Article pubs.acs.org/accounts
Organic Micro/Nanoscale Lasers Wei Zhang,†,‡ Jiannian Yao,†,‡ and Yong Sheng Zhao*,†,‡ †
Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
‡
CONSPECTUS: Micro/nanoscale lasers that can deliver intense coherent light signals at (sub)wavelength scale have recently captured broad research interest because of their potential applications ranging from on-chip information processing to high-throughput sensing. Organic molecular materials are a promising kind of ideal platform to construct highperformance microlasers, mainly because of their superiority in abundant excited-state processes with large active cross sections for high gain emissions and flexibly assembled structures for high-quality microcavities. In recent years, ever-increasing efforts have been dedicated to developing such organic microlasers toward low threshold, multicolor output, broadband tunability, and easy integration. Therefore, it is increasingly important to summarize this research field and give deep insight into the structure−property relationships of organic microlasers to accelerate the future development. In this Account, we will review the recent advances in organic miniaturized lasers, with an emphasis on tunable laser performances based on the tailorable microcavity structures and controlled excited-state gain processes of organic materials toward integrated photonic applications. Organic π-conjugated molecules with weak intermolecular interactions readily assemble into regular nanostructures that can serve as high-quality optical microcavities for the strong confinement of photons. On the basis of rational material design, a series of optical microcavities with different structures have been controllably synthesized. These microcavity nanostructures can be endowed with effective four-level dynamic gain processes, such as excited-state intramolecular charge transfer, excited-state intramolecular proton transfer, and excimer processes, that exhibit large dipole optical transitions for strongly active gain behaviors. By tailoring these excited-state processes with molecular/crystal engineering and external stimuli, people have effectively modulated the performances of organic micro/nanolasers. Furthermore, by means of controlled assembly and tunable laser performances, efficient outcoupling of microlasers has been successfully achieved in various organic hybrid microstructures, showing considerable potential for the integrated photonic applications. This Account starts by presenting an overview of the research evolution of organic microlasers in terms of microcavity resonators and energy-level gain. Then a series of strategies to tailor the microcavity structures and excited-state dynamics of organic nanomaterials for the modulation of lasing performances are highlighted. In the following part, we introduce the construction and advanced photonic functionalities of organic-microlaser-based hybrid structures and their applications in integrated nanophotonics. Finally, we provide our outlook on the current challenges as well as the future development of organic microlasers. It is anticipated that this Account will provide inspiration for the development of miniaturized lasers with desired performances by tailoring of excited-state processes and microcavity structures toward integrated photonic applications.
1. INTRODUCTION Microlasers, in which the stimulated emission process takes place in microcavity nanostructures (Figure 1), can generate intense coherent light at micro/nanoscale, exhibiting great potential in optical communication, sensing, three-dimensional (3D) imaging, and data storage.1 Meanwhile, it is also indispensable for the development of photonic integrated circuits (PICs), which may overcome the fundamental limitations of silicon-based electronic circuits such as the heat effect and processing speed.2 The first microlasers with dimensions approaching several lasing wavelengths were realized in GaInAs quantum wells in the 1980s.3 Since then, numerous optical materials down to the wavelength scale have been widely explored to construct such microlasers. Over the past decades, inorganic semiconductors such as ZnO, CdS, and © XXXX American Chemical Society
GaAs have exhibited great progress in the development of miniaturized lasers with high performances.4 However, these microlasers often suffer from some inevitable drawbacks, including mechanical rigidity, high-cost processing, and limited tunability.5 Organic materials, with weak intermolecular interactions and broad emissive bands, provide an ideal alternative for the fabrication of such compact lasers.6 In fact, since optically driven lasing was demonstrated in dye-doped polymers in 1967 and molecular crystals in 1972,7 organic solid-state (especially single-crystal) lasers have been a hot research field toward the development of portable or on-chip light sources with wide Received: May 2, 2016
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DOI: 10.1021/acs.accounts.6b00209 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 1. Illustration of micro/nanoscale lasers with pump source, microcavity feedback, and stimulated emission of the gain medium.
