Ultracompact Pseudowedge Plasmonic Lasers and Laser Arrays

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Ultra-compact pseudowedge plasmonic lasers and laser arrays Yu Hsun Chou, Kuo-Bin Hong, Chun-Tse Chang, Tsu-Chi Chang, Zhen-Ting Huang, Pi Ju Cheng, Jhen-Hong Yang, Meng-Hsien Lin, TZY-RONG LIN, Kuo-Ping Chen, Shangjr Gwo, and Tien-Chang Lu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03956 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Ultra-compact pseudowedge plasmonic lasers and laser arrays

Yu-Hsun Chou1, Kuo-Bin Hong1, Chun-Tse Chang1, Tsu-Chi Chang1, Zhen-Ting Huang2, Pi-Ju Cheng3, Jhen-Hong Yang4, Meng-Xian Lin5, Tzy-Rong Lin2, 6, KuoPing Chen7, Shangjr Gwo5, 8, and Tien-Chang Lu1* 1.

Department of Photonics, National Chiao Tung University, Hsinchu 30010, Taiwan

2.

Department of Mechanical and Mechatronic Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan

3.

Academia Sinica, Research Center for Applied Sciences, Taipei 11529, Taiwan

4.

Institute of Photonic System, National Chiao Tung University, Tainan 71150, Taiwan

5.

Department of Physics, National Tsing-Hua University, Hsinchu 30013, Taiwan

6.

Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan

7.

Institute of Imaging and Biomedical Photonics, National Chiao Tung University, Tainan 71150, Taiwan

8.

National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan *Corresponding author e-mail address: [email protected]

Concentrating light at the deep subwavelength scale by utilizing plasmonic effects has been reported in various optoelectronic devices with intriguing phenomena and functionality. Plasmonic waveguides with a planar structure exhibit a twodimensional degree of freedom for the surface plasmon; the degree of freedom can be further reduced by utilizing metallic nanostructures or nanoparticles for surface plasmon resonance. Reduction leads to different lightwave confinement capabilities, which can be utilized to construct plasmonic nanolaser cavities. However, most theoretical and experimental research efforts have focused on planar surface plasmon polariton (SPP) nanolasers. In this study, we combined nanometallic structures intersecting with ZnO nanowires and realized the first laser emission 1

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based on pseudowedge SPP waveguides. Relative to current plasmonic nanolasers, the pseudowedge plasmonic lasers reported in our study exhibit extremely small mode volumes, high group indices, high spontaneous emission factors, and high Purell factors beneficial for the strong interaction between light and matter. Furthermore, we demonstrated that compact plasmonic laser arrays can be constructed, which could benefit integrated plasmonic circuits.

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The ability of surface plasmons to break the optical diffraction limit provides a new means of investigating the interactions between light and matter;

1–5

this ability also

impels the use of plasmonic effects in various applications such as high-definition lithography,6–8 ultrafast spectroscopy,9–10 full-color subwavelength imaging,11 biosensing and chemical sensing,12–13 quantum information,14 and integrated plasmonic circuits.15 A novel type of laser involving a planar surface plasmon polariton (SPP) on the interface between a metal and an insulator has been demonstrated; the cavity size is on a deepsubwavelength scale.16–25 Lasers operated by a localized surface plasmon (LSP) with metallic nanoparticles surrounded by insulators have been reported, although observation of a single LSP nanolaser emitter is challenging.26–28 A plasmonic waveguide should be able, depending on the metallic structures, to have three different levels of freedom for lightwaves: two, one, or zero degrees29–31 (examples are shown in Supporting Information, Figure S1a–c). Amid a proliferation of research efforts, various studies have addressed planar SPP nanolasers, but fewer studies have focused on the characteristics of lower dimension SPP nanolasers.32–37 This disparity may be because low-dimensional SPP waveguides or cavities are required to have wedge, groove, metallic nanowire, or nanoparticle structures with highly accurate shapes to prevent scattering loss from the waveguides4. Moreover, the crystal quality of metal is reportedly an essential factor for SPP propagation and oscillation, and this crystal quality could be more crucial in lowdimensional SPP waveguides.26–28,32–42 In this study, we investigated ZnO plasmonic nanolasers on a pseudowedge SPP waveguide formed on a high-quality Ag crystal carved with a subwavelength metallic grating using a focused ion beam system. The pseudowedge plasmonic nanolasers and laser arrays were unambiguously demonstrated 3


