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Keywords:

  • lasers;
  • nanowires;
  • H-aggregates;
  • organic semiconductors;
  • photonics

Organic semiconductors are of current interest in photonic applications,1 because of their chemically tuneable optoelectronic properties and their ability to self-assemble for bottom-up fabrication.2, 3 Optically pumped organic lasers have been demonstrated in a variety of resonator geometries,4 such as microcavity,5 micro-ring,6 distributed feedback (DFB),7 and photonic bandgap structures.8 In these cases, stimulated emission takes place from the lowest electronic excited state |10〉 to the first vibronic replica |01〉 of the ground state, exhibiting a lasing threshold that depends on both amplification and loss processes.4, 9–11 The unoccupied |01〉 state in thermal equilibrium facilitates the population inversion. However, the concomitant radiative loss of the exciton reservoir to the |00〉 state increases the required population inversion density threshold; meanwhile, the intrinsic ground state self-absorption represents a major channel of optical losses.4, 12 As a matter of fact, electrically driven organic lasers remain a great challenge, partially due to the high lasing-threshold observed so far.7, 13 Therefore, the development of organic gain materials with optimized energy levels that help decrease the lasing threshold is of crucial importance.

Nanowire lasers are promising for applications ranging from on-chip optical communication to high-throughput sensing.1–3, 14, 15 Recently, crystalline nanowires of organic semiconductors have shown capabilities in both photon waveguiding and charge transporting properties.9–11 Still, even though these nanowires are ultra-small in two-dimensions, the axial cavity defined between the two wire-end-faces has to be ca. 10 μm to build up enough gain for lasing. Here, we prepared rectangular slab-nanocrystals (SNCs) of 1,4-dimethoxy-2,5-di[4′-(methylthio)styryl]benzene (TDSB),16 in which H-aggregation is advocated by tight co-facial molecular packing. Due to the exciton–vibration coupling, the optically allowed |10〉 [RIGHTWARDS ARROW] |0n〉 (n ≥ 1) transitions make H-aggregated SNCs of TDSB highly emissive with a solid-state quantum yield of 0.81; meanwhile, the optically forbidden |10〉[LEFT RIGHT ARROW] |00〉 transitions not only reduce the self-absorption effect but also minimize the direct radiative loss of the exciton reservoir to the |00〉 state. The two lateral-faces of SNCs constitute a high quality (cavity quality factor (Q) ≈ 1000) built-in Fabry–Pérot (FP) cavity at the wavelength scale, in which a lasing threshold as low as 100 nJ cm−2 was achieved. Moreover, the laser light generated in the ultra-small radial cavity of SNCs can propagate along its length up to hundreds of micrometers, making them attractive building blocks for miniaturized photonic circuits.

TDSB SNCs were prepared via a self-assembly method by injection of 100 μL of a stock solution (5 mM) in tetrahydrofuran (THF) into 2.0 mL of hexane under stirring.17 Scanning electron microscopy (SEM) results (Figure 1a and Figure S1, Supporting Information) confirm that as-prepared SNCs have a rectangular cross-section with a width (W) of 0.5−2 μm uniformly distributed along the entire length (L) of 10−30 μm. Individual SNCs were also imaged by atomic force microscopy (AFM), revealing that their outer surfaces present a molecular-scale flatness around a few nanometers (Figure S2, Supporting Information). Moreover, the ratio of height (H) to width is found to be H:W ≈ 2−3:4 (Figure S2). The upper inset in Figure 1a displays the selected area electron diffraction (SAED) pattern recorded by directing the electron beam almost perpendicular to the flat surface of a single SNC (lower inset). Monoclinic TDSB crystals (CCDC No. 675387) belong to the space group of P21/c, with cell parameters of a = 8.16 Å, b = 5.42 Å, c = 26.15 Å, α = γ = 90°, and β = 97.35°.16 Therefore, the squared and circled sets of spots in the SAED pattern are due to {010} and {001} Bragg reflections with d spacing values of 5.5 and 26 Å, respectively, suggesting that SNCs are single crystals grown preferentially along the crystal [010] direction. This is consistent with X-ray diffraction (XRD) measurements (Figure 1b), in which only diffraction peaks corresponding to crystal planes with k = 0, such as equation image and high-order equation image, and equation image peaks, are observed, indicating the abundance of these crystal facets on the surfaces of SNCs (Figure 1a and Figure S3, Supporting Information).

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Figure 1. a) High-magnification SEM image of SNCs. The top inset shows the TEM image of a single SNC with the corresponding SAED pattern shown in the bottom inset. The squared and circled spots are due to {010} and {001} Bragg reflections. b) XRD profiles of SNCs and powder. c)Photoluminescence (PL) microscopy image of ensemble SNCs on a glass substrate excited with un-focused UV light (330–380 nm).