Figure 2. Schematic diagram of the structure−property relationship for organic microlasers.
tunability, high mechanical flexibility, and good processability.8,9 Organic molecules readily form ordered nanomaterials through a mild self-assembly process driven by the weak intermolecular interactions.10 These nanomaterials that possess regular shapes and larger refractive indexes than the environment (silica, mica, etc.) can act as low-loss waveguides and high-quality microresonators, which are essential for realizing low-threshold lasers.11 Moreover, the microcavity effects can be effectively modulated by tailoring the shape and size of the materials. For example, by means of crystal design, onedimensional (1D) nanowires and 2D nanoplates have been controllably fabricated, demonstrating a shape-controlled microcavity effect, i.e., 1D structures usually function as Fabry−Pérot (FP) resonators, while 2D structures function as whispering-gallery-mode (WGM) cavities.12 Besides the flexible microcavity structures, high-gain, widely tunable stimulated emissions are also an inherent superiority of organic-based microlasers.13 This can be attributed to the abundant and efficient excited-state gain processes in organic materials.6 In general, the gain process of organic dyes is a quasi-four-level structure, which is beneficial for easier population inversions, larger stimulated cross sections, and lower reabsorption losses.14 Therefore, low-threshold stimulated emissions have been commonly achieved in organic material systems.9,15 More importantly, some peculiar excitedstate processes such as intramolecular charge transfer (ICT) and excimer emission can be introduced into these molecular materials to help to realize widely tunable microlasers based on cooperative gain processes between two/multiple excited states.6 In the past decade, considerable efforts have been devoted to investigating such organic microlasers toward low threshold, flexible integration, and broad tunability by tailoring the microcavity and energy-level structures.16−18 Compared with their inorganic counterparts,1,4 organic microlasers usually exhibit lower processing costs, greater material versatility, lower power thresholds, and wider wavelength-tunable ranges (UV to NIR),14 which recently have received wide attention. However, a comprehensive understanding of the field, especially the structure−property relationship, to date remains so unclear and limited that further research seems to be stagnant and perplexed. This Account presents a systematic overview of the recent advances in organic microlasers, with an emphasis on the relationships among the molecular assemblies, microcavity structures, excited-state processes, and laser performances (Figure 2). First, we discuss how the organic molecules form various kinds of regular microstructures, which can act as efficient microcavities. Next, we introduce the ways that these microcavity nanostructures are endowed with four-level excited-
state processes for efficient population inversion. Subsequently, we present the design and construction of organic-microlaserbased hybrid structures toward integrated nanophotonic applications. Finally, our perspectives on the current challenges as well as the future development of organic microlasers are provided.
2. CONTROLLED MOLECULAR ASSEMBLY FOR EFFICIENT MICROCAVITY RESONATORS Gain media and microcavities are basic elements to construct microlasers (Figure 1). The gain medium is capable of amplifying the incoming light by stimulated emission, where a photon stimulates the exciton transition to generate a second photon with the same phase and frequency.13 Meanwhile, the microcavity provides the selective feedback for light inside the gain medium to enhance the stimulated emission of specific wavelengths. In common laser systems, they are usually two separated modules, so the gain process usually requires an external cavity, thus enlarging the size of the laser. Molecular nanomaterials with large active cross sections and regular shapes can simultaneously function as gain media and optical microcavities, providing an effective avenue to realize ultracompact mirrorless lasers.2 Many conjugated organic small molecules (Figure 3, top) and luminescent polymers (Figure 3, bottom) possess efficient four-level fluorescence processes with high quantum yields,5 making them ideal candidates for gain media of microlasers. For example, the π-conjugated phenylethylene-related distyrylbenzene (DSB) molecule shows strong blue fluorescence with a yield approaching 100% in monomer solutions (∼65% in crystals), which has been applied as a kind of excellent gain medium.9,19 By enlargement of the π-conjugation length or the introduction of intramolecular charge/proton transfer structure, the gain spectral range can be effectively extended to the nearinfrared (NIR), demonstrating a commendable versatility and tunability.9 These π-conjugated molecules with weak intermolecular interactions can self-assemble into regular nanostructures such as nanowires and nanodisks, which may act as fine optical microcavities to provide feedback and mode selection for the stimulated emission (Figure 4).20 The intermolecular interactions, including π−π stacking, hydrogen bonds, and van der Waals forces, are critical driving forces in the assembly process and greatly influence the nanostructure morphology, which determines the final microcavity effects. Generally speaking, single-direction intermolecular force would drive the molecules to assemble into 1D structures, such as nanowires and nanobelts, that usually function as FP-type microcavities.21 Meanwhile, the flexibility of 1D molecular nanomaterials B
DOI: 10.1021/acs.accounts.6b00209 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 3. Molecular structures of some typical organic materials used in the construction of microlasers.
