- Email: [email protected]
Enhanced optical properties in a polarization-matched semiconductor plasmonic nanocavity
Accepted Manuscript Enhanced optical properties in a polarization-matched semiconductor plasmonic nanocavity Y. Hou PII: DOI: Reference:
S0167-577X(18)31761-0 https://doi.org/10.1016/j.matlet.2018.11.002 MLBLUE 25224
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
22 August 2018 25 October 2018 1 November 2018
Please cite this article as: Y. Hou, Enhanced optical properties in a polarization-matched semiconductor plasmonic nanocavity, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.11.002
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced optical properties in a polarization-matched semiconductor plasmonic nanocavity Y. Hou,* Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom
Email: [email protected]
Abstract: Plasmonic nanocavity is a novel platform to investigate fundamental light-matter interactions. In such a cavity, the working media are localized or propagating surface plasmons, which are transverse magnetic polarized electromagnetic waves forming at a metal/dielectric interface. Therefore, the carrier dynamics and optical properties of a plasmonic nanocavity are strongly dependent on the polarization matching between the comprising emitter and the plasmonic cavity modes. Here we demonstrate a plasmonic nanocavity with an InGaN/GaN multiquantum well emitter, fabricated by cost-effective post-growth approaches from a commercially available epiwafer. The polarization state of the emission light from the single nanorod is the same as that of plasmonic cavity mode. As a consequent, significantly enhanced optical properties of the plasmonic nanocavity has been observed, confirmed by time-resolved and power-dependent micro-photoluminescence measurements. This work provides a promising insight into the study on light-matter interaction in a plasmonic nanocavity.
Keywords: GaN, nanorod, nanocavity, polarization, surface plasmon.
1
1. Introduction: A plasmonic nanocavity is the basic component for surface plasmon (SPs) amplification by stimulated emission of radiation (spaser),1 which has been regarded as a prospective coherent light source for next generation information technology. Owing to its ultrasmall mode volume and fast speed, a plasmonic nanocavity is a promising platform to develop a number of photonic devices in additional to spasers, such as sensors, optical switches or modulators.
2–6
Generally speaking, a plasmonic nanocavity is composed of a dielectric
material placed in vicinity of noble metals.
7,8
Linear or non-linear optical process can be
generated by placing emitters or optical gain materials in the nanocavity by launching localized or propagating SPs depending on the architecture of the design.
9
However,
launching SPs by coupling the incident light is conditional due to their native polarization states. Theoretically, only transverse magnetic (TM) polarized light can be coupled into SPs, whereas the light with a polarization perpendicular to this will be missed. Therefore, the orientation of an emitter in the plasmonic nanocavity critically determines the coupling efficiency and the final performance of whole system. Instead of changing the configuration of plasmonic nanocavity, extremely limited attention has been paid on modifying the emitter so that the polarization of the emitting light matches the requirement of SPs. In the early work on experimentally demonstrated spasers, single compositional wurtzite semiconductor nanowires or co-axial structures have been utilized as the emitters,7,10–14 where the emitted light is transverse electric (TE) polarization dominated due to the selection rule. As a result, the performance of plasmonic nanolaser is limited to at least one of the conditions including pulsed pumping, low-temperature working or high-threshold. Recently, Chikkaraddy and co-workers reveal that by matching the polarization of the emitter in a plasmonic nanocavity, optical strong coupling can be achieved at room temperature,
15
demonstrating great potential of plasmonic nanocavity in nano- and
quantum photonics. In present work, we report a semiconductor plasmonic nanocavity 2
utilizing a muti-quantum well (MQW) embedded single nanorod as the emitter. The single nanorod itself demonstrates an emission with TM dominated polarization, the same polarization state as SPs in the plasmonic nanocavity. Via this kind of “polarization matching” coupling, the exciton energy in the single nanorod emitter is sufficiently directed into the plasmonic cavity modes, leading to greatly enhanced optical properties. 