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A Highly-Efficient Single Segment White Random Laser Golam Haider, Hung-I Lin, Kanchan Yadav, Kun-Ching Shen, Yu-Ming Liao, Han Wen Hu, Pradip Kumar Roy, Krishna Prasad Bera, Kung-Hsuan Lin, Hsien-Ming Lee, Yit-Tsong Chen, Fu-Rong Chen, and Yang-Fang Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03035 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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A Highly-Efficient Single Segment White Random Laser

Golam Haider1,2,3,4†, Hung-I Lin3†, Kanchan Yadav2,5, Kun-Ching Shen6, Yu-Ming Liao1,3, HanWen Hu3, Pradip Kumar Roy3, Krishna Prasad Bera2,3, Kung-Hsuan Lin7, Hsien-Ming Lee 8, YitTsong Chen5, Fu-Rong Chen1, and Yang-Fang Chen3,9*

Affiliations 1

Department of Engineering and System Science, National Tsing Hua University, Hsinchu-300,

Taiwan 2

Nano-Science and Technology Program, Taiwan International Graduate Program, Academia

Sinica. Taipei-115, Taiwan 3 Department

of Physics, National Taiwan University, Taipei-106, Taiwan

4 J Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague

8, Czechia 5 Department

of Chemistry, National Taiwan University, Taipei-106, Taiwan

6

Research Center for Applied Sciences, Academia Sinica. Taipei-115, Taiwan

7

Institute of Physics, Academia Sinica. Taipei-115, Taiwan

8

Institute of Chemistry, Academia Sinica. Taipei-115, Taiwan

9

Advanced Research Center for Green Materials Science and Technology, National Taiwan

University, Taipei-106, Taiwan

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ABSTRACT Production of multi-color or multiple wavelength lasers over the full visible-color spectrum from a single chip device has widespread applications, such as super-bright solid-state lighting, color laser displays, light-based version of Wi-Fi (Li-Fi), and bio-imaging, etc. However, designing such lasing devices remains a challenging issue owing to the material requirements for producing multi-color emissions and sophisticated design for producing laser action. Here we demonstrate a simple design and highly-efficient single segment white random laser based on solution-processed NaYF4:Yb/Er/Tm@NaYF4:Eu core-shell nanoparticles assisted by Au/MoO3 multilayer hyperbolic meta-materials. The multi-color lasing emitted from core-shell nanoparticles covering the red, green, and blue, simultaneously, can be greatly enhanced by the high photonic density of states with a suitable design of hyperbolic meta-materials, which enables to decrease the energy consumption of photon propagation. As a result, the energy up-conversion emission is enhanced by ~ 50 times with a drastic reduction of the lasing threshold. The multiple scatterings arising from the inherent nature of the disordered nanoparticle matrix provide a convenient way for the formation of closed feedback loops, which is beneficial for the coherent laser action. The experimental results were supported by the electromagnetic simulations derived from the finitedifference time-domain (FDTD) method. The approach shown here can greatly simplify the design of laser structures with color-tunable emissions, which can be extended to many other material systems. Together with the characteristics of angle free laser action, our device provides a promising solution towards the realization of many laser-based practical applications.

KEYWORDS: white random laser, single segment multicolor laser, up-conversion, hyperbolic meta-materials, high photonic density of states 2 ACS Paragon Plus Environment

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Light-emitting diodes (LEDs) based solid-state lighting (SSL) devices are the most efficient practical source of high color quality for lighting and display applications. Unfortunately, the performance of the LEDs is significantly limited because of the "efficiency droop".1,

2

In

contrast, laser diodes can serve as a potential alternative due to its brighter, energy efficient, and more accurate and vivid colors for display applications.3 Display system composed of lasers has shown its potential to cover as much as 70 percent more colors than the current standard display industry.1 However, a multi-color or multi-wavelength laser with the wavelength spanning throughout the visible-spectrum is beyond the capacity of a single material. Even though to circumvent this hurdle is very difficult, producing white laser has been the focus of great interest during the past few years, and the progress in this guideline has been reported by several groups.48

But owing to the challenges of multicolor emitting materials and its suitable design to derive the

laser action, the reports of white laser action with emission wavelength covering the whole visible range are only a few.4, 8 In most of the cases, the emission blocks that produce the primary colors Red, Green, and Blue (RGB) are spatially separated, which are very bulky, inefficient, complicated, and costly. Notably, a monolithic white laser based ZnCdSSe alloys has been successfully demonstrated recently by C. Z. Ning’s group,4 which has attracted a great deal of attention. However, the device was grown based on a very dedicated strategy by exploiting the interplay among vapor-liquid-solid, vapor-solid and dual ion exchange mechanisms. This approach is very difficult to be implemented to other material systems, and, it is well known that the long-term stability of II-VI semiconductor devices is a serious concern for practical application.9- 12 Random lasers possess a major advantage over regular lasers because of their cost-effective production due to their simplified fabrication processes.13- 16 The high-precision methods that are needed to design the ultraprecise micro-cavities, for example in the case of conventional 3 ACS Paragon Plus Environment

