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Wavelength tunable plasmonic lasers based on intrinsic self-absorption of gain material Qing Zhang, Qiuyu Shang, Jia Shi, Jie Chen, Rui Wang, Yang Mi, Wenna Du, Chao Shen, Renmin Ma, Xiaohui Qiu, Xinfeng Liu, and Tze Chien Sum ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00757 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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Wavelength tunable plasmonic lasers based on intrinsic self-absorption of gain material Qing Zhang,1* Qiuyu Shang,1 Jia Shi,2 Jie Chen,2 Rui Wang,2 Yang Mi,2 Wenna Du,2 Chao Shen,3 Renmin Ma,4 Xiaohui Qiu,2 Xinfeng Liu2*, Tze Chien Sum5* 1

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China

2

Division of Photonics, CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center of Excellence for Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China 3

State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China

4

State Key Lab for Mesoscopic Physics and School of Physics, Peking University, Beijing 100871, China

5

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 *To whom correspondence should be addressed, Email address: [email protected],

[email protected] and [email protected]

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Abstract Multicolor plasmonic nanowire lasers enable below diffraction-limit directional optical waveguiding and amplification vital to the development of next generation compact on-chip optical communications, super-resolution imaging, and display technologies etc. However, progress in developing these compact lasers for different wavelengths is severely curtailed by the few complex fabrication methods available. In this work, we demonstrate wavelength-tunable plasmonic nanowire lasers by leveraging the intrinsic optical self-absorption of the gain medium. The plasmonic lasing wavelength is tunable from 465 to 491 nm by simply adjusting the nanowire length – i.e., by approximately 76% over the interval width of the emission spectrum. The Purcell factor of plasmonic nanowire cavity decreases with increasing nanowire length; while the propagation loss increases from 1020 to 8354 cm-1 with decreasing nanowire diameter – exhibiting a plasmonic-photonic mode transition at diameters around 120-150 nm. Importantly,

our findings

advance the fundamental

understanding and provide a new approach for engineering wavelength tunable plasmonic lasers. Keywords: Plasmonic lasers, CdS Nanowire, Self-absorption, Wavelength tunable, Surface plasmon polariton

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In the last two decades, semiconductor nanowires have unleashed their potential as building blocks for integrated on-chip optical and optoelectronic devices, such as waveguide, laser, detector, etc.1-5 However, constrained by the optical diffraction law, the dimensions of nanowire photonic devices is still above half the wavelength of light.6,

7

One viable approach would be to leverage hybrid metal-semiconductor

plasmonic nanostructures, where the optical energy could be stored in the collective electron oscillations at a metal-dielectric interface8, also known as surface plasmons. This permits further shrinking of photonic devices into the sub-wavelength region. Similar to photonic lasers, plasmonic lasers are based on the amplification of surface plasmons by stimulated emission of radiation with the semiconductor also serving as the gain material. To date, plasmonic lasers have been experimentally demonstrated through several approaches involving the amplification of both localized surface plasmons (LSPs) in nanoparticle geometry and propagating surface plasmon polaritons (SPPs) in planar structures.9-18 Hybrid metal-semiconductor plasmonic nanowire lasers enable ultra-small below diffraction-limit coherent light sources, which not only provides an interesting platform for investigating light-matter interactions and quantum optics, but is also highly relevant for applications in ultra-compact fast photonic circuits, on-chip information processing, data storage, super-resolution biomedical imaging.9, 19-21 Nevertheless, a key challenge is to realize multicolor and broadband plasmonic nanolaser sources to meet the divergent needs of these technological applications. The extrinsic methods available to modulate the laser properties are few and far between (e.g., varying composition of ternary 3 / 25


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semiconductors, which either relies on complicated fabrication procedures or introduces inevitable defects undesirable for eventual device performance). For plasmonic nanolasers, a semiconductor nanowire gain medium is used to compensate the high SPP propagation absorption loss in metallic nanocavities via the near-field coupling of exciton and surface plasmons. The output lasing wavelengths from the plasmonic nanocavity is determined by both the plasmonic resonance frequencies and emission wavelengths of the gain medium. In the LSP system such as single and periodic arrays of metallic nanocavities, lasing occurs around LSP resonance frequency.22,23 While in hybrid plasmonic waveguide lasers, which support wide-band SPP, lasing modes vary with the gain spectra of semiconductors. Therefore, the excitonic emission properties of the nanowire dominate the behavior of plasmonic lasers. Emulating semiconductor photonic lasers, multicolor plasmonic nanolasers over the visible range were obtained directly by tuning the elemental compositions in a single InxGa1-xN@GaN core-shell nanowire.24 Apart from this wavelength tuning approach, we had previously demonstrated two other methods to realize multicolor nanowire photonic lasers. Both focus on tuning the excitonic properties of the gain material. Our first approach involves utilizing the intrinsic exciton self-absorption to achieve a redshift of the nanowire laser wavelengths by simply increasing the nanowire length.25 Our second approach utilizes the plasmon enhanced Burstein-Moss effect (see Supporting information, Figure S1) to achieve a blue shift of the nanowire laser wavelengths.26 In shorter nanowire photonic lasers, Burstein-Moss effect dominates; while for longer nanowire lasers, the self-absorption effect plays a leading 4 / 25


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role. In the hybrid plasmonic nanowire laser system, both effects could be further enhanced under the influence of strong exciton-plasmon interactions and would be non-trivial. Nonetheless, these concepts are rarely explored in the plasmonic nanowire laser system.

In this work, we demonstrate wavelength-tunable plasmonic nanowire lasers by leveraging the intrinsic self-absorption property of the gain material. Due to the self-absorption of the gain material, the emission spectra will redshift over the bandage and extend to the Urbach tail region at the lower energy side. The Urbach tail is defined as the exponential tail near the fundamental optical absorption edge (see Figure 1a), which originates from the fluctuation in electronic energy bands because of the strong exciton-phonon coupling in undoped semiconductors.27-29 This effect is significantly enhanced in the one-dimensional (1-D) nanowire system compared to their bulk counterparts due to the stronger phonon-exciton coupling under confinement.30-34 Due to the intrinsic absorption from the Urbach tail, the emission spectrum will be red-shifted as propagation occurs within the nanowire cavity, as illustrated in Figure 1b. This means that the gain profile which is used to amplify the nanowire plasmonic laser will also red-shift with increasing nanowire length. Wavelength-tunable plasmonic nanowire lasers can thus be easily achieved by simply adjusting the nanowire length. Results and Discussion Figure 1c shows a schematic of the plasmonic nanowire laser, which consists of a 5 / 25


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high quality single crystalline cadmium sulfide (CdS) nanowire (see Figure S2), a silver (Ag) film (see Figure S3) and a thin spacer layer SiO2 between CdS nanowire and Ag film (see Figure 1c). The detailed fabrication methods for CdS nanowires, Ag film and SiO2 spacer layer can be found in previous literature.35, 36 The CdS NWs are dispersed into isopropyl alcohol via ultra-sonication and then transferred onto Ag film using a drop-casting method.24 The length and diameter of the CdS nanowire are determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements, respectively. To inhibit photonic lasing, CdS nanowires with diameters less than the cut-off dimension of photonic modes (~120-150 nm) are selected for plasmonic laser devices. The thickness of SiO2 spacer layer is approximately 5-10 nm. This ensures effective exciton-plasmon energy transfer while preventing emission quenching between semiconductor CdS nanowire and metallic Ag film. A hybrid plasmonic mode can be supported along the interface of CdS nanowire and silver film (Figure 1d). In this case, light could be concentrated into an extremely small volume, i.e. several nanometers by storing its energy into collective oscillations of free electrons or surface plasmons. Photonic nanowire lasers containing CdS nanowire placed on quartz substrate are also fabricated for comparison.25 The plasmonic and photonic laser devices are optically pumped by a pulsed femtosecond laser with excitation wavelength of 400 nm (see Methods section). The excitation beam is beam-expanded to cover the whole laser device to increase injection efficiency and reduce heating effects. The scattered emission from the two end-facets of

the

nanowires

are

collected

and

measured

by

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a

reflection

mode

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micro-photoluminescence spectrometer (see Methods and Figure S4). All the laser devices are cooled down to 77 K to increase the gain of CdS nanowire and to compensate the plasmon propagation losses.

