Sebastian Bauerschmidt

A gold-nanotip optical fiber for plasmon-enhanced near-field detection

A wet-chemical etching and mechanical cleaving technique is used to fabricate gold nanotips attached to tapered optical fibers. Localized surface plasmon resonances (tunable from 500 to 850 nm by varying the tip dimensions) are excited at the tip, and the signal is transmitted via the fiber to an optical analyzer, making the device a plasmon-enhanced near-field probe. A simple cavity model is used to explain the resonances observed in numerical simulations.

From Arrays of THz Antennas to Large-Area Emitters

Arrays of coherently driven photomixers with antenna (antenna emitter arrays, AEAs) have been evaluated as a possibility to overcome the power limitations of individual conventional photomixers with antenna (“antenna emitters”, AEs) for the generation of continuous-wave (CW) THz radiation. In this paper, “large area emitters” (LAEs) are proposed as an alternative approach, and compared with AEAs. In this antenna-free new scheme of photomixing, the THz radiation originates directly from the acceleration of photo-induced charge carriers generated within a large semiconductor area. The quasi-continuous distribution of emitting elements corresponds to a high-density array and results in favorable radiation profiles without side lobes. Moreover, the achievable THz power is expected to outnumber even large AEAs. Last not least, the technological challenge of fabricating LAEs appears to be significantly less demanding.

Gain Enhancement by Dielectric Horns in the Terahertz Band

A new geometry for the design of antennas in the Terahertz band is presented. The structure is based on a horn antenna etched in the substrate and fed with a planar printed antenna used for generation of terahertz radiation, designed for the 200 GHz to 3 THz range. For the proposed antenna, the energy distribution through the substrate is reduced towards an increase in the gain of the system, at least, 8 dB in a 1:10 bandwidth. The structure has been measured showing the expected behavior in the low band.

Dramatic Raman Gain Suppression in the Vicinity of the Zero Dispersion Point in a Gas-Filled Hollow-Core Photonic Crystal Fiber.

In 1964 Bloembergen and Shen predicted that Raman gain could be suppressed if the rates of phonon creation and annihilation (by inelastic scattering) exactly balance. This is only possible if the momentum required for each process is identical, i.e., phonon coherence waves created by pump-to-Stokes scattering are identical to those annihilated in pump-to-anti-Stokes scattering. In bulk gas cells, this can only be achieved over limited interaction lengths at an oblique angle to the pump axis. Here we report a simple system that provides dramatic Raman gain suppression over long collinear path lengths in hydrogen. It consists of a gas-filled hollow-core photonic crystal fiber whose zero dispersion point is pressure adjusted to lie close to the pump laser wavelength. At a certain precise pressure, stimulated generation of Stokes light in the fundamental mode is completely suppressed, allowing other much weaker phenomena such as spontaneous Raman scattering to be explored at high pump powers.

Continuous wave Terahertz emitter arrays for spectroscopy and imaging applications

We report on arrays of THz-emitters based on n-i-pn-i-p-superlattice photomixers for imaging and spectroscopic applications. The output power of a n-i-pn-i-p superlattice photomixers recently has reached nearly 1 μW at 1 THz with a broadband antenna. There are no fundamental physical limitations at this stage for further improvement. Tunable continuous wave (CW) THz-sources for imaging and spectroscopy are highly desired tools for security and environmental applications. In particular, most stand-off imaging applications require a rather high THz power to allow for a sufficient dynamic range, and a narrow illumination spot size for high spatial resolution. Both goals can be reached by using an array of mutually coherent photomixers. We have simulated beam patterns for an arbitrary number of mutually coherent single sources with respect to a small beam size and high peak intensity. Here, we confirm the simulations experimentally by an array of 4 sources with a 4 inch THz optics. The beam profile is measured in the target plane at a stand-off distance of 4.2 m. As a result, the beam diameter is reduced by a factor of 6 and the peak intensity is enhanced by a factor of close to (4)2 = 16, in excellent agreement with our simulations. Such an arrangement allows not only for high resolution stand-off imaging but also for spectroscopic investigations at stand-off distances. The THz frequency can be tuned over more than a decade (i.e. 0.1 to 2.5 THz) by tuning the wavelength of the mixing lasers. The spectral linewidth of the THz sources is only limited by the linewidths of the mixing lasers and can be made extremely narrow. A straightforward demonstration is achieved by water vapor spectroscopy in laboratory air with a single source.

Supercontinuum up-conversion via molecular modulation in gas-filled hollow-core PCF.

We report on the efficient, tunable, and selective frequency up-conversion of a supercontinuum spectrum via molecular modulation in a hydrogen-filled hollow-core photonic crystal fiber. The vibrational Q(1) Raman transition of hydrogen is excited in the fiber by a pump pre-pulse, enabling the excitation of a synchronous, collective oscillation of the molecules. This coherence wave is then used to up-shift the frequency of an arbitrarily weak, delayed probe pulse. Perfect phase-matching for this process is achieved by using higher order fiber modes and adjusting the pressure of the filling gas. Conversion efficiencies of ~50% are obtained within a tuning range of 25 THz.

Arrayed free space continuous-wave terahertz photomixers

We present free space coherent arrays of continuous-wave terahertz (THz) photomixers and compare the results to on-chip arrays. By altering the relative phases of the exciting laser signals, the relative THz phase between the array elements can be tuned, allowing for beam steering. In addition, the constructive interference of the emission of N elements leads to an increase of the focal intensity by a factor of N2 while reducing the beam width by ∼N(-1), below the diffraction limit of a single source. Such array architectures strongly improve the THz power distribution for stand-off spectroscopy and imaging systems while providing a huge bandwidth at the same time. We demonstrate this by beam profiles generated by a 2×2 and a 4×1 array for a transmission distance of 4.2 m. Spectra between 70 GHz and 1.1 THz have been recorded with these arrays.

Sub-THz and THz Photonic generation with continuous tunability using gain switching based optical frequency comb generators and n-i-p-n-i-p superlattice photomixers

In this work we present a compact and versatile way to generate sub-THz and THz signals using an OFCG based on a Photonic Local Oscillator responsible to generate the two wavelengths to be mixed in the photomixer based on a Gain Switching (GS) Scheme and the use of Phase Modulation; and n-i-p-n-i-p superlattice photomixers. The proposed scheme is able to cover a wide span with continuous tunability. The description of the different components of the system (Photonic Local Oscillator and Photomixer) is covered as well as some preliminary data.

Efficient optical up-conversion by coherent sum-frequency generation for highly sensitive terahertz detection

Detection of weak THz signals is very challenging due to the extremely low energy of one THz photon (temperature equivalent of 48K). Frequency up-conversion to the near infra-red (NIR) and subsequent detection of the up-converted photon with sensitive, low-noise detectors is one way to achieve this task, albeit suffering form extremely poor conversion rates. Whispering gallery mode resonators (WGMRs), however, support well confined modes with high quality factors (Q) and extremely high field intensities inside the resonator, thus allowing efficient non-linear conversion rates.

Generation of DUV/VUV Raman frequency comb via molecular modulation in a H 2 -filled Kagomé-PCF

Emission of narrowband pulses down to the VUV (184 nm) is demonstrated in a hydrogen-filled kagomé-PCF pumped at 266 nm. Intermodal Raman scattering facilitates the parametric excitation of short wavelengths in the all-normal dispersion regime.