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Figure 2a depicts the diffused reflection absorption and photoluminescence (PL) spectra of ensemble SNCs placed on a glass substrate. For comparison, the absorption and PL spectra of monomers in THF solution are also included. The absorption spectrum of the monomers exhibits a broad featureless peak at 404 nm, while its PL spectrum is clearly vibrationally structured with an apparent sub-band spacing of 1200–1300 cm−1 with contributions arising mainly from vinyl stretching modes (top panel of Figure 2a).18 As compared with the monomer absorption, the absorption spectrum of SNCs exhibits a slightly blue-shifted maximum at 397 nm, with additional bands around 442 nm (2A) and 461 nm (1A) due to aggregate states in SNCs (bottom panel of Figure 2a). The PL spectrum of SNCs is dominated by the 0−1 transition: the intensity ratio between 0−1 and 0−0 emissions is around 12:1 (bottom panel of Figure 2a). This is a specific fingerprint of H-aggregates of π-conjugated oligomers with a herringbone molecular packing.19, 20 (TDSB molecules are stacked co-facially along the crystal b-axis with the shortest separation about 3.4 Å in SNCs. The longitudinal and transverse displacements between neighboring molecules are 0.3 and 3.6 Å, respectively (Figure S3).) For such H-aggregates composed of non-rigid molecules, such as TDSB, only nodeless excitons can be optically excited from the vibrationless ground state, as indicated by 1A and 2A transitions in Figure 2b.19 These photo-generated excitons then relax rapidly into the lowest energy |10〉 state by emission of vibrational phonons (upper waved arrow in Figure 2b). Although the 0−0 emission, i.e., |10〉 [RIGHTWARDS ARROW] |00〉, is optically forbidden, the 0−n sideband emissions, i.e., |10〉 [RIGHTWARDS ARROW] |0n〉, n ≥ 1, are allowed (Figure 2a and b).19, 20 In most cases, herringbone H-aggregates of π-conjugated oligomers, such as oligothiophenes and oligophenylenes in thin films and crystals, are characterized by poor PL quantum yields (ϕ).20 However, due to exciton–vibration coupling, highly ordered H-aggregated SNCs of TDSB are strongly emissive in the soild-state with ϕSMC = 0.81 ± 0.05, which is only slightly smaller than that of monomers ϕm= 0.96 ± 0.03.16 To obtain further information on the nature of the excited states, we performed time-resolved PL measurements. The monomer PL at 452 nm decays monoexponentially with a lifetime (τ) of τm= 1.33 ± 0.06 ns (Figure S4, Supporting Information), so does the SNC PL at 500 nm with τSNC= 1.56 ± 0.06 ns. Therefore, the radiative decay rates (k) of TDSB monomers and SNCs are calculated to be km = 0.72 ± 0.04 ns−1 and kSNC = 0.52 ± 0.05 ns−1, respectively, according to the equation, k = ϕ/τ.21 The fact that kSNC < km is also consistent with the H-aggregation model.19 Remarkably, the absence of |10〉 [LEFT RIGHT ARROW] |00〉 transitions due to H-aggregation results in negligible overlap between the absorption and the PL spectra of SNCs thus minimizing the self-absorption effect (Figure 2a).

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Figure 2. a) Absorption (dash) and normalized PL (solid) spectra of assembled SNCs (lower) placed on a glass substrate and monomers in THF solution (upper). b) Energy level diagram for the optical transitions of TDSB H-aggregates. Blue energy levels correspond to nodeless excitons accessed by photon absorption, which then vibrationally relax to the lowest energy |10〉 node excitons as indicated by the upper waved arrow. Because of optical rules, the |10〉 [LEFT RIGHT ARROW] |00〉 transitions are forbidden. The weakly observed 0−0 emission in bottom panel of (a) is due to the presence of a low density of side disorders in single-crystalline SNCs.

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The SNCs exhibit strong green PL under excitation of un-focused UV light (330−380 nm), with typical features of an active optical waveguide, such as bright PL spots at the tips and weaker PL from the bodies (Figure 1d). This suggests that the SNCs are able to absorb the excitation light and propagate the PL along the 1D direction, leading to bright tips when PL exits the SNC.9, 11 In order to investigate the microcavity effect, we characterized isolated single SNCs using a home-made optical microscope equipped with a 50 × 0.9 NA objective (Scheme S1, Supporting Information). The second harmonic (λ = 400 nm, pulse width 150 fs) of a 1 KHz Ti:sapphire regenerative amplifier was focused to a 2 μm diameter spot to excite the middle part of the selected single SNC on a 2D movable table. Spatially resolved PL spectra along the SNC body with a resolution ∼1 μm were then collected underneath by using a 3D-movable objective coupled to an optical fiber and detected using a liquid-nitrogen cooled charge-coupled device (CCD).