Figure 2a shows the emission spectra from a plasmonic nanowire laser device (CdS length ~12 µm, diameter ~115 nm, SiO2 thickness ~5 nm) as the pump fluence is increased from 8.3 to 118 µJ/cm2 (bottom to up). The light input-light output (L-L) plot and full width at half maximum (FWHM) of the lasing peak at 486 nm are extracted and plotted in Figure 2b. The SEM (panel I) and optical image (panel II) of the as-fabricated nanowire laser devices are shown in Figure 2a inset. At relatively low pump fluence (˂55 µJ/cm2), a broad emission peak (with FWHM ~7 nm) is observed, which is ascribed to the spontaneous emission of CdS nanowire. With increasing pump fluence from 55 to 75 µJ/cm2, a narrower peak with FWHM of ~3.3 nm appears above the broad spontaneous emission peak that is attributed to amplified spontaneous emission. Lasing occurs when the pump fluence is over 75 µJ/cm2. The relation between the pump fluence versus output intensities can be well-fitted by a classic rate equation fitting model (see Supporting information, Figure S5) with a spontaneous emission coupling factor (β ) of ~0.21. However, for photonic CdS nanowire lasers, the corresponding spontaneous emission coupling factor of β is ~0.028 (see Supporting information, Figure S6). β, which is indicative of the spectra match between the gain emission and the longitudinal cavity SPP mode, is approximately one order of magnitude larger than that reported in photonic nanowire lasers. For example, in GaAs and ZnO photonic nanowire lasers, the value of β is 7 / 25


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reported to be 0.015 and 0.016, respectively.37, 38 Oulton et al also demonstrated that

β factor increases from 0.026 for photonic nanowire laser to 0.38 for plasmonic nanowire lasers.11 Recently, using molecular-beam-epitaxial grown silver film Lu et al reported a nearly 100% spontaneous coupling efficiency (β =1) as a result of strong spatial confinement of electromagnetic fields inside the Al2O3 area, which leads to an ultralow lasing threshold, i.e., continuous wave pumped laser devices.24 In our case, the observed laser with β value of ~0.21 is postulated to be from plasmonic mode lasing (more evidences will be presented and discussed in the following paragraphs).

As propagating losses (e.g., Ohmic and radiative losses) are compensated by semiconductor gain, hybrid plasmonic modes resonate and oscillate between the two end facets of CdS nanowire. When optical pump energy is above threshold, plasmonic lasing can be realized by the longitudinal plasmonic cavity modes. To observe oscillations of plasmonic mode, far-field emission images of plasmonic nanowire lasers are recorded with pump fluence below (50 µJ/cm2, panel III, Inset Figure 2a) and above threshold (110 µJ/cm2, panel IV, Inset Figure 2a). In a plasmonic laser, strong emission can be observed at the two end-facets below plasmonic lasing threshold, which contrasts with photonic nanowire lasers.37 In as-prepared CdS nanowire photonic lasers, unidirectional spontaneous emission dominates weak weak pumping and before population inversion is achieved. The directional emission behavior of plasmonic nanowire lasers is due to strong confinement of plasmonic modes along semiconductor-metal interface. Clear interference fringes can be seen above the lasing threshold, suggesting that coherent radiation is generated in the NW 8 / 25


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cavity.

To clearly distinguish between plasmonic and photonic lasing, polarization dependent lasing spectroscopy is also performed. The lasing emission scattering from the two end-facets of nanowires are collected and analyzed. In plasmonic nanowire lasers, the lasing polarization is parallel to the long axis of nanowire (z-direction); while for the photonic laser, the lasing polarization is perpendicular to the long axis of nanowire.1 As shown in Figure 2c, in CdS nanowire-SiO2-Ag devices, lasing emission along the long axis of the nanowire is much stronger than perpendicular components, indicating plasmonic lasing. In contrast, the photonic laser has a dominant lasing emission perpendicular to the long axis of the nanowire (Supporting Information Figure S7).

Our previous work shows that the gain profile of a semiconductor nanowire redshifts with increasing nanowire length due to the intrinsic self-absorption effect. This effect gives rise to the nanowire length dependent spontaneous emission and lasing wavelength. Here we demonstrate that the plasmonic nanowire lasing wavelength can also be tuned by this self-absorption effect. To minimize the plasmonic lasing wavelength shifts caused by the difference of nanowire diameter, CdS nanowires with length L from 26.4 to 5.2 µm and comparable diameter (110±10 nm) are selected. The lasing spectra of the as-prepared CdS nanowire-SiO2-Ag film plasmonic lasers are shown in Figure 2d. The lasing wavelength red-shifts from 465 to 491 nm when the nanowire length increases from 5.2 to 26.4 µm. The FWHM for 9 / 25


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the plasmonic laser peaks are ~3.3 nm, which is comparable with previous values (~2-4 nm).12 The lasing FWHM is relatively larger than their photonic laser counterpart (~2 nm) because of high metallic losses for the plasmonic mode. Another distinctive feature noted is the increasing threshold of plasmonic lasing in longer nanowire device. This length dependent behavior is contrary to that for photonic lasers, where a longer nanowire laser exhibits lower threshold. This indicates that the loss mechanism in the plasmonic lasers is different from their photonic counterparts.

