Letter pubs.acs.org/journal/apchd5
High-Yield Plasmonic Nanolasers with Superior Stability for Sensing in Aqueous Solution Suo Wang,†,§ Bo Li,†,§ Xing-Yuan Wang,† Hua-Zhou Chen,† Yi-Lun Wang,† Xiao-Wei Zhang,† Lun Dai,†,‡ and Ren-Min Ma*,†,‡ †
State Key Lab for Mesoscopic Physics and School of Physics, Peking University, Beijing 100871, China Collaborative Innovation Center of Quantum Matter, Beijing 100871, China
‡
S Supporting Information *
ABSTRACT: Plasmonic nanolasers with strong field confinement beyond the diffraction limit are an emergent tool for various applications, ranging from on-chip optical interconnectors to biomedical sensing and imaging. However, despite the rapidly advanced research in plasmonic nanolasers, there is no study on their stability and yield yet. Here, we systematically study the stability and yield of plasmonic nanolasers and reveal that surface passivation is crucial for them to operate in biocompatible aqueous solution with high stability and yield. We further demonstrate passivated plasmonic nanolasers as refractive index sensors. The figure of merit of intensity sensing is ∼8000, which is about 40 times higher than a state-of-the-art surface plasmon resonance sensor. Our results hold promise for practical applications of plasmonic nanolasers in super-resolution imaging, ultrasensitive sensing, and detection. KEYWORDS: plasmonic nanolasers, spasers, high yield, stability, surface plasmon resonance sensing, loss compensation
A
on their stability and yield yet, which is apparently crucial for any of their practical applications. In this work, we systematically study the stability and yield of plasmonic nanolasers. We show that the as-fabricated plasmonic nanolasers based on CdS and CdSe gain materials can be stably operated in an ambient environment, while they barely survived in aqueous solution due to photochemical reactions. We employ a step of surface passivation, which improved significantly the yield of stable plasmonic nanolasers to be over 68%. We further demonstrate these passivated plasmonic nanolasers as refractive index sensors, which have a superior performance than that of state-of-the-art surface plasmon resonance (SPR) sensors.
nanoscale laser spot can significantly enhance light− matter interactions and has been widely employed in various advanced technologies spanning from ultradense data storage and nanolithography to SERS and super-resolution imaging.1,2 Conventional lasers cannot directly generate nanoscale laser spots due to the diffraction limit. The nanoscale laser spot is usually obtained from the focus of a conventional laser by a plasmonic lens. In 2003, a new class of quantum amplifiers, named a spaser or plasmonic nanolaser, was proposed where the surface plasmons can be amplified to lasing state by stimulated emission of radiation.3 Plasmonic nanolasers are no longer limited by the diffraction limit of photons and thus can directly generate laser spots at the nanoscale. Until now, the fundamental physics of plasmonic nanolasers is rather well understood and widely investigated.4−16 The next phase of plasmonic nanolaser research, i.e., the study of its practical application, remains challenging. Recently, plasmonic nanolasers have been employed in applications of on-chip photonic circuits and sensing.17−19 However, there is no study © XXXX American Chemical Society
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RESULTS AND DISCUSSION We first studied the stability and yield of plasmonic nanolasers with CdS as gain material.5,7,17,18,20,21 The device consists of a Received: April 29, 2017 Published: May 26, 2017 A
DOI: 10.1021/acsphotonics.7b00438 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 1. Performance of three kinds of plasmonic nanolasers in aqueous solution. (a) CdS nanosquare plasmonic nanolasers cannot lase in water. (b) CdSe nanosquare plasmonic nanolasers can lase but usually only last for tens of seconds. (c) Passivated CdSe nanosquare plasmonic nanolasers can lase stably in water. The overlapped spectra indicate no observable lasing intensity decrease. Top panels in (a)−(c): device schematics of side view. Bottom panels in (a)−(c): emission intensity traces.
