The saturation and power scaling of terahertz radiation produced by large-aperture photoconducting antennas under high electric fields and high optical fluences are described. From the saturation behavior, a large-aperture transmitter can be designed to produce the maximum pulse energy of terahertz radiation for a given photoconductor, optical pulse energy and electric field.

We extend Kuizenga and Siegman's theory on active mode locking to the case of broad bandwidth continuous-wave tunable lasers; in particular, we analyze the effects of intracavity group-velocity dispersion and bandwidth control caused by intracavity optical elements, such as the solid-state hosts of the gain medium, modulators, and birefringent tuning filters. In contrast with simpler theoretical models on active mode locking which predict a monotonic decrease in the laser pulsewidth with increasing intracavity bandwidth, the present theory predicts that in the presence of significant group-velocity (material) dispersion (as is particularly relevant for the active mode locking of broad band solid-state lasers), there exists an optimal value of the intracavity bandwidth at which the shortest pulsewidths can be obtained. This optimal value of the intracavity bandwidth is inversely proportional to the square root of the intracavity group-velocity dispersion, i.e., to square-root[d2P / d-lambda-2\lambda = lambda-a], where P is the round-trip optical path length of the laser cavity, lambda-is the optical wavelength, and lambda-a is the center wavelength of the laser radiation. The results of this theory are compared with our previous experiments on active mode locking of a CW Ti:Al2O3 laser; for (d2P / d-lambda-2\lambda = lambda-a) = 4.86 x 10(3)-mu-m-1 and a near optimal intracavity bandwidth of 13.8 THz, a nearly transform-limited pulsewidth of 7.2 +/- 0.3 ps was observed in the experiments, in agreement with a value of 7.9 ps predicted by the above theory. The theory also predicts the possibility of obtaining pulsewidths less than 4 ps from appropriately designed mode-locked CW Ti:Al2O3 lasers without the use of intracavity dispersion compensating elements; pulse-widths of subpicosecond duration should be obtainable directly from actively mode-locked Ti:Al2O3 lasers with suitable dispersion compensation.

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.

We describe the saturation properties of ultrafast pulsed electromagnetic radiation generated by large-aperture photoconducting antennas as a function of optical excitation fluence. We present a theory that predicts this effect. The amplitude saturation of the radiation has been observed from 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 antenna 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.

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.

The recent observation of the optical rectification of femtosecond electromagnetic waves provides an alternative optical technique to characterize the depletion fields at semiconductor surfaces and interfaces 1 The basic physical mechanism of the optical rectification is the electromagnetic radiation from the transient photocurrent within the depletion width. The outward rectified field E1 radiated from a bare semiconductor surface can be expressed as follows.

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.

Large-aperture photoconducting antennas have been used to both generate and detect ultrafast submillimeter electromagnetic radiation. When photoconductively driven by optical pulses with energies in the range of 10-4 J/cm2, emitting antennas have produced far-infrared peak electric fields that are comparable to the bias field. Large-aperture emitters of semi-insulating InP with various electrode spacings (1,4, and 10 mm) have been measured. In contrast with previously used dipole antennas, which have a 5 μm radiation-damaged silicon-on-sapphire photoconducting gap, our new detectors have electrodes spaced 0.1-1 mm apart and also have a larger dynamic range.

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.

A large variety of semiconductor samples have been used to generate highly directional, optically steerable, and diffraction-limited ultrafast electromagnetic radiation. Samples have been selected from III-V, II-VI, and group IV semiconductors, including single-crystal, polycrystalline, and amorphous structures. We have compared the optically induced radiated fields from different semiconductor surfaces (interfaces), including an air/semiconductor interface, a metal/semiconductor interface, a p-n junction (solar cell), and a strain-induced piezoelectric layer. In addition, we report the temperature dependence of optically induced electromagnetic pulses. When the surface depletion width was tuned to cross the optical absorption length by varying the sample temperature, a dramatic change of the radiation waveform from the narrow-bandgap semiconductors was observed. We also observed a three-fold rotation symmetry of the radiation from (111)-oriented samples. Finally, we describe the conversion efficiency of optically induced electromagnetic radiation from these samples.

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.

For numerous applications, including investigation of material and device properties of various optical and optoelectronic devices (such as those composed of GaAs and related III-V compounds), there has been a long-existing need for stable, turn-key, high repetition rate sources of short pulse (sub-nanosecond to sub-picosecond) laser radiation in the near infrared, particularly those that are continuously tunable in the 0.7 µm to 1.0 µm range. Although this need is partially satisfied by external cavity diode lasers, and partially by synchronously and passively mode-locked near infrared dye lasers1, problems such as limited tunability, poor long-term stability (deterioration in a few weeks of continuous usage), liquid handling problems, short term instabilities (due to dye jet fluctuations), etc., create the need for alternative solutions, such as those based on tunable solid-state laser materials