2. Material and methods: Fig.1a displays the schematic of the nanocavity, consisting of a single InGaN/GaN MQW based nanorod on a plasmonic substrate which comprises a thin SiO2 dielectric layer (10 nm) on a thick silver film. The InGaN/GaN MQWs and the whole nanorod act as the emitter and a Fabry-Pérot (F-P) nanocavity, respectively. The SiO2 coated Ag were fabricated by deposition methods on Si substrate with a square root roughness (RMS) of 1.6 nm in a scanning area of 5×5 μm2 measured by atomic force microscopy. The nanorod utilized for the nanocavity were fabricated from a standard light emitting diode (LED) wafer with InGaN/GaN MQWs by means of a combination of dry-etching and self-organised nickel nano-masks as shown in Fig.1b. Details of the fabrication method can be found elsewhere.16 The fabrication of such a blue LED wafer into a nanorod array structure can lead to a high optical gain due to the alleviated quantum confined stark effect compared with the as-grown sample.17 Fig.1c shows a typical SEM image of the single nanorod on the plasmonic substrate with a length of ~2 μm, demonstrating straight sidewalls and a pair of well-defined parallel facets. These ensure both minimised optical mode leakage and good optical confinement of the nanocavity. Fig.1d shows a high resolution transmission electron microscopy (TEM) image of the single nanorod, displaying a 250 nm p-type GaN region, 9 pairs of InGaN/GaN MQWs (InGaN well: 2.9 nm /GaN barrier: 13.4 nm) and n-type GaN region. 3. Results and Discussion: Fig.1e shows the mode profile in the nanocavity calculated using standard finite difference time domain simulations, demonstrating that the electric fields is strongly confined 3
into the 10 nm SiO2 layer between the nanorod and the Ag film. The effective mode area is as small as 0.07( / 2) 2 , substantially enhancing the exciton-SPs cavity modes interaction. Based on the geometry of the nanocavity, it is expected that the emission light is TM polarized,7,10
confirmed
by
room
temperature
polarization-dependent
micro-
photoluminescence (µ-PL) measurements. Fig.2a displays the normalized PL intensity of the bare single nanorod (no any exciton-SPs coupling, labelled as Ph-rod) as a function of polarization angle plotted in polar coordination system, where the thick green line represents the single nanorod. The highest PL intensity is along the nanorod axis labelled as c (namely, E//c) while the lowest normal to the nanorod axis (i.e., E⊥c), confirming that the PL emission from the single nanorod is dominated by TM polarization. The degree of polarization (P) is 0.52, calculated by P ( I // I ) /( I // I ) . Here I// and I⊥ are the PL intensities along E//c and E⊥c directions, respectively. The other two samples, which are a single nanorod without F-P cavity on plasmonic substrate (with exciton-SPs coupling but no cavity modes, labelled as SPWG) and the nanocavity (spaser), demonstrate the same polarization states, with the degree of 0.59 and 0.75 (Fig. 2b and Fig. 2c), respectively. Obviously, due to the amplification of TM polarized light as a result of exciton-SPs interaction, the degree of polarization was greatly enhanced. Fig. 2d presents a comparison of emission spectra from the nanocavity along E//c and E⊥c. On E//c, a sharp spike at 437 nm with periodic peaks is observed, recognized as longitudinal cavity modes. In comparison, such features do not manifest themselves on E⊥c. In order to have a deeper insight into the coupling between excitons and plasmonic nanocavity, time-resolved micro-PL (μ-TRPL) measurements have been performed on the three samples with a polarizer aligned along E//c at room temperature, where a 375 nm pulsed diode laser with a pulse width of 50 ps was used as an optical pumping source. Initial emission spectral measurements have been performed on the three samples at room temperature using a polarizer aligned along E//c with a power density of 2.8 mW/cm2 (Fig.3a). 4
The emission features with extremely narrow line width and periodic mode spacing can be only observed in the nanocavity, whilst as expected there is no any cavity modes observed from other two samples. Although the SP-WG sample does not exhibit cavity modes due to the lack of F-P facets, there indeed exits exciton-SPs coupling, leading to significantly enhanced spontaneous emission compared with the Ph-rod. Consequently, the integrated intensity of the SP-WG is 5.8 times higher than that of the Ph-rod (Fig.3a). Furthermore, compared with the Ph-rod, the emission intensity from the plasmonic nanocavity is substantially enhanced by a factor of 12 with a massive reduction in full width at half maximum (FWHM) from 22 to 5 nm as shown in Fig. 3a. These results agree well with the observations from polarization characterizations. Fig. 3b shows the μ-TRPL decay traces of the three samples. All the decay times are well obtained through properly fitting the decay traces. For the Ph-rod, a standard bi-exponential model is used, described by the equation bellow, 18
I (t ) A1 exp( t / 1 ) A2 exp(t / 2 )
(1)
where A1 and τ1 (A2 and τ2) represent the fast (slow) decay components, and fast decay time is also the PL decay time, τpl. However, this model is no longer valid in the case of SP-WG and spaser, where a third term is introduced to characterize the exciton-SPs coupling, 19,20
I (t ) A1 exp( t / 1 ) A2 exp( t / 2 ) A3 exp( t / 3 )
(2)
where τ3 is the decay time related with SPs, denoted as τp in the SP-WG and τc in the plasmonic nanocavity to distinguish with each other. As expected, the three samples demonstrate completely different behaviours. The plasmoic nanocavity shows an extremely fast decay ( c =159 ps) due to exciton-SPs cavity mode coupling. In remarkable contrast, the SP-WG sample exhibits a decay time p of 822 ps, which is still 6.3 times faster than the decay of the Ph-rod ( pl =5.161ns), owing to general exiton-SPs coupling.
