B. B. Hu

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.

500 GHz electrically steerable photoconducting antenna array

When illuminated by a train of properly spaced ultrashort optical pulses, an array of short photoconducting dipole antennas emits a submillimeter wave beam which can be electrically steered by varying the periodicity of the voltage bias applied to the individual antenna elements.

Temperature dependence of femtosecond electromagnetic radiation from semiconductor surfaces

We present observation of temperature dependence of optically induced femtosecond electromagnetic radiation from several semiconductors. When the sample temperature decreases from 280 to 80 K, over 4 times and 21 times increases of the radiated peak field and radiated energy have been found. A large shift of the frequency spectrum of the radiation was observed when the InSb sample temperature was tuned over this range.

Optically induced femtosecond electromagnetic pulses from GaSb/AlSb strained‐layer superlattices

The strain generated electric field in a 〈111〉 oriented misfit superlattice provides a suitable bias for generating optically induced femtosecond electromagnetic radiation. We have measured the electromagnetic radiation from GaSb/AlSb strained‐layer superlattices and GaSb thin films; extremely fast electromagnetic pulses from 〈111〉 oriented superlattices have been observed. Because the quantum well structure in the superlattice samples limits the pulse duration of the transient photocurrent, it is possible to generate electromagnetic radiation having a pulse duration comparable with the photocarrier transit time across the quantum well.

Femtosecond response of electro‐optic poled polymers

We investigate the ultrafast electro‐optic response and sensitivity of a poled side chain polymer film via the electro‐optic sampling technique. A 760 fs rise‐time electrical transient is observed corresponding to a bandwidth of 460 GHz. We believe this to be device limited and not due to limitations in the speed of response of the polymer.

Electrically controlled frequency scanning by a photoconducting antenna array

The radiation frequency of an electromagnetic wave from a spatial periodically biased photoconducting antenna array can be electrically scanned by varying the periodicity of the bias voltage on the antennas. Over 900 GHz frequency tuning bandwidth has been demonstrated.