided in Figure 2g. The simulated distribution of the x-


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and z- components of the electric field for the fundamental plasmonic-waveguide mode, which is the only mode found for a cavity width smaller than 120 nm (see details in Discussion), are shown in Figure 2h and indicate that the main electric field component is in the z-direction. The z-direction coincides with the observed polarization in the experiment (along with the cavity axis), thus confirming that plasmonic-waveguide lasing is obtained. The simulated electric field distributions of photonic and plasmonic-waveguide modes in xz plane are provided in the Supporting Information. Plasmonic lasing characterization of the nanolaser at high temperature Figure 3a shows the emission spectrum of the nanolaser with w = 100 nm (plasmonic-waveguide mode lasing) pumped at room temperature (293 K) for three different pump energy densities. Below the threshold pump energy density, spontaneous emission is observed, and above it a strong lasing peak is observed. The lasing peak at λ = 376.7 nm red-shifts from the lasing peak observed at 147 K. In Figure 3b, the emission spectrum of the nanolaser below and above the threshold for operating temperatures of 310, 320, and 330 K is shown. The same emission intensity scale is used to allow for comparison of the emission intensities. As the temperature increases, the lasing threshold increases (Pin = 0.35 mJ/cm2 at 330 K) and the lasing peak redshifts (λ = 382.0 nm at 330 K). The inset of Figure 3b shows the lasing peak for a pump energy density far above the threshold. Lasing of the nanolaser is obtained at 330 K, which demonstrates the possibility to operate the nanolaser device at high temperatures. The variation in the lasing peak wavelength of the nanolaser with operating temperature from 147 to 330 K is shown in Figure 3c. The inset of Figure 3c shows a clear red-shift of the lasing peak as the temperature increases from 165 to 235 K. The lasing wavelength of the nanolaser is found to be lower than


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that of exciton absorption, and higher than that of exciton-exciton emission and electron-hole plasma emission.27,37−40 Discussion The photonic lasing mode and plasmonic-waveguide lasing mode of the monolithically fabricated nanolasers are studied using the calculated effective index neff (Figure 4a) and the confinement factor (Figure 4b) as a function of the cavity width for a ZnO cavity height of 120 nm and an Al layer thickness of 30 nm. The electric field distributions of the three optical modes of the cavity with a width of w = 140 nm are shown in Figure 4c. The HE11 mode (black) shown in Figure 4a is the only photonic mode found for a cavity width smaller than 250 nm and has a cutoff width of w = 125 nm, where the effective index becomes as small as the refractive index of the sapphire substrate. Two plasmonic modes are observed in the investigated range of widths. The fundamental plasmonic mode (dark violet) shows no cutoff down to a cavity width of w = 60 nm, whereas the high-order plasmonic mode (light violet) is not observed for cavity widths smaller than 125 nm. The results shown in Figure 4a for the ZnO cavity with h = 120 nm and w = 100 nm suggest that the lasing behavior observed in Figures 2e-g and Figure 3 corresponds to the fundamental plasmonic mode. The confinement factor is found to be larger for the fundamental plasmonic mode than for the photonic mode. The reason for the plasmonic mode has a higher confinement factor is that the cavity height of h = 120 nm is close to the cutoff size of the photonic mode (125 nm), and hence the photonic mode is poorly confined (see Supporting Information). Finally, the confinement factor of the high-order plasmonic mode is smaller than that of the fundamental mode because the maximum electric field enhancement of the higher order mode is at the edges of the cavity as opposed to in the center, as shown in Figure 4c.


