Janis A. Valdmanis

Subpicosecond carrier lifetime in GaAs grown by molecular beam epitaxy at low temperatures

Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time‐resolved‐reflectance techniques, a sub‐picosecond (<0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at ∼200°C and annealed at 600 °C. With the same material as a photoconductive switch we have measured electrical pulses with a full‐width at half‐maximum of 0.6 ps using the technique of electro‐optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of ∼120–150 cm2/V s. GaAs grown by MBE at 200 °C and annealed at 600 °C is also semi‐insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of subpicosecond photoconductive applications.

Terahertz attenuation and dispersion characteristics of coplanar transmission lines

Experimental verification of analytic formulas for the dispersion and the attenuation of electrical transient signals propagating on coplanar transmission lines is presented. The verification is done in the frequency domain over a terahertz range although the experiments are in the time domain. The analytic formulas are obtained from fits to the full-wave analysis results. It is quantitatively verified that the full-wave steady-state solutions can be directly applied to the transient time-domain propagation experiments. Subpicosecond electrical pulses and an external electrooptic sampling technique are used to obtain the time-domain propagation data. From the Fourier transforms of the time-domain data both the attenuation and the phase information as a function of frequency are extracted. The dispersion and the attenuation characteristics are investigated for both coplanar waveguide and coplanar strip transmission lines. The investigation is carried out on both semiinsulating semiconductor and dielectric substrate materials. No observable losses caused by the semiconductor material are indicated. >

Two-dimensional imaging through diffusing media using 150-fs gated electronic holography techniques

We combine 150-fs holographic gating with specifically adapted electronic holography techniques to produce transmission images of objects embedded in diffusing material. The technique captures, without the use of scanning procedures, complete two-dimensional images with 150-fs temporal resolution, thereby yielding submillimeter spatial resolution through diffusing objects several centimeters thick.

Imaging through scattering media with holography

Various holographic methods for imaging through scattering media such as biological tissue are described. The methods utilize light of either reduced spatial coherence or reduced temporal coherence.

Experimental characterization of external electrooptic probes

The accuracy and invasiveness of various external LiTaO/sub 3/ electrooptic probe (EEP) geometries are investigated experimentally. The EEPs studied are used for microwave coplanar transmission-line measurements. It is shown that commonly employed EEP geometries can cause substantial inaccuracies in their measurements and that these inaccuracies can be related to specific geometric parameters. The major cause of distortions in pulse measurements was found to be attributable to electromagnetic radiation coupling into the LiTaO/sub 3/ crystal and resonating between the top and bottom crystal interfaces. The authors suggest eliminating the deleterious bulk resonance effects due to the EEP material bulk by using a thin LiTaO/sub 3/ crystal that will act as a single lumped element. The thickness that is expected to be optimal should be about the extent of the guided mode confinement near the conductors. A thicker than optimum cryst al will exhibit resonance, and a thinner one will have reduced sensitivity due to a reduced signal integration path. Thinned crystals possess the additional advantages of reduced thermal drift and reduced stray signal pickup from adjacent lines.<<ETX>>

Electronic holography and speckle methods for imaging through tissue using femtosecond gated pulses.

Electronic holography and speckle interferometry are combined with femtosecond gating techniques to form images of absorbing structures embedded in organic tissue. The method takes advantage of the inherent instability of living tissue.

External electro-optic integrated circuit probing

Abstract An external electro-optic measurement system with subpicosecond resolution has been developed. This electro-optic sampling system is designed to operate as a non-contact probe of voltages in electrical devices and circuits with modified wafer-level test equipment and no special circuit preparation. Measurements demonstrate the systems ability to probe continuous and pulsed signals on microwave integrated circuits on arbitrary substrates with single-micron spatial resolution. We also discuss the application of external electro-optic sampling to various aspects of time-domain circuit studies, including the generation of short electrical test pulses using novel photoconductive techniques and the propagation of pulses on interconnects.

Ultrahigh-bandwidth vector network analyzer based on external electro-optic sampling

Abstract We report the development of an ultrahigh-bandwidth vector network analyzer useful for small-signal characterization of high-speed semiconductor devices. It employs 100-fs optical pulses for making terahertz-bandwidth electro-optic measurements of electrical signals, as well as for sub-picosecond, photoconductive, electrical-stimulus-signal generation. High-bandwidth coplanar strip transmission lines are used for signal transmission. A 0.15 × 50 μm gate AlGaAs/InGaAs/GaAs HFET has been characterized over a bandwidth of 100 GHz using this network analyzer. A comparison with conventional RF network analyzer measurements performed to 40 GHz demonstrated good agreement throughout this bandwidth. Such measurements of the actual device characteristics across their entire operating frequency range should improve device development and incorporation into active circuits.