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at the intersections between ZnO nanowires and Ag gratings. The optical microscope (OM) images shown in Figure 1a illustrate the fabricated metal grating with a period of 500 nm and a notch width of 80 nm. The schematics of a Ag notch under y-polarized excitation and a bright OM image of Ag gratings are shown in Figure 1b. In the y-polarized excitation, the grating areas showed different colors with respect to the flat Ag area. The measured reflectivity spectra showed almost no features in the x-polarized direction but several distinct dips in the y-polarized direction due to the interaction of plasmonic modes (Supporting Information Figure S3). Although metal gratings can confine the SPP fields that couple to the gain materials,43–44 the Bloch reflection is reportedly the main laser feedback mechanism that requires a relatively large area to form a laser cavity.45–47 To understand the SPP characteristics of metal gratings, we calculated the reflectance spectra of y-polarized light with different periods from 350 to 500 nm, as shown by the color maps in Figure 1c. The upper two modes in the visible region can be shifted by varying the grating period. However, as the grating period increased, the lower two modes existing in the ultraviolet region remained at the same wavelength. To further verify the modes shown in Figure 1c, we calculated the electric field intensities of a metal grating with a period of 500 nm for wavelengths λ = 344, 366, 468, and 707 nm, as shown in Figure 1d. The leftmost two figures of Figure 1d belong to the upper two modes shown in Figure 1c, which exhibited Bloch mode patterns with weak mode confinement effects. The electric fields for the lower two modes in Figure 1c are shown in the rightmost two figures of Figure 1d. The fields were highly localized at the edges of the Ag notch, one at the top and one at the bottom, corresponding to different plasmonic modes. Modes at the top edges were also termed as pseudowedge plasmons 4


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with strong localization characteristics.32 Therefore, the top edges of the notch in the Ag grating can be used to support pseudowedge SPP waves, and pseudowedge SPP nanolasers can be realized when the ZnO nanowires are placed on top of the notches and exposed to laser gain.

To demonstrate single-device laser operation assisted by the pseudowedge SPP mode with an extremely small mode volume, we proposed a laser structure with a ZnO nanowire placed on a Ag grating. The laser cavity length was then determined by the intersection between the ZnO nanowire and the edges of the notch, as shown in the inset of Figure 2, forming a pseudowedge SPP laser or laser array; this significantly differs from the laser cavity defined by the nanowire length in the planar SPP nanolaser structure.18,20–25,29 In this study, we applied ultra-high-quality colloidal Ag flakes as the metal layer to minimize surface scattering and ohmic losses, and we coated a 3 nm thick Al2O3 layer onto the surface to prevent vulcanization and control metal losses (Supporting Information Figure S2).

The measured photoluminescence (PL) emission spectra at 77 K for the ZnO nanowires on the Ag grating are shown in Figure 3a. The corresponding light in–light out (L–L) curves with linewidth variation are shown in Figure 3b, which clearly reveals lasing behavior above the threshold pumping power with a sudden reduction in emission linewidth. Notably, only a single lasing peak emission was observed from the device, which differed from the multiple-mode laser emission in planar SPP nanolasers (Supporting Information Figure S4); this is because the effective cavity length was 5


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determined by the contact length of the ZnO nanowire with the top edge of the notch. A short cavity length (approximately 50 nm) results in single-longitudinal-mode operation. In addition, we used modified rate equations to investigate the proportion of light coupling into the laser mode,23,25 which is known as the spontaneous emission β factor. The β value fitted by the measured L–L curves was 0.8. Such a high spontaneous emission factor suggests a remarkably compact cavity, with the corresponding plasmonic nanolasers exhibiting strong light–matter interaction.