To understand the length dependent lasing behavior, systematical studies on optical waveguiding in CdS nanowire-SiO2-Ag structure is conducted. The self-absorption effects are validated by remote-excitation of the photoluminescence as shown in Figure 3. A focused 405 nm laser beam is used to excite different locations of the CdS nanowire. The emission from one of the CdS nanowire end-facets are collected and analyzed by spectrometer. It should be noted that the spectra profile of the emission from the nanowire end facet is dependent on the cavity length and independent of the position of the excitation point (see Supporting information, Figure S8). Therefore, we excited the center of the plasmonic devices and monitored the emission from the end facet of the nanowire, as shown in Figure 3 inset. By measuring the out-coupled spectra for the plasmonic devices with different lengths, the gain profile for the length-dependent CdS/SiO2/Ag structure can be plotted. Figure 3a shows the normalized photoluminescence (PL) spectra collected at the excitation point (B) and the left facet (A) of nanowire for a representative plasmonic device with CdS length of ~20 µm and diameter of 120 nm. Clear redshift of the 10 / 25


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emission peak can be observed from 480 nm (at point B) to 491 nm (at point A). The spectrum measured at point B is the emission without any propagation, in contrast with that at point A, which had undergone propagation. Due to the self-absorption process, the high-energy side of the emission profile was absorbed, leaving behind an emission profile with a red-shifted main peak. To capture the length-dependent redshift behavior, a series of plasmonic devices are prepared with comparable diameter (110±10 nm) and their emission spectra from one facet of the nanowire collected. The corresponding spectra for nanowires of different lengths are measured and presented in Figure 3b. With increasing nanowire length, a large red-shift of the peak wavelength (marked with red crosses) due to the length dependent intrinsic self-absorption is observed. The red-shift trend of the emission peak is similar to that for the plasmonic laser when altering the length of nanowire. Hence, it is suggested that the plasmonic wavelength tunability is qualitatively related to the intrinsic property of self-absorption. Next, quantitative analysis is conducted to correlate the self-absorption processes in these plasmonic devices with theoretical calculations.

Theoretically, the absorption coefficient α of a semiconductor can be described by the following equations after taking into consideration the contributions from the Urbach tail,29, 39, 40 exp − , ≥ = exp − , E <

(1)

where A0 is a constant (2×105 cm-1 (eV)-1/2), which represents the saturation of α(E) for energy above the bandgap energy; Eg is the band gap energy of the gain material; 11 / 25


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kBT is the thermal energy at 77 K; σ is a dimensionless phenomenological fitting parameter, which is an indicator of the degree of disorder in crystalline materials,41 σ ~1 was used in our calculation due to the good crystallinity of the CdS nanowire; the energy E = hv with v the photon frequency, h is the Planck’s constant, and Ecr = Eg+kBT/2σ is the crossover energy. Based on the absorption coefficient, the relationship between original intensity I0(hv) and the intensity Ix(hv) after propagating a distance, x, inside the cavity can be expressed by the following equation. In this process, the redshift of the emission peak is observed to be due to the intrinsic self-absorption of gain material:

I x ( hv ) =

(1 − R ) [1 − exp ( − xα (hv) )] I xα (hv)

0

( hv )

(2)

where x is the distance between the excitation point and the collection point, R is the surface reflectance of the sample, E = hv is the photon energy.42 By combing equations 1 and 2, the propagation length (i.e., the CdS nanowire length) dependent emission spectra were calculated as shown in Figure 3c. For clearer comparison with the experimental data for different lengths of nanowires, both the experimental and theoretical data were plotted together (see Figure 3c, the gray triangles are the fitting data; the blue circles are the experimental data extracted from Figure 3b). From Figure 3c, the experimental and theoretical data of the emission peak positions overlap very well, suggesting that the intrinsic self-absorption mechanism correctly describes of the length dependent redshift of the gain material. However, it should be noted in Figure 3c that there are large deviations between the PL spontaneous emission peaks and the plasmonic lasing peaks (solid green squares, data extracted 12 / 25


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from Figure 2d). Compared to the spontaneous emission peak energy, evident blue-shift is observed for the plasmonic laser modes for CdS nanowire devices with a specific length. The blue-shift behavior (see Supporting information, Figure S8) is much more obvious in the shorter nanowire devices compared to those that are longer. We would like to emphasize that these two sets of experiments (PL emission profile peaks and the lasing peaks) were performed under different experiment conditions: the emission profiles are measured using a CW-laser with relatively weak pump fluence; while femtosecond laser pulses are needed to obtain lasing from plasmonic nanowire devices due to the large losses in the system. At higher pump fluence (especially for the plasmonic laser system, the threshold for lasing is one order larger than that of photonic laser system), there is a combined effect of carrier screening and band filling (both blue shifting), and the competing effect of bandgap renormalization (red shifting). Given the evident blueshift observed in our experiments, the first two effects dominate in our case, leading to the deviations between PL emission and lasing wavelength (Figure 3c). With these evidences, wavelength-tunable plasmonic nanowire lasers based on intrinsic self-absorption of semiconductor nanowire are clearly demonstrated.

