Compact terahertz radiation source using a bimodal laser and plasmonic photomixer
Terahertz waves offer unique opportunities in many fields. These include chemical sensing, material characterization, non-destructive diagnosis, medical imaging, and security screening. For many of these applications, however, practical feasibility is limited by low radiation power, narrow bandwidth, and the bulky nature of existing terahertz radiation sources.1
To address these limitations, we have recently demonstrated a compact terahertz source based on a novel two-section digital distributed feedback (D-DFB) laser diode and plasmonic photomixer. Our system generates terahertz radiation with 0.15–3THz frequency tunability, 2MHz linewidth, and less than 5MHz frequency stability over one minute, at power levels that can be used in practical imaging and sensing applications. We achieve terahertz wave generation through difference frequency generation, by pumping the plasmonic photomixer with two output optical beams from a two-section D-DFB laser diode.
Our two-section D-DFB laser diode—see Figure 1(a)—is a standard ridge waveguide laser diode, with a ridge width of 2.5um, which we fabricated on an indium phosphide substrate. The laser active region consists of five compressively strained aluminum gallium indium arsenide quantum wells with a well thickness of 5nm. The laser cavity is 700μm long and divided into two sections (with lengths of 400 and 300μm, respectively) that are separated by a 2μm-wide etched trench. The output beam is emitted from the first of these sections (section 1).
To achieve dual-mode output wavelengths, different features are etched into the ridge waveguide of each section to select two dynamically stable single lasing modes.2, 3 When both lasers are operated together, the coupling between the two laser cavities results in the generation of light at two wavelengths.4 In the meantime, we can tune the separation between the wavelengths by varying the temperature of the overall structure and the currents applied to the laser sections. The output spectra of the two-section D-DFB laser, at three specific operating points, are shown in Figure 1(b). With these operating conditions, we achieve spectral separations of 0.15, 1.62, and 2.99THz, and linewidths of about 1MHz.1
We used a plasmonic photomixer to convert the optical beam from the two-section D-DFB laser to terahertz radiation.5, 6 The photomixer comprises an ultrafast photoconductor, with plasmonic contact electrode gratings, integrated with a logarithmic spiral antenna on an erbium arsenide:indium gallium arsenide substrate, as shown in Figure 2(a). When the optical beam from the two-section D-DFB laser is incident on the anode plasmonic contact electrodes, a large fraction of the photogenerated carriers is generated in close proximity to the contact electrodes (as a result of the excitation of surface plasmon waves). By concentrating a major portion of the incident optical beam near the plasmonic contact electrodes, a large number of the photogenerated electrons drift to the anode plasmonic contact electrodes on a sub-picosecond timescale to efficiently contribute to terahertz radiation.5–11 We then feed the induced photocurrent to the logarithmic spiral antenna to generate terahertz radiation at the beating frequency of the two main spectral peaks of the two-section D-DFB laser. We use a fiber amplifier with a 2% duty cycle to amplify the laser output before coupling it to the plasmonic photomixer.
The measured terahertz radiation power, as a function of frequency, is shown in Figure 2(b). These results indicate a broad radiation frequency tuning range of 0.15–3THz. At an average optical pump power of 100mW, we achieