key: cord-0131815-e3m65yfg authors: Dhawan, Amit Raj; Nasilowski, Michel; Wang, Zhiming; Dubertret, Benoit; Maitre, Agnes title: Efficient single-emitter plasmonic patch antenna fabrication by novel deterministic in situ optical lithography using spatially modulated light date: 2021-10-14 journal: nan DOI: nan sha: f676cdcb8467a58aea512a4d03ab2c4cd01aa0e2 doc_id: 131815 cord_uid: e3m65yfg Single-emitter plasmonic patch antennas are room-temperature deterministic single photon sources, which exhibit highly accelerated and directed single photon emission. However, for efficient operation these structures require three-dimensional nanoscale deterministic control of emitter positioning within the device, which is a demanding task, esp. when emitter damage during fabrication is a major concern. To overcome this limitation, our deterministic room-temperature in situ optical lithography protocol uses spatially modulated light to position a plasmonic structure non-destructively on any selected single-emitter with three-dimensional nanoscale control. In this paper we analyze the emission statistics of such plasmonic antennas that embed a deterministically positioned single colloidal CdSe/CdS quantum dot that highlight acceleration and brightness of emission. We demonstrate that the antenna induces a 1000-fold increase in the emitter absorption cross-section, and under high pumping, these antennas show nonlinearly enhanced emission. The possibility of deterministically controlling the local environment of an emitter to make its emission faster, the capability of collecting more emitted photons, and the effectiveness of exciting a system with only one photon will benefit several optical quantum technologies [1] . The highly optimized light-matter interactions in devices with these features will highlight the involved physics [2] and find application in fields like single photon generation [3, 4, 5] , quantum plasmonics [6, 7] , and optical antennas [8, 9, 10] . Advances in microlithography [11] produced superior electronics and helped attain the first quantum revolution, and the developments in nanofabrication [12] are paving the way for the second quantum revolution [ 13] . By means of Purcell effect [14] , structures such as optical cavities [15] and plasmonic antennas [16, 17] , modify light emission. Their optimal operation necessitates precise spatial positioning of the emitter with respect to its environment, which is a challenging task. Although electron-beam lithography obtains higher resolution than optical lithography, it requires more expensive and specialized equipment and working conditions. Moreover, the beam electrons can damage the emitter. Optical lithography is a cheaper and convenient solution that achieves good writing resolution without electron exposure. However, the intense light required to perform lithography locally above the emitter can photodegrade it. Our fabrication protocol [18] resolves this problem by selectively not exposing the emitter to intense light during the lithography as it uses a custom-designed donut laser beam. At room-temperature and under ambient working conditions, we have employed this protocol to position a plasmonic patch on any selected CdSe/CdS QD with nanometric vertical and horizontal precision, and fabricate plasmonic patch antennas with remarkable emission properties. Core/shell QDs [19] , vacancy centers in nanodiamonds [20] , and defects in two-dimensional materials [21, 22] are deterministic room-temperature single photon sources. Coupling them to optical or plasmonic cavities can make their emission brighter and faster. Optical cavities require very precise spectral matching, which makes them unsuitable for broadband emitters like colloidal QDs at room-temperature. However, such broadband emitters can couple very efficiently to plasmonic antennas due to their wide spectral resonance and low volume, thus making them promising room-temperature single photon sources. This has been discussed [17, 23, 24] and demonstrated [25, 26] recently. Embedding the QD between the nano-spaced metal-dielectric interfaces couples its radiation to generate surface plasmon polaritons (SPPs) at the interfaces [17] , which creates very high electromagnetic field around the QD and accelerates its emission. As the patch thickness is less than the skin depth of the field, the SPP-coupled interaction in the antenna emits photons from the patch as shown in Figure 1 (a). Here we describe the deterministic fabrication of single-emitter patch nanoantennas and demonstrate that the antenna can dramatically modify QD emission, which is illustrated by high recombination rate and fluorescence enhancement, increased absorption cross-section, and the nonlinear emission of the antenna. The metallic losses in plasmonic antennas that reduces their radiative efficiency [27] can be mitigated by their acceleration and directionality of emission, thus achieving faster and brighter emission. We show that a plasmonic antenna can increase the absorption cross-section of a QD by more than three orders, and lead to highly nonlinear emission under low power excitation. A single QD is selected and embedded in a dielectric layer sandwiched between a plasmonic metal film and a patch to obtain a single-emitter plasmonic patch antenna, as illustrated in a. A transmission electron microscopy image (b) of the colloidal CdSe/CdS QDs used in this work, and c, monoexponentially fitted emission decays of four QDs in a homogeneous medium. d Optical setup used for the generation of spatially modulated light using a reflective phase-only spatial light modulator. PBS is a polarizing beam splitter. Conventional photolithography [28] utilizes light of appropriate wavelength and intensity to locally expose the photoresist, which is then chemically etched. Using in situ photolithography, Belacel et al. [10] demonstrated deterministic fabrication of plasmonic patch antennas that contain an aggregate of colloidal QDs. This method cannot be extended straightforwardly to room-temperature deterministic fabrication of single-emitter antennas because the fluorescence of a single QD is considerably lower than that of an aggregate, and therefore it is masked by the luminescence of the photoresist. The high light intensity required to observe a QD leads to unintended exposure (chemical modification due to light) of the photoresist above it and makes localized lithography impossible. Our protocol avoids this problem by using a resist bi-layer which has low luminescence and is not exposed like conventional photoresists during the process. The bi-layer resist stack consisting of a lift-off resist (LOR® [29] ) and polymethyl methacrylate (PMMA) does not rely upon conventional exposure because it is evaporated by intense laser light during the laser etching process [30] . However, the intense light of the fundamental laser mode required to remove the resist bi-layer above a single QD usually results in photobleaching it. We resolve this problem by using spatially modulated laser light with a donut profile (Figure 1d ), which does not expose the QD to light during the lithography, and the carefully designed intensity profile of the laser mode leads to a successful positioning a plasmonic patch centered/off-centered above the QD with a lateral precision of ±50 nm. The QD can be positioned vertically in the antenna with a precision of ±3 nm using spin-coated thin films. This lithography protocol can work with a variety of emitters-single or aggregates-such as QDs, vacancy centers in nanodiamond, molecules, and defects in two-dimensional materials. This article describes the use of this protocol to deterministically select and position a single colloidal CdSe/CdS QD ( Figure 1b) inside a plasmonic patch antenna as illustrated in Figure 1a . Light-matter interactions in nanophotonic devices can be optimized by deterministic control over emitter selection and device fabrication, and the described lithography protocol is capable of achieving this. The antennas discussed in this paper were fabricated at roomtemperature and under atmospheric pressure, but the fabrication protocol can be used at low temperature and in vacuum conditions. Compared to our other protocol [25] that utilizes a laser with wavelength tunability to maximize absorption of light by the resist and minimize QD photobleaching, this method excels at preventing emitter damage as no light is seen by the QD during the laser etching step. Moreover, by modifying the phase map on the spatial light modulator (SLM) screen, a variety of high-resolution shapes and patterns can be created. The The performance of these antennas depends on the dipolar orientation of the QD, the dielectric spacer thickness around the QD, the position of the QD with respect to the patch, and the size of the patch. This protocol allows optimal control over all these parameters. In this study, we controlled all these parameters except QD orientation [31] . In homogeneous PMMA medium and under low intensity off-resonance excitation (405 nm laser pulsed at 2.5 MHz), a QD from the studied batch typically emitted single photons with a monoexponential decay of characteristic lifetime of about τ = 90 ns (Figure 1c) . Coupling a QD to the antenna changes its decay rate ( Figure 3 ) and its single photon emission characteristics. Most of the spontaneous decay response curves of these antennas can be fitted with a biexponential function with decay constants τ fast (multiexciton) and τ slow (exciton). Removing photon events corresponding to the fast decay through post-processing leaves mainly exciton recombination events, which are single photon emission events and therefore show lower 2 (0) . This is demonstrated in Figures 3d, e, where filtering out the fast component of the antenna decay (blue trace), leaves mostly exciton events (orange trace), which exhibits a negligible zero delay peak in the photon coincidence curve (Figure 3e , low 2 (0) curve), thus validating the above hypothesis about the studied systems. An excited QD decays in a cascade from the multiexciton states to the exciton state and finally to the ground state as depicted in Figure 3f . Auger processes, which are non-radiative and quench multiexciton emission, only influence multiexciton transitions such as biexciton to exciton, and are included in the multiexciton to exciton rate Γ MX = Γ MX non-Auger + Γ MX Auger , where Γ MX Auger and Γ MX non-Auger denote the Auger and non-Auger multiexciton rates. Auger processes are inherently less efficient in these relatively large QDs due their size [32] . Purcell effect changes QD decay characteristics when it is coupled to the antenna. As the Purcell effect acts only on non-Auger electromagnetic transitions, Auger processes become even more ineffective in an antenna [25, 33] . This makes these antennas prone to multiexciton emission as demonstrated by the The distinct rapidity of multiexciton emission allows for temporal filtering via post processing as shown in Figure 3d , e. The possibility of temporal filtering can be exploited to make such antennas better single photon sources [34] . As long as the exciton quantum yield of the QD is stable, the antenna emission can be switched very rapidly between single photon and multiphoton emission by varying the excitation intensity-this effect can find application in optoelectronic technologies [35] . The widefield fluorescence images of Figure 4a demonstrate the enhancement in brightness due to a patch antenna. Both the images were acquired under similar conditions using continuous wave mercury lamp illumination at 438±12 nm. The top image of Figure 4a was collected after removing the resist bi-layer above the QD (at step c of Figure ) . We note that the QD fluorescence signal collected with this acquisition time is Time-correlated single photon counting on a Hanbury Brown-Twiss optical measurement setup reveals that the emission characteristics of these antennas change considerably as the excitation intensity is varied. As these antennas involve plasmonically coupled large QDs, they are prone to multiexciton emission [25] . Exciting the bright antenna of Figure 4a more intensely resulted in a larger contribution of multiexcitons to its emission. The low average power of the pulsed laser (405 nm laser at 2.5 MHz in fundamental mode) required to excite this antenna demonstrate its efficiency (Figure 5a ) and its sensitivity to nonlinear emission transition. At low power excitation (<0.5 nW), the antenna emission is mainly due to excitons and its g 2 (0) remains around 0.3 as shown by in the graph corresponding to P1 (Figure 5a, b) , and curve follows a typical 1 − −β trend, where is the average power of the laser and β is some coefficient (here β = 9.2 nW 1 ). However, around point P2 and beyond, the emission becomes highly nonlinear and the trend was noted as α , where α = 5.3 in this case. Analyzing QD emission as a function of pump intensity, Zhang et al. have reported a very similar trend [36] ; here we observe higher nonlinearity and a larger absorption crosssection due to the antenna. Plasmonic interactions provide efficient ways of trapping and propagating light [37, 38, 39] , which increases the effective absorption cross-section of these antennas [40, 41] . Our measurements reveal a 1000-fold increase in the absorption cross-section due to the antenna. We calculated the absorption cross-section of the antenna using the fitting parameter β as σ abs = spot ℎ λ rep β, where spot is the area of the laser spot focused by the objective, From P2 to P4, 2 (0) increases from 0.5 to 1 showing stronger multiexciton emission, and the lifetime reduces significantly because multiexciton recombination is considerably faster. This nonlinear emission suggests a superradiance effect [42, 36] , where localized dipolar emitters emit collectively. Although the antenna contains a single QD, the multiple states created in the QD under higher excitation can be thought of as interacting dipolar emitters localized in it similar to coupled emitters in Dicke superradiance [43] . Another way to view this superradiance effect is as a plasmonic Dicke effect [44] , where the cooperative emission from the antenna is due to the resonant energy transfer between the emitter and the plasmons rather than between multiple emitters. Such nonlinearity in emission introduced at a slightly higher power results in a substantial increase in brightness and recombination rate, thus creating an efficient and fast photon source. a Detected photon intensity