Microcavity Laser Based on a Single Molecule Thick High Gain Layer Alexander Palatnik, Hagit Aviv, and Yaakov R. Tischler* Bar-Ilan University, Ramat-Gan 5920002, Israel ABSTRACT: The ability to confine excitons within monolayers has led to fundamental investigations of nonradiative energy transfer, super-radiance, strong light−matter coupling, high-efficiency lightemitting diodes, and recently lasers in lateral resonator architectures. Vertical cavity surface emitting lasers (VCSELs), in which lasing occurs perpendicular to the device plane, are critical for telecommunications and large-scale photonics integration, however strong optical self-absorption and low fluorescence quantum yields have thus far prevented coherent emission from a monolayer microcavity device. Here we show lasing from a monolayer VCSEL using a single molecule thick film of amphiphilic fluorescent dye, assembled via Langmuir−Blodgett deposition, as the gain layer. Threshold was observed when 5% of the molecules were excited (4.4 μJ/cm2). At this level of excitation, the optical gain in the monolayer exceeds 1056 cm−1. High localization of the excitons in the VCSEL gain layer can enhance their collective emission properties with Langmuir−Blodgett deposition presenting a paradigm for engineering the high gain layers on the molecular level. KEYWORDS: microcavities, organic materials, dye lasers, monolayers light. The first organic VCSEL was based on dye in solution,24 and later solid-state devices using fluorescent polymers and semiconducting host−guest systems25,26 were demonstrated. Planar microcavities not only provide the optical feedback in VCSELs but also play an important role in modifying light− matter interactions in both weak (Purcell effect)27 and strong coupling limits.28,29 In the weak coupling regime, the small active volume of the microcavity restricts the available photonic modes leading to changes in spontaneous emission rates. This effect was demonstrated in planar microcavities.30−32 The position of the emitters inside the microcavity has a strong influence on the emission properties.33 A larger Purcell effect and pronounced modification to nonradiative energytransfer rates was experimentally observed when thin films of a few molecular layers of organic materials were positioned at the antinode of the electromagnetic field.34−36 However, until now in VCSEL devices, the organic gain material has been distributed throughout the microcavity volume in order to minimize concentration quenching and thereby preserve the fluorescence quantum yield. Even for small molecules and polymers that have been designed to have reduced concentration quenching, thus far, distributed gain architectures have been adopted in order to prevent high local excitation densities
onolayers of optically active excitonic materials find a wide range of applications spanning photographic sensitization, photocatalysis, chirality-selective synthesis, dye-sensitized solar cells, light-emitting diodes (LEDs), fluorescence-based chemical sensors, and lasers in lateral configurations based on transition-metal dichalcogenides (TMDC).1−6 There are many ways to make monolayers of excitonic materials: self-assembly,7,8 covalent attachment,9 Langmuir adsorption,10 CVD,11,12 PVD,13 exfoliation,14 colloidal synthesis,15 and Langmuir−Blodgett (LB) deposition16 among others. Being one of the first practical techniques for monolayer deposition, LB remains widely used.17−19 In LB, amphiphilicity of the deposited material or a co-surfactant is required to form a thin film at the air−water interface that can then be easily transferred to a target substrate.20 The ability to control film density and molecular orientation makes LB a natural choice for applications involving organic monolayers such as J-aggregate formation, energy-transfer-based light harvesting layers, monolayer OLED’s, and fluorescence-based chemical sensors,21−23 in all of which such molecular level control can influence the electrical, optical, and chemical properties of the monolayer constituents. In vertical cavity surface emitting lasers (VCSELs), lightemitting material capable of supporting population inversion is situated inside an optical microcavity consisting of two planar mirrors that are separated by a small distance (on the order of a wavelength) carefully chosen to resonantly trap the emitted
M
© 2017 American Chemical Society
Received: December 2, 2016 Accepted: April 5, 2017 Published: April 5, 2017 4514
DOI: 10.1021/acsnano.6b08092 ACS Nano 2017, 11, 4514−4520
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ACS Nano which can lead to strong optical losses via exciton−exciton annihilation. Here using LB deposition to assemble a monolayer of the amphiphilic fluorescent dye, Lissamine rhodamine B sulfonyl didodecyl amine (LRSD), we form the gain layer of a VCSEL that is one molecule in thickness, as shown in Figure 1. We
deposited on a convex substrate with a radius of curvature of 4.5 m. We chose to form the microcavity by mechanically sandwiching the dielectric mirrors, instead of by deposition of the top mirror, in order to preserve the optical properties of the LRSD monolayer and to obtain a high Q factor resonator. This approach to cavity fabrication enables facile integration of the microcavity from two dielectric mirrors fabricated separately in optimized conditions.38,39
RESULTS Optical and morphological characterizations confirm the single molecule thickness of the LRSD layer (Figure 2c,d). The LRSD molecules were transferred from the LB trough onto a PMMA film at a surface pressure of 22 mN/m, which according to the isotherm of Figure 2a corresponds to an average area of 1.6 nm2 per molecule situated at the air−water interface. The absorption and emission spectra of the transferred layer are shown in Figure 2b. The layer has an absorption peak at 575 nm where it absorbs 0.06% of the incident light and an emission peak at 588 nm. From a comparison to the absorption of the same organic dye in thin film with known concentration, we determined the areal density to be 2.9 × 1013 molecules × cm−2. Equivalently, each molecule of LRSD occupies on average an area of 3.45 nm2, thus indicating that a monolayer of LRSD deposits onto the PMMA surface with a transfer ratio of 46%. The AFM scan of LRSD coated PMMA in Figure 2c,d shows that the dye molecules are concentrated in ∼1.5 nm high domains, which are a single molecule in thickness, about 5 μm × 5 μm in size and covering about 50% of the total PMMA surface area. Taken together, from the absorption spectrum and AFM scan, we conclude that a single monolayer of LRSD transfers to PMMA with islands of uniform dye packing at a surface density of about 1.7 nm2 per molecule. The peak in the emission spectrum at 588 nm indicates that the LRSD monolayer thus formed is not highly concentrated compared to LRSD monolayers transferred onto glass and onto PMMA at higher surface pressures (above 30 mN/m), which exhibit a red-shifted emission spectrum closer to 600 nm.37 We compared the spontaneous emission lifetimes of LRSD in a monolayer form to Rhodamine 6G doped in a PMMA matrix and to LRSD in both solution and matrix configurations (Figure 3). The lifetime of the LRSD monolayer is 3.2 ns (Figure 3d), which is similar to Rhodamine 6G in a PMMA matrix, and to LRSD dissolved with PMMA in a chloroform solution, Figure 3a,c, respectively, while the lifetime of LRSD doped into a PMMA matrix is 0.2 ns (Figure 3b). Likewise, the measured quantum yield of the LRSD monolayer is QY = 0.55 ± 0.1, while for LRSD in the PMMA matrix, it is only QY = 0.07 (for comparison, the QY = 0.75 for Rhodamine 6G in PMMA matrix). The low QY and shortened lifetime of LRSD in matrix form is caused by concentration quenching likely due to micelle formation of the amphiphilic LRSD molecules. In contrast, Rhodamine 6G remains uniformly dispersed in the PMMA matrix and thus exhibits a high quantum yield and longer lifetime, as does LRSD in chloroform solution. Similarly, the LRSD monolayers show high QY and a lifetime comparable to Rhodamine 6G in solution. Therefore, no lifetime shortening typical for exciton−exciton annihilation or concentration quenching is observed for the monolayer. Fluorescence intensity of the monolayer did not change as a function of the polarization of a linearly polarized excitation beam, hence the in plane dipole moments appear to be randomly oriented on the micron length scale.
