Letter pubs.acs.org/NanoLett
Chip-Based All-Optical Control of Single Molecules Coherently Coupled to a Nanoguide Pierre Türschmann,† Nir Rotenberg,† Jan Renger,† Irina Harder,† Olga Lohse,† Tobias Utikal,† Stephan Götzinger,‡,† and Vahid Sandoghdar*,†,‡ †
Max Planck Institute for the Science of Light, Staudtstraße 2, D-91058 Erlangen, Germany Friedrich Alexander University Erlangen-Nuremberg, D-91058 Erlangen, Germany
‡
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S Supporting Information *
ABSTRACT: The feasibility of many proposals in nanoquantum-optics depends on the efficient coupling of photons to individual quantum emitters, the possibility to control this interaction on demand, and the scalability of the experimental platform. To address these issues, we report on chip-based systems made of one-dimensional subwavelength dielectric waveguides (nanoguides) and polycyclic aromatic hydrocarbon molecules. We discuss the design and fabrication requirements, present data on extinction spectroscopy of single molecules coupled to a nanoguide mode, and show how an external optical beam can switch the propagation of light via a nonlinear optical process. The presented architecture paves the way for the investigation of many-body phenomena and polaritonic states and can be readily extended to more complex geometries for the realization of quantum integrated photonic circuits. KEYWORDS: On-chip single photon source, nonlinear optics, quantum optics, hybrid systems, subwavelength nanoguides, evanescent coupling
N
carbons (PAH) such as pentacene or dibenzoterrylene (DBT) can be nearly indefinitely photostable when embedded in organic crystals (see Figure 1a).14,15 Furthermore, PAHs can have very stable and narrow resonances at superfluid helium temperature, giving access to scattering cross sections close to the ideal value of 3λ2/2π.16 Altogether, these properties lead to efficient light-matter interactions that allow for functionalities such as single-molecule transistors17 and few-photon nonlinear quantum optics.18 Recently, several groups have presented interesting results on the coupling of organic molecules to nanoguides.2,5,19 However, the employed fabrication strategies are not easily compatible with low-loss and efficient coupling to narrow-band molecules. Here, one should keep in mind that success in the realization of cascadable interactions depends on the suppression of scattering losses caused by crystal edges or sample roughness. In this Letter, we report on a fabrication protocol that does justice to this important requirement and present results on the coherent coupling of single molecules to an on-chip nanoguide. Furthermore, we show that the molecule’s emission can be controlled by an external optical beam via its intrinsic nonlinearity. These building blocks can be scaled up to a molecular quantum network with high optical densities for
anoscopic sources of photons, switches, and memories are in high demand as building blocks for quantum networks, which have been the subject of intense research in the past decade.1 Equally important are solutions for efficient “wiring” of these elements. Furthermore, real applications of such circuits will require a high degree of scalability, making it almost imperative that the physical system be realized in the solid state and on a chip. An attractive approach to address these issues is to couple solid-state quantum emitters to a onedimensional subwavelength waveguide (nanoguide), which acts as an optical wire or bus. Practical implementations of nanoguides depend on the particularities of the material system of choice and have been realized as nanofibers2−5 or on-chip waveguides.6−9 While the latter technology readily lends itself to the coupling of quantum dots7 and color centers,6,8,9 there are fabrication challenges that must be met to successfully integrate organic molecules with chip-based inorganic nanoguides.10−12 Overcoming these difficulties, however, is particularly desirable because chip architectures allow for the incorporation of micro- and nanoelectrodes to tune the resonance frequencies of the individual molecules via the Stark effect and can host feedback microstructures such as photonic crystals. Organic dye molecules are used in a large number of applications in physics, chemistry, and technology and are particularly well-known for the crucial role they play in fluorescence nanoscopy.13 Although conventional dye molecules suffer from strong phonon coupling and prohibitive photobleaching, it turns out that polycyclic aromatic hydro© 2017 American Chemical Society
Received: May 15, 2017 Revised: June 28, 2017 Published: July 3, 2017 4941
DOI: 10.1021/acs.nanolett.7b02033 Nano Lett. 2017, 17, 4941−4945
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Nano Letters
Figure 2. (a) Schematic of the optical setup. S, sample; TCA, telecentric lens assembly; AL, aspheric lens; SM, scanning mirror; BS, beam splitter; F, spectral filter; APD, avalanche photodiode. (b) Extinction (green) and Stokes-shifted fluorescence (red) spectra recorded through the output and input grating ports, respectively, (see inset lower right) as a function of the excitation laser frequency. The central inset shows a zoom of the extinction spectrum.
