Coherent two-dimensional micro-spectroscopy: An investigation of plasmon propagation

A well-known bottleneck for cutting-edge nano-electronic circuits which enable broadband data-processing, is their miniaturization. In the last decade the opto-electronic approach of using surface plasmon-polaritons [1] became a promising concept to achieve this goal due to the deep subwavelength confinement [2] of electromagnetic fields and their propagation velocity near the speed of light. By combining fs laser pulses with optical or photoemission electron microscopy, several spatio-temporally coupled processes in designed nanostructures could be demonstrated, e.g., coherent control of plasmon propagation in nano-circuits [3] and strong coupling of widely separated nano-antennas [4]. Nano-antennas have the unique ability to channel far-field radiation to sub-wavelength dimensions. The resulting strongly confined and enhanced electromagnetic fields boost nonlinear optical effects at the nanoscale [5]. For this purpose, we introduce coherent two-dimensional (2D) micro-spectroscopy which probes the nonlinear optical response of the nano-antennas with sub-micron spatial resolution [6]. An LCD-based pulse shaper in 4f geometry is used to create collinear trains of 12-fs visible/NIR laser pulses in the focus of a numerical aperture of a 1.4 immersion-oil microscope objective [7]. We motivate this new method for getting nonlinear third-order information of the ultrafast dynamics of plasmon propagation via phase cycling, e.g., for the local spatial investigation of the strong coupling between a transition metal dichalcogen-ide (TMD) monolayers and a nano-antenna on top of it. References [1] M. L. Brongersma et al. “The case for plasmonics”. Science, 328 (5977): 440-441, 2010. [2] J.A. Schuller et al. “Plasmonics for extreme light concentration and manipulation”. Nat. Mater., 9 (3):1 93-204, 2010. [3] C. Rewitz et al. “Coherent Control of Plasmon Propagation in a Nanocircuit”. Phys. Rev. Applied., 1: 014007, 2014. [4] M. Aeschlimann et al. “Cavity-assisted ultrafast long-range periodic energy transfer between plasmonic nanoantennas”. Light-Sci. Appl., 6: e17111, 2017. [5] B. Metzger et al. “Ultrafast Nonlinear Plasmonic Spectroscopy: From Dipole Nanoantennas to Complex Hybrid Plasmonic Structures”. ACS Photonics, 3 (8):1336–1350, 2016 [6] S. Goetz et al. “Coherent two-dimensional fluorescence micro-spectroscopy”. Opt. Express, 26 (4):3915-3925, 2018 [7] M. Pawłowska et al. “Shaping and spatiotemporal characterization of sub-10-fs pulses focused by a high-NA objective”. Opt. Express, (22):31496-31510, 2014</p

Revised Method for Overcoming Visibility Reduction in Hanbury-Brown Twiss Experiments using Random Phase Modulation

    Random Phase Modulation (RPM) is a method to generate a pseudo-thermal light source (PTLS) for testing setup and fitting routines in HBT experiments. By illuminating a rotating ground glass disc (GGD) with a laser, RPM imprints µs to ms long coherence times, overcoming the limitations of natural thermal light sources. We rotated the GGD at different frequencies. We found the coherence time shifts following theoretical expectations for high frequencies, and deviating at low frequencies, leading to a revision of the formula for finite line broadening of the laser. We determined the first order coherence time of the laser to 0.3620±0.0063 μs, which is not accessible by conventional g² methods. The light field was found to thermalize for frequencies >35 Hz with a g² value of 1.42. We found a shift of the peak center of the bunching curves with frequency, leading to a deduced lateral misalignment of 3 µm. RPM was shown to be highly accurate for checking coherence properties and detecting lateral misalignments in HBT setups.</p

Determining Coherence Properties of Quantum Light Sources: A Study on Experimental Jitter and Power Dependency

Timing jitter of a single-photon device is essential for determining the coherence properties of quantum light sources. However, Jitter measurements were often not performed at the ultra-diluted light level present for quantum light sources. We investigate the timing jitter of a single-photon device sing a time-to-digital converter (TDC) and HBT interferometry to measure the distribution of delay between start and stop pulses. We determined the Gaussian jitter of a single single channel of the TDC to 7.12 ps, while HBT interferometry showed an excitation power dependency. The functional form of the pulsed histogram was determined using an exponentially modified Gaussian function. The exponential part showed minor effects for low count rates approaching a Lorentz shape count rate, consistent with photons getting converted deeper in the junction. These findings are crucial for understanding the underlying physics of jitter and accurately performing second-order correlation measurements. </p

Quantum Control Spectroscopy of Competing Reaction Pathways in a Molecular Switch

Excitation with shaped femtosecond laser pulses is a logical extension of coherent two-dimensional (2D) spectroscopy. Here we combine quantum control and information from 2D spectroscopy to analyze the initial steps in three competing reaction pathways of an isomerizing merocyanine dye. Besides the achievement of control objectives, we show how excitation with tailored pulses can be used to retrieve photochemical information that is inaccessible or experimentally demanding to obtain with other approaches