5
Power dependent spectral measurements have been conducted on the nanocavity using a polarizer aligned along E//c, as shown in Fig.4a. Even under the lowest excitation power, a clear and narrow emission with FWHM < 6 nm can still be observed at 437 nm, while the PL emission of the Ph-rod exhibits a typical FWHM > 15 nm (not shown here). Such a narrow FWHM in the nanocavity leads to a quality factor of 73, very similar to the reported values. 10,11
Fig.4b shows a light-light (L-L) plot of the SPs cavity mode at 437 nm, described in a
log-log scale. As a result of spontaneous emission with the polarization of E//c strongly coupled into SPs cavity mode, no distinct threshold in L-L plotting is observed in the plasmonic nanocavity. In other words, β factor of the nanocavity, defined as spontaneous emission filtered into cavity modes, is 1. In the meantime, the lasing peak exhibits a tiny blueshift of 0.7 nm, indicating the fast exhaustion of photo-generated carriers. The FWHM as a function of excitation power density has also been provided in Fig.4b, which shows that the FWHM remains less than 6 nm with increasing the excitation power density, indicating a high coherent degree of the emission light. A slight reduction in FWHM has been observed under high excitation power densities, but the FWHM narrowing doesn’t follow Schawlow-Townes relationship, similar to previous reports in a thresholdless spaser.
21,22
These optical
characteristics suggest a thresholdless spaser is possible to be realized by using “polarization matching” technic, but it subjected to further verification by, e.g., second order photon correlation measurements.10,22 In addition, it is worth pointing out that the emission wavelength of device is 437 nm, which approaches the SPs frequency of Ag/GaN, around ~410 nm,
23
indicating a maximized energy transfer from the excitons in the InGaN/GaN
MQWs to the SPs. The multi-peak features observed in both Fig.4a and Fig. 4c are due to F-P modes, showing a mode spacing of ~17 nm, which agrees well with the calculated value of a mode spacing (17.9 nm) based on the cavity length of 2 µm and the refractive index of 2.55 for GaN.
6
In a remarkable contrast, under identical conditions except for the polarizer aligned to E⊥c, only typical spontaneous emission is observed with FWHM>16 nm (Fig.4c). Fig. 4d shows the corresponding L-L plot in a sub-linear manner, along with broadening of the FWHM of the emission peak from 16 to 22 nm with increasing an excitation power density from 570 W/cm2 to 11.46 kW/cm2. Simultaneously, the emission peak exhibits a large blueshift of 6 nm with increasing excitation power density as a result of carrier screening effect, typically observed on standard InGaN/GaN MQWs as spontaneous emission. 4. Conclusion: In summary, we designed a plasmonic nanocavity by utilisation of a MQWs-in-rod structure fabricated from a commercially available LED epiwafer. The emission light of the single nanorod has the same polarization state as the SPs modes, which leads to a high coupling efficiency between the emitter and the plasmonic nanocavity, confirmed by timeresolved and power-dependent μ-PL characterizations. The improved coupling strength by such “polarization matching” scheme eventually leads to a massive enhancement in both polarization and light intensity, along with a remarkable line width narrowing. These discoveries reveal “polarization matching” is crucial in weak-coupling regime in addition to the previous findings in strong-coupling regime,
15
which is important to develop
thresholdless spasers, high-efficiency sensors and fast-speed optical switches.
7
References: 1
D. Bergman and M. Stockman, Phys. Rev. Lett. 90, 27402 (2003).
2
Y.-H. Chou, K.-B. Hong, C.-T. Chang, T.-C. Chang, Z.-T. Huang, P.-J. Cheng, J.-H. Yang, M.-H. Lin, T.-R. Lin, K.-P. Chen, S. Gwo, and T.-C. Lu, Nano Lett. 18, 747 (2018). 3
Y. Deng, G. Cao, H. Yang, G. Li, X. Chen, and W. Lu, Sci. Rep. 7, 10639 (2017).
4
F. Geng, Y. Zhang, Y. Yu, Y. Kuang, Y. Liao, Z. Dong, and J. Hou, Opt. Express 20, 26725 (2012). 5
H.P. Paudel and M.N. Leuenberger, Nano Lett. 12, 2690 (2012).
6
B. Špačková, P. Wrobel, M. Bocková, and J. Homola, Proc. IEEE 104, 2380 (2016).
7
R.F. Oulton, V.J. Sorger, T. Zentgraf, R.-M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, Nature 461, 629 (2009). 8
M.A. Noginov, G. Zhu, A.M. Belgrave, R. Bakker, V.M. Shalaev, E.E. Narimanov, S. Stout, E. Herz, T. Suteewong, and U. Wiesner, Nature 460, 1110 (2009). 9
M.T. Hill and M.C. Gather, Nat. Photonics 8, 908 (2014).