Far-field characteristics can reveal the distinct features of these pseudowedge plasmonic lasers. The polarization direction of the laser emission was perpendicular to the grating, as shown in the inset of Figure 3c. This was in sharp contrast to the sample on the Ag without any grating, where all the lasing peaks were polarized along the nanowire direction, as shown in the inset of Figure 3f, indicating that they belong to the fundamental planar SPP mode.18,20–25,29 Because of the characteristics of the pseudowedge SPP on the Ag gratings, the polarization direction of the laser remained perpendicular to the Ag gratings, regardless of the nanowire direction, enabling precise mode control of these coherent nanoscale emitters (Supporting Information S5–7). Emission patterns shown in the inset of Figure 3c unambiguously demonstrate that the four pseudowedge SPP nanolasers were located exactly at the notches of the Ag gratings below the ZnO nanowire; therefore, the nanolaser array can be formed by adjusting the ZnO nanowire length. Three and one pseudowedge SPP nanolasers are clearly exhibited in Figure 3d,e. In comparison, the inset of Figure 3f shows two emission spots from a planar SPP nanolaser in which coherent light is scattered from two facets at the end of a 6


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nanowire.

To further understand the characteristics of these plasmonic nanolasers, we calculated the dispersion relations of fundamental SPP modes for ZnO nanowires on Ag with and without notches, as shown in Figure 4a. The pseudowedge SPP mode existing in the sample with a notch demonstrated a high group index (ng = 165) and high momentum to efficiently couple with ZnO excitons. The insets in Figure 4a are the corresponding mode profiles for two types of SPP waveguides. Figure 4b shows the mode profile observed from the top; this mode profile was obtained using a three-dimensional simulation model. The figure reveals a strong localized effect on the top edges of notches. The pseudowedge SPP wave packets were trapped in the contact region between the hexagonal ZnO nanowires, yielding an extremely small mode area Am = 3.5 × 10−5 λ2 and mode volume Vm = 1.5 x 10−5 λ3, which is 100 times smaller than that of a planar SPP nanolaser and is the smallest mode volume ever reported23. An in-plane pseudowedge SPP wave can leak out at discontinuous boundaries and be detected at the top, as shown in Figure 4c. To verify the polarization direction of the far-field radiation intensity, the polarization curve of a nanolaser was calculated and is shown in Figure 4d. The black line demonstrates the polarization direction of emission, which is shown to be perpendicular to the notch (Supporting Information Figure S5–6).

To investigate the robustness of these plasmonic nanolasers, the operating temperature was raised. The temperature-dependent emission spectra for nanolasers operated just above the threshold pumping power are shown in Figure 5a. A red shift of lasing peak 7


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follows the Varshni trend of ZnO excitons.48 The excellent Ag crystal quality raised the operating temperature of the plasmonic nanolasers to 220 K, despite the nanocavity having exhibited notable metal loss (Supporting Information Figure S8–9). As shown in Figure 5b, the threshold pumping power for the plasmonic nanolasers increased rapidly during the heating process, potentially due to heat dissipation and imperfections of the grating structures. Figure 5c shows the carrier lifetime of the ZnO nanowires on different templates; the lifetimes of the ZnO nanowires on the flat Ag and Ag gratings were 23 and 19 ps, respectively. The carrier lifetime of the measured sample can be expressed as follows:

1

τ

=

F

τr

+

1

τ nr

+

1

τ quench

The radiative lifetime τ r and the nonradiative lifetime τ nr were estimated from the timeresolved photoluminescence measurements of ZnO nanowires on sapphire substrates;

τ quench

refers to the quenching time of an exciton. In this study, the values of the estimated

Purcell factor F for the ZnO nanowires on the flat Ag and Ag gratings were 12 and 15, respectively (Supporting Information Figure S10). Simulated local Purcell factors of the planar and pseudowedge SPP nanolasers are shown in Figure 5d. Extremely high local Purcell factors were observed at the top edges of notches. The calculated effective Purcell factors inside the ZnO nanowire lying on the Ag with and without grating were 22 and 17, respectively (Supporting Information Figure S12). The slight differences between the measured and calculated Purcell factors were due to the influence of quenching and nonradiative recombination caused by the imperfections of the grating structures and the strong confinement of the electric field at wedges that further deteriorated the 8


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propagation loss. Such differences indicate that the performance of pseudowedge SPP nanolasers could still be improved with fine-tuned nanofabrication technology.