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Figure 4d shows the variation of the pump energy density of the lasing threshold at room temperature (293 K) with the cavity width for the range of 80 to 240 nm. The height and the length of the cavities are kept constant at h = 120 nm and l = 30 μm, respectively. The inset on the right in Figure 4d shows the corresponding variation in the approximate gain threshold values (that is, the propagation loss, estimated from the imaginary part of the mode eigenvalue, divided by the confinement factor). The threshold values for both the approximate gain and the measured lasing increase slightly as the width decreases as long as the cavity width remains larger than the cutoff width of the photonic mode. The threshold energy density then jumps to a higher value once the cavity width reaches w = 100 nm, where only the fundamental plasmonic mode exists. This suggests that the lasing here is single-mode plasmonic lasing in the subwavelength regime. The inset on the left of Figure 4d shows the energy density values for the room-temperature lasing threshold of five fabricated photonic lasing cavities and five fabricated plasmonicwaveguide lasing cavities. The average threshold values are 0.06 mJ/cm2 and 0.20 mJ/cm2 for the photonic lasing cavities and plasmonic-waveguide lasing cavities, respectively. Note that the lasing threshold of the photonic mode of the monolithically fabricated nanolaser is larger than the threshold shown for nanowire-based lasers.26 However for the plasmonicwaveguide mode lasing of the monolithically fabricated nanolaser, the low interface scattering at the metal-semiconductor interface and the efficient photon and exciton-plasmon energy transfer make it possible to achieve a threshold value only roughly three times larger than that of the photonic mode. The ratio between plasmonic and photonic lasing thresholds for this structure is less than the lasing threshold ratio between the hybrid photonic-plasmonic mode and the photonic mode in nanowire-based lasers.26 This enables the on-chip monolithically fabricated


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nanolaser to achieve lasing with a threshold close to that of the nanowire-based lasers that supported hybrid photonic-plasmonic modes.


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METHODS Fabrication Process. The ZnO layer was fabricated using a pulsed laser deposition system on a sapphire substrate by Nanovation SARL.41 The photoluminescence measurement of the as-deposited ZnO layer is shown in Supporting Information. An electron beam resist (ZEP520A, Zeon Corporation, Tokyo, Japan) was spin coated onto the ZnO layer and patterned with an electron beam lithography system (F7000S-VD02, Advantest, Tokyo, Japan) to define the nanolaser patterns. Then, an Al layer was deposited with an ion beam deposition system. After the Al deposition, a lift-off process was performed to remove the resist together with the top Al layer. The Al pattern on ZnO was used as a mask for the Ar plasma etching of the ZnO using an inductively-coupledplasma etching system (Ulvac, Inc, CE-300I, Chigasaki, Japan). The final structures consist of ZnO/Al cavities of different widths and lengths. Optical Measurement Systems. The nanolaser was pumped by a 343 nm 250 fs laser pulse from an optical parametric amplifier (Orpheus-HP, Light Conversion, Vilnius, Lithuania) with a repetition rate of 75 kHz. The pump light was focused onto the nanolaser cavity by an objective lens with a magnification of 20 and a numerical aperture of 0.45. The diameter of the spot was estimated to be 40 μm (see Supporting Information). A dichroic mirror was used to separate the pump light (λ = 343 nm) from the emitted light (λ > 360 nm). The emitted light was collected by the same objective lens, coupled to an optical fiber and then analyzed using a spectrometer (IsoPlane 160, Princeton Instruments, Trenton, USA). To analyze the polarization of the emitted light, the polarizer was placed in the microscope’s infinity space before the light was coupled to the optical fiber. To study the lasing


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characteristics at high and low temperatures, a temperature controlled microscope stage (10084L, Linkam Scientific Instruments, Tadworth, UK) with continuous liquid nitrogen flow was used. Simulation details. The simulated absorption distributions were calculated using the finite-difference time-domain technique (FullWAVE, RSoft Design Group, Ossining, USA). The refractive index of ZnO and sapphire are modeled according to literature values.42,43 The complex permittivity of Al is described according to the Lorentz-Drude dispersion model.44 For the mode calculations, perfectly matched layer boundary conditions were used at the edges of the simulated domain. The absorption is defined by

Im

′ |

′,

| , where E is the electric field and ε is

the permittivity. The effective refractive index of the modes, electric field distributions of the modes, and lasing propagation behavior were computed using the finite-element method (COMSOL Multiphysics, COMSOL, Inc., Burlington, USA). Numerical Calculations The effective index is calculated as the real parts of the mode eigenvalue. The propagation loss can be obtained with the imaginary part of the eigenvalue. The mode area is defined as the ratio of the total energy of the mode and maximum energy density ( ∬