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semiconductor lasers, are not required because the feedback closed loops can be easily fulfilled by multiple scatterings when the emitted light travels in the random matrix of the active gain media. The broad angular distribution of output emission of random lasing devices makes it in principle an ideal candidate for SSL and display applications.13- 16 In addition, the active gain media can be deposited on arbitrary substrates.16, 17 Indeed, a stretchable random laser with tunable coherent loops has been demonstrated recently.16,

18

Therefore, random lasers provide an excellent

alternative for a wide range applications, including speckle-free image, lighting, data communication, and biosensors, etc.13, 16, 19, 20 There also exist preliminary attempts to demonstrate white lasers from random laser action, but the efficiency is rather limited.21- 23 On the other hand, lanthanide-based up-conversion nanoparticles (UCNPs) has been used for a variety of applications, including bio-imaging, data storage, multi-color displays, and photovoltaic devices owing to their remarkable properties of photon energy upconversions.24- 31 But, low up-conversion quantum efficiency remains the major shortcoming of all the UCNPs.24- 26 Several complementary research strategies have been explored, including surface functionalization, host lattice manipulation, and high-power excitation scheme to reduce the surface quenching effect.32-

34

Nevertheless, a well-established approach to suppress unwanted radiative

recombination of intermediate states is greatly desired. One of the useful approaches is to synthesize core-shell nanoparticles, which not only enable to passivate surface defects but also can enrich the emission spectrum from the nanoparticles.30 Importantly, the emission color can be tuned based on the concentration ratio of the constituent elements in the core and shell.30 Recently, meta-materials that emerge towards the realization of several intriguing optical phenomena by sophisticated manipulation of material design using many existing materials, accomplish paramount breakthroughs, such as invisible cloak,35, 36 sub-wavelength imaging,37- 39 4 ACS Paragon Plus Environment

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and meta-lens.40, 41 Among these meta-materials, a special class of design, defined as hyperbolic meta-materials (HMMs), which is composed of alternately arranged metal and dielectric layers of proper composition or well-aligned metal nanowires embedded inside a dielectric medium,42, 43 has attracted a substantial attention in optoelectronic device applications. These HMMs are categorized by the dispersion relation with hyperbolic shape in momentum space, i.e.,

2 c2



k x2  k y2





k z2

 ||

, while the sign of the effective permittivity (ε) or permeability (μ) behaves the

reversed direction for their optical tensor, e.g.,     || green. This can be understood from the inherent emission process as depicted in Figure 3e. The blue and green lines are originated from Tm3+ and Er3+ ions in the core of the nanoparticle. In comparison with green emission, the blue emission involves absorption of more photons. Hence, the blue line has the higher threshold. As the red emission is from the shell, it requires an intersystem crossing. Hence, it also has a higher threshold value as compared to the green. On the other hand, as the Eu3+ ions have superior emission strength than Tm3+ ions,30 the threshold for the red line is lower than the blue line. It is important to note that the lanthanide-based UCNPs possess a nonlinear emission behavior. In order to discern the nonlinearity due to the inherent nature of up-conversion materials and the lasing phenomena, we compared the change of emission intensities of the


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material with respect to the pumping power density as shown in Supplementary Figure 7, where the more dramatic change of integrated intensities of the sharp peaks indicates the super-linearity is not due to the inherent property of the UCNPs. To further confirm the lasing behavior, we have performed excited state carrier lifetime measurement. Interestingly, it is observed that the carrier lifetime possesses a sub-linear change above the threshold value of the pump power as shown in Figure 3f. It is a well-known phenomenon that the radiation density is proportional to the number of transition per unit time.50 Thus, the obtained change of the carrier lifetime is consistent with the observed change in emission radiation density (integrated intensity). This intriguing observation provides an excellent evidence in support of lasing behavior from the device. We interpret the observed lasing behavior as a random laser action,13- 15 where the highly porous UCNP layer acts as the optical feedback matrix (Figure 2a). In order to solidify this argument, we have shown the angular dependence of the emission spectra as shown in Supplementary Figure 8. Quite interestingly, the emission profile does not show a significant dependence on the angle between sample and spectrometer, which provides a firm evidence of random lasing phenomena.13, 15, 16 Unlike the conventional lasers with only single or few modes, random laser typically exhibits a large number of sharp peaks due to the fact that there exist many possible routes for the formation of coherent closed loops. Therefore, the spatial profile of random lasers is very different from the conventional lasers, in which random lasers show angle free emission. The temporal coherence length (L) of the laser is found to be ~ 100 µm, using the expression L = c/(n Δf), where (Δf), c, and n are the frequency bandwidth, the velocity of light at vacuum, and the refractive index of the medium, respectively. The lasing phenomena from up-conversion materials reveal a distinct feature of the material, which can be attributed to the formation of highly porous geometry of the UCNPs that provides strong optical feedbacks to the optically active gain media of UNCP


