Measurement of Terahertz Optical-Beat Frequency Using High-Order Harmonics of Microwave in a Photoconductive Device

A method for measuring frequencies of the terahertz (THz) radiation emitted by the antenna mounted on the photoconductive (PC) device is presented. Two laser beams with slightly different frequencies irradiate the PC device, producing a beat current of 1？THz in the photocurrent. A microwave signal is applied to the antenna electrode. The frequency of the THz wave is measured using the homodyne detection of the optical beat with the high-order harmonics of the microwave. It is being investigated that the high-order harmonics are produced by the PC device owing to its nonlinearity. Periodic peaks generated by the homodyne detection were observed in the photocurrent, as the microwave was swept from 16 to 20？GHz with a power of ？40？dBm. Using the peak frequencies, the THz-wave frequency was determined to be ？GHz. The measurement error is estimated to be less than 0.43？GHz. The proposed method realizes a compact frequency meter in the THz region. 1. Introduction A method for measuring frequencies of continuous wave (cw) terahertz (THz) radiation is one of key technologies for various applications of THz waves, for example, THz spectroscopy, radioastronomy, and future communication systems. Frequencies of the THz wave are usually measured using a Fourier-transformed interferometer which is applicable to both the cw and pulsed THz radiation. Various types of interferometers in the THz region are used, for example, a Martin-Puplett interferometer [1] and scanning Fabry-Perot etalon [2]. Frequency resolution of the interferometer is limited by the scanning range of the path difference of the THz wave. Longer scanning range is necessary to obtain higher frequency resolution. For example, the scanning range of 1 meter is necessary to obtain the frequency resolution of 0.15？GHz. Such an interferometer is generally large compared with the THz generation/detection system itself and is not suitable for practical use. The cw THz wave can be generated by a photomixing technique [3]. When two cw laser beams with slightly different optical frequencies irradiate the antenna gap fabricated on the photoconductive (PC) device, an optical-beat current at a difference frequency ( ) is produced between the electrodes (i.e., the antenna). The THz wave with the same frequency is emitted from the antenna by dipole radiation. One method to know the frequency of the THz wave is by utilizing the locked difference frequency of the two laser beams of the light sources [4, 5]. However, the setup required to lock the difference frequency of the two laser beams is apt to increase system