Figure 3a shows the PL spectra of an isolated SNC (W = 1.2 μm and L = 18 μm), collected at the excitation point, as a function of pump intensity varied using a series of metallic neutral density filters. At low pump density of P = 108 nJ cm−2, the PL spectrum is dominated by a broad spontaneous emission (green curve). When the pump density exceeds a threshold from P = 290 (red) to 405 nJ cm−2 (blue), strong laser emission develops as a set of sharp peaks on the top of the 0−1 transition. The full width at half maximum (FWHM) of the 0−1 peak dramatically decreases from 18 nm below the threshold to 5 nm above the threshold. The inset of Figure 3a shows the integrated intensities of the 0−1 peak as a function of P, showing clearly a threshold at Pth = 220 nJ cm−2. The intensity dependence is fitted to a power law xp with p = 0.44 ± 0.02 below threshold, indicating a sublinear regime where bimolecular quenching (exciton–exciton annihilation) dominates,5 whereas a superlinear increase is found above the threshold with p = 2.43 ± 0.03. In sharp contrast, the 0−2 band remains in the sublinear regime both below and above the threshold. To further verify the lasing action, we investigated the PL lifetimes with a streak camera using the same detection geometry (Figure 3b). The SNC PL follows single exponential decay with τSNC = 1.56 ± 0.06 ns at a very low excitation density of P = 0.05 Pth. Upon increasing the pump intensity to P = 0.22 Pth and/or 0.61 Pth, bi-exponential PL decay takes place with the short component ascribed to the presence of bimolecular quenching. Above the threshold, for example, at P = 1.62 Pth, the PL decay time always collapses to < 10 ps and is limited by the resolution of our apparatus. This short PL lifetime above the threshold suggests an effective stimulated process from the reservoir of |10〉 excitons to the |01〉 state.22

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Figure 3. a) PL spectra of a single SNC with W = 1.2 μm and L = 18 μm, measured as a function of excitation density at 400 nm. Inset: integrated area of the 0−1 peak as a function of pump density. The lasing threshold is identified as the intersection between the sublinear and superlinear regions. b) Typical PL decay profiles of SNCs monitored at 500 nm, showing the evolution from un-quenched, to strongly quenched, to lasing, with increasing the pumping laser power. Above the threshold the decay time always collapses to < 10 ps, indicating an effective stimulated process.

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The middle curve in Figure 4d is the enlarged, high- resolution version of the lasing spectrum in Figure 3a at P = 405 nJ cm−2, corresponding to a SNC with W = 1.2 μm. It can be seen that periodic interference peaks are observed as evidence of the cavity longitudinal modes with a spacing of Δλ = 0.76 nm. Moreover, these interference peaks exhibit a FWHM of ca. 0.5 nm at 500 nm, giving rise to Q as high as 1000. Previously, axial FP cavities along the length of nanowire crystals had been demonstrated for both inorganic and organic semiconductors, due to reflections of guided light at both flat wire-end-faces.9, 23 In these cases, cavity resonances were well-resolved in the spectra recorded at tip positions but undetectable from the bodies.9, 23 In order to understand the influence of SNC sizes on cavity effect, we investigated a number of individual SNCs with W = 1.6 (Figure 4a), 1.2 (Figure 4b), and 0.9 μm (Figure 4c), and 3−5 SNCs for each W covering L = 5−20 μm. i) Thresholds between 100 and 800 nJ cm−2 for typical lasing features were observed at both body and tip positions of the SNC lasers studied, including superlinear output intensity, spectral linewidth narrowing, and appearance of cavity resonances. ii) The cavity mode pattern is only related to the width of the SNC studied and independent of its length, for example, Δλ = 1.03 nm for W = 0.9 μm SNCs (top curve in Figure 4d), and Δλ = 0.58 nm for W = 1.6 μm SNCs (bottom curve in Figure 4d). It is known that the mode spacing (Δλ) at λ for a FP cavity of length L is given by Equation 1,9, 23

  • equation image((1))
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Figure 4. a−c) High-magnification SEM image of individual SNCs with W = 0.9, 1.2, 1.6 μm (L ≈ 20 μm). d) High-resolution PL spectra of laser emission recorded above threshold for SNCs corresponding to those shown in (a−c). e) A schematic picture of the SNC radial cavity along its width. f) The mode spacing Δλ at λ = 500 nm versus 1/W of SNCs, showing clearly a linear relationship. g,h) The dark-field and PL micro-images of two interconnected SNCs. i) Spatially resolved PL spectra of laser emission upon excitation at the circled region labeled in (g) and (h) using 400nm, 150 fs light above threshold, recorded at the circled point (middle), tip-1 (bottom), and tip-2 (top), respectively (see g and h). Figures (g−i) demonstrate a prototype circuit, where the laser light generated in the local radial cavity of SNCs can propagate along its length and couple into other SNCs.