10
Y.-J. Lu, J. Kim, H.-Y. Chen, C. Wu, N. Dabidian, C.E. Sanders, C.-Y. Wang, M.-Y. Lu, B.-H. Li, X. Qiu, W.-H. Chang, L.-J. Chen, G. Shvets, C.-K. Shih, and S. Gwo, Science 337, 450 (2012). 11
C.-Y. Wu, C.-T. Kuo, C.-Y. Wang, C.-L. He, M.-H. Lin, H. Ahn, and S. Gwo, Nano Lett. 11, 4256 (2011). 12
Y.-J. Lu, C.-Y. Wang, J. Kim, H.-Y. Chen, M.-Y. Lu, Y.-C. Chen, W.-H. Chang, L.-J. Chen, M.I. Stockman, C.-K. Shih, and S. Gwo, Nano Lett. 14, 4381 (2014). 13
T.P.H. Sidiropoulos, O. Hess, S. a Maier, and R.F. Oulton, Nat. Phys. 10, 870 (2014).
14
Q. Zhang, G. Li, X. Liu, F. Qian, Y. Li, T.C. Sum, C.M. Lieber, and Q. Xiong, Nat. Commun. 5, 4953 (2014). 15
R. Chikkaraddy, B. de Nijs, F. Benz, S.J. Barrow, O.A. Scherman, E. Rosta, A. Demetriadou, P. Fox, O. Hess, and J.J. Baumberg, Nature 535, 127 (2016). 16
Y. Hou, P. Renwick, B. Liu, J. Bai, and T. Wang, Sci. Rep. 4, 5014 (2014).
17
J. Bai, Q. Wang, and T. Wang, J. Appl. Phys. 111, 113103 (2012).
18
B. Liu, R. Smith, J. Bai, Y. Gong, and T. Wang, Appl. Phys. Lett. 103, 101108 (2013).
19
K. Okamoto, I. Niki, A. Scherer, Y. Narukawa, T. Mukai, and Y. Kawakami, Appl. Phys. Lett. 87, 71102 (2005). 20
W. Zhou, M. Dridi, J.Y. Suh, C.H. Kim, D.T. Co, M.R. Wasielewski, G.C. Schatz, and T.W. Odom, Nat. Nanotechnol. 8, 506 (2013). 21
M. Khajavikhan, a Simic, M. Katz, J.H. Lee, B. Slutsky, A. Mizrahi, V. Lomakin, and Y. Fainman, Nature 482, 204 (2012). 22
S.T. Jagsch, N.V. Triviño, F. Lohof, G. Callsen, S. Kalinowski, I.M. Rousseau, R. Barzel, J.-F. Carlin, F. Jahnke, R. Butté, C. Gies, A. Hoffmann, N. Grandjean, and S. Reitzenstein, Nat. Commun. 9, 564 (2018). 23
K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, Nat. Mater. 3, 601 (2004).
8
Figures captions: Figure 1. (a) Schematic structure of the plasmonic nanocavity, where the nanorod containing InGaN/GaN MQWs is placed on a 10nm SiO2 spacer on a 100 nm thick Ag layer; (b) Crosssectional SEM image of the nanorod arrays fabricated by a self-organised Ni nano-mask approach; (c) Typical SEM image of a single nanorod (with ~2 μm in length and ~200 nm in diameter) placed on the Ag layer, where a pair of parallel and well-defined facets and smooth sidewalls can be observed; (d) Dark filed TEM image of a single nanorod showing 250 nm pGaN, 9 pairs of InGaN/GaN MQWs and n-GaN; and (e) Electric field profile of the nanocavity at a wavelength of 437nm, where the electric fields are tightly confined at the interface between the SiO2 spacer and the silver. Figure 2. PL intensity as a function of polarization angle in a polar coordination system for the samples of an InGaN/GaN MQW based single nanorod on a SiO2 layer (a), single nanorod without F-P facets on plasmonic substrate (b), and the plasmonic nanocavity (c), where the thick green lines indicate the position of the nanorod; (d), the emission spectra of the nanocavity coupled MQWs with polarizer aligned parallel and normal to the nanorod. Figure 3. (a) Comparison of the emission spectra from the plasmonic nanocavity, and a single nanorod without F-P facets but on a plasmonic substrate (SP-WG) and a single nanorod on a SiO2 layer (Ph-rod); (b) μ-TRPL decay traces of the three samples. In both cases, all measurements are performed under identical conditions, namely, using a polarizer aligned along the nanorod rod axis, i.e., E//c, at room temperature. The excitation power density used is 2.8 kW/cm2. Figure 4. PL spectra measured with using a polarizer on a condition of E//c (a) and E⊥c (c) as function of excitation power density. Corresponding L-L curve and FWHM are plotted as a function of excitation power density measured on the condition of E//c (b) and E⊥c (d), respectively. The dash-lines are guides to eyes.
9
10
11
12
13
Highlights:
A high performance plasmonic nanocavity coupled with multi-quantum wells fabricated by cost-effective dry-etching methods.
The carrier dynamic of the emitter coupled with the plasmonic nanocavity greatly enhanced owing to a “polarization matching” process.