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materials, as shown in Figure 2a.32 Importantly, it is a well-known fact that the absorption and emission spectra of UCNPs are well separated as they up-convert the infrared photons to the visible photons, while they show a selective absorption in visible range.27, 29, 30, 51, 52 This is because the excited energy levels of UCNPs are very narrow, as shown in Figure 3e. Thus, during the process of multiple reflections, the photons carrying different energies can contribute to the emission of the respective energies. These phenomena provide a major advantage to circumvent the existing shortcomings of previously designed white laser system where the emission blocks need to be spatially separated in order to avoid the absorption of higher energetic emitted photons in the lower energy photons emitter. Note that, the term single segment is to emphasize that the emission of the white light arises from the UCNPs contained in a single block, which is very different from the previous studies of Ning et. al.4 and Chang et. al.,53 where different emission blocks have been used for the production of photons carrying different energies and a resultant emission from different blocks produces the white light. In general, semiconductor materials have a specific band gap, which will produce a particular color of light and the radiation of a higher energy can be strongly absorbed by the material. Thus, it is a challenging issue to design a multicolored laser using a single block consisting of different semiconductor materials, as the light in the gain medium will face a strong absorption loss in the cavity. On the other hand, the individual UCNP can produce multiple colors in a single block owing to the existence of ladder-like multiple narrow energy levels, and thus it supports the amplification of other photons for the production of the laser action. Hyperbolic meta-material induced robust enhancement of photon-energy up-conversion efficiency It is known that up-conversion materials have a very poor emission quantum yield.24, 26, 34 12 ACS Paragon Plus Environment

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Hence, the emission enhancement for up-converting materials is of great importance for the interest of both scientific research and practical application. Multiple strategies have been reported during the past few years to enhance the up-conversion efficiency.33, 34 However, a great effort is still required to make a substantial improvement for its practical application. Unlike conventional plasmonic materials, hyperbolic metamaterials have superior advantages of high-k propagating waves that produce volume plasmon polaritons due to the interlayer surface plasmon coupling, which has been shown to be capable of producing strong enhancement of spontaneous emission.42, 54

Moreover, broadband response profile, frequency tunability, and most importantly the simple

fabrication process that is useful for large-scale production with high uniformity, make it an attractive material for optoelectronic device applications.42, 54 To demonstrate the enhancement of up-conversion emission, we have designed the devices on three different HMM samples as discussed above. The up-conversion emission spectra recorded at a constant pump power density on different substrates along with the reference sample are depicted in Figure 4a. Interestingly, the emission spectra on different substrates possess a dramatic change. For HMM 1, it was observed that the up-conversion emission factor can be as high as > 50 times for 4S3/2 → 4I15/2 and 2H

11/2

→ 4I15/2 of Er3+ transitions. The other transitions responsible for red and blue lines are found

to be enhanced by ~ 40 and 35 times, respectively. HMM 2 can assist a robust enhancement of ~ 50 and 30 times for the red and green lines, respectively, which mainly originate from 5D1 → 7F5 and 5D2 → 7F6 transitions of Eu3+ and 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 of Er3+, respectively. On the other hand, HMM 3 on an average can enhance the transition lines in the visible range only ~ 10 times. The variation of the resultant color of different devices has been estimated by calculating the Commission Internationale d'Eclairage (CIE) coordinate corresponding to the emission spectra as shown in Figure 4b. We can clearly see that the CIE index (0.333, 0.334) for the HMM 1


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sample matches well with the definition of white light. To consolidate our observation, we have performed the carrier lifetime measurement as shown in Supplementary Figure 9. It is observed that the lifetime of the carrier for green and red lines are reduced by 45 % and 24 %, respectively for HMM 1. Meanwhile, HMM 2 can assist ~ 30 % and 50 % reduction of carrier lifetime for green and red emission, respectively. Whereas, HMM 3 induces ~ 5- 10 % change of the carrier lifetime. These intriguing facts of strong emission enhancement and rapid reduction of carrier lifetime can be well interpreted based on the simulation results as discussed below. Hyperbolic meta-material assisted lasing phenomenon We have carefully studied the lasing action of the UCNP assisted by the HMM substrates. The sharp pronounced peaks on the individual emission lines shown in Figure 4a resemble the occurrence of random lasing phenomena as reported in many previous reports.13-

15, 32

The

corresponding dependences of the emission spectra on pumping power density are depicted in Figure 5a, 5b, and 5c, respectively, and the change of the integrated emission intensity, the output power and spectral linewidth of HMM 1, HMM 2 and HMM 3 samples are shown in Supplementary Figure 10, 11 and 12, respectively. Notably, the lasing action intensity and the output power assisted by HMMs becomes more pronounced as shown in Figure 4a. In addition, the lasing threshold for the reference sample is found to be ~ 0.5- 0.6 kW cm-2, as shown in Figure 3b, whereas, for the HMM samples it is found to be ~ 0.1 kW cm-2, as shown in Supplementary Figure 10, 11, and 12. Thus, the HMM substrates can generate ~ 80 % reduction of the lasing threshold than the observed threshold value for the reference sample on SiO2/Si substrate. Quite interestingly, based on the different fill-fraction of the HMMs, the color of the emission spectra can be tuned as shown in Figure 4b. The device on HMM 1 produces a perfect white light 14 ACS Paragon Plus Environment