Figure 4. Typical microcavity structures for organic microlasers.
provides opportunities to construct excellent WGM microrings by connecting the two ends of 1D nanostructures under the driving of external forces.22 In addition, two kinds of balanced intermolecular forces along different directions would induce 2D molecular stacking to form microdisks, which usually supply a WGM resonance.12 It is worth mentioning that, the good compatibility of dye molecules with polymers has also inspired researchers to fabricate various dye-doped flexible microstructures by solution-processing strategies, such as inkjet printing,18 electrospinning,23−25 and nanoimprinting,26 for use as polarized waveguides and high-quality microcavities.27 For instance, by electrospinning and nanoimprinting, distributed feedback (DFB)-based fiber resonators have been controllably prepared (Figure 4), showing a facile way to achieve low-threshold single-mode lasers.26 Besides, other methods and mechanisms to construct 1D nanostructures, such as epitaxial growth, template methods, vapor deposition, and solution self-assembly, have also been reported elsewhere.2 Herein we mainly discuss some fundamental strategies and concepts concerning how organic dyes assemble or are processed into various regular microcavities.
Figure 5. (A) 1D growth morphology of DBPT molecules along the c axis through π−π interactions. (B) Microscopy image of the DBPT nanowires. Inset: waveguide images of a single wire. (C) Modulated waveguide spectra with different wire lengths. (D) Electric field distribution of a DPBT nanowire FP-type cavity. Reproduced with permission from ref 21. Copyright 2011 American Chemical Society.
zine (DBPT),21 driving the molecules to stack along the c-axis direction, which results in the formation of 1D nanostructures. The 1D nanostructures have a regular morphology with smooth surfaces and flat end faces (Figure 5B), which help to efficiently reflect and confine the guided photoluminescence (PL) to form a high-quality microcavity. Therefore, the waveguided PL spectra exhibit a length-dependent modulation effect (Figure 5C), suggesting an axial FP-type microcavity effect from the reflectivity of the two flat end facets (Figure 5D). This provided an ideal platform to realize two-photon-pumped lasing in a single nanowire.
2.1. Self-Assembled Organic Microwires for Fabry−Pérot Microcavities
Organic nanowires, with efficient 1D waveguides, are a kind of key building block to construct flexible PICs.2 Their good light confinement effect and flat ends/lateral surfaces further contribute to a strong microcavity effect for lasing.20 Therefore, organic active nanowires can function as not only interconnected waveguides but also coherent light sources for PICs. Diverse strategies, especially liquid-phase self-assembly, have been widely explored to fabricate such wire-shaped structures. Moreover, the relevant microcavity properties have also been systematically studied. In our previous work, a kind of typical FP-type nanowire resonator was controllably prepared by means of intermolecular-interaction-induced solution self-assembly. As shown in Figure 5A, strong π−π interactions exist between the molecules of a large π-conjugated two-photon fluorescent dye, 2-(N,Ndiethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-tria-
2.2. Organic Microrings and Microdisks for Whispering-Gallery-Mode Resonators
High-quality (Q, defined as the ratio of the lasing wavelength to the corresponding line width) microcavities are extremely important for engineering of light−matter interactions and the realization of low-threshold lasers.28 Unfortunately, organic nanowires usually exhibit very small Q factors (