The proposed “polarization matching” plasmonic nanocavity serving as an excellent platform to study light-matter interactions.
14
S0167-577X(18)31761-0 https://doi.org/10.1016/j.matlet.2018.11.002 MLBLUE 25224
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
22 August 2018 25 October 2018 1 November 2018
Please cite this article as: Y. Hou, Enhanced optical properties in a polarization-matched semiconductor plasmonic nanocavity, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.11.002
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced optical properties in a polarization-matched semiconductor plasmonic nanocavity Y. Hou,* Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom
Email: [email protected]
Abstract: Plasmonic nanocavity is a novel platform to investigate fundamental light-matter interactions. In such a cavity, the working media are localized or propagating surface plasmons, which are transverse magnetic polarized electromagnetic waves forming at a metal/dielectric interface. Therefore, the carrier dynamics and optical properties of a plasmonic nanocavity are strongly dependent on the polarization matching between the comprising emitter and the plasmonic cavity modes. Here we demonstrate a plasmonic nanocavity with an InGaN/GaN multiquantum well emitter, fabricated by cost-effective post-growth approaches from a commercially available epiwafer. The polarization state of the emission light from the single nanorod is the same as that of plasmonic cavity mode. As a consequent, significantly enhanced optical properties of the plasmonic nanocavity has been observed, confirmed by time-resolved and power-dependent micro-photoluminescence measurements. This work provides a promising insight into the study on light-matter interaction in a plasmonic nanocavity.
Keywords: GaN, nanorod, nanocavity, polarization, surface plasmon.
1
1. Introduction: A plasmonic nanocavity is the basic component for surface plasmon (SPs) amplification by stimulated emission of radiation (spaser),1 which has been regarded as a prospective coherent light source for next generation information technology. Owing to its ultrasmall mode volume and fast speed, a plasmonic nanocavity is a promising platform to develop a number of photonic devices in additional to spasers, such as sensors, optical switches or modulators.
2–6
Generally speaking, a plasmonic nanocavity is composed of a dielectric
material placed in vicinity of noble metals.
7,8
Linear or non-linear optical process can be
generated by placing emitters or optical gain materials in the nanocavity by launching localized or propagating SPs depending on the architecture of the design.
9
However,
launching SPs by coupling the incident light is conditional due to their native polarization states. Theoretically, only transverse magnetic (TM) polarized light can be coupled into SPs, whereas the light with a polarization perpendicular to this will be missed. Therefore, the orientation of an emitter in the plasmonic nanocavity critically determines the coupling efficiency and the final performance of whole system. Instead of changing the configuration of plasmonic nanocavity, extremely limited attention has been paid on modifying the emitter so that the polarization of the emitting light matches the requirement of SPs. In the early work on experimentally demonstrated spasers, single compositional wurtzite semiconductor nanowires or co-axial structures have been utilized as the emitters,7,10–14 where the emitted light is transverse electric (TE) polarization dominated due to the selection rule. As a result, the performance of plasmonic nanolaser is limited to at least one of the conditions including pulsed pumping, low-temperature working or high-threshold. Recently, Chikkaraddy and co-workers reveal that by matching the polarization of the emitter in a plasmonic nanocavity, optical strong coupling can be achieved at room temperature,
15
demonstrating great potential of plasmonic nanocavity in nano- and
quantum photonics. In present work, we report a semiconductor plasmonic nanocavity 2
utilizing a muti-quantum well (MQW) embedded single nanorod as the emitter. The single nanorod itself demonstrates an emission with TM dominated polarization, the same polarization state as SPs in the plasmonic nanocavity. Via this kind of “polarization matching” coupling, the exciton energy in the single nanorod emitter is sufficiently directed into the plasmonic cavity modes, leading to greatly enhanced optical properties. 