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spectrum, which is superimposed with sharp pronounced peaks on every emission lines, and it can be phrased as ‘white laser’. On the contrary, the device composed of HMM 2 and HMM 3 possess the appearance of a stronger red emission. Intuitively, the terminology ‘white’ and ‘laser’ seems self-contradictory because a white light implies a broad spectrum and laser possesses a narrow bandwidth. However, it can be well understood if the emission contains red, green and blue laser actions, simultaneously. It is worth noting that the laser action observed here covering a wide spectral range of more than 200 nm in a single light emitting device, which is the largest value ever reported. The threshold value of the lasing phenomena also shows a strong dependence on the fill factor of the underlying HMM substrate. The threshold for the green line follows a sequence of HMM 1 < HMM 2 < HMM 3; whereas for red line it follows HMM 2 < HMM 1 < HMM 3. This interesting feature can be understood based upon the underlying mechanism of the emission enhancement as discussed below. Considering a uniform emission towards all the directions perpendicular to the tophemisphere on the sample HMM1, at an illumination intensity 0.66 kW-cm2, the optical-to-optical conversion efficiencies (output power/input power) are found to be ~5.0×10-5, ~3.8×10-5 and ~0.6×10-5 for red, green and blue lines, respectively. Whereas, keeping the pumping power same, the efficiency for the reference samples are found to be ~1.03×10-6, ~0.77×10-6 and ~0.2×10-6 for red, green and blue lines, respectively. Note that the input power is the power of 980 nm pumping laser which produces the red, green, and blue lines simultaneously as the output. Hence, the obtained optical-to-optical conversion efficiency is different from the external quantum efficiency. The term “highly-efficient” used here is to emphasize several aspects of our design. Primarily, the enhancement of the photon energy up-conversion in lanthanide-based materials based on the multiple reflections of the emitted photons in the porous matrix, as well as with the assistance of


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the hyperbolic meta-materials. Because the typical quantum efficiency of up-conversion nanoparticles is very low only around 1%, it is very difficult to observe laser action, which is in stark contrast with the high quantum efficiency of more than 90% for conventional semiconductor nanoparticles. Our work represents a very important demonstration for the white light laser action covering a wide spectral range derived from up-conversion nanoparticles, which is very rare and difficult to achieve. We therefore used the term “highly efficient” to highlight the main result of our study. In addition, the design of our device is very cost-effective as compared to other semiconductor materials based white random laser emitters. We believe that the following improvements to our approach can be introduced to implement enhanced device efficiency. Coating the UCNPs on patterned HMM substrates following the method as discussed by Shen et. al., would be an efficient way to enhance the efficiency of the device.45, 46 On the other hand, introducing strong scattering centers such as metal (for example, Ag, Au) or dielectric nanoparticles (for example, SiO2) at the light amplification matrix can produce an efficient closed loop of optical feedback, which can improve the device performance. Fundamental theory The emission from the UCNP can be enhanced by the underneath HMM substrates owing to the additional factor of the excited high-k modes of the HMM that allows high PDOS. The emission from the UCNP is related to the transition rate occurred between the UCNP and the environment, which can be expressed by the Fermi’s golden rule. Assuming the initial state  i and the final state  f , the transition rate from the initial (i) → final (f) state can be expressed by the following expression,55

 fi 

2 |  i |H'| i |2  (k ) , h

(1) 16 ACS Paragon Plus Environment

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where, h is Plank’s constant,  (k ) is the density of state, and   f |H'| i | is the perturbation Hamiltonian ( H' ) between the initial state and the final state. Notably, the density of states is mainly determined by the wave vectors (  (k )  k 3 ), while the wave vectors for the HMM can be arbitrarily extended compared with the isotropic medium for the limited wave vectors. As a consequence, theoretically, the emission spectra within the hyperbolic dispersion resembles higher transition rates for both the spontaneous and stimulated emissions compared with the isotropic medium counterpart. The emission of the UCNP can be enhanced by the plasmonic-based nanoparticle (e.g., Ag or Au) owing to the increase of excitation rate by local field enhancement, where the emission wavelength of UCNP matches with the resonance frequency of the noble nanoparticles. Furthermore, the enhanced emission excitation rate can also be attributed to the coupling effect of the UCNP with the broadband resonance by the HMM substrates, which will effectively increase both the nonradiative and radiative decay rates. Compared with other plasmonic-based systems, the metal component of our proposed HMM substrate does not directly contact the UCNP, in which an 8 nm MoO3 capping layer was deposited on top of the HMM substrate. This capping layer will prevent the undesirable quenching effect, and therefore the enhanced factor can be significantly larger. The decay rate for the dipole emitter is  HMM = vacuum   spp   high  k , where  vacuum and  spp are the decay rates in vacuum and surface plasmon polariton (SPP) modes of meta-materials, respectively, while  high  k only exists in the HMMs owing to the high-k modes.43, 44 Moreover,  high  k is given by  high  k 

