Sandra Wolff

One- and two-dimensional photonic crystal microcavities in single crystal diamond

Diamond is an attractive material for photonic quantum technologies because its colour centres have a number of outstanding properties, including bright single photon emission and long spin coherence times. To take advantage of these properties it is favourable to directly fabricate optical microcavities in high-quality diamond samples. Such microcavities could be used to control the photons emitted by the colour centres or to couple widely separated spins. Here, we present a method for the fabrication of one- and two-dimensional photonic crystal microcavities with quality factors of up to 700 in single crystal diamond. Using a post-processing etching technique, we tune the cavity modes into resonance with the zero phonon line of an ensemble of silicon-vacancy colour centres, and we measure an intensity enhancement factor of 2.8. The controlled coupling of colour centres to photonic crystal microcavities could pave the way to larger-scale photonic quantum devices based on single crystal diamond.

Flexible, all-polymer microelectrode arrays for the capture of cardiac and neuronal signals.

Microelectrode electrophysiology has become a widespread technique for the extracellular recording of bioelectrical signals. To date, electrodes are made of metals or inorganic semiconductors, or hybrids thereof. We demonstrate that these traditional conductors can be completely substituted by highly flexible electroconductive polymers. Pursuing a two-level replica-forming strategy, conductive areas for electrodes, leads and contact pads are defined as microchannels in poly(dimethylsiloxane) (PDMS) as a plastic carrier and track insulation material. These channels are coated by films of organic conductors such as polystyrenesulfonate-doped poly(3,4-ethylenedioxy-thiophene) (PEDOT:PSS) or filled with a graphite-PDMS (gPDMS) composite, either alone or in combination. The bendable, somewhat stretchable, non-cytotoxic and biostable all-polymer microelectrode arrays (polyMEAs) with a thickness below 500 μm and up to 60 electrodes reliably capture action potentials (APs) and local field potentials (LFPs) from acute preparations of heart muscle cells and retinal whole mounts, in vivo epicortical and epidural recordings as well as during long-term in vitro recordings from cortico-hippocampal co-cultures.

Polarization-independent active metamaterial for high-frequency terahertz modulation

We present a polarization-independent metamaterial design for the construction of electrically tunable terahertz (THz) devices. The implemented structure consists of an array of gold crosses fabricated on top of an n-doped gallium arsenide (GaAs) layer. Utilizing THz time-domain spectroscopy, we show that the electric resonance and thus the transmission properties of the cross structure can be tuned by an externally applied bias voltage. We further demonstrate the fast amplitude modulation of a propagating THz wave for modulation frequencies up to 100 kHz.

Fourier-optical transverse mode selection in external-cavity broad-area lasers: experimental and numerical results

Broad-area lasers (BALs) with external Fourier-optical cavities with spatial filter for transverse mode selection have been studied experimentally and theoretically. The transverse mode structure of BALs is modeled using a three mirror cavity approach. The model accounts for gain saturation, carrier diffusion, and anti-guiding effects. Near- and far-field distributions are calculated and compared to experimental results. Transverse mode selection is achieved for BALs with a residual front facet reflectivity of 10% at low pump currents. For BALs having a very low front facet reflectivity of 0.001%, transverse modes can be selectively excited up to pump currents more than 200% above laser threshold. Calculations show that BALs having a 0.001%-antireflection coating with an external Fourier-optical cavity high above threshold can operate in a self-Q-switched-like mode with pulse durations of 2-4 ns and repetition rates of 100-200 MHz. Experimental results obtained with a hybrid integrated-optical external cavity for transverse mode selection are also presented.

Fourier-optical selection of higher order transverse modes in broad area lasers

A partially and a highly antire ection coated broad area laser are operated in an external cavity Fourier-optical 4f set-up to experimentally investigate transverse mode selection. The external cavity consists of a lens and a spatial frequency filter. Running freely the lasers show non-stationary filamentation. Placing the spatial filter unit directly onto the optical axis gives cw fundamental mode operation and a transverse shift of the spatial filter in the plane of the active region allows for selective excitation of higher order modes.

Intracavity stabilization of broad area lasers by structured delayed optical feedback.