2. Material and methods: Fig.1a displays the schematic of the nanocavity, consisting of a single InGaN/GaN MQW based nanorod on a plasmonic substrate which comprises a thin SiO2 dielectric layer (10 nm) on a thick silver film. The InGaN/GaN MQWs and the whole nanorod act as the emitter and a Fabry-Pérot (F-P) nanocavity, respectively. The SiO2 coated Ag were fabricated by deposition methods on Si substrate with a square root roughness (RMS) of 1.6 nm in a scanning area of 5×5 μm2 measured by atomic force microscopy. The nanorod utilized for the nanocavity were fabricated from a standard light emitting diode (LED) wafer with InGaN/GaN MQWs by means of a combination of dry-etching and self-organised nickel nano-masks as shown in Fig.1b. Details of the fabrication method can be found elsewhere.16 The fabrication of such a blue LED wafer into a nanorod array structure can lead to a high optical gain due to the alleviated quantum confined stark effect compared with the as-grown sample.17 Fig.1c shows a typical SEM image of the single nanorod on the plasmonic substrate with a length of ~2 μm, demonstrating straight sidewalls and a pair of well-defined parallel facets. These ensure both minimised optical mode leakage and good optical confinement of the nanocavity. Fig.1d shows a high resolution transmission electron microscopy (TEM) image of the single nanorod, displaying a 250 nm p-type GaN region, 9 pairs of InGaN/GaN MQWs (InGaN well: 2.9 nm /GaN barrier: 13.4 nm) and n-type GaN region. 3. Results and Discussion: Fig.1e shows the mode profile in the nanocavity calculated using standard finite difference time domain simulations, demonstrating that the electric fields is strongly confined 3
into the 10 nm SiO2 layer between the nanorod and the Ag film. The effective mode area is as small as 0.07( / 2) 2 , substantially enhancing the exciton-SPs cavity modes interaction. Based on the geometry of the nanocavity, it is expected that the emission light is TM polarized,7,10
confirmed
by
room
temperature
polarization-dependent
micro-
photoluminescence (µ-PL) measurements. Fig.2a displays the normalized PL intensity of the bare single nanorod (no any exciton-SPs coupling, labelled as Ph-rod) as a function of polarization angle plotted in polar coordination system, where the thick green line represents the single nanorod. The highest PL intensity is along the nanorod axis labelled as c (namely, E//c) while the lowest normal to the nanorod axis (i.e., E⊥c), confirming that the PL emission from the single nanorod is dominated by TM polarization. The degree of polarization (P) is 0.52, calculated by P ( I // I ) /( I // I ) . Here I// and I⊥ are the PL intensities along E//c and E⊥c directions, respectively. The other two samples, which are a single nanorod without F-P cavity on plasmonic substrate (with exciton-SPs coupling but no cavity modes, labelled as SPWG) and the nanocavity (spaser), demonstrate the same polarization states, with the degree of 0.59 and 0.75 (Fig. 2b and Fig. 2c), respectively. Obviously, due to the amplification of TM polarized light as a result of exciton-SPs interaction, the degree of polarization was greatly enhanced. Fig. 2d presents a comparison of emission spectra from the nanocavity along E//c and E⊥c. On E//c, a sharp spike at 437 nm with periodic peaks is observed, recognized as longitudinal cavity modes. In comparison, such features do not manifest themselves on E⊥c. In order to have a deeper insight into the coupling between excitons and plasmonic nanocavity, time-resolved micro-PL (μ-TRPL) measurements have been performed on the three samples with a polarizer aligned along E//c at room temperature, where a 375 nm pulsed diode laser with a pulse width of 50 ps was used as an optical pumping source. Initial emission spectral measurements have been performed on the three samples at room temperature using a polarizer aligned along E//c with a power density of 2.8 mW/cm2 (Fig.3a). 4
The emission features with extremely narrow line width and periodic mode spacing can be only observed in the nanocavity, whilst as expected there is no any cavity modes observed from other two samples. Although the SP-WG sample does not exhibit cavity modes due to the lack of F-P facets, there indeed exits exciton-SPs coupling, leading to significantly enhanced spontaneous emission compared with the Ph-rod. Consequently, the integrated intensity of the SP-WG is 5.8 times higher than that of the Ph-rod (Fig.3a). Furthermore, compared with the Ph-rod, the emission intensity from the plasmonic nanocavity is substantially enhanced by a factor of 12 with a massive reduction in full width at half maximum (FWHM) from 22 to 5 nm as shown in Fig. 3a. These results agree well with the observations from polarization characterizations. Fig. 3b shows the μ-TRPL decay traces of the three samples. All the decay times are well obtained through properly fitting the decay traces. For the Ph-rod, a standard bi-exponential model is used, described by the equation bellow, 18
I (t ) A1 exp( t / 1 ) A2 exp(t / 2 )
(1)
where A1 and τ1 (A2 and τ2) represent the fast (slow) decay components, and fast decay time is also the PL decay time, τpl. However, this model is no longer valid in the case of SP-WG and spaser, where a third term is introduced to characterize the exciton-SPs coupling, 19,20
I (t ) A1 exp( t / 1 ) A2 exp( t / 2 ) A3 exp( t / 3 )
(2)
where τ3 is the decay time related with SPs, denoted as τp in the SP-WG and τc in the plasmonic nanocavity to distinguish with each other. As expected, the three samples demonstrate completely different behaviours. The plasmoic nanocavity shows an extremely fast decay ( c =159 ps) due to exciton-SPs cavity mode coupling. In remarkable contrast, the SP-WG sample exhibits a decay time p of 822 ps, which is still 6.3 times faster than the decay of the Ph-rod ( pl =5.161ns), owing to general exiton-SPs coupling.