Figure 1. (a) Molecular structures of host material AC and dopant DBT. (b) Schematics of a nanoguide architecture where single DBT molecules are evanescently coupled to a subwavelength TiO 2 waveguide (red) with a cross section of 200 × 200 nm2 and a total length of 50 μm. On each side of the waveguide an adiabatic taper interfaces the guided mode via an outcoupler grating to free-space modes. The TiO2 nanoguide is supported on a fused silica (gray) substrate and embedded in DBT-doped anthracene (green). The structure is encapsulated using an upper glass substrate with integrated reservoir (see SI for details).
beams are sent through each arm of the resulting dual microscope, allowing us to address single molecules within the inhomogeneous spectrum by resonant excitation via the Fourier-limited zero-phonon line (00ZPL) that connects the ground vibrational levels of the electronic ground (|g, v = 0⟩) and excited (|e, v = 0⟩) state.14 The upper state can decay via the resonant 00ZPL or via red-shifted transitions to |g, v ≠ 0⟩ levels. Here, the nanoguide geometry provides convenient simultaneous access to both the incoherent red-shifted fluorescence and the coherent signal, which can be detected in transmission and reflection of the waveguide or via light scattered into nonguided modes using sensitive cameras and avalanche photodiodes. The green and red spectra in Figure 2b present the coherent transmission and the reflected red-shifted fluorescence signal of a nanoguide, respectively. The transmission spectrum was recorded by sending a laser beam on the input grating and detecting the transmitted light at the output grating where it is scattered into the objective, while the reflected red-shifted fluorescence was detected simultaneously via the input grating. The red-shifted spectrum shows more than 20 sharp peaks, each of which we associate with a single molecule. However, only about half of the peaks in the red-shifted spectrum correspond to dips in the resonant transmission (green curve). We attribute the missing resonances in transmission to molecules that lie on the input grating but with low coupling to the nanoguide mode. The inset of Figure 2b displays a zoom of two extinction dips in the resonant transmission spectrum, revealing sharp spectral features with Lorentzian line shapes. The observed full width at half-maximum (fwhm) of Γ0 = 30 MHz corresponds to the natural line width of the 00ZPL in DBT/AC and is a robust indication for the coherent coupling of single molecules to the nanoguide mode. The extinction dips in the transmission signal
studying novel polaritonic effects20−22 or as a linear quantum simulator.23 Our fabrication approach exploits the capillary flow of a molten liquid organic matrix in nanoscopic channels and subsequent crystallization upon controlled cooling.4,24 This process allows us to uniformly cover a subwavelength TiO2 nanoguide (n = 2.4) supported on a fused silica chip (n = 1.46) with a DBT-doped anthracene (AC) crystal (n = 1.7), ensuring low-loss propagation (see Figure 1a). A schematic representation of our nanoguide with cross section of 200 × 200 nm2 and 50 μm length is sketched in Figure 1b. As we show, several tens of molecules lie within the evanescent tail of the guided mode and, consequently, couple to the nanoguide. Here, the coupling efficiency β is defined as the fraction of the power radiated by the emitter into the nanoguide mode. Numerical simulations show that β can reach up to 33% for a linear dipole at the interface between AC and TiO2. This geometry allows us to address each molecule individually, either through the waveguide using integrated grating couplers or from free space, providing clean low-background signals (see Supporting Information). We demonstrate the coherent coupling of single organic dye molecules to nanoguides by using the experimental setup for low-temperature spectroscopy and microscopy diagrammed in Figure 2a. The chip samples are mounted inside a helium bath cryostat operating at 1.7 K and can be positioned in the substrate (x−y) plane using slip-stick piezo sliders. Two aspheric lenses with numerical aperture NA = 0.77 were used to access the sample from opposite sides, whereby one of them could be positioned in all three dimensions, and the other one was adjustable along z. Narrow-band (Δν < 1 MHz) laser 4942
DOI: 10.1021/acs.nanolett.7b02033 Nano Lett. 2017, 17, 4941−4945
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Nano Letters reach up to 7.2%, corresponding to a coupling efficiency β = 7.3% if we assume a combined Franck−Condon/Debye− Waller factor of 0.5 (see Supporting Information). The observed narrow line widths indicate the high quality of the sample and are especially noteworthy because close vicinity to interfaces often compromises the spectral stability and purity of guest emitters in solid-state matrices.24,25 To explore the molecule-nanoguide system and its functionality further, we consider the single molecule marked at its resonance frequency in the inset of Figure 2b by two arrows. In Figure 3a, we show a wide-field fluorescence image
function g(2) shown in Figure 3c. A strong antibunching at zero delay of 0.01 confirms that the fluorescence stems from a single molecule. Moreover, the theoretical fit at a Rabi frequency of 0.9Γ0 lets us determine a molecular excited-state lifetime of 5 ns, setting an upper bound to the scattering rate. The image obtained in a spatially resolved reflection scan in Figure 3b helps visualize the chip geometry. By correlating the images in (a) and (b), we found the molecule to lie at about 360 nm from the nanoguide edge. The coupling efficiency expected in theory for this emitter-nanoguide spacing agrees well with the measured β of 7.3%. One can gain external control on the coupled emitternanoguide system by exploiting the large inherent nonlinearity of PAHs.18 Figure 4a shows clear changes to the transmission
Figure 3. (a) Wide-field fluorescence image when a single molecule was excited by a focused laser beam from the side (see upper schematics). (b) Geometry of the nanoguide structure imaged in reflection. A cross shows the position of the molecule imaged in (a). (c) Second-order cross-correlation measurement of the red-shifted fluorescence detected through the two grating ports while the molecule was excited as in (a). The g(2)(0) = 0.01 is limited by detector noise.
where the molecule was excited from the side in the focus of a laser beam. The three bright regions denote the Stokes-shifted fluorescence of the molecule detected directly at its location and through the two grating ports. The absence of fluorescence along the nanoguide in Figure 3a verifies the high quality of the waveguide and negligible scattering losses, which would have otherwise led to fluorescence leackage at different locations. Lack of fluorescence background also confirms a good quality of our organic crystal and the absence of broad molecular transitions. Because the transition dipole of the molecule is linearly polarized, our nanoguide-molecule system must act like a perfect 50:50 single photon beam splitter for incident light. From the amplitudes of the light on the two gratings (Figure 3a) we calculate an off-chip splitting ratio of 47:53. We attribute this slight difference to fabrication imperfections of the grating coupler. By recording the fluorescence scattered by the molecule into the waveguide mode in a start−stop coincidence configuration, we obtained the second-order cross-correlation
Figure 4. (a) Nanoguide transmission as a function of pump-molecule and pump−probe frequency detunings for fixed probe and pump Rabi frequencies of Ωprb = 0.1 Γ0 and Ωpmp = Γ0, respectively. (b) Exemplary nanoguide transmission spectra for three different pump intensities Ωpmp = 0.5 MHz (blue), Ωpmp = 43 MHz (green), Ωpmp = 77 MHz (red) at a fixed pump-molecule detuning of −13 MHz. The vertical dashed lines point to the strongest attenuations (black) and amplifications (red) of the probe beam. (c) Extinction signal (black) and coherent amplification (red) of the probe beam as a function of Rabi frequency and incoming photon number of the pump beam. The gray regions correspond to expected variations caused by a very small spectral diffusion less than 0.2Γ0. 4943
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integrated quantum circuitry. In addition, molecular chip architectures ideally lend themselves to interfacing with other chip-based materials such as color centers8 or rare-earth ions28,29 to realize hybrid quantum systems.
of a weak probe beam as it propagates through the nanoguide and interacts with a single molecule in the presence of a pump beam that can be detuned relative to both the probe and the molecular resonance. Here, the pump strength was chosen to correspond to a moderate Rabi frequency of Ωpmp = Γ0 while the probe was kept weak at Ωprb = Γ0/10. For large pump− probe detunings, two spectral features are apparent along the pump−molecule detuning (vertical line cut in Figure 4a): A probe beam resonant to the molecule is attenuated by 7% due to extinction (blue) while the transmission signal is enhanced (red) by 3% if the pump beam is resonant to the molecule but the probe beam is off-resonant. The increase in the transmission is, however, predominantly caused by the resonant scattering of the pump into the waveguide mode that is also present without probe beam. We can account for this effect by normalizing the probe signal to its value far from the molecular resonance for a given pump-molecule detuning. This allows one to extract the nonlinear interaction between probe and pump mediated by the molecule. A near-resonant pump beam (horizontal line in Figure 4a) can perfectly suppress the extinction of the probe. Next, we explore the nonlinear evolution of the transmission in Figure 4b where three exemplary transmission curves for a fixed pump−molecule detuning of −13 MHz and pump Rabi frequencies of 0.02 Γ0 (blue), 1.4 Γ0 (green), and 2.6 Γ0 (red) are shown. These spectra illustrate that the control beam can alter the resonant transmission in three ways, which depend on the detuning of the probe photons from the molecular resonance. First, the pump beam can be used to AC-Stark shift the molecular resonance, decreasing the transmission at lower frequencies as the pump power increases. Second, if the probe frequency is held constant at the molecule resonance frequency (black dashed line), the attenuation of the transmission is altered, decreasing as the pump power increases. Lastly, when the probe frequency is higher than that of the molecular resonance (red dashed line), the pump beam can coherently amplify the probe signal. Here, we observe a maximum amplification of 0.3%. These data are summarized in Figure 4c, where we plot the extinction and coherent amplification as a function of the pump Rabi frequency or the number of pump photons, respectively. The shadowed regions of the solid curves show the corresponding calculated quantities, allowing for a small spectral instability less than 0.2Γ0, which occurred over several hours during these measurements (see Supporting Information). In this Letter, we showed that single organic molecules can be integrated into on-chip nanoguides, resulting in an efficient and low-background platform for quantum optical applications. The coherent interaction of single molecules with the guided mode allows for all-optical nonlinear control of the transmission of the coupled system using an external laser beam. This option is particularly attractive because it can be implemented with a diffraction-limited spatial resolution and at speeds of up to hundreds of megahertz. The demonstration that nanofabricated inorganic structures can be uniformly covered by high quality organic crystals opens realistic prospects for investigating polaritonic and quantum manybody phenomena while controlling the contribution of each individual emitter. Furthermore, bringing molecular quantum optics to on-chip waveguide systems allows for the use of optical circuit elements such as beam splitters, routers, and interferometers26 that in combination with novel devices such as on-chip superconducting detectors27 will open the field of
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02033. Details on sample fabrication, the scattering efficiencies of the emitters and the nanoguide gratings as well as on the nonlinear experiments (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Pierre Türschmann: 0000-0002-2491-8276 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Harald Haakh for fruitful discussions and help with theoretical design and Maksim Schwab for his contributions to the mechanical components. This work was supported by an Alexander von Humboldt professorship, the Max Planck Society, and the European Research Council (Advanced Grant SINGLEION).
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REFERENCES
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