The influence of structured delayed optical feedback (SDOF) on a broad area laser is investigated experimentally. SDOF is realized with a miniature-sized convex external mirror. The setup takes into account the small time scales involved in semiconductor laser dynamics by employing short external resonator lengths. Careful choice of the feedback parameters leads to a narrow single-lobe farfield even at high pump currents. The experimental results confirm earlier microscopic dynamic simulations by O. Hess et al. predicting that SDOF might be capable of stabilizing the emission of broad area lasers.

Fabrication of ridge waveguides in LiNbO 3

We have fabricated ridge waveguides in lithium niobate with sidewall roughness of 14 nm (rms) and sidewall angles of more than 71°. The use of thick electroplated metal masks for reactive ion etching (RIE) makes it possible to manufacture ridge structures with several microns in height. For light confinement towards the substrate we investigate direct heterobonding techniques. Due to the expected low transmission losses we envision future applications in the field of quantum optics.

Prototyping all-polymer bioelectrical signal transducers

For historical reasons, the signal transduction interface of bioelectronic devices is commonly based on metals or inorganic (semi-)conductors. This also applies to application areas where artificial components such as biomedical screening devices, in vitro microelectrode arrays and in vivo neuroprosthetics come into direct contact with biological tissue. In a proof-of-concept microelectrode array design study, we present an alternative all-polymer approach for the low-cost fabrication of bioelectrical signal transduction devices with adjustable flexibility, electrical impedance and transparency. The fabrication process entailed three steps. Firstly, by means of a replica-moulding strategy, different types of transparent polymers were microstructured by two-level SU-8 masters to create vias for contact pads and electrodes, and indentations for interconnecting microchannels. Secondly, recesses in the insulating polymer sheets were filled with conductive polymer composites based on quasi-transparent polystyrenesulfonate doped poly(3,4-ethylenedioxy-thiophene) (PEDOT:PSS). In a last step, the passive microelectrode arrays were backside-insulated by a second layer of a transparent polymer. The electrical properties of the resulting polymer microelectrode arrays were characterized by impedance spectroscopy, baseline noise measurements and recordings of bioelectrical signals from acute preparations of chicken cardiomyocytes. Biocompatibility was tested with in vitro cultures of cortical neurons derived from embryonic chicken.

Air-Suspended SU-8 Polymer Waveguide Grating Couplers

We present SU-8 polymer waveguide grating couplers for TE mode with enhanced refractive index contrast by employing air as upper and lower cladding layer. Numerical simulations predict a coupling loss of 4.9 dB, indicating a coupling efficiency of 32% at a maximum spectral response of 1550 nm. Based on a polymer lamination method, air-suspended waveguide grating couplers were successfully fabricated and characterized. Due to current limitations in the fabrication process, the perturbation of the experimental grating couplers is weaker than the original simulation. However, transmission measurements have shown that a single grating coupler exhibits approximately 8 dB coupling loss at a center wavelength of 1557 nm, indicating a coupling efficiency of 16% with respect to a single-mode optical fiber. The demonstrated parallel fabrication method employing the widely used SU-8 photoresist makes the polymer waveguide grating couplers very attractive for a wide range of low-cost polymer photonic applications.

Self Q-switching and mode locking in broad area lasers subject to transverse mode selective feedback

Numerical and experimental results of output dynamics investigations of AR-coated broad area lasers (BALs) above laser threshold are presented. The BALs are subject to feedback from a free-space external Fourier-optical 4f-setup with a spatial reflective filter in the Fourier-plane for transverse mode selection. It is shown theoretically and experimentally that under certain pump current conditions the BALs are operating in a repetitive self-pulsation mode. Pulse duration is approx. 1 ns at repetition rates of 200 to 500 MHz. Using the same setup active mode-locking of a BAL is achieved experimentally. Pulse durations of 103 ps are obtained. The Gaussian-like fundamental and higher order transverse modes up to mode No. 4 can be adjusted while the laser is operating in a mode-locked state. Experimentally, the simultaneous combination of mode-locking, transverse mode selection, and pulse shaping of a BAL in a modified 4f-setup implementing a spectral filter is investigated. Employing an optimized spectral sinc-like function as amplitude and phase filter the mode-locked BAL emits nearly square-shaped pulses with a pulse duration of 705 ps, while running close to the Gaussian-like transverse mode.