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We demonstrate two self-referencing optical frequency combs with a repetition rate of 19.8 MHz, which is the lowest ever reported, using nonlinear-polarization-evolution mode-locked Er:fiber lasers. The two combs are phase-locked with a common CW laser at 1.5 um using high-speed feedback controls with servo bandwidths of 380 kHz. Over 40 absorption lines of the n 1 + n 3 vibration band of 13C2H2 are observed by dual-comb spectroscopy using those combs. We resolve the observed Doppler-limited lines with a 19.8 MHz frequency spacing (repetition rate), which is currently the smallest frequency spacing achieved, to the best of our knowledge.

A new technique called deep-lebel Fourier spectroscopy (DLFS), is proposed. This is an efficient and convenient method for evaluating deep livels in semiconductors. The technique is presented with a frequency space analysis for the case of exponential capacitance transients. Taking the Fourier transforms of the transient capacitance values, the deep level emission rate is determined in sequence from various numerical calculations of the Fourier coefficients. This method makes it possible to analyze the deep level accurately and automatically without judgements of the peaks in the spectra. Computer simulation and experiments using the method for a p-type Si Schottky diode are demonstrated. Finally, a comparison is made with the DLTS method.

The photon energy dependence of the transient photocurrent in Cr-doped semi-insulating GaAs materials was investigated by photo-deep-level Fourier spectroscopy (photo-DLFS) and photosensitivity transient spectroscopy (PSTS). From the experimental results, it was found that photo-DLFS signals change systematically according to the change in the photon energy of monochromatic light; fourteen deep levels were detected. It is shown that measurements taken while continuously varying the photon energy are very useful in studying specified deep levels in cases where the signals of two or more deep levels overlap and in detecting the many deep levels by increasing the change in the occupancy probability at each deep level. Also, a photoquenching effect is shown to occur within the new photon energy range, 0.85-0.92 eV, by PSTS.

The effect of crystalline defects induced in annealed p-type CZ silicon on deep levels has been investigated by transmission electron microscopy and deep level transient spectroscopy. The majority carrier traps, located at Ev+0.47, Ev+ 0.55 and Ev+0.63 eV, have been found in Schottky diodes which contained oxide precipitates (?1011cm-3), dislocation loops (?1010cm-3) and stacking faults (3×109cm-3), respectively. These trap densities are about 1015cm-3. To explain the difference between defect density and trap density, these levels are ascribed to the defect strain. A simple defect strain model is proposed.

A new technique called photo-deep-level Fourier spectroscopy (photo-DLFS) for evaluating deep levels in semi-in-sulating materials is proposed. It is presented with a frequency space analysis that makes it possible to analyze the deep-level parameters, without correction for the complicated temperature dependence in the current-transient equation, using numerical calculation of the Fourier coefficients taken from the Fourier transforms of the transient current values. Seven deep levels were detected in semi-insulating undoped LEC GaAs crystals using this method. Also, a comparison is made with the photo-DLTS method.