The strong localized fields in the gap region between the base of the nanowire and the metal layer (see Figure 1d) in plasmonic lasers provide an excellent platform for investigating light-matter interactions. When an emitter is in close proximity to the gap region, the surface plasmon induced strong electric field modifies its emissive behavior, leading to reduced lifetime of spontaneous emission. The ratio between the 13 / 25


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modified and free-space emission rates is known as the Purcell factor. Determining the Purcell factor provides an estimate of the system losses. To minimize the heating and exciton‒exciton scattering effects, time-resolved PL decay transients (as shown in Figure 4a) were measured under weak pumping conditions (with the laser source being an oscillator (Mira 900F), average power at 50 µW). The reference sample is a photonic CdS device that is dispersed on quartz substrate with a diameter of 220 nm. Its lifetime is almost the same as the bulk CdS crystal, which is defined as the free-space lifetime of CdS and is taken to be the reference sample (see Supporting information, Figure S9). Length dependent Purcell factors from different plasmonic devices (from 3 µm to 20 µm) with the same gap of 5 nm can be calculated using the lifetime ratio between reference sample and the lifetime of plasmonic devices. The results are plotted in Figure 4b. The Purcell factor increases with decreasing nanowire length in the plasmonic devices. From the literature, the Purcell factor (F) can be expressed in the following equation F =

Γ g 3 λ 3  Q  43 ( )   . It indicates that Γ0 4π 2 n  V 

F has a linear relationship with the Q/V, where Q is the quality factor of the hybrid cavity mode and V is the cavity mode volume. Therefore, the increased Purcell factor should be due to the larger Q/V brought upon by decreasing the cavity length. The observed behavior is consistent with a previous report by Ma et al,12 where the increase in Purcell effect with the decrease in cavity side length was ascribed to the reduction in mode volume whilst possessing a high quality factor. Based on theoretical calculations, the Purcell factor with different cavity lengths can also be described by the following expression F ≅ F∞ [1 + β Lcsch(α L / 2)] , where F∞ is the 14 / 25


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known enhancement factor for long cavities, is the round-trip cavity loss and

β is

a fitting parameter.12 The best fit using the above equation in Figure 4b reveals the

important parameter of system loss ~8130 cm-1. This value is comparable with the

system loss in CdS square cavity plasmonic laser system with a loss parameter ~6323 cm-1 reported in reference 12. The comparable values in these two systems also validate the plasmonic mode lasing in our current system.

To compare the system loss between plasmonic and photonic devices, the propagation loss in these two types of devices are investigated using the same experimental method as described in the inset of Figure 3a. In contrast to simply monitoring the emission profile, here the facet emission intensities are collected and analyzed to estimate the total system loss using the following description I = I0e-αL, where L is the propagation length and α is the propagation loss. Figure 5a shows the propagation distance dependent normalized output intensities (red dots) for a representative CdS plasmonic device with diameter ~320 nm of CdS nanowire. The inset images of Figure 5a show the corresponding images with different excitation positions of the nanowire device. Using the same method, different diameters of nanowire plasmonic devices were also collected as shown in Figure 5b. Fitting using the above exponential decay equation, the values of α obtained are: ~1020, 1705, 2832 and 8345 cm-1 for the CdS nanowires with diameters of ~320, 215, 154 and 110 nm, respectively. The propagation loss increases with decreasing CdS nanowire diameter. Specifically, for the plasmonic device with CdS nanowire diameter of 110 nm, the propagation loss is around ~8345 cm-1. This value is very close to the value 15 / 25


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(~8130 cm-1) estimated from fitting the Purcell factors in this device, illustrating the consistency of these two methods. The nanowire diameter dependent propagation loss (α) was plotted in Figure 5c. There exists a sharp increase in the α value when the diameter of nanowire is around 135 nm (the gray area). This is because when the nanowire is below ~135 nm (area I in Figure 5c), pure plasmonic modes exist due to the forbiddenness of the photonic modes, leading to a much larger propagation loss. However, when the nanowire diameter is larger than 135 nm (area III in Figure 5c), both plasmonic and photonic modes can be supported, therefore a relatively low loss should be expected. It also indicates that only pure plasmonic mode lasing are expected

in

thinner

CdS

nanowire

based

devices

for

these

semiconductor-dielectric-metal structures.

