Thin film lithium niobate optical modulators allow modulation of optical signals up to several THz due to perfect phase matching between RF signal and optical signal that can be achieved using thin film devices. The platform uses a ridge waveguide fabricated by direct etching of lithium niobate thin film fabricated on silicon substrates. The lithium niobate thin film has been developed and optimized in our facility. Transmission spectrum of fabricated micro-ring resonators on this platform shows a linewidth of approximately 7 pm corresponding to a Q value of 2.2×105 and an optical waveguide loss of 2 dB/cm. A coupling loss of -5 dB per coupler is obtained using grating couplers. Measured fiber to fiber insertion loss of the device is -10 dB. The measured 3-dB optical bandwidth of the fiber to fiber optical coupler is 45 nm. A Mach-Zehnder modulator consisting of two MMIs and 6 mm long arms were designed and fabricated on X-cut thin film of lithium niobate. Measured Vπ of the device is 7.5 V at low frequencies (i.e. 10KHz) for a device with 7 μm gap between the electrodes. The measured half-wave voltage-length product, Vπ.L, is equal to ~4.5 V.cm. High speed measurement results of the device response are presented. A THz electric field of 10kV/mHz0.5 is detected with a low dynamic range OSA and it is estimated that a THz electric field with a strength as low as ~100V/mHz0.5 is detectable by modulating the optical signal using these modulators.
Indium nitride (InN) is a recently discovered source of broadband terahertz (THz) frequency radiation. Emission of THz-radiation
occurs upon irradiation of InN with femtosecond (fs) near-infrared laser (nir) pulses. As a narrow band gap
semiconductor, InN is an exciting material for future time-domain THz-spectroscopy and THz-imaging systems powered
by femtosecond fiber lasers operating at communication wavelengths (1550nm) and directly diode-laser pumped
femtosecond solid state lasers emitting in the 1000-1600nm wavelengths range. Advantages of InN as THz-emitter are
strong intrinsic electric fields, potentially low intrinsic carrier concentrations and a very low probability of intervalley
scattering of photocarriers. Recent results on the impact of n- and p-type dopants on THz-radiation emission from InN
thin films are discussed. Emission of THz-radiation from silicon (Si) -doped and native n-type InN increases with
mobility as expected for transient photocurrents as primary mechanism of terahertz radiation emission. Doping of InN
with magnesium (Mg) enhances the emission of THz-radiation compared to doping of InN with Si. This is experimental
evidence for Mg acting as an electrically active acceptor in InN. THz- radiation emission from InN:Si is weaker than
emission of THz-radiation from native n-type InN because of increased electron concentrations due to Si being an
electrically active donor in InN.
We report an experimental study on THz-emission from narrow band gap semiconductors excited by femtosecond nearinfrared
laser pulses. In particular, we studied GaSb, Ga1-xInxSb and InN. The Ga1-xInxSb material system enables the
study of the influence of carrier concentrations on THz emission process in narrow band gap semiconductors. The study
demonstrates the existence of a compromise between the positive effect of high electron temperature provided by narrow
band gap materials and the negative effect of high intrinsic carrier concentrations. The influence of the majority and
minority carrier types and concentrations on THz emission strength was investigated using GaSb:Te. By varying the
majority and minority carrier type and carrier concentrations over three orders of magnitude the THz emission
mechanism in GaSb can be tuned from being dominated by the photo-Dember effect to being dominated by surface field
acceleration. Within each regime photo-Dember based THz emission and surface field acceleration based THz emission
are maximized under specific majority and minority carrier concentrations. Strong advantages of InN as THz-emitter
over other narrow band gap materials, are strong intrinsic electric fields, low intrinsic carrier concentration and most
importantly, very low probability of intervalley scattering.
We investigate the size of transient artificial pores as a function of the incident laser intensity in femtosecond nearinfrared laser opto-injection into single living Bovine aortic endothelial cells (BAECs). Molecules ranging in size from 457 dalton to 500kD were used in size exclusion experiments. We found that the threshold laser intensity for pore creation was in dependence of the size of the molecule.
Time-domain Terahertz (THz) spectroscopy and imaging is currently evaluated as a novel tool for medical imaging and diagnostics. The application of THz-pulse imaging of human skin tissues and related cancers has been demonstrated recently in-vitro and in-vivo. With this in mind, we present a time-domain THz-transmission study of artificial skin. The skin samples consist of a monolayer of porous matrix of fibers of cross-linked bovine tendon collagen and a glycosaminoglycan (chondroitin-6-sulfate) that is manufactured with a controlled porosity and defined degradation rate. Another set of samples consists of the collagen monolayer covered with a silicone layer. We have measured the THz-transmission and determined the index of refraction and absorption of our samples between 0.1 and 3 THz for various states of hydration in distilled water and saline solutions. The transmission of the THz-radiation through the artificial skin samples is modeled by electromagnetic wave theory. Moreover, the THz-optical properties of the artificial skin layers are compared to the THz-optical properties of freshly excised human skin samples. Based on this comparison the potential use of artificial skin samples as photo-medical phantoms for human skin is discussed.
We develope thin-film Ni-NiO-Ni tunneling diodes with a minimum contact area of 0. 1 2 im2
and a width of the antenna arm of 0.35 jim. With these diodes we detect cw as well as pulsed
10 pm CO2-laser radiation.
We manufacture thin-film Ni-NiO-Ni diodes for the detection of 10 p,m CO-laser radiation with the
aid of electron and UV lithography and sputtering techniques. The minimum dimensions are 0.4 tm.
The thickness of the oxide layers is of the order of 50 A. According to theory, the response of MOM
diodes to infrared radiation should be proportional to the second derivative d21/dV2 of dc I-V
characteristics of these diodes. However, part of the response of the MOM diodes originates in the
thermal heating by the infrared radiation. Consequently, we have measured the dc I-V characteristics
as well as the response to cw 10 .tm-CO2-laser radiation of Ni-NiO-Ni diodes at room temperature
for different orientations of the diodes with respect to the incident laser radiation and its polarisation.
For Ni-NiO-Ni diodes we have observed a cosine dependence of the signal on the angle of the
polarisation versus the antenna which represents the proper non-thermal nonlinear response of the
diode. At the moment, the coupling factor is of the order of 70.
The MOM diodes and their thin-film metal-strip antennas are attached to Si wafers. The high
refractive index of bulk Si at 10 tm wavelength diminishes thç coupling of the 10 p.m laser radiation
to the MOM-diode by the attached thin-film metal strip antenna. Therefore, we reduce the thickness of
the Si wafer on the back of the MOM diodes and their antennas to about 5p.m by etching. Thus, we
increase the transmission of the Si wafer under the metal-strip antenna and improve the antenna
performance.
Furthermore, we try to increase the sensitivity of the diodes by further reduction of their minimum
dimensions, which can be achieved by direct writing with the electron beam of the electron-beamlithography
facility at Centre Suisse d'Electronique et de Microtechnique, Neuchâtel, Switzerland.
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