⁄max

). The confinement factor is defined by the ratio of the electric

energy in the gain material and the total electric energy of the mode (Γ ∬

∬

). The approximate gain threshold value is defined as the

propagation loss divided by the confinement factor. The local Purcell factor calculation, for the study of the enhancement of the exciton recombination rate, is based on the enhancement of the emission rate of radiation modes and the surface plasmon polariton (SPP) modes FSPP, which is considered as the main contribution of recombination rate. The Purcell factor of a dipole


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emitter16,45 is in the nanocavity is simplified as

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1

,

,

⁄2, where Fspp

is the SPP enhanced emission rate with the orientation along the direction. Fspp is given by 3

⁄2

where ng is the group index of the surface plasmon

mode, λ is the wavelength, ncavity is the refractive index of the cavity material, and ASPP is the mode area based on the direction.


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FIGURES

Figure 1. Monolithically fabricated plasmonic-waveguide nanolasers. (a) Schematic diagram of the monolithically fabricated ZnO/Al nanolasers. ZnO cavities with Al top layers are directly grown and fabricated on a sapphire substrate with well-controlled alignment. A 343-nm laser source is used to pump the cavities from the back side of the substrate. Plasmonic lasing can be achieved and waveguided along the ZnO cavities beyond the diffraction limit. (b) Scanning electron microscopy (SEM) image of the ZnO/Al cavity on the sapphire substrate. Inset: Highmagnification SEM images of the cavities supporting photonic lasing (top) and plasmonic lasing (bottom). (c) Optical microscopy image of the ZnO/Al nanocavities array. The cavities are fabricated with different lengths and widths. Inset: Dark-field microscopy image of the cavity. The scale bar is 10 μm. (d) Optical microscopy images of lasing at room temperature for two different cavity lengths. The scale bar is 10 μm. (e) Calculated electric field distributions of the plasmonic mode of the ZnO/Al nanolaser with a cavity width of w = 80 nm (left) and a ZnO nanowire with a diameter of 110 nm on an Al surface (right). (f) Calculated absorption distribution of the ZnO/Al cavity for pumping from top and bottom (left) and that of the ZnO nanowire on the Al surface (right).


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Figure 2. Photonic and plasmonic lasing characteristics of the nanolaser at 147 K. (a,e) Emission spectra at 147 K of (a) a ZnO/Al photonic cavity with a cavity width of w = 140 nm and (e) a plasmonic cavity with a smaller width of w = 100 nm. The emission spectra are shown at the lasing threshold, together with the spectra below and above the threshold. (b,f) Output emission intensity versus the pump energy density for (b) the photonic cavity and (f) the plasmonic cavity. (b) and (f) are both shown on a log−log scale. (c,g) The lasing spectra polarized in the 0°, 45°, and 90° directions of (c) the photonic cavity and (g) the plasmonic cavity. Inset: The polarization directions are defined as the angle between the electric field of the emission light and the cavity axis. (d,h) Simulated distributions of the x- and z-components of the electric field for (d) the photonic mode and (h) the plasmonic mode.


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Figure 3. Plasmonic lasing of the nanolaser from low temperature to high temperature. (a) Emission spectra of the monolithically fabricated plasmonic cavity at room temperature (293 K). The spectra are measured at the lasing threshold (Pth), together with the spectra below and above the threshold. (b) The emission spectra for the plasmonic cavity at 310 K (left), 320 K (center), and 330 K (right). The scale of the emission intensity plot for the spectra recorded at the three different temperatures is the same. Inset: The lasing spectrum of the plasmonic cavity at 330 K well above Pth. (c) Wavelength and corresponding energy of the lasing peak versus the operating temperature. Inset: The lasing spectra for 165 K, 198 K, and 235 K.


