A novel technique, that combines optical heterodyning and near-field optics, was developed for highly localized of millimeter waves in ultrafast devices. The technique relies on evanescent coupling of the interfering laser to a small are of the device, by means of a near-field fiber optic probe. The applicability of the technique was first validated by measurements on heterojunction photo transistors up to 100GHz. Later, scanned measurements at 63GHz were performed on two ultrafast device structures, namely low temperature GaAs photoconductive switches and InP-based high electron mobility transistors. The response characteristics were rich in structures that revealed important details of the device dynamics.
A new generation of InP based 50 nm gate-length pseudomorphic high electron mobility transistors was employed in continuous wave optical mixing experiments to generate millimeter waves to extremely high frequencies. Direct radiation of optical mixed signals was demonstrated at 212 GHz. This, to our knowledge, is the highest frequency optically generated millimeter wave radiation ever reported for three terminal devices. Newly developed G-band probes and a horn antenna were used for radiation measurements. A signal to noise ratio of approximately 20 dB was obtained that indicates significant response of our devices at these frequencies. A broadband three wave detection scheme was used to further extend optical mixing to 238 and 267 GHz. Optical time domain and electrical characterization were carried out to reveal the high frequency capabilities of these devices. A photoresponse time of 6.9 psec was measured using a picosecond electrooptic sampling setup. Cut-off frequencies of 228 GHz and maximum oscillation frequencies of 124 GHz were obtained in S-parameter measurements. Specific examples of applications in communications and spectroscopy were investigated.
The high speed response of 50-nm gate AlInAs/GaInAs/InP pseudomorphic high electron mobility transistors (HEMTs) have been used in optical mixing experiments to generate difference frequencies to 211 GHz from two continuous wave laser beams. A 16 dB signal to noise ratio was achieved. To our knowledge, this is the highest frequency optical mixing ever obtained for three-terminal devices. A broadband three wave mixing technique was employed to detect the optically mixed signals at these high frequencies. This scheme involves the nonlinear interaction of the optically generated signal with a millimeter wave signal electrically injected at the gate. The resulting signal, downshifted to W band, was radiated into the waveguide input of an external millimeter wave receiver system. To demonstrate the wide tunability of our system a sweep of frequencies from 160 - 190 GHz was performed. The HEMTs exhibited a relatively flat response with signal to noise ratios of greater than 12 dB. Ultrafast response of the HEMTs as indicated by cw mixing results was also characterized in the time domain using a picosecond electro-optic sampling system. To illustrate the use of the HEMTs in optical millimeter wave systems, optically mixed signals at 97 GHz, both continuous wave and modulated, were radiated into free space using a horn antenna. Modulation was obtained by injecting a baseband signal into the gate of the HEMT. Electrical characterization of the devices yielded cut-off frequencies of 228 GHz and a maximum oscillation frequency of 124 GHz.
The generation of 84 GHz radiation was demonstrated using a mode-locked semiconductor laser (MLSL) pumped heterojunction bipolar transistor (HBT). The passively mode-locked MLSL was biased appropriately utilizing two diode laser drivers (current sources). Mode-locked behavior was achieved in a colliding pulse mode, resulting in a pulse repetition rate frequency of approximately equals 84 GHz. The mode-locked behavior was confirmed by utilizing both an interferometer-based correlation measurement and an optical spectrum analyzer. The MLSLO was then used to pump an HBT that was specially designed for optical pumping (a 10 mm X 10 mm window was fabricated in the HBT), allowing efficient optical excitation of the device. HBT-radiated MMW signals as high as 20 dB (above the noise floor) were achieved at approximately equals 84 GHz.