5
Power dependent spectral measurements have been conducted on the nanocavity using a polarizer aligned along E//c, as shown in Fig.4a. Even under the lowest excitation power, a clear and narrow emission with FWHM < 6 nm can still be observed at 437 nm, while the PL emission of the Ph-rod exhibits a typical FWHM > 15 nm (not shown here). Such a narrow FWHM in the nanocavity leads to a quality factor of 73, very similar to the reported values. 10,11
Fig.4b shows a light-light (L-L) plot of the SPs cavity mode at 437 nm, described in a
log-log scale. As a result of spontaneous emission with the polarization of E//c strongly coupled into SPs cavity mode, no distinct threshold in L-L plotting is observed in the plasmonic nanocavity. In other words, β factor of the nanocavity, defined as spontaneous emission filtered into cavity modes, is 1. In the meantime, the lasing peak exhibits a tiny blueshift of 0.7 nm, indicating the fast exhaustion of photo-generated carriers. The FWHM as a function of excitation power density has also been provided in Fig.4b, which shows that the FWHM remains less than 6 nm with increasing the excitation power density, indicating a high coherent degree of the emission light. A slight reduction in FWHM has been observed under high excitation power densities, but the FWHM narrowing doesn’t follow Schawlow-Townes relationship, similar to previous reports in a thresholdless spaser.
21,22
These optical
characteristics suggest a thresholdless spaser is possible to be realized by using “polarization matching” technic, but it subjected to further verification by, e.g., second order photon correlation measurements.10,22 In addition, it is worth pointing out that the emission wavelength of device is 437 nm, which approaches the SPs frequency of Ag/GaN, around ~410 nm,
23
indicating a maximized energy transfer from the excitons in the InGaN/GaN
MQWs to the SPs. The multi-peak features observed in both Fig.4a and Fig. 4c are due to F-P modes, showing a mode spacing of ~17 nm, which agrees well with the calculated value of a mode spacing (17.9 nm) based on the cavity length of 2 µm and the refractive index of 2.55 for GaN.
6
In a remarkable contrast, under identical conditions except for the polarizer aligned to E⊥c, only typical spontaneous emission is observed with FWHM>16 nm (Fig.4c). Fig. 4d shows the corresponding L-L plot in a sub-linear manner, along with broadening of the FWHM of the emission peak from 16 to 22 nm with increasing an excitation power density from 570 W/cm2 to 11.46 kW/cm2. Simultaneously, the emission peak exhibits a large blueshift of 6 nm with increasing excitation power density as a result of carrier screening effect, typically observed on standard InGaN/GaN MQWs as spontaneous emission. 4. Conclusion: In summary, we designed a plasmonic nanocavity by utilisation of a MQWs-in-rod structure fabricated from a commercially available LED epiwafer. The emission light of the single nanorod has the same polarization state as the SPs modes, which leads to a high coupling efficiency between the emitter and the plasmonic nanocavity, confirmed by timeresolved and power-dependent μ-PL characterizations. The improved coupling strength by such “polarization matching” scheme eventually leads to a massive enhancement in both polarization and light intensity, along with a remarkable line width narrowing. These discoveries reveal “polarization matching” is crucial in weak-coupling regime in addition to the previous findings in strong-coupling regime,
15
which is important to develop
thresholdless spasers, high-efficiency sensors and fast-speed optical switches.
7
References: 1
D. Bergman and M. Stockman, Phys. Rev. Lett. 90, 27402 (2003).
2
Y.-H. Chou, K.-B. Hong, C.-T. Chang, T.-C. Chang, Z.-T. Huang, P.-J. Cheng, J.-H. Yang, M.-H. Lin, T.-R. Lin, K.-P. Chen, S. Gwo, and T.-C. Lu, Nano Lett. 18, 747 (2018). 3
Y. Deng, G. Cao, H. Yang, G. Li, X. Chen, and W. Lu, Sci. Rep. 7, 10639 (2017).
4
F. Geng, Y. Zhang, Y. Yu, Y. Kuang, Y. Liao, Z. Dong, and J. Hou, Opt. Express 20, 26725 (2012). 5
H.P. Paudel and M.N. Leuenberger, Nano Lett. 12, 2690 (2012).
6
B. Špačková, P. Wrobel, M. Bocková, and J. Homola, Proc. IEEE 104, 2380 (2016).
7
R.F. Oulton, V.J. Sorger, T. Zentgraf, R.-M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, Nature 461, 629 (2009). 8
M.A. Noginov, G. Zhu, A.M. Belgrave, R. Bakker, V.M. Shalaev, E.E. Narimanov, S. Stout, E. Herz, T. Suteewong, and U. Wiesner, Nature 460, 1110 (2009). 9
M.T. Hill and M.C. Gather, Nat. Photonics 8, 908 (2014).
