D. H. Auston

Generation of femtosecond electromagnetic pulses from semiconductor surfaces

We have generated electromagnetic beams from a variety of semiconductors. When a bare semiconductor wafer was illuminated by femtosecond optical pulses, electromagnetic waves radiate from the surface and form collinear diffraction‐limited electromagnetic beams in the inward and outward directions. The amplitude and phase of the radiated field depend on carrier mobility, the strength and polarity of the static internal field at the semiconductor surface.

Terahertz beam generation by femtosecond optical pulses in electro-optic materials

We have observed subpicosecond pulses having terahertz bandwidths generated from electro‐optic materials by using femtosecond optical pulses. The time‐resolved radiation signal clearly indicates that the terahertz radiation comes from the regions within the coherence length near the surfaces of electro‐optic materials under the phase‐mismatched condition. When the high‐energy femtosecond laser pulses are used, this method provides a new way to generate large amplitude broad bandwidth terahertz radiation efficiently.

Saturation properties of large-aperture photoconducting antennas

The authors describe the saturation properties of ultrafast pulsed electromagnetic radiation generated by large-aperture photoconducting antennas as a function of optical excitation fluence. A theory that predicts this effect is presented. The amplitude saturation of the radiation has been observed form antennas incorporating GaAs, InP, and radiation damaged silicon-on-sapphire consistent with theoretical expectations. The radiated fields were measured directly with a time resolution of 0.6 ps with the use of a large-aperture antennas as a detector. From these experimental studies, information about the high-speed response (i.e., the transient carrier mobility in the first few picoseconds after optical excitation) of the photoconductors incorporated in the antenna can be obtained under conditions of high applied electric fields and optical fluences. >

Coherent detection of freely propagating terahertz radiation by electro‐optic sampling

We report the demonstration of an electro‐optic sampling technique that allows for the detection of freely propagating terahertz radiation. Coherent sampling is performed in a poled polymer device that is physically separated from the emitter. The poling electrodes in the sampling element are found to have an integrating effect on the incident terahertz field. The shot noise limited minimum detectable field in the polymer is 100 (mV/cm)/√Hz. We discuss methods by which the sensitivity may be significantly enhanced.

Free‐space radiation from electro‐optic crystals

We describe a technique to extract electro‐optic Cherenkov radiation from a LiTaO3 crystal into free space. This permits the generation of collimated beams of terahertz radiation into free space and overcomes previous limitations imposed by total internal reflection.

Optically steerable photoconducting antennas

When illuminated by ultrashort optical pulses, large aperture planar photoconductors are shown to radiate a directional electromagnetic pulse which can be steered by varying the angle of incidence of the optical beam.

Subpicosecond electromagnetic pulses from large-aperture photoconducting antennas

The radiation efficiency and physical mechanism for the generation of subpicosecond electromagnetic pulses from large-aperture photoconducting antennas are evaluated. The rise time of the photocurrent is found to be more important than the fall time in determining the relative sensitivity of photoconducting materials. Indium phosphide is found to be almost 10 times more sensitive than radiation-damaged silicon-on-sapphire.

Optically induced electromagnetic radiation from semiconductor surfaces

Ultrafast electromagnetic radiation induced by a femtosecond laser beam from a semiconductor provides determination of the impurity doping concentration, carrier mobility, sign, and strength of the depletion field near the semiconductor surface.

Generation of tunable narrow‐band THz radiation from large aperture photoconducting antennas

We report a novel technique for the generation of tunable narrow‐band submillimeter wave electromagnetic radiation in free space. This is accomplished by mixing two linearly chirped optical pulses with a variable time delay on a biased large aperture GaAs photoconducting antenna. The center frequency of the THz radiation varies linearly with the delay between the two chirped pulses from dc to ∼1 THz.

Power scaling of large‐aperture photoconducting antennas

We describe the power scaling and saturation properties of large‐aperture planar photoconducting antennas which emit and detect ultrashort terahertz electromagnetic pulses. At high optical fluences, the radiated electric field saturates at a value comparable to the bias field in agreement with a simple model of the radiation mechanism.