Figure 2. Yield of stably operated plasmonic nanolasers with and without passivation. (a, b) Continuous trace of emission spectra of CdSe nanosquare plasmonic nanolasers without a Al2O3 passivation layer (a) and with a Al2O3 passivation layer (b). (c) Yield of CdSe nanosquare plasmonic nanolasers without and with Al2O3 passivation.
nanolasers (see Methods) and tested them first in ambient conditions (Supporting Information, S3). These devices show continuous lasing over hours without detectable degradation. We then put these devices in water. Over 50 devices have been tested; however, none of them show a lasing signal even for a
CdS nanosquare on top of a Ag thin film separated by a 5 nm SiO2 insulator layer (Supporting Information, S1). The feedback mechanism is based on the total internal reflections of a metal−insulator−semiconductor plasmonic gap mode (Supporting Information, S2).7,22 We fabricated CdS plasmonic B
DOI: 10.1021/acsphotonics.7b00438 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 3. Passivated plasmonic nanolasers for refractive index sensing. (a) Light−light curve of a typical device lasing in solutions with refractive indexes of 1.36404 and 1.3380. (b) Line width evolution with the pump power. There is a significant line width narrowing effect from a fwhm of ∼25 nm below threshold to ∼0.35 nm above. (c) Time-resolved emission of the device at the spontaneous emission region (black) and stimulated emission region (blue). The instrument response function is also shown (red). (d) Above the lasing threshold, the lasing emission peak shows a clear shift with the refractive index of the introduced solution. Such a shift keeps constant with the change of the pump power. In panels (a), (b), and (d), data points in red represent the device in solution with a refractive index of 1.36404, while data points in olive represent the device in solution with a refractive index of 1.3380.
emission lasts only about 300 s. In stark contrast, the lasing emission of a passivated device shows no observable degradation in the measured 3600 s, as shown in Figure 2b. For devices without passivation, 15 out of 73 total devices measured show lasing signal at the beginning of pump, and three devices show stable operation with no observable degradation in 600 s (Figure 2c left). For the passivated devices, 65 out of 85 total devices measured show lasing signal at the beginning of pump. Out of these 65 lasing devices, 58 devices show stable operation with no observable degradation in 600 s (Figure 2c, right). We can see that the overall yield of stable operation was dramatically improved from 4.1% to 68.2% by surface passivation. A high yield of stable operation can help plasmonic nanolasers to make a step toward real world applications, especially for those applications relying on near-field effects such as sensing and imaging.18,19,25−27 In the following, we show that these passivated plasmonic nanolasers can serve as high-performance refractive index sensors in an aqueous solution environment. The working principle follows the recently reported sensors based on lasing-enhanced surface plasmon resonance (LESPR).18,19 Compared to conventional SPR sensors, LESPR sensors have a much narrower resonance line width due to the loss compensation by stimulated emission. Although the LESPR sensors have demonstrated superior performance compared to their passive counterparts in air and alcohol, it has not been studied if they can work in a biocompatible aqueous solution environment.
