P. Haring Bolivar

Low-frequency active surface plasmon optics on semiconductors

A major challenge in the development of surface plasmon optics or plasmonics is the active control of the propagation of surface plasmon polaritons (SPPs). Here, we demonstrate the feasibility of low-frequency active plasmonics using semiconductors. We show experimentally that the Bragg scattering of terahertz SPPs on a semiconductor grating can be modified by thermal excitation of free carriers. The transmission of SPPs through the grating at certain frequencies can be switched completely by changing the temperature less than 100°C. This semiconductor switch provides a basis for the development of low-frequency surface-plasmon optical devices.

Analysis of the propagation of terahertz surface plasmon polaritons on semiconductor groove gratings

We have investigated the propagation of terahertz (THz) surface plasmon polaritons (SPPs) on gratings formed by grooves structured in silicon wafers. These gratings exhibit a stop gap where SPPs are Bragg scattered. We observe a strong effect of the groove depth on the characteristics of the stop gap. To quantify the scattering strength of these structures we obtain the SPP attenuation length by measuring the transmission as a function of the number of grooves. We also determine the effective refractive index of the Bragg gratings using two different approaches: by measuring the transmittance through the gratings at different angles of incidence and from the phase of the transmitted THz pulses. The two approaches give results in good agreement.

High-power solid-state cw dye laser

In the present paper we describe a high-power tunable solid-state dye laser setup that offers peak output power up to 800 mW around 575 nm with excellent long-time power stability and low noise level. The spectral width of the laser emission is less than 3 GHz and can be tuned over more than 30 nm. A nearly circular mode profile is achieved with an M(2) better than 1.4. The device can be integrated in a compact housing (dimensions are 60 × 40 × 20 cm(3)). The limitation of long-time power stability is mainly given by photo decomposition of organic dye molecules. These processes are analyzed in detail via spatially resolved micro-imaging and spectroscopic studies.

THz spectroscopy with ultrahigh sensitivity

Summary form only given.Although THz radiation has been used successfully for several applications the signal-to-noise ratio is mostly restricted to 10/sup 4/ to 10/sup 6/ in intensity. Especially for thin films of material the absorption and time delay of a THz pulse are mostly too low to be detected. We show a new method to enhance the sensitivity of THz spectroscopy to relative changes of 10/sup -10/ in intensity by shaking a sample through the focus of a THz beam and using lock-in detection on the shaking frequency.

Diminishing relative jitter in electrooptic sampling of active mm-wave and THz circuits

In this work a novel approach in synchronization of electrooptic sampling systems for the ultra-broadband characterization of active mm-wave and THz devices is presented. The relative time jitter between sampled circuit and probing electrooptic head is eliminated by using a femtosecond laser system both as the generator of CW driving the device under test as well as the impulsively probing element. Previous ultra-broadband approaches were applicable to passive components driven by THz impulses, only. The presented system is more generally applicable to active mm-wave and THz components driven by conventional CW electronic sources. Broadband analysis on silicon nonlinear transmission line elements up to a frequency of 300 GHz is presented in order to illustrate the capabilities of the concept.

Long-range guided THz radiation by thin layers of water

We propose a novel method to guide THz radiation with low losses along thin layers of water. This approach is based on the coupling of evanescent surface fields at the opposite sides of the thin water layer surrounded by a dielectric material, which leads to a maximum field amplitude at the interfaces and a reduction of the energy density inside the water film. In spite of the strong absorption of water in this frequency range, calculations show that the field distribution can lead to propagation lengths of several centimeters. By means of attenuated total reflection measurements we demonstrate the coupling of incident THz radiation to the long-range surface guided modes across a layer of water with a thickness of 24 μm. This first demonstration paves the way for THz sensing in aqueous environments.

Terahertz responsivity enhancement and low-frequency noise study in silicon CMOS detectors using a drain current bias

We report on the study of the enhancement of responsivity and properties of low-frequency noise in silicon 0.25 µm CMOS transistor-based detectors for terahertz radiation under applied dc source-to-drain current. We find that at signal modulation frequencies above 50 kHz the signal-to-noise ratio becomes independent from applied current, whereas the responsivity of detectors can be enhanced up to three times. We present quantitative results of noise measurements in the frequency range from 600 Hz to 1 MHz and currents up to the saturation current.

Efficient, robust, and scale-invariant decomposition of Raman spectra

Raman spectroscopy is used to identify unknown constituent minerals and their abundances since Raman spectra convey characteristic information about the samples chemical structure. We present a novel method to identify constituting pure minerals in a mixture by comparing the measured Raman spectra with a reference database. Our method comprises of two major components: A novel scale-invariant spectral matching technique, that allows to compare measured spectra with the reference spectra from the database even when the band intensities are not directly comparable and an iterative unmixing scheme to decompose a measured spectrum into its constituent minerals and compute their abundances.

High-power CW tunable solid state dye lasers: from the visible to UV

We describe a high power CW solid-state dye laser setup. With perylene orange in PMMA as gain medium an output power up to 800 mW at 576 nm and a tuning range between 565 and 595 nm is reached. The laser output shows good long time power stability. The durability can be adjusted by variation of the pump power. A feedback loop controls the laser output. At a setpoint of e.g. 100 mW, the laser output can be provided for more than eight hours with a low noise level (RMS < 10%). The spectral width of the laser emission is less than 3 GHz and can be tuned over more than 30 nm. A circular mode-profile is achieved with M2 < 1.4 [1]. Via intra-cavity second harmonic generation more than 1 mW of 290 nm UV-radiation is achieved. As nonlinear element a 7 mm BBO (Beta-Barium Borate) crystal is used. The UV laser radiation can be tuned over 10 nm. The theoretical limit of UV output is estimated to 3.5 mW. To our knowledge we present the first tunable CW polymer UV laser. While the output stability at the fundamental wavelength is reasonably good, in the UV region a significant enhancement of the noise level is observed. In addition to this the long time stability is reduced to few minutes. The limitation is mainly given by the photo-decomposition of the organic dye molecules.

THz spectroscopy of bovine serum albumin solution using the long-range guided mode supported by thin liquid films

We demonstrate THz spectroscopy of mM BSA solutions using the long-range guided mode as an alternative approach to traditional transmission and reflection concepts. The cm propagation lengths pave the way for integrating field-enhancing resonating structures.