2 2 |  || |   , where d is the distance between the UCNP and the 8hd 3 1 |  || |  

HMMs. 17 ACS Paragon Plus Environment

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We further discussed the iso-frequency curves to realize the out-coupling configurations for the HMM samples as shown in Supplementary Figure 13. For the HMM 1 sample, the dispersion shows hyperboloid from S4 to S7 transition lines. The dispersion for the HMM 2 sample shows that the hyperboloid shape only occurs at S7 transition line, while for the HMM 2 sample, all the dispersion shapes are elliptic. When light passes through the UCNPs, it is scattered and then spreads out to numerous directions into the HMM substrates, leading to induce more wave-vector modes and out-couple the energy. These pronounced high wave-vector modes will contribute to the increased photonic density of states, resulting in a significant transition rate to enlarge the optical gain and achieve the random lasing action. This phenomenon will also suppress the optical loss from the metal components. On the other hand, when the dispersion is elliptic in the isofrequency curve, the wave-vector modes are limited, which will not have the above-mentioned benefits. Therefore, the induced wave-vector modes do have a major contribution to the occurrence of the observed laser action and lifetime measurements for they can correlate well with each other and can be interpreted in a consistent way. Numerical simulation The multiple emission line enhancements from UCNP can be attributed to the Purcell factor from the HMM samples with hyperbolic dispersion for the high photonic density of states (PDOS).56 The Purcell factor can be physically interpreted as the enhancement of the emission rate influenced by the surroundings with the dipole radiation based on the spontaneous emission dynamics.56 Figure 6a depicts the theoretically calculated Purcell factors with a dipole above the

1 2 HMM substrate, which can be determined by Fiso  F  F|| , where ( F ) and ( F|| ) are the dipole 3 3 perpendicular and parallel to the substrate.57 The calculated Purcell factor here is to emphasize the


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importance of the light-matter interactions for the interpretation of the enhanced phenomena in our measurement.46 Our calculation was obtained with a dipole source fixed at 10 nm above the substrates. Besides, to emphasize the significance of the near-field coupling enhancement, we also simulated the Purcell factors within 50 nm marked on a logarithmic scale as shown in Supplementary Figure 13. In general, within 5 nm, the Purcell factors can be as high as ~500 times. Supplementary Figure 14 also shows the calculated Purcell factors with a dipole source fixed at 100 nm above the substrate, which is in the same order of magnitude of the thickness of the UCNPs porous film. And, the obtained result still shows an enhancement factor of two times. Therefore, the major enhancement is dominated by the near-field coupling effect. Interestingly, based on the fill-fraction, the Purcell factor shows it peak value at different wavelength regime. Here, considering the exponential decay of the Purcell factor, we average the whole Purcell factors within the sample on top of the HMM substrates from Supplementary Figure 14 as shown in Supplementary Table 1. As a result, for HMM 1, the peaks S3 and S4 corresponding to the transition 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 of Er3+ get strongly amplified (more than 50 times), and the average Purcell factor is 60 times. Whereas, the other peaks are amplified lesser. The high value of the Purcell factor in the longer wavelength regime HMM 2 results a robust enhancement of the enhancement factor is ~ 50 and 30 times for 5D1 → 7F5 and 5D2 → 7F6 transitions of Eu3+ and 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 of Er3+, respectively, and the average Purcell factors are 61 and 40 times, which will tune the resultant emission color to reddish. The small value of the Purcell factor in the visible region for HMM 3 will result in a lesser emission enhancement. This less enhancement may be due to the emission regions, which are not in the hyperbolic dispersion. By the same token, as the Purcell factor is directly related to the transition rate from the ground state to the excited state, the lifetime corresponding the emission line gets shorter. The influence of the


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reflectance effect of the four substrates has been estimated to explore the role of the reflection from the substrate in the emission enhancement as shown in Supplementary Figure 15. And the result shows that the emission enhancement is dominated by the effect of the Purcell factor. The occurrence of the lasing phenomenon is due to the multiple scattering of the emitted photons in the porous UCNP matrix, which enables the formation of coherent closed loops and results in the random lasing action. Thus, a clear understanding of the scattering cross-section of UCNP influenced by different substrates is very crucial. In order to stand out the scattering process in the composite, we have calculated the efficiency (

 scat Ascat

) as shown in Figure 6b, where  scat is

the scattering cross-section and Ascat is the scattering cross-section area.58 Similar to the Purcell factor, the scattering efficiency can be tuned to different wavelength regime based on the suitable choice of the HMM structure. In the case of HMM 1, in addition to the transition rate enhancement due to the Purcell effect, the strong scattering efficiency will drive the lasing spectrum more prominent and reduce the lasing threshold. The discussion of scattering efficiency is to realize the out-coupling effect of the energy on the environment rather than being annihilated as a form of evanescent field by the metal component of the multilayer substrates. We further emphasize here that our proposed design of different multilayer components can easily achieve the multiple wavelength enhancement with a significant enhancement ratio. Compared with the previous reports using the sophisticated nano-patterned HMM structures by focused ion beam technology and electron beam lithography,59- 61 our method of using the porous structure of UCNP clusters can serve as the microcavity thus providing an efficient platform for photon scattering for the propose of optical feedback to occur the random lasing action. This combination of the porous structure of UCNP clusters and the HMM substrates can effectively interact with each other to


