Enhanced Continuous-wave Trahertz emission by nano-electrodes in a photoconductive photomixer
Semiconductor materials used as PCA-based photomixers must exhibit high resistivity, high carrier mobility and ultrashort carrier lifetime. Low-temperature-grown GaAs (LT GaAs) has been shown to have such characteristics 14–18 . The samples used in our experiment had a 1-mm-thick LT GaAs layer grown on a 1-mm- thick AlAs layer on a semi-insulating GaAs substrate using molecular beam epitaxy (MBE). The LT GaAs has a resistivity of 5× 10 7 V cm, carrier mobility of 5,000 cm 2 V 21 s 21 and sub- picosecond carrier lifetime. The samples were fabricated into c.w.
terahertz photomixers by using photolithography and electron-beam lithography for the patterning of the micro-antenna structure and nano-electrode active region, respectively, followed by electron- beam metal deposition and lift-off. The fabrication process is described in detail in the Supplementary Methods. Schematic drawings and the dimensions of the devices are presented in Fig. 1. The width and separation of the electrode fingers of the active region are 100 nm and 300 nm, respectively, the same for both interdigitated and tip-to-tip nanogap electrodes. Plan-view scanning electron
microscopy (SEM) images of the photomixer with interdigitated electrodes and tip-to-tip nanogap electrodes are presented in Fig. 2. Both devices have a modified meander antenna, which has similar effective radiating parts as a simple dipole antenna, but better impedance matching.
The devices were tested at the same d.c. bias voltage of 15 V and excited by two tunable distributed feedback (DFB) lasers. The tunable lasers had central emission wavelengths of 852 nm and 855 nm, respectively, and a total output power of 90 mW. The terahertz wave emitted from the GaAs substrate side was coupled to a silicon hyper-
hemispherical lens and measured by vacuum Fourier-transform infrared spectroscopy (FTIR) with a liquid
helium-cooled silicon bolometer detector. Details of the measurement set-up are presented in Supplementary Fig. 1. Selected emission spectra of the tip-to-tip nanogap photomixer as recorded by the FTIR system are shown in Fig. 3a. Thex-axis isthewavenumber,with 33.33 cm 21 approximately equal to 1 THz in frequency. The tip-to-tip nanogap photomixer emission spectra from 7 cm 21 to 33 cm 21 are presented in the plot. Emissions were actually recorded
up to 53 cm 21 , but these are not presented due to the large intensity difference with the spectrum below 33 cm 21 . The inset of Fig. 3a shows the emission spectra of the interdigitated photomixer; these have a similar shape to that of the tip-to-tip nanogap photomixer,but with much lower intensity. The peak position of the terahertz emission in Fig. 3a is equal to the laser offset. As an example, for a peak position of 11.7 cm 21 , the laser offset frequency would be
0.35 THz. The characteristic of a c.w. terahertz photomixer emission, with its very narrow linewidth determined by the linewidth of the pump lasers (,10 MHz in our case), is promising for applications in high-resolution spectroscopy. The recorded emission spectra linewidth is limited by the resolution of the FTIR system(�6 GHz). The emission spectrum of a mercury lamp is also included as reference; the mercury lamp has an almost flat broadband emission across the measured range.
To obtain the output powerof the c.w. terahertz photomixers, we used the calibration method as described in the Supplementary Methods. The measured c.w. terahertz emission powers of the photomixer with interdigitated electrodes and tip-to-tip nanogap electrodes are shown in Fig. 3b. Significant enhancement in the output power is observed across a bandwidth of 1.3 THz for the tip-to-tip nanogap electrode photomixer compared to the interdigitated photomixer. The output power of the photomixer with the interdigitated electrode configuration fell below the detection limit at 0.75 THz.
However, the photomixer with tip-to-tip electrode configurations emitted even stronger output power at its highest oper-
ating frequency of 1.6 THz than the highest peak power from the interdigitated electrode photomixer at 0.3 THz. The total enhancement of the output power was approximately two orders of magnitude across the range. At 400 GHz, the enhancement from the nanogap photomixer was more than three orders of magnitude. These measurement results clearly demonstrate the advantages of using planar terahertz antennas coupled with a tip-to-tip nanogap active region, resulting in highly efficient c.w. terahertz photomixers.
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