In summary, we demonstrated laser operation based on the pseudowedge SPP mode. Compared with a planar SPP nanolaser with a lateral cavity defined by dielectric materials, we showed that the pseudowedge SPP nanolasers exhibited an ultracompact effective mode volume that is 100 times smaller than that of a planar SPP nanolaser and that a strong dispersion relation of the pseudowedge SPP mode produced a small group velocity of 1/165 the speed of light. Therefore, the extracted spontaneous emission factor could be as high as 0.8. Moreover, pseudowedge SPP modes can be generated at predefined metal structures; this technology can be used to construct densely integrated nanolaser arrays. The presented observation of the pseudowedge SPP nanolaser characteristics fills a gap in academic knowledge of SPP nanolasers and provides a new pathway to manipulate the optical field at a deep subwavelength scale; this can be applied in high-density plasmonic circuits.

Acknowledgement

Authors acknowledge the help of Profs. S. C. Wang, H. C. Kuo, Mr. T. Y. Chang at National Chiao Tung University. Author Miss Pi-Ju Cheng would like to thank Dr. ShuWei Chang at the Research Center for Applied sciences, Academia Sinica for his insightful discussion and support. Author Prof Tzy-Rong Lin expresses his deepest gratitude to his Father, Mr. Hsing-Chung Lin, for his cultivating parenting, and frequently encouraging during his research, and shows his endless love to his Father by this paper. 9


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This work was partially supported by the Ministry of Education Aim for the Top University program and by Minister of Science and Technology (MOST) under Contract Nos. MOST 104-2221-E-009 -096 -MY3 and MOST 104-2923-E-009 -003 -MY3, and MOST 103-2221-E-019-028-MY3, and MOST 105-2221-E-019-049-MY3.

Contribution Y. H. C., K. B. H., T. R. L. and T. C. L. initiated the study. K. B. H., C. T. C., T. C. C., M. X. L. and S. J. G designed and prepared the samples. Y. H. C., C. T. C., K. P. C. and J. H. L. performed the optical experiments. K. B. H., Z. T. H., P. J. C. and T. R. L performed the numerical simulation and modelling. Y. H. C., K. B. H., P. J. C., T. R. L., K. P. C., S. J. G. and T. C. L. analysed the experiment data. Y. H. C., K. B. H. and T. C. L. wrote the manuscript. Y. H. C and K. B. H. equally contributed to this work. All authors discussed the results and commented on the manuscript.

Corresponding Authors Correspondence and requests for materials should be addressed to Tien-Chang Lu ([email protected])

Method Summary Fabrication. To synthesize single-crystalline Ag, we used the Pt-nanoparticle-catalyzed 10


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and NH3OH-controlled polyol reduction method (for details, see Refs. 49–51). The Ag surface was then coated with 3 nm thick Al2O3 for protection from oxidation. The grating structures on the Ag single crystals were fabricated by using a focused ion beam system (FEI Nova 200). These structures were milled at a 30 kV acceleration voltage with an ion beam current of 50 pA. The ZnO nanowires were synthesized by hydrothermal methods23–25 and were composed of a hexagonal crystalline structure grown along the cdirection with lengths ranging from 0.2 to 3 µm. The side length of the hexagonal cross section ranged from 20 to 40 nm. A small nanowire cross section ensures no photonic mode in nanoscale waveguides23. The ZnO nanowires were drop cast on both Ag flakes and gratings. Measurement system. The ZnO nanowires were scanned with a scanning electron microscope before the optical measurement process to confirm the positions of the ZnO nanowires precisely so that no other ZnO nanowires appeared within the pumping beam area. To detect the signal emitted from the nanolaser, the fabricated samples were mounted in a cryogenic chamber with pressure below 10−6 mbar, and a charge-coupled device (CCD) camera was used to verify the position of the grating. The nanowires were pumped by a Nd:YVO4 355 nm pulse laser with a 1 kHz repetition rate and a 0.5 ns pulse duration. A 100× near-ultraviolet (UV) infinity-corrected objective lens with a working distance of 1 cm and N.A. of 0.55 was used for concentrating the incident laser beam. The focused spot size was approximately 15 µm in diameter. Light emitted from the nanolaser was collected by the objective lens and transmitted through a 600 µm core UV optical fiber. A 320 mm single monochromator attached to a nitrogen-cooled CCD was used to analyze the collected signals from the nanolaser. The polarization values were 11