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achieve the strong enhancing effect. For the pure UCNPs, the calculated scattering efficiency is less than 10-2, which means it is hard to form a highly convenient way to form the closed feedback loops. For the UCNP on top of the HMM substrates, the scattering efficiency keeps at around 0.1 in the general wavelength region. Below the wavelength region of 600 nm, the HMM 1 shows even a pronounced 2 order of magnitude stronger than the pure UCNP. This strong scattering effect from the interaction between the HMM substrate and UCNP is beneficial to form the closed feedback loops. Besides, compared with other commonly used noble materials (e.g., Ag or Al) as calculated scattering efficiency as shown in Supplementary Figure 16, which shows the scattering efficiency in the shorter wavelength regions with a weak efficiency, especially for the Al/MoO3 multilayer structures. Therefore, the proposed Au/MoO3 multilayer structures are suitable for the generation of the highly-efficient white random laser with a single segment. The realization of the far-field angular |E|2 distributions is very important as it is related to several intriguing factors that play a major role to assist the lasing phenomena. The simulated scattering field intensities with multi-wavelength regions for XY, XZ, and YZ planes are shown in Figure 7, where X and Y (Z) refer to the scattering direction parallel (perpendicular) to the substrate. The understanding of |E|2 distributions on the XY plane is important since the formation of closed loops dominates the possibility for generating random lasing action. Moreover, the XZ and YZ planes are the observation planes for the far-field detection, i.e., the left-hand side of the white-dash line is the north sphere. For the HMM samples, note that |E|2 can be out-coupled to the far-field rather being annihilated in between the multilayers owing to Ohmic loss. Compared to the reference sample, HMMs produce a stronger |E2| value in the XY plane, which signifies a better closed-loop formation that enables to assist the lasing action. On the other hand, for XZ and YZ plane of the reference sample, the |E2| trends downwards. In contrast, for the scattered |E|2 out-


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coupled to the far-field region enhancing the transition rate with the assistance of PDOS associated to the high-k modes of HMM.54 To make a clearly readability, Supplementary Figure 17, 18, 19, and 20 plot the far-field angular |E|2 distributions in different transition lines in polar coordinates for the reference, HMM 1, HMM 2, and HMM 3 samples, respectively. For the reference sample, the majority (more than 80 %) of the |E|2 is scattered to the forward direction instead to the backward (far-field) for the formation of the closed-loop path. For the HMM 1 sample, most of the |E|2 distributions are out-coupled to the far-field due to the pronounced transition rate from the high wave-vector modes rather than being confined in the multilayers. This significant |E|2 distribution is realized as the effect of volume plasmon polariton (VPP),62 which arises from the multi-coupling effects of surface plasmon polariton (SPP) in between the interfaces of Au and MoO3. Then, a strong feedback will provide the UCNP to make the gain and overcome its loss easily for the random lasing action. For the HMM 2 sample, since the entire transition lines are not exactly located at the regions of hyperbolic dispersion, the |E|2 distributions are not as pronounced as the HMM 1 sample. However, for the HMM 3 sample, all of the transition lines are not in the regions of hyperbolic dispersion, the |E|2 distributions are relatively weak and even more than 40 % of the |E|2 distributions will be trapped and then annihilated inside the multilayer components owing to the ohmic loss from the metal material. Thus, the robust enhancement of up-conversion emission strength and the occurrence of enhanced lasing phenomena are justified. In the end, we would like to stress here the discussed out-coupling configurations, the Purcell factors, scattering efficiency, and the far-field angular |E|2 distributions are correlated well with each other. As a whole, the HMM structure provides a higher chance to induce the high wavevector modes in the regions of hyperbolic dispersion. Then, this effect will make the pronounced photonic density of states and cause the increased transition rates as well as the emission intensity


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based on the Fermi’s golden rule. Furthermore, the scattering efficiency for the UCNPs located on top of the HMM substrate is even more than 2 order of magnitude stronger than the pure UCNP. Together with the strong out-coupled far-field angular |E|2 distributions, they are extremely beneficial for the formation of the closed loops to easily achieve the random lasing action. Our proposed design is to present a proof of concept for the physical interactions between the HMM and UCNPs to easily obtain a broadband and higher enhancement factor. This aspect can be used for all the semiconductor nanoparticles, which are highly competitive candidates to become the future light emitters and many other optoelectronic technologies. CONCLUSION In summary, we have successfully demonstrated a highly efficient single segment white random laser based on the integration of HMM and UCNPs. The observed laser action covers a wide spectral range of more than 200 nm, which is the largest value ever reported. A combination of several important factors makes our demonstration possible. The existence of high PDOS from the HMM substrates is very useful to drive lasing action by the amplification of radiative transition. The emission enhancement of up-conversion > 50 times with this simple architecture is intriguing, which shows a great potential for practical applications. It also induces a drastic reduction of the lasing threshold. Moreover, the hyperbolic dispersion can be tuned to lead the emission enhancement in the desired spectral regime. In addition, the concentration ratio of the emitting constituent materials in the up-conversion core-shell nanoparticles also plays a decisive role to achieve white light emission. Furthermore, the inherent nature of the formation of closed loops due to multiple scatterings when the emitted light traveling in random active gain media is a key to relax the rigid requirement of the ultraprecise resonant cavity. A theoretical simulation has been performed to support our experimental observation of the enhanced white laser action. We stress 23 ACS Paragon Plus Environment