Conclusions In summary, a systematic study of semiconductor nanowire dimension dependent plasmonic nanowire lasing properties is presented. By adjusting the length of CdS nanowire, the plasmonic laser wavelength can be tuned from 465 to 491 nm due to intrinsic self-absorption. Moreover, with decreasing nanowire length, the Purcell factor increases, suggesting that the mode loss in plasmonic lasers is smaller in the shorter nanowires. Furthermore, optical propagation losses in individual plasmonic nanowire lasers is obtained, which increases from 1020 to 8354 cm-1 as the nanowire diameter decreases from 300 nm to 90 nm. A plasmonic and photonic mode transition around ~120-150 nm is demonstrated. Our results are not only important for

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advancing the fundamental understanding to realize multi-color, low threshold plasmonic laser, but are also critical for establishing the basic design rules needed for engineering high performance plasmonic lasers.

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Methods Plasmonic laser device preparation. High optical quality CdS nanowires were grown using chemical vapor deposition (CVD) of CdS powders on Si substrates by self-assembly from a 10-nm Au film seeding layer leading to random nanowire diameters ranging from 50 to 500 nm. CdS nanowires were deposited from IPA solution by spin-coating onto pre-prepared Ag films with varying SiO2 thicknesses of 5, 10, 20 and 50 nm, as well as the control devices of nanowires on a quartz substrate. Photoluminescence

and

lasing

characterizations.

A

frequency-doubled,

mode-locked Ti-sapphire regenerative amplifier laser (Coherent, Legend) was used to pump the plasmonic and photonic lasers (pump wavelength 400 nm, repetition rate 1 kHz, pulse length 120 fs). An objective lens (20×, numerical aperture 0.35) was used to focus the pump beam to a 40-µm-diameter spot on the sample. All experiments were carried out at low temperature, 77 K, using a liquid-nitrogen-cooled cryostat (Janis Research). Individual spectra were recorded using a spectrometer with a resolution of 0.30 nm and an electrically-cooled charge coupled device (CCD, Princeton Instruments). Time-resolved photoluminescence measurements. The lifetime measurements were conducted under very low pump conditions to avoid heating and exciton–exciton scattering effects using a streak camera system (Optronics GmbH, time resolution ~10 ps); the laser source used here is a frequency-doubled mode-locked Ti-sapphire oscillator laser (pump wavelength of 400 nm, repetition rate 76 MHz, pulse length 120 fs). A 425 nm long pass filter was used to filter out ambient light and pass light from the CdS exciton line near 488 nm. Simulation details. Simulations of plasmonic mode distributions were first performed in 1D (z direction) using the eigenmode method (MODE Solutions from Lumerical Solutions), and the simulated effective mode index was used to obtain the 2D (x and y directions) mode distributions using the finite-difference time-domain (FDTD) method (FDTD Solutions from Lumerical Solutions). 18 / 25


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Acknowledgements X.F.L thanks the support from the Ministry of Science and Technology (No.2016YFA0200700),

National

(No.21673054),

Research

Key

Natural

Science

Program

of

Foundation Frontier

of

China

Science,

CAS

(No.QYZDB-SSW-SYS031). Q.Z. acknowledges the funding support from the Ministry of Science and Technology (2017YFA0304600; 2017YFA0205700), National Natural Science Foundation of China (No. 61774003), open research funding program of the state key laboratory of low-dimensional quantum physics (KF201604) and one-thousand talent program from the Chinese government. T.C.S. acknowledges the support from the Ministry of Education Academic Research Fund Tier 1 grants RG101/15

and

RG173/16,

and

Tier

2

grants

MOE2014-T2-1-044,

MOE2015-T2-2-015 and MOE2016-T2-1-034. C.S thanks the support from NSFC Grant No. 11404324. We sincerely thank the strong support from Prof. Q. Xiong at NTU. Author contributions T.C.S, X.L and Q. Z conceived the idea and designed the experiments. Q.Z, Q. S, J.S, J.C, R.W, Y. M, W. D, C. S prepared the samples and performed optical absorption, PL spectroscopy measurements and AFM measurements. Q.Z and X.L conducted the lasing and time-resolved PL measurement. Q.Z, X.L performed the simulations. All the authors discuss the results and write the manuscript. T.C.S, X. L and Q.Z led the project. Additional information The Supporting Information is available free of charge on the ACS Publications website. Illustration of the BM effects on the band gap energy of CdS NW; TEM characterization of CdS NW; AFM characterization of Ag film surface; Schematic of the home-made optical setup; Light-pump curves fitted by using a rate equation analysis; Photonic laser behavior in CdS nanowire; Polarization dependent emission spectra in photonic CdS nanowire lasers; Excitation position irrelevant emission profile in one fixed cavity; Lifetime measurements of CdS NW on quartz and 19 / 25


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SiO2/Ag substrate. Correspondence and requests for materials should be addressed to T.C.S, X.L or Q.Z. Competing financial interest: The authors declare no competing financial interest.