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measured at the tops of the samples. Carrier lifetime was measured by a streak camera (Hamamatsu) when the samples were pumped by a 76 MHz Ti:Sapphire laser with its operating frequency tripled to 266 nm. The laser pulse width was less than 1 ps; the timeresolved system response was limited to 6 ps. Simulation. Two- and three-dimensional simulations were performed using the COMSOL RF module with the finite element method. We used the eigenvalue solver to determine the eigenmodes of the nanowires formed on the dielectric/metal substrates and grating structures. The computational domains were enclosed by scattering boundary conditions, to absorb the scattered field and minimize the reflection from boundaries. For the simulations, the refractive indices of ZnO, Al2O3, and Ag were taken from Refs. 52– 54, respectively. For calculating the local Purcell factor, we must estimate the electromagnetic energy inside the entire computational domain of the SPP mode for a specific wavelength (e.g., 375 nm) by using the surface integral operator. Subsequently, we swept the wavelength of the excited field to discover the dispersion curves of desired fundamental SPP modes (e.g., planar SPP mode and pseudowedge SPP mode) and further calculated the slope of the dispersion curve at the specific wavelength to obtain the group index, ng = neff − λ (dneff d λ ) , where neff is the effective mode index. Finally, the local Purcell factor of the SPP mode was applied to predict the optical and lasing characteristics of the ZnO nanolasers. The expression of the local Purcell factor and detailed descriptions are provided in the Supplementary Information of Refs. 18 and 23.

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Figure 1 | Structures and characteristic of metal gratings. (a) Optical microscope images of gratings on one Ag flake. The inset shows Ag flakes on a glass substrate. (b) Schematic of a Ag notch under y-polarized excitation and bright OM image of Ag metal gratings. (c) Calculated y-polarized reflectance map of a Ag grating covered with a 3 nm Al2O3 layer as a function of period; the corresponding notch width and depth were set to 80 nm. In the calculation, two modes existed in the UV region, and the wavelength remained constant as the period of the grating increased. The wavelengths of the other two modes were located in the visible region shifted by varying the grating period. (d) Calculated electric field intensities of a metal grating with a period of 500 nm for wavelengths of 344, 366, 468, and 707 nm; these wavelengths are marked by dotted lines in c.

Figure 2 | Structure of pseudowedge SPP nanolaser. Schematic of a ZnO nanowire lying on a Ag grating covered with 3 nm thick Al2O3, where the stimulated emission was scattered from the boundaries of the pseudowedge SPP cavity. 18


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Figure 3 | Optical characteristics of plasmonic nanolasers. (a) Emission spectra of a pseudowedge SPP nanolaser pumped from low to high power at 77 K. Only a single peak emission was observed from the lasing spectra. (b) Red spheres corresponding to the L–L curve at the emission peak of 371.8 nm, fitted with a black curve using modified rate equations. The emission peak linewidth for increasing pumping power is indicated by blue spheres. (c) Scanning electron microscope image of a ZnO nanowire lying on a Ag grating with a period of 500 nm. The lower-left inset shows a cross-sectional image of the Ag grating. The 2.1 µm long nanowire crosses four notches of the Ag grating, corresponding to four emission spots above the threshold shown in the upper-left inset. Red dashed lines represent the notch positions. The polarization shown in the upper-right inset is perpendicular to the grating; it is independent of the nanowire orientation. Emission profiles of the ZnO nanowires across (d) three notches and (e) one notch. When the length of ZnO nanowire was shorter than 1 µm, a single emission spot was observed, indicating single pseudowedge SPP nanolaser operation. (f) Scanning electron microscope image of the measured planar SPP nanolaser; the upper-left inset shows the emission image above the threshold. Clear interference patterns indicate the coherence of the emission. The upper-right inset shows the direction of polarization at the top measurement position; the polarization angle is shown to be parallel to the nanowire.