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here that the demonstration of white laser based on a single segment with such a cost-effective fabrication process is very useful for the realization of energy efficient devices in multiple practical applications spanning from the display, lighting, data storage, energy harvesting to biomedical areas. Notably, our approach can be easily implemented in many other material systems towards the development of not yet realized high-performance optoelectronic devices.

METHODS/EXPERIMENTAL Fabrication of HMM superlattice A SiO2/Si wafer with a 150 nm thick SiO2 layer is used as the substrate to design the multilayer. At first, the substrate was cleaned by a sonicator in acetone, isopropyl alcohol, and deionized (DI) water in a sequence to remove the unwanted contaminations. Then, the substrate was heated on a hotplate for 10 min at 80 °C. After that, 6 pairs of Au/MoO3 multilayer structures were alternately deposited on the pre-cleaned SiO2/Si substrate using thermal evaporation of Au and MoO3 under < 5 x 10-7 Torr pressure. It is important to keep the deposition rates fixed at 0.3 Å/s in order to avoid the non-uniformity in the deposited layers. After the deposition of the MoO3 layer, the chamber was kept in high vacuum conditions ~ 1 x 10-7 Torr for 30 minutes to make enhance the quality of the metal layers as we used raw MoO3 powder for the deposition of the MoO3 layer. Finally, another 8 nm MoO3 capping layer was deposited on the multilayers. The raw materials, Au and MoO3 were commercially available ultra-pure, which were purchased from Sigma Aldrich. Optical and structural characterizations The electron microphotograph of the UCNPs has been recorded on a JEOL JEM-1400 (120 kV) transmission electron microscopy (TEM). For taking SEM pictures, we first used the focused ion beam (FIB) system (Helios Nanolab™ 600 DualBeam™) to prepare the sample for cross24 ACS Paragon Plus Environment

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sectional studies. The multilayer was cut using a gallium ion source under the operating voltage of 30 kV and the current of 50 pA in order to avoid the intermixing of the layers. Finally, we used field emission scanning electron microscopy (FE-SEM) (JSM-6500F) to image the cross-section with a 52° tilt angle. The PL and laser action were performed under the excitation of a continuous wave (CW) diode laser of 980 nm with a tunable power range of 0.5–8 W (ChangChun New Industry, China) and the spectra were recorded using a Horiba Jobin Yvon TRIAX 320 spectrometer connected in a micro-PL setup. The spectral resolution of the spectrometer is 0.06 nm. The external pumping laser was carefully focused at the focus point of the micro-PL with an incident angle of 45°. Fluorescence lifetime measurement of UCNPs In order to monitor the charge carrier dynamics in the UCNPs on the device, we employed the fluorescence lifetime measurement using a frequency-domain method,63- 64 where a mechanical chopper was employed at a frequency of ~1 KHz to modulate the 980 nm excitation pulses. The photocurrent was measured from the avalanche photodiode (APD, Thorlabs, Inc.) with a lock-in amplifier. The phase delays between the fluorescence and the modulated excitation light were recorded as a function of modulation frequency. The fluorescence decay is assumed to be an exponential function (e-t/τ) with the phase delay of Φ = tan-1(2πfτ)), where f and τ represent the modulation frequency and lifetime, respectively.65 Numerical simulation The numerical simulations were carried out using commercial electromagnetic software (Lumerical FDTD Solutions) to analyze the response from our proposed design. In this study, the refractive indices of Au and Ag used are from Palik et. al.,66 and MoO3 and UCNP are inserted from Lajaunie et. al. 67 and Vogel et. al., (Optical Simulation of Up-conversion Nanoparticles for 25 ACS Paragon Plus Environment

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Solar Cells, Thesis) respectively. The polarization of the electric field is set along the x-direction with the polarization of the magnetic-filed along the y-direction; thus, the light propagates along the z-direction. Here, the perfectly matched layer is used to cover around all dimensions to absorb the light with the minimal reflections, and then the undesired artificial simulation results can be avoided. The mesh is set as 1 nm to obtain the higher accurate simulation results.