second. Figure. 1a, bottom, shows a typical spontaneous emission intensity trace of a CdS plasmonic nanolaser in water. We can see a very fast degradation in its emission intensity. In less than 20 seconds, the intensity drops more than an order of magnitude. Another kind of plasmonic nanolasers are then tested with CdSe as gain material and Au as substrate (Figure 1b). We choose this material combination for two reasons: (1) The CdSe emits at 700 nm, which is within the biochemical window wavelength;23 (2) Au is much more stable than Ag although with higher ohmic loss. We found that while this kind of plasmonic nanolaser can lase stably in ambient conditions, most of them cannot operate stably in water. The lasing signals usually last for only about tens of seconds (Figure 1b, bottom). The fast degradation of both CdS- and CdSe-based plasmonic nanolasers in water is due to the reaction of photogenerated holes with active sites at the crystal surface.24 To improve the stability of plasmonic nanolasers, a layer of Al2O3 is employed to passivate the device from photochemical reactions. Atomic layer deposition (ALD)-grown Al2O3 (7 nm) was deposited conformably on the CdSe plasmonic nanolaser (Figure 1c) (see Methods). As shown in Figure 1c, bottom, the device can lase in water, and there is no observable intensity decrease after 270 s of continuous exciting. The yield of stable operation is studied statistically on CdSe plasmonic nanolasers with and without passivation by measuring 158 devices (Figure 2). Figure 2a shows a typical decay process of the emission spectrum of a CdSe plasmonic nanolaser in water without a passivation layer. The lasing C
DOI: 10.1021/acsphotonics.7b00438 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 4. Calibration curve and figure of merit of a passivated plasmonic nanolaser for sensing. (a) Red: calibration curve of the device. Black: the peak line width does not change with the refractive index. (b) Full lasing spectra of the device in the five solutions with varied refractive indexes. (c) Intensity detection figure of merit of the device.
From Figure 4, the Sλ and FOMλ are obtained to be about 22 nm/RIU and 51, respectively. The FOMλ is higher than the theoretical calculated FOMλ for LSPR sensors (∼20) and comparable to that of state-of-the-art SPR sensors (∼50) (Supporting Information, S5).28 The intensity detection figure of merit of our device is ∼8000, as shown in Figure 4c, which is more than 40 times higher than state-of-the-art SPR sensors at the same wavelength (Supporting Information, S6).29
The performance of passivated plasmonic nanolasers as refractive sensors is studied by monitoring the lasing emission wavelength shift with the change of its surrounding refractive index. A mixed solution of water and glycerine is introduced to the devices in a chamber, where the different mixture ratios give different refractive indexes. Figure 3 shows a device lasing in solutions with refractive indexes of 1.36404 and 1.3380. The device has a tilted height of 210 nm to 1.09 μm, a length of 5.8 μm, and a width of 2.7 μm. The lasing behaviors of the device in both solutions are almost identical in an S-shaped light−light curve (Figure 3a) and line width evolution (Figure 3b). Figure 3c shows the time-resolved emission of the device, indicating a significant reduction in emission lifetime from the spontaneous to stimulated emission region. Above the lasing threshold, the lasing emission peak shows a clear shift with the refractive index of the introduced solution. Such a shift keeps constant with the change of the pump power, showing a reliable detection at varied pump levels of the device (Figure 3d). The performance of the device is further studied in solutions with five refractive indexes. Figure 4a shows the refractive index dependence of the peak wavelength shifts (calibration curve) and the peak line width of the lasing spectra after the onset of the lasing. Figure 4b shows the full lasing spectra of the device in the five solutions with varied refractive indexes. We can see that the peak wavelength shift has a linear dependence on the refractive index change, while the peak line width remains almost the same, which indicates that the change of refractive index results in only the shift of the lasing wavelength but not the lasing state. The simple linear relation of lasing wavelength shift with the change of environment refractive index is an advantage for the postprocessing in practical applications and leads to a minimum distinguishable quantity in refractive index of 0.00385 (Supporting Information, S4). Two key parameters, sensitivity and figure of merit, are employed to characterize the sensor performance quantitatively. The sensitivity of a sensor is defined as the rate of change of the information parameter (wavelength λ or intensity I here) magnitude measured with the analyte refractive index change Δn. An important dimensionless quantity defined as the figure of merit (FOMP) is further used, which takes into account the sharpness of the resonance and thus can be used as a comparative parameter between different kinds of sensing devices. The sensitivity Sλ and FOMλ of the wavelength sensing can be calculated according to S λ = Δλ/Δn and FOMλ =
Δλ / Δn fwhm
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CONCLUSION In conclusion, we demonstrate that (I) two kinds of typical plasmonic nanolasers can be stably operated in ambient environment while barely surviving in aqueous solution; (II) a step of surface passivation can improve significantly the yield of plasmonic nanolasers with a superior stability of over 68%; (III) the passivated plasmonic nanolasers can serve as refractive index sensors in a biocompatible environment with superior performance than that of state-of-the-art SPR sensors. The surface passivation method used here can be employed in other nanolaser systems to improve the stability and yield. Our results will help plasmonic nanolasers to make a step toward real world applications, especially for those applications relying on nearfield effects such as sensing and imaging.