Figure 1. (a) Monolayer VCSEL formed from planar and curved dielectric mirrors with a monolayer of dye molecules situated between them to provide optical gain. (b) Experimental setup to obtain photoluminescence spectra and fluorescence lifetime measurements. (c) A monolayer of the amphiphilic Rhodamine dye, LRSD, adsorbed to the surface of the PMMA λc/4n spacer layer.
synthesized LRSD by functionalizing Lissamine Rhodamine B sulfonyl chloride (LRSC) with didodecylamine via a straightforward sulfonation reaction37 and deposited the dye onto the water subphase of the LB trough from a 1 mg/mL chloroform solution. We designed the LRSD molecule to have two alkane chains in order to render it suitable for compression into monolayers at the air−water interface and simultaneously minimize concentration quenching. Indeed, despite a high surface density, equivalent to about 1 M in a bulk thin film, monolayers of LRSD exhibit minimal concentration quenching, with the quantum yield being as high as 0.55 ± 0.1 and the fluorescent lifetime being comparable to LRSD dissolved in chloroform, namely 3.2 and 3.3 ns, respectively. We incorporated the LRSD monolayer into the VCSEL architecture (Figure 1a) using the following procedure. First, we spin-coated a thin film of poly(methyl methacrylate) (PMMA) on top of a highly reflective dielectric mirror (R > 99.9% in the spectral range of 525−645 nm, custom coating by Altechna Ltd.). Next, we used LB to deposit the monolayer onto the PMMA coated mirror (see Methods for details of the LB procedure). The monolayer was transferred on retraction of the substrate from the water subphase, in order to ensure a single monolayer coating. Finally, to complete the microcavity, a PMMA layer was coated on a second dielectric mirror that was then mechanically pressed on top. Thus, the two PMMA coatings serve as spacer layers between the dielectric mirrors, and together they define the resonant wavelength of the cavity, λc. We spin-coated both PMMA layers to have a thickness of λc/4n, to form a λ/2 cavity, and to position the LRSD monolayer at the antinode of the cavity. To ensure a large contact area between the mirrors surfaces, the top mirror was 4515
DOI: 10.1021/acsnano.6b08092 ACS Nano 2017, 11, 4514−4520
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Figure 2. (a) Isotherm of surface pressure versus mean molecular area of LRSD dye. (b) Absorption and emission spectra of an LRSD monolayer transferred at 22 mN/m onto a PMMA-coated substrate. (c) AFM topographic image of the LRSD monolayer deposited on PMMA. (d) Height profile of the AFM scan.
Figure 3. (a) Relaxation of the Rhodamine 6G dye emission in PMMA host matrix at concentration of 0.05% (w/w) (3 ns). (b) Relaxation of the LRSD dye emission in PMMA host matrix at concentration of 0.05% (w/w) (0.2 ns). (c) Relaxation of emission of LRSD dye and PMMA dissolved in chloroform (3.3 ns). (d) Relaxation of the LRSD dye emission in monolayer (3.2 ns).
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Figure 4. (a) Output power as a function of input energy density for polarization following excitation polarization (∥) and polarization transverse to excitation polarization (⊥). (b) Parallel polarization spectrum below the threshold (0.5Eth) and above the threshold (5Eth).
Figure 5. (a) Cavity line width for different resonances, measured by exciting small spot with 457 nm CW laser using 100× objective lens. (b) Blue circles are lasing threshold for devices with different resonance wavelengths, and green squares are lifetime of the emission in the cavity and in free space.
When the monolayer VCSEL devices were optically excited by a nanosecond pulsed laser, we observed a transition to lasing. The lasing threshold, output power dependence, and emission spectrum are shown in Figure 4a for a microcavity tuned to λc = 592 nm. At an excitation energy density of 4.4 μJ/ cm2, we observe a threshold-like increase in the slope of the output emission having the same polarization as the excitation beam. In contrast, the cross-polarized output retains the same slope as more energy is pumped into the device. Above threshold, at an excitation density of 5Eth, the emission has about a 50% degree of polarization, and it is co-polarized with the pump, a phenomenon observed in solution and solid-state organic VCSELs.38−40 Above threshold, a narrow spectral line appears at the cavity resonance wavelength as shown in Figure 4b, with a line width that is