Figure 4 | Models of a plasmonic nanolaser constructed from a ZnO nanowire placed on an Al2O3/Ag notch. (a) Dispersion relations of planar and pseudowedge SPP nanolasers, as determined using a planar mode solver for comparison. For the planar SPP 19


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nanolaser, the cross section consisted of a ZnO hexagon oriented toward the c-direction sitting on the Ag layer with a 3 nm thick Al2O3 layer. The planar SPP wave was localized at the bottom of ZnO and propagated along the x-direction. However, for the pseudowedge SPP nanolaser, the ZnO nanowire was placed on top of and perpendicular to the notch. The pseudowedge SPP wave was localized at the top edges of the notch and propagated along the x-direction. This figure illustrates that the effective index and group index of the pseudowedge SPP wave were higher than those of the planar SPP wave. The insets are the corresponding mode profiles for two types of nanolasers, showing extremely small mode areas for the pseudowedge SPP nanolasers. (b) Near-field crosssectional image of the Fabry-Pérot-like pseudowedge surface plasmonic mode existing in the nanocavity, as obtained using a three-dimensional mode solver. The ZnO nanowire was placed at a 50° off-angle to the notch. The resonant pseudowedge SPP wave packets were extremely close to the top edges of the notch, corresponding to the distinct emission spots shown in Figure 3c–e whenever the ZnO nanowire intersected the notches of the Ag gratings. The out-coupling percentage of the pseudowedge SPP wave from the FabryPérot cavity was estimated to be 2.2%. (c) Radiation profile of a plasmonic nanolaser outcoupling to the air. The color map displays the y–z plane cross-sectional image of an electric field radiating from the nanolasers. (d) Calculated polarization curve of the plasmonic nanolaser from the top direction. The black line indicates the electric field intensity variation with the polarization angle. The green and gray stripes represent the ZnO nanowire and Ag notch, respectively. The polarization direction of the laser emission was perpendicular to the notch.

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Figure 5 | Temperature characteristics and carrier dynamics. (a) Lasing spectra of pseudowedge SPP nanolasers for a temperature range of 77–220 K. (b) Threshold pumping power versus temperature for the pseudowedge SPP nanolasers. The corresponding characteristic temperature (T0) was 50 K. (c) Carrier dynamics of ZnO nanowires on flat Ag and Ag gratings measured at 77 K. The black solid squares are traces for a ZnO nanowire placed on a sapphire substrate. The 297 ps decay time represents the intrinsic exciton decay time; the blue triangles and red spheres are traces for ZnO nanowires placed on flat Ag and on Ag gratings, respectively. The significantly reduced decay time indicates the strong plasmonic effect on ZnO excitons. (d) Simulated local Purcell factors of planar and pseudowedge SPP nanolasers.18 The maximum local Purcell factor was located at the bottom of the ZnO hexagon next to the dielectric/metal interface for the planar SPP nanolaser, and it was located at the bottom of the ZnO nanowire, close to the top edges of the notch, for the pseudowedge SPP nanolaser.

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FIG.1 a

b y-pol.

grating 100μm

c

d


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Fig.2


2

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Fig.3 a

c

b 2

Intensity (arb. units)

87MW/cm

2


71MW/cm

2

45MW/cm

120 100 80 300 60 40 20 0 270 20 40 60 80 240 100 120

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600 500 400 300 200 100 100 200 300 400 500 600

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OM

OM ACS Paragon Plus Environment

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Fig.4

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Nano Letters

a

planar SPP

b

pseudowedge SPP

planar SPP

2D SPP

pseudo-wedge SPP

z

1D SPP y Notch

c

d

z x

y ACS Paragon Plus Environment

x

Fig.5 Nano Letters

a

Threshold pumping power (MW/cm2)

b Normalized intensity (arb. units)

220K 150K 100K 77K

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365

370 375 380 Wavelength (nm)

385

390

700 600 500 400 300 200 100 0 60

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77K

planar SPP

297 ps

23 ps

0.1

ZnO NW ZnO on Ag grating ZnO on Ag

19 ps 0

200 Time (ps)

400


pseudowedge SPP

Ultracompact Pseudowedge Plasmonic Lasers and Laser Arrays

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