10
Y.-J. Lu, J. Kim, H.-Y. Chen, C. Wu, N. Dabidian, C.E. Sanders, C.-Y. Wang, M.-Y. Lu, B.-H. Li, X. Qiu, W.-H. Chang, L.-J. Chen, G. Shvets, C.-K. Shih, and S. Gwo, Science 337, 450 (2012). 11
C.-Y. Wu, C.-T. Kuo, C.-Y. Wang, C.-L. He, M.-H. Lin, H. Ahn, and S. Gwo, Nano Lett. 11, 4256 (2011). 12
Y.-J. Lu, C.-Y. Wang, J. Kim, H.-Y. Chen, M.-Y. Lu, Y.-C. Chen, W.-H. Chang, L.-J. Chen, M.I. Stockman, C.-K. Shih, and S. Gwo, Nano Lett. 14, 4381 (2014). 13
T.P.H. Sidiropoulos, O. Hess, S. a Maier, and R.F. Oulton, Nat. Phys. 10, 870 (2014).
14
Q. Zhang, G. Li, X. Liu, F. Qian, Y. Li, T.C. Sum, C.M. Lieber, and Q. Xiong, Nat. Commun. 5, 4953 (2014). 15
R. Chikkaraddy, B. de Nijs, F. Benz, S.J. Barrow, O.A. Scherman, E. Rosta, A. Demetriadou, P. Fox, O. Hess, and J.J. Baumberg, Nature 535, 127 (2016). 16
Y. Hou, P. Renwick, B. Liu, J. Bai, and T. Wang, Sci. Rep. 4, 5014 (2014).
17
J. Bai, Q. Wang, and T. Wang, J. Appl. Phys. 111, 113103 (2012).
18
B. Liu, R. Smith, J. Bai, Y. Gong, and T. Wang, Appl. Phys. Lett. 103, 101108 (2013).
19
K. Okamoto, I. Niki, A. Scherer, Y. Narukawa, T. Mukai, and Y. Kawakami, Appl. Phys. Lett. 87, 71102 (2005). 20
W. Zhou, M. Dridi, J.Y. Suh, C.H. Kim, D.T. Co, M.R. Wasielewski, G.C. Schatz, and T.W. Odom, Nat. Nanotechnol. 8, 506 (2013). 21
M. Khajavikhan, a Simic, M. Katz, J.H. Lee, B. Slutsky, A. Mizrahi, V. Lomakin, and Y. Fainman, Nature 482, 204 (2012). 22
S.T. Jagsch, N.V. Triviño, F. Lohof, G. Callsen, S. Kalinowski, I.M. Rousseau, R. Barzel, J.-F. Carlin, F. Jahnke, R. Butté, C. Gies, A. Hoffmann, N. Grandjean, and S. Reitzenstein, Nat. Commun. 9, 564 (2018). 23
K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, Nat. Mater. 3, 601 (2004).
8
Figures captions: Figure 1. (a) Schematic structure of the plasmonic nanocavity, where the nanorod containing InGaN/GaN MQWs is placed on a 10nm SiO2 spacer on a 100 nm thick Ag layer; (b) Crosssectional SEM image of the nanorod arrays fabricated by a self-organised Ni nano-mask approach; (c) Typical SEM image of a single nanorod (with ~2 μm in length and ~200 nm in diameter) placed on the Ag layer, where a pair of parallel and well-defined facets and smooth sidewalls can be observed; (d) Dark filed TEM image of a single nanorod showing 250 nm pGaN, 9 pairs of InGaN/GaN MQWs and n-GaN; and (e) Electric field profile of the nanocavity at a wavelength of 437nm, where the electric fields are tightly confined at the interface between the SiO2 spacer and the silver. Figure 2. PL intensity as a function of polarization angle in a polar coordination system for the samples of an InGaN/GaN MQW based single nanorod on a SiO2 layer (a), single nanorod without F-P facets on plasmonic substrate (b), and the plasmonic nanocavity (c), where the thick green lines indicate the position of the nanorod; (d), the emission spectra of the nanocavity coupled MQWs with polarizer aligned parallel and normal to the nanorod. Figure 3. (a) Comparison of the emission spectra from the plasmonic nanocavity, and a single nanorod without F-P facets but on a plasmonic substrate (SP-WG) and a single nanorod on a SiO2 layer (Ph-rod); (b) μ-TRPL decay traces of the three samples. In both cases, all measurements are performed under identical conditions, namely, using a polarizer aligned along the nanorod rod axis, i.e., E//c, at room temperature. The excitation power density used is 2.8 kW/cm2. Figure 4. PL spectra measured with using a polarizer on a condition of E//c (a) and E⊥c (c) as function of excitation power density. Corresponding L-L curve and FWHM are plotted as a function of excitation power density measured on the condition of E//c (b) and E⊥c (d), respectively. The dash-lines are guides to eyes.
9
10
11
12
13
Highlights:
A high performance plasmonic nanocavity coupled with multi-quantum wells fabricated by cost-effective dry-etching methods.
The carrier dynamic of the emitter coupled with the plasmonic nanocavity greatly enhanced owing to a “polarization matching” process.
The proposed “polarization matching” plasmonic nanocavity serving as an excellent platform to study light-matter interactions.
14