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References (1) Oulton, R. F.; Sorger, V. J.; Genov, D. A.; Pile, D. F. P.; Zhang, X. A Hybrid Plasmonic Waveguide for Subwavelength Confinement and Long-Range Propagation. Nat. Photon. 2008, 2, 496-500. (2) Pyayt, A. L.; Wiley, B.; Xia, Y. N.; Chen, A.; Dalton, L. Integration of Photonic and Silver Nanowire Plasmonic Waveguides. Nat. Nanotech. 2008, 3, 660-665. (3) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Room-Temperature Ultraviolet Nanowire Nanolasers. Science 2001, 292, 1897-1899. (4) Johnson, J. C.; Choi, H. J.; Knutsen, K. P.; Schaller, R. D.; Yang, P. D.; Saykally, R. J. Single Gallium Nitride Nanowire Lasers. Nat. Mater. 2002, 1, 106-110. (5) Hayden, O.; Agarwal, R.; Lieber, C. M. Nanoscale Avalanche Photodiodes for Highly Sensitive and Spatially Resolved Photon Detection. Nat. Mater. 2006, 5, 352-356. (6) Yan, R.; Gargas, D.; Yang, P. Nanowire Photonics. Nat. Photon. 2009, 3, 569-576. (7) Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. D. Nanoribbon Waveguides for Subwavelength Photonics Integration. Science 2004, 305, 1269-1273. (8) 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, 027402. (9) Hill, M. T.; Marell, M.; Leong, E. S. P.; Smalbrugge, B.; Zhu, Y.; Sun, M.; van Veldhoven, P. J.; Geluk, E. J.; Karouta, F.; Oei, Y.-S.; et al. Lasing in Metal-Insulator-Metal Sub-Wavelength Plasmonic Waveguides. Opt. Express 2009, 17, 11107-11112. (10) 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, 1110-1112. (11) Oulton, R. F.; Sorger, V. J.; Zentgraf, T.; Ma, R.-M.; Gladden, C.; Dai, L.; Bartal, G.; Zhang, X. Plasmon Lasers at Deep Subwavelength Scale. Nature 2009, 461, 629-632. (12) Ma, R.-M.; Oulton, R. F.; Sorger, V. J.; Bartal, G.; Zhang, X. Room-Temperature Sub-Diffraction-Limited Plasmon Laser by Total Internal Reflection. Nat. Mater. 2011, 10, 110-113. (13) Marell, M. J. H.; Smalbrugge, B.; Geluk, E. J.; van Veldhoven, P. J.; Barcones, B.; Koopmans, B.; Nötzel, R.; Smit, M. K.; Hill, M. T. Plasmonic Distributed Feedback Lasers at Telecommunications Wavelengths. Opt. Express 2011, 19, 15109-15118. (14) Lakhani, A. M.; Kim, M.-k.; Lau, E. K.; Wu, M. C. Plasmonic Crystal Defect Nanolaser. Opt. Express 2011, 19, 18237-18245. (15) Lu, Y.-J.; Kim, J.; Chen, H.-Y.; Wu, C.; Dabidian, N.; Sanders, C. E.; Wang, C.-Y.; Lu, M.-Y.; Li, B.-H.; Qiu, X.; et al. Plasmonic Nanolaser Using Epitaxially Grown Silver Film. Science 2012, 337, 450-453. 21 / 25