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METHODS Materials Growth. CdS and CdSe nanosquares were synthesized via the chemical vapor deposition method.30 CdS (99.995%) and CdSe (99.99%) powders were used as the source, respectively. Si wafers covered with 10 nm thick Au catalysts were used as the substrates. Before heating, high-purity argon (Ar) was used to clean the quartz tube inside a tube furnace for 90 min. For CdS, the furnace was rapidly heated to 850 °C under a constant Ar flow with a rate of 200 sccm. Then a quartz boat loaded with CdS powder and Si substrates was inserted into the tube. The synthesis duration was 2 h. For CdSe, the growth temperature and duration were set to be 700 °C and 0.5 h under a high-purity Ar flow with a flow rate of 100 sccm. Device Fabrication. The CdS nanosquares were transferred onto the SiO2/Ag (5 nm/500 nm) substrates, which were deposited by magnetron sputtering, and the CdSe nanosquares were transferred to the MgF2/Au (5 nm/200 nm) substrates that were deposited by e-beam evaporation. Some of the CdSe nanosquare samples were coated with a 7nm-thick Al2O3 thin film through ALD growth. Atomic Layer Deposition of Al2O3. The 7 nm Al2O3 was deposited by an ALD-P-100B (produced by SVT Associates). Deionized water and trimethylaluminum were used as source, and the growth temperature was 120 °C. We set the
respectively, where fwhm is the full width
at half-maximum of a lasing peak. The sensitivity SI and FOMI of the intensity sensing can be calculated according to SI = ΔI(λ)/Δn(λ) and FOMI = max
ΔI(λ) / Δn(λ) I(λ)
, respectively.28 D
DOI: 10.1021/acsphotonics.7b00438 ACS Photonics XXXX, XXX, XXX−XXX
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atmospheric pressure to 0.2 Torr. The growth rate was 1.4 Å per cycle. Lasing and Stability Characterization. The CdS and CdSe devices were measured under a picosecond pump laser (λpump = 405 nm, repetition rate 3.8 MHz, pulse length 3 ps) and a nanosecond pump laser (λpump = 532 nm, repetition rate 1 kHz, pulse length 4.5 ns), respectively. A 20× objective lens (NA = 0.4) was used to focus the pump beam to a ∼10 μm diameter spot onto the sample and to collect the luminescence. LESPR Characterization. We characterize the performance of our sensors in a chamber with two ports for changing of solutions with varying refractive indexes and an optical window for both pumping and signal collection. The solution was a mixture of water and glycerine of various ratios. All experiments were carried out at room temperature.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00438. Morphology of nanosquare plasmonic nanolaser (Section S1), feedback mechanism of nanosquare plasmonic nanolaser and mode profile (Section S2), lasing spectrum and light−light curve for CdS plasmonic nanolaser in ambient conditions (Section S3), calibration curve of the passivated plasmonic nanolaser (Section S4), wavelength sensitivity and FOM of SPR sensors (Section S5), intensity sensitivity and FOM of SPR sensors (Section S6) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail (R.-M. Ma):
[email protected]. ORCID
Suo Wang: 0000-0001-5386-9170 Lun Dai: 0000-0002-6317-6340 Author Contributions §
S. Wang and B. Li contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 11574012, 61521004, and 11474007), the “Youth 1000 Talent Plan” Fund, and National Basic Research Program of China (No. 2013CB921901).
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REFERENCES
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