ACKNOWLEDGMENTS This work was financially supported by the “Advanced Research Center for Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006), the Ministry of Science and Technology in Taiwan (MOST 107-3017-F-002-001), the Ministry of Education Youth and Sport of Czech Republic (projects CZ.02.2.69/0.0/0.0/16_027/0008355 and LTC18039). G.H., K.Y. Y.M.L. and K. P. B. thank the support of Nanoscience and Technology Program, Taiwan International Graduate Program, Academia Sinica. G.H. thanks Dr. Martin Kalbáč of J. Heyrovsky Institute of Physical Chemistry of the ASCR, v.v.i, Prague 8, Czech Republic. The authors thank Janis Liu for the visual design of White-RL. AUTHOR INFORMATION Corresponding Author † The authors contribute equally. Correspondence and requests for materials should be addressed to Y. F. C. *Email (Y.F. Chen): [email protected] Author contributions 26 ACS Paragon Plus Environment

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G.H. and Y.F.C. conceived the project. G.H. designed the devices and performed the experiments. H.I.L. performed the theoretical calculations. K.Y., H.M.L., and Y.T.C. synthesized the UCNPs and performed the structural characterizations. Y.M.L., H.W.H., P. K. R, K.C.S., and K. P. B. assisted the optical characterizations. K.H.L. and G.H. performed the lifetime measurement. G.H., H.I.L., F.R.C., and Y.F.C. wrote the manuscript with the input from all the authors.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Supplementary Figures (1-20), References (1-6) The authors declare no conflict of interest.

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Figure 1| Morphology of the constituent materials. a, Transmission electron microscopy image of highly monodispersed NaYF4:Yb/Er/Tm@NaYF4:Eu core-shell nanocrystals showing the size of 20 ± 0.5 nm. b, c, and d, Cross-sectional scanning electron microscopy (SEM) images of the hyperbolic metamaterial samples. The corresponding metal and dielectric layer thicknesses of the compositions are 16 nm /16 nm, 9 nm / 23 nm, and 5 nm / 27 nm, respectively.


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Figure 2| Device structure. a, Schematic illustration of the device structure. The UCNPs were coated on top of HMM samples. Under the excitation of continuous 980 nm IR laser, the emission spectrum produces multiple emission lines covering visible range. As a result, white laser action is observed. Printed with the permission from Janis Liu. b, Cross-sectional SEM image of the composite heterostructure. The inset is the top view of the UCNP, which shows the porous structure of the sample.


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Figure 3| White laser demonstration from up-conversion nanoparticles. a, Emission spectra from UCNPs under the excitation of 980 nm laser. b, c, and d, are the pump power density dependent integrated intensity, output power, and full width at half maxima (FWHM) for red (655 – 665 nm), green (540 – 545 nm) and blue (473 – 480 nm) emission from the upconversion 36 ACS Paragon Plus Environment

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nanoparticles on the SiO2 substrate, respectively. e, Schematic illustration of photon up-conversion mechanism in a NaYF4:Yb/Er/Tm@NaYF4:Eu core-shell nanocrystal. f, The variation of carrier lifetime

on

pumping

power

density

corresponding


to

red

and

green

emissions.

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Figure 4| Hyperbolic metamaterial induced enhancement of up-conversion emission. a, Emission spectra from UCNP/HMM composites under the excitation of 980 nm laser at a constant pumping power density of 0.66 kW cm-2. b, Plot of the calculated CIE coordinates (Ref: (0.332, 0.331; HMM 1: (0.333, 0.334); HMM 2, (0.334, 0.354); HMM 3: (0.334, 0.0.342)).


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Figure 5| Laser action spectra from the samples with hyperbolic metamaterial. a, b, and c, depict the incident power density dependence for the emission spectra of the samples HMM 1, HMM 2, and HMM 3, respectively.


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Figure 6| Calculated Purcell factors and scattering efficiency. a, Calculated Purcell factors with a dipole source are fixed at 10 nm above the substrates for reference, HMM 1, HMM 2, and HMM 3 samples. b, Theoretical calculation of scattering efficiencies for a UCNP on different substrates. Based on the structure a UCNP, the simulation is taken into account the packing of UCNPs.


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Figure 7 | Far-field angular |E|2 distributions for XY, XZ, and YZ planes. a, b, c, d, e, f, g, h, i, j, k, and l are the scattered |E|2 intensities for the SiO2/Si and Au-based HMMs, which are arranged in a matrix form, in which each row corresponds to the plane (XY, YZ or XZ) and each column stands for a particular sample. The mark of the x-axis, phi, is radian. The white-dash line is located at  , which is the equatorial plane. The left (right)-hand side area of the white-dash line is the north (south) sphere.


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TABLE OF CONTENT

Highly-efficient single segment white random laser based on solution-processed NaYF4:Yb/Er/Tm@NaYF4:Eu core-shell nanoparticles assisted by Au/MoO3 multilayer hyperbolic metamaterials. The multi-color lasing emitted from core-shell nanoparticles covering the red, green, and blue, simultaneously, can be greatly enhanced by the high photonic density of states with a suitable design of hyperbolic metamaterials, which enables to decrease the energy consumption of photon propagation. Printed with the permission from Janis Liu.


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A Highly-Efficient Single Segment White Random ... - ACS Publications

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