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Figure 4. Photonic and plasmonic modes and the lasing thresholds for the nanolasers. (a) The effective index and (b) the confinement factor of the fundamental plasmonic mode (dark violet line), the high-order plasmonic mode (light violet line), and the photonic mode (black line) versus the cavity width. The cutoff width of the photonic mode and low-index plasmonic mode is around w = 125 nm. The height of the ZnO cavity and the thickness of the Al layer are 120 nm and 30 nm, respectively. (c) Calculated electric field distributions of the fundamental plasmonic mode (top), high-order plasmonic mode (middle), and photonic mode (bottom) with a cavity width of w = 140 nm. (d) The energy density of the lasing threshold at room temperature versus the cavity width for the fabricated nanolasers. Left Inset: Lasing threshold measurements of five photonic lasing cavities and five plasmonic lasing cavities, showing the average threshold values are 0.06 mJ/cm2 and 0.20 mJ/cm2 for photonic lasing and plasmonic lasing, respectively. Right Inset: Approximate gain threshold of the above three modes as a function of the cavity width (right).


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ASSOCIATED CONTENT Supporting Information. Fabrication details of the plasmonic-waveguide nanolaser, experimental setup, emission spectra and optical microscopy images of the photonic lasers, mode selection of monolithically fabricated nanocavities, Purcell factor of plasmonic and hybrid photonic-plasmonic mode in the nanolaser, polarization of photonic and plasmonic-waveguide mode lasing, and simulation of propagation of the plasmonic-waveguide mode and the coupling to the on-chip plasmonic waveguide. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †These authors contributed equally. Funding Sources This work was supported through Japan Society for the Promotion of Science KAKENHI Grant Numbers (17H03229, 17K18867, 18K13799), Japan. A part of this work was conducted in Center for Nano Lithography & Analysis, The University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan. ACKNOWLEDGMENT


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The authors would like to extend our grateful appreciation to Professor Makoto KuwataGonokami, Professor Junji Yumoto and Dr. Kuniaki Konishi from School of Science, The University of Tokyo for important discussion and technical support. The authors would also like to acknowledge helpful discussions in nanofabrication with Dr. Eric Lebrasseur from Center for VLSI Design and Education Center, The University of Tokyo. REFERENCES 1. Bergman, D. J.; Stockman, M. I. Surface Plasmon Amplification by Stimulated Emission of Radiation: Quantum Generation of Coherent Surface Plasmons in Nanosystems. Phys. Rev. Lett. 2003, 90 (2), 4. 2. Berini, P.; De Leon, I. Surface Plasmon-Polariton Amplifiers and Lasers. Nat. Photonics 2012, 6 (1), 16–24. 3. Ma, R. M.; Oulton, R. F.; Sorger, V. J.; Zhang, X. Plasmon Lasers: Coherent Light Source at Molecular Scales. Laser Photonics Rev. 2013, 7 (1), 1–21. 4. Hill, M. T.; Gather, M. C. Advances in Small Lasers. Nat. Photonics 2014, 8 (12), 908–918. 5. Wang, D.; Wang, W.; Knudson, M. P.; Schatz, G. C.; Odom, T. W. Structural Engineering in Plasmon Nanolasers. Chem. Rev. 2018, 118 (6), 2865–2881. 6. Noginov, M. A.; Zhu, G.; Belgrave, A. M.; Bakker, R.; Shalaev, V. M.; Narimanov, E. E.; Stout, S.; Herz, E.; Suteewong, T.; Wiesner, U. Demonstration of a Spaser-Based Nanolaser. Nature 2009, 460 (7259), 1110–1112. 7. Nezhad, M. P.; Simic, A.; Bondarenko, O.; Slutsky, B.; Mizrahi, A.; Feng, L.; Lomakin, V.; Fainman, Y. Room-Temperature Subwavelength Metallo-Dielectric Lasers. Nat. Photonics 2010, 4 (6), 395–399. 8. Ma, R. M.; Oulton, R. F.; Sorger, V. J.; Bartal, G.; Zhang, X. Room-Temperature Sub-


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On-Chip Monolithically Fabricated Plasmonic-Waveguide Nanolaser

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