Universität Stuttgart
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Item Open Access Tailored nanocomposites for 3D printed micro-optics(2020) Weber, Ksenia; Werdehausen, Daniel; König, Peter; Thiele, Simon; Schmid, Michael; Decker, Manuel; Oliveira, Peter William de; Herkommer, Alois; Giessen, HaraldItem Open Access Long-term stability of capped and buffered palladium-nickel thin films and nanostructures for plasmonic hydrogen sensing applications(2013) Strohfeldt, Nikolai; Tittl, Andreas; Giessen, HaraldOne of the main challenges in optical hydrogen sensing is the stability of the sensor material. We found and studied an optimized material combination for fast and reliable optical palladium-based hydrogen sensing devices. It consists of a palladium-nickel alloy that is buffered by calcium fluoride and capped with a very thin layer of platinum. Our system shows response times below 10 s and almost no short-term aging effects. Furthermore, we successfully incorporated this optimized material system into plasmonic nanostructures, laying the foundation for a stable and sensitive hydrogen detector.Item Open Access Surface- and tip-enhanced resonant Raman scattering from CdSe nanocrystals(2015) Sheremet, Evgeniya; Milekhin, Alexander G.; Rodriguez, Raul D.; Weiss, Thomas; Nesterov, Maxim; Rodyakina, Ekaterina E.; Gordan, Ovidiu D.; Sveshnikova, Larisa L.; Duda, Tatyana A.; Gridchin, Victor A.; Dzhagan, Volodymyr M.; Hietschold, Michael; Zahn, Dietrich R. T.Surface- and tip-enhanced resonant Raman scattering (resonant SERS and TERS) by optical phonons in a monolayer of CdSe quantum dots (QDs) is demonstrated. The SERS enhancement was achieved by employing plasmonically active substrates consisting of gold arrays with varying nanocluster diameters prepared by electron-beam lithography. The magnitude of the SERS enhancement depends on the localized surface plasmon resonance (LSPR) energy, which is determined by the structural parameters. The LSPR positions as a function of nanocluster diameter were experimentally determined from spectroscopic micro-ellipsometry, and compared to numerical simulations showing good qualitative agreement. The monolayer of CdSe QDs was deposited by the Langmuir–Blodgett-based technique on the SERS substrates. By tuning the excitation energy close to the band gap of the CdSe QDs and to the LSPR energy, resonant SERS by longitudinal optical (LO) phonons of CdSe QDs was realized. A SERS enhancement factor of 2 × 10 3 was achieved. This allowed the detection of higher order LO modes of CdSe QDs, evidencing the high crystalline quality of QDs. The dependence of LO phonon mode intensity on the size of Au nanoclusters reveals a resonant character, suggesting that the electromagnetic mechanism of the SERS enhancement is dominant. Finally, the resonant TERS spectrum from CdSe QDs was obtained using electrochemically etched gold tips providing an enhancement on the order of 10 4 . This is an important step towards the detection of the phonon spectrum from a single QD.Item Open Access Short-range surface plasmonics: localized electron emission dynamics from a 60-nm spot on an atomically flat single-crystalline gold surface(2017) Frank, Bettina; Kahl, Philip; Podbiel, Daniel; Spektor, Grisha; Orenstein, Meir; Fu, Liwei; Weiss, Thomas; Horn-von Hoegen, Michael; Davis, Timothy J.; Meyer zu Heringdorf, Frank-Joachim; Giessen, HaraldItem Open Access Nearly diffraction limited FTIR mapping using an ultrastable broadband femtosecond laser tunable from 1.33 to 8 µm(2017) Mörz, Florian; Semenyshyn, Rostyslav; Steinle, Tobias; Neubrech, Frank; Zschieschang, Ute; Klauk, Hagen; Steinmann, Andy; Giessen, HaraldMicro-Fourier-transform infrared (FTIR) spectroscopy is a widespread technique that enables broadband measurements of infrared active molecular vibrations at high sensitivity. SiC globars are often applied as light sources in tabletop systems, typically covering a spectral range from about 1 to 20 µm (10 000 - 500 cm−1) in FTIR spectrometers. However, measuring sample areas below 40x40 µm2 requires very long integration times due to their inherently low brilliance. This hampers the detection of ultrasmall samples, such as minute amounts of molecules or single nanoparticles. In this publication we extend the current limits of FTIR spectroscopy in terms of measurable sample areas, detection limit and speed by utilizing a broadband, tabletop laser system with MHz repetition rate and femtosecond pulse duration that covers the spectral region between 1250 - 7520 cm−1 (1.33 - 8 µm). We demonstrate mapping of a 150x150 µm2 sample of 100 nm thick molecule layers at 1430 cm−1 (7 µm) with 10x10 µm2 spatial resolution and a scan speed of 3.5 µm/sec. Compared to a similar globar measurement an order of magnitude lower noise is achieved, due to an excellent long-term wavelength and power stability, as well as an orders of magnitude higher brilliance.Item Open Access Mass-producible micro-optical elements by injection compression molding and focused ion beam structured titanium molding tools(2020) Ristok, Simon; Roeder, Marcel; Thiele, Simon; Hentschel, Mario; Guenther, Thomas; Zimmermann, André; Herkommer, Alois; Giessen, HaraldItem Open Access Electron-driven photon sources for correlative electron-photon spectroscopy with electron microscopes(2020) Christopher, Joshua; Taleb, Masoud; Maity, Achyut; Hentschel, Mario; Giessen, Harald; Talebi, NahidElectron beams in electron microscopes are efficient probes of optical near-fields, thanks to spectroscopy tools like electron energy-loss spectroscopy and cathodoluminescence spectroscopy. Nowadays, we can acquire multitudes of information about nanophotonic systems by applying space-resolved diffraction and time-resolved spectroscopy techniques. In addition, moving electrons interacting with metallic materials and optical gratings appear as coherent sources of radiation. A swift electron traversing metallic nanostructures induces polarization density waves in the form of electronic collective excitations, i.e., the so-called plasmon polariton. Propagating plasmon polariton waves normally do not contribute to the radiation; nevertheless, they diffract from natural and engineered defects and cause radiation. Additionally, electrons can emit coherent light waves due to transition radiation, diffraction radiation, and Smith-Purcell radiation. Some of the mechanisms of radiation from electron beams have so far been employed for designing tunable radiation sources, particularly in those energy ranges not easily accessible by the state-of-the-art laser technology, such as the THz regime. Here, we review various approaches for the design of coherent electron-driven photon sources. In particular, we introduce the theory and nanofabrication techniques and discuss the possibilities for designing and realizing electron-driven photon sources for on-demand radiation beam shaping in an ultrabroadband spectral range to be able to realize ultrafast few-photon sources. We also discuss our recent attempts for generating structured light from precisely fabricated nanostructures. Our outlook for the realization of a correlative electron-photon microscope/spectroscope, which utilizes the above-mentioned radiation sources, is also described.Item Open Access Photo-induced ionization dynamics of the nitrogen vacancy defect in diamond investigated by single-shot charge state detection(2013) Aslam, Nabeel; Waldherr, Gerald; Neumann, Philipp; Jelezko, Fedor; Wrachtrup, JörgThe nitrogen-vacancy centre (NV) has drawn much attention for over a decade, yet detailed knowledge of the photophysics needs to be established. Under typical conditions, the NV can have two stable charge states, negative (NV-) or neutral (NV0), with photo-induced interconversion of these two states. Here, we present detailed studies of the ionization dynamics of single NV centres in bulk diamond at room temperature during illumination and its dependence on the excitation wavelength and power. We apply a recent method which allows us to directly measure the charge state of a single NV centre, and observe its temporal evolution. We find that the steady-state NV− population is always ⩽75% for 450–610 nm excitation wavelength. In combination with saturation measurements, we show that the optimal excitation wavelength is around 510-540 nm. Furthermore, the relative absorption cross-section of NV- is determined for 540-610 nm, revealing a double-peak structure. Finally, the energy of the NV- ground state of 2.6 eV below the conduction band is measured. These results reveal new insights into the charge state dynamics of the NV centre.Item Open Access Detuning dependent Rabi oscillations of a single molecule(2019) Rezai, Mohammad; Wrachtrup, Jörg; Gerhardt, IljaA single organic dye molecule at cryogenic conditions is resonantly excited in a confocal microscope. Under strong laser illumination it undergoes Rabi oscillations. Mathematically, this was well described and had been experimentally implemented. These oscillations can be measured as side-bands on their resonance fluorescence, e.g. in the Mollow-Triplet. An alternative method is to research this effect by an analysis of the single molecule anti-bunched photon statistics. This has been performed in this work. Here we research on the detuning dependence of this signal-it is experimentally demanding since the utilized laser might drift or single emitters are not necessarily spectrally stable enough, such that the spectrum can be measured indefinitely. We therefore apply a measurement technique in which the photon correlation signal is acquired in detuning dependent steps. This is performed by continuous laser sweeps over the single molecule excitation spectrum. A single recording of the anti-bunched photons takes 20-50 ms. After approx. 1 h of repetitive laser detunings a full anti-bunching curve is reconstructed for each spectral position. An alternative technique with 100 ns laser pulses allows us to acquire a set of comparable data. Our study is derived from a single dibenzanthanthrene molecule with a natural linewidth of 2π×16 MHz. It emits under resonant excitation more than 380.000 photons per second. Under spectral detuning, Rabi-oscillations are observed up to ΩRabi = 2π×160 MHz.Item Open Access Alignment-free difference frequency light source tunable from 5 to 20 µm by mixing two independently tunable OPOs(2020) Mörz, Florian; Steinle, Tobias; Linnenbank, Heiko; Steinmann, Andy; Giessen, HaraldTunable mid-infrared ultrashort lasers have become an essential tool in vibrational spectroscopy in recent years. They enabled and pushed a variety of spectroscopic applications due to their high brilliance, beam quality, low noise, and accessible wavelength range up to 20 µm. Many state-of-the-art devices apply difference frequency generation (DFG) to reach the mid-infrared spectral region. Here, birefringent phase-matching is typically employed, resulting in a significant crystal rotation during wavelength tuning. This causes a beam offset, which needs to be compensated to maintain stable beam pointing. This is crucial for any application. In this work, we present a DFG concept, which avoids crystal rotation and eliminates beam pointing variations over a broad wavelength range. It is based on two independently tunable input beams, provided by synchronously pumped parametric seeding units. We compare our concept to the more common DFG approach of mixing the signal and idler beams from a single optical parametric amplifier (OPA) or oscillator (OPO). In comparison, our concept enhances the photon efficiency of wavelengths exceeding 11 µm more than a factor of 10 and we still achieve milliwatts of output power up to 20 µm. This concept enhances DFG setups for beam-pointing-sensitive spectroscopic applications and can enable research at the border between the mid- and far-IR range due to its highly efficient performance.