Ultrafast non‐linear 2D microspectroscopy reveals coherent phonon‐mediated intra‐ and intervalley exciton interaction in an individual SWCNT

Further developments in molecular electronics, adressing SWCNTs, would benefit strongly from insights in the spatio‐temporal evolution of molecular processes. Ultrafast non‐linear techniques provide tracking of energy transfer pathways e.g., mediated via electron‐phonon coupling [1]. A comprehensive way to observe these dynamics is coherent 2D fluorescence microspectroscopy [2]. This method is a generalization of transient absorption spectroscopy with frequency‐resolved pump and probe steps, combined with spatially‐resolved optical microscopy. This provides, e.g., to observe the phonon‐mediated formation and annihilation dynamics of initially bright and dark‐state excitons due to the strong exciton‐phonon coupling on the femtosecond timescale. Here, we utilize the third‐order 2D signal for monitoring the trapped intra‐ and intervalley exciton interaction in a SWCNT [3,4]. To this end, an transform‐limitted LCD‐shaped four‐pulse sequence is focused on an (6,4) SWCNT and the fluorescence is detected as a function of inter‐pulse time delays and phases. [1] Graham, M. et al., Nano Lett. 12, 813–819 (2012). [2] Goetz, S. et al., Opt. Express 26, 3915–3925 (2018). [3] Secchi, A. et al., Phys. Rev. B 88 (2013). [4] Kislitsyn, D. et al., J. Phys. Chem. Lett. 5, 3138–3143 (2014).</p

Ultrafast non-linear 2D micro-spectroscopy reveals coherent phonon-mediated intra- and intervalley exciton interaction in an individual SWCNT

Poster presentation from  NT19: International Conference on the Science and Application of Nanotubes and Low-Dimensional Materials 21-26 July 2019, Würzburg, Germany We perform nonlinear fluorescence-based two-dimensional spectroscopy(F-2DES) on (6,4) SWCNTs at the single molecule level. We highllight the possibilities with the single molecule setup for fluorescence based experiments at ultra-diluted light levels. We provide evidence for single-molecular investigation by various methods. We identifiy an energetic substructure in the phonon sideband of individual SWCNTs, observed with F-2DES measurements. We evalaute the significance of the peak structure in F-2DES data, by the application of statistical inference methods. We relate our findings of the energetic peak positions to polaron formation, induced by phonon-mediated intervalley dynamics. We explain F-2DES data by Liouville pathways. We further observe vacancy relaxation of laser induced defects in SWCNTs and verify our findings with help of a temperature model </p

Analytic Optimization of Near-Field Optical Chirality Enhancement

We present an analytic derivation for the enhancement of local optical chirality in the near field of plasmonic nanostructures by tuning the far-field polarization of external light. We illustrate the results by means of simulations with an achiral and a chiral nanostructure assembly and demonstrate that local optical chirality is significantly enhanced with respect to circular polarization in free space. The optimal external far-field polarizations are different from both circular and linear. Symmetry properties of the nanostructure can be exploited to determine whether the optimal far-field polarization is circular. Furthermore, the optimal far-field polarization depends on the frequency, which results in complex-shaped laser pulses for broadband optimization

Analytic Optimization of Near-Field Optical Chirality Enhancement

We present an analytic derivation for the enhancement of local optical chirality in the near field of plasmonic nanostructures by tuning the far-field polarization of external light. We illustrate the results by means of simulations with an achiral and a chiral nanostructure assembly and demonstrate that local optical chirality is significantly enhanced with respect to circular polarization in free space. The optimal external far-field polarizations are different from both circular and linear. Symmetry properties of the nanostructure can be exploited to determine whether the optimal far-field polarization is circular. Furthermore, the optimal far-field polarization depends on the frequency, which results in complex-shaped laser pulses for broadband optimization

Analytic Optimization of Near-Field Optical Chirality Enhancement

We present an analytic derivation for the enhancement of local optical chirality in the near field of plasmonic nanostructures by tuning the far-field polarization of external light. We illustrate the results by means of simulations with an achiral and a chiral nanostructure assembly and demonstrate that local optical chirality is significantly enhanced with respect to circular polarization in free space. The optimal external far-field polarizations are different from both circular and linear. Symmetry properties of the nanostructure can be exploited to determine whether the optimal far-field polarization is circular. Furthermore, the optimal far-field polarization depends on the frequency, which results in complex-shaped laser pulses for broadband optimization

Analytic Optimization of Near-Field Optical Chirality Enhancement

We present an analytic derivation for the enhancement of local optical chirality in the near field of plasmonic nanostructures by tuning the far-field polarization of external light. We illustrate the results by means of simulations with an achiral and a chiral nanostructure assembly and demonstrate that local optical chirality is significantly enhanced with respect to circular polarization in free space. The optimal external far-field polarizations are different from both circular and linear. Symmetry properties of the nanostructure can be exploited to determine whether the optimal far-field polarization is circular. Furthermore, the optimal far-field polarization depends on the frequency, which results in complex-shaped laser pulses for broadband optimization