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where [n−λ(dn/dλ)] is the group velocity refraction index (n is the phase refractive index and dn/dλ is the dispersion relation). Figure 4f presents a plot of the mode spacing Δλ at λ = 500 nm versus 1/W of the SNCs studied, demonstrating clearly a linear relationship. This confirms that different from previously reported 1D-nanowire axial cavities,3, 9, 11, 14, 15, 23 our SNCs operate along its width as radial cavities. That is, the pair of lateral-faces of SNCs can function as two reflectors and provide strong self-cavity optical confinement (Figure 4e).

As TDSB is two-photon active,16 we are able to cut the SNCs into small pieces by moving a tightly focused 800 nm laser light across its body, offering opportunities for future two-photon processing of TDSB SNCs according to practical needs. By shortening the selected SNC (Figure S5, supporting Information), we found that the lasing pattern of the broken parts did not change, further supporting the assignment of SNC radial cavities. In this way, the smallest SNC that still exhibits lasing action (nearly single mode lasing) was found to have L ≈ 2 μm (restricted by the spatial resolution of our optical microscope) and W = 450 nm, representing the smallest nanowire lasers ever reported (Figure S6, Supporting Information).

Note that the group velocity refraction index, [n−λ(dn/dλ)], calculated using Equation 1 based on the data presented in Figure 4d, is enormously high around 150. This is probably due to the formation of exciton-polaritons (EPs) as a result of strong coupling of excitons with cavity photons in the nanometer-scale SNC radial cavities,23–25 although the actual mechanism needs to be further clarified by detailed comparison of the dispersion in angular momentum space (in-plane wavevector k//, Figure 4e) through angle-resolved characterizations (PL, absorption, and reflection).5, 26 The group velocity refraction index, [n−λ(dn/dλ)] = 150, means that the signal velocity is only 1/150 of the light speed in vacuum,23, 24 enabling SNCs capable of propagating the polariton light and steering them on the nanometer scale. Figure 4g and h show bright-field and PL images of two interconnected SNCs: the horizontal one with W = 1.0 μm and L = 80 μm; the perpendicular one with W = 2.0 μm and L = 50 μm. The middle curve in Figure 4i shows the lasing spectrum recorded upon local excitation of the horizontal SNC at the circled region as shown in Figure 4g and h, while the top and bottom curves indicate the spectra collected at tip-1 and 2, respectively. It can be seen that the laser light generated in the local radial cavity at the circled region can propagate along the same SNC and leak out of tip-1. Moreover, laser light propagated along the left side of the horizontal SNC can couple into the perpendicular one over an angle of 105° and finally leak out of tip-2. This long propagation distance and the reduced signal velocity can be beneficial for the sensitivity of sensing applications due to increased interaction time. Moreover, the local radial cavity paves the way for integrating SNCs into light-emitting transistors (LET) as electrically driven devices, where the light-emitting zone (cavity region) can be distantly separated from the SNC/electrode contacts.

In conclusion, we demonstrate that highly emissive H-aggregated organic semiconductors can be superior gain materials for laser applications, in which the absence of |10〉 [LEFT RIGHT ARROW] |00〉 transitions minimize the self-absorption induced optical loss and avoid direct radiative depopulation of the exciton reservoir to the |00〉 state. The two lateral-faces of SNCs self-assembled from TDSB constitute a high-quality (Q ≈ 1000) FP cavity along its width. The optimized energy level of H-aggregates and the high quality radial cavity of SNC led to a lasing threshold as low as 100 nJ cm−2, above which lasing action was evidenced by features, such as superlinear output intensity, spectral narrowing, emission decay collapse, and appearance of cavity resonances. Moreover, the laser light generated in the ultra-small SNC radial local cavity (0.5−2 μm) can propagate along its length up to hundreds of micrometers, making them attractive as a coherent light source for miniaturized photonic circuits.

Supporting Information

  1. Top of page
  2. Supporting Information
  3. Acknowledgements
  4. Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

  1. Top of page
  2. Supporting Information
  3. Acknowledgements
  4. Supporting Information

This work was supported by the National Natural Science Foundation of China (Nos. 20925309, 21190034, 21073200, 21073079, J1103307), the National Basic Research Program of China (973) 2011CB808402, 2012CB933102, the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP. 20110211130001), and the Fundamental Research Funds for the Central Universities and 111 Project. Supporting Information is available online from Wiley InterScience or from the author.

Supporting Information

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  2. Supporting Information
  3. Acknowledgements
  4. Supporting Information

Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors.

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