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Figure captions Figure 1∣ ∣Wavelength tuning mechanism and the configuration of nanowire based plasmonic laser. (a) Absorption coefficient of a semiconductor with bandgap Eg versus the energy. The Urbach tail dominates the absorption near but below the bandgap. (b) When the CdS nanowire was excited at the left end with laser source, the emission light propagates inside the nanowire cavity, the emission intensity and color (wavelength) change with the propagation distance. (c) Schematic diagram of the plasmonic nanowire laser structure. Single CdS nanowire was dispersed on the SiO2-covered flat Ag film. The scanning electron microscopy (SEM) image on the upper right shows a representative plasmonic nanolaser device, the scale bar is 100 nm inset. (d) The simulated electric filed distribution of a blue plasmonic nanowire laser (wavelength ~489 nm). The cross-sectional filed plot along the yellow broken line illustrates the strong overall confinement in the gap region. The resonant field is tightly confined in the 5 nm thickness of SiO2 layer. The plasmonic cavity mode (red color) is formed between the nanowire and the Ag film and confined inside the SiO2 layer. Figure 2∣ ∣Characterization of the plasmonic laser and its lasing wavelength tunability. (a) Emission spectra for a 12 µm long CdS nanowire (with CdS/SiO2/Ag structure) with a diameter of ~115 nm which is photoexcited with different pump fluence ranging from 8.3 µJ/cm2 to 118 µJ/cm2, showing the transition to lasing action. Inset images from bottom to top: (I) optical microscope image, (II) SEM image, far-field image on CCD camera with pump fluence of (III) 50 µJ/cm2 (below threshold) and (IV) 110 µJ/cm2 (above threshold), respectively. The scale bar shown inset is 5 µm. (b) Characteristic L-L plots of pump fluence dependent integrated intensity and emission line width measured from single CdS nanowire plasmonic laser (solid blue diamond). The solid lines are fitted using a multi-mode lasing model as in reference 34. In this model, our fitting parameter β, is related to the gain saturation of individual longitudinal laser modes and their lateral mode area. The fitted β value is ~0.21. (c) Polarization-dependent lasing spectra for a CdS plasmonic nanowire laser with the polarization oriented parallel (black, 0 degree) and perpendicular (red, 90 degree) to the long axis of CdS nanowire. Inset is a polar plot of the lasing emission intensity. (d) The lasing peak wavelength (with different excitation fluence) measured on different length of CdS nanowires (with comparable diameter of ~110±10 nm) and the pump fluences are ~25, 35, 55, 70, 85 and 95 µW/cm2, respectively. Figure 3 ∣ Mechanism of tunable plasmonic lasers based on intrinsic self-absorption of gain material. (a) Emission spectra of CdS nanowire on SiO2/Ag film collected at point A (one end of the NW) and B (the excitation position) point. The CdS nanowire was excited in the center of the nanowire with a CW-laser with a wavelength of 405 nm. Inset top shows the schematic diagram of the configuration; inset bottom shows the dark field optical image of the CdS nanowire device. (b) For CdS nanowires of different lengths (from 1.3 to 30.2 µm), the emission spectra 24 / 25


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measured at point A, and all the CdS nanowires are excited in the center. (c) The theoretical calculation of the emission spectra for different propagation length using the equations 1 and 2 described in the main text. The emission peak positions (blue dots) measured at point A as shown in b. The gray triangles represent the calculated peaks after considering the Urbach tail absorption in CdS nanowires. The green squares are the lasing peaks for CdS nanowires of different lengths (data extracted from figure 2d). Figure 4∣ ∣Observation of the Purcell effect and the system loss estimation. (a) Time-resolved photoluminescence decay transients were recorded with increased nanowire length (from 2.2 to 18.2 µm). The solid lines are fitted using a single lifetime decay equation. (b) The Purcell factors determined from a numerical fit of the emission rate measurements in a. The green solid line is a simple theoretical model, taking into account the numerous emission processes, which yields an SPP loss of ~8130 cm-1. Figure 5 ∣Diameter dependent system loss in nanowire based plasmonic devices. (a) The CW laser (wavelength 473 nm) focused on different parts of the single CdS nanowire and the corresponding PL intensity was recorded at the left output ends (spots in the in green dashed squares). The red solid experimental data are the relative integrated intensity (normalized by using the 3 µm distance point intensity) for different lengths. Inset is the real-color PL emission photographs of the CdS nanowire, with local excitation at different spots along its length. The scale bar in the inset image is 5 µm. (b) Propagation distance dependent PL integrated intensity (integral area) at the left end for the different diameters (320, 215, 154 and 95 nm) of CdS nanowire deposited on the SiO2/Ag film. The experimental data are fitted into an exponential decay function as shown inside I = I0e-αL, α is the propagation loss. From the fitting, the values of α are 1020, 1705, 2832 and 8345 cm-1, respectively. (c) Diameter dependent propagation loss in plasmonic nanowire devices. Area I is the pure plasmonic mode propagation region, area III is hybrid photonic and plasmonic mode propagation region, area II is the transition region from hybrid modes to pure plasmonic modes.

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