vs excitation power displays the transition from exciton emission to multiexciton emission (b), which is further highlighted by the emission decay and the normalized photon correlation curves for points P1-P4. The described deterministic nanolithography protocol helped us realize single-emitter plasmonic patch antennas with very high control over emitter positioning inside the antenna. The fabricated antennas showed directional and rapid single photon emission, and demonstrated nonlinear increase in brightness due to the high optical confinement inside the antenna and the high electron-hole pair confinement inside the nanoemitter. The dramatic increase in the absorption cross-section noted in plasmonic antennas opens opportunities for engineering these effects to create efficient light nanosources. Further investigation into the factors behind this effect, and controlling these factors by precise lithography will allow ways to overcome plasmonic losses, and create bright and fast emission sources. Moreover, a high absorption cross section is boon to applications that rely on light absorption such as solar energy conversion. In our future studies, we will investigate if the nonlinear emission of such patch antennas present opportunities that will benefit quantum technologies. For laser etching on the sample, a resist bi-layer was deposited. Initially, lift off resist LOR ® 5A was spin-coated at 7000 rpm for 40 s and the sample was baked at 150°C for 2 minutes to obtain a film of thickness 450 nm, and then a 10 nm thick layer of PMMA was spin-coated using a solution of 0.5% [m/m] PMMA in toluene. Finally, the sample was baked at 150°C for 2 minutes. The protocol is detailed in the paper. Photonic quantum technologies Exploring the Quantum: Atoms, Cavities, and Photons Single-photon sources Efficient source of single photons: A single quantum dot in a micropost microcavity Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals A single-photon transistor using nanoscale surface plasmons Quantum plasmonics Optical antennas Photodetection with active optical antennas Controlling spontaneous emission with plasmonic optical patch antennas Microlithography: Science and Technology, Second Edition, ser. Optical science and engineering Nanofabrication: Principles, Capabilities and Limits The second quantum revolution Spontaneous emission probabilities at radio frequencies Optical microcavities Plasmonic nanoantennas: Fundamentals and their use in controlling the radiative properties of nanoemitters Optical patch antennas for single photon emission using surface plasmon resonances Optical lithography process adapted for a sample comprising at least one fragile light emitter Core/shell semiconductor nanocrystals Diamond photonics Quantum emission from hexagonal boron nitride monolayers Single photon sources in atomically thin materials Probing the mechanisms of large Purcell enhancement in plasmonic nanoantennas Revisiting quantum optics with surface plasmons and plasmonic resonators Extreme multiexciton emission from deterministically assembled single-emitter subwavelength plasmonic patch antennas Ultrafast room-temperature single photon emission from quantum dots coupled to plasmonic nanocavities Design of highly efficient metallo-dielectric patch antennas for single-photon emission Principles of Lithography LOR lift-off resists Laser lithography on resist bi-layer for nanoelectromechanical systems prototyping Measurement of three-dimensional dipole orientation of a single fluorescent nanoemitter by emission polarization analysis Quantization of multiparticle Auger rates in semiconductor quantum dots Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics Purification of single photons by temporal heralding of quantum dot sources Multiexcitons in semiconductor nanocrystals: A platform for optoelectronics at high carrier concentration New insights into the multiexciton dynamics in phase-pure thick-shell CdSe/CdS quantum dots Plasmonics for improved photovoltaic devices Plasmonic nanostructure design for efficient light coupling into solar cells Differential reflectivity spectroscopy on single patch nanoantennas Surface Plasmon Enhanced, Coupled and Controlled Fluorescence Absorption and Scattering of Light by Small Particles Biexciton versus exciton lifetime in a single semiconductor quantum dot Coherence in spontaneous radiation processes Cooperative emission of light by an ensemble of dipoles near a metal nanoparticle: The plasmonic dicke effect The authors thank Jean-Paul Hugonin for insightful discussions, Willy Daney de Marcillac for spectrometric measurements, Loic Becerra and Stéphan Suffit for vapor deposition, and Bruno Gallas for ellipsometry experiments. This work was supported by DIM NanoK funding through the project PATCH and by ANR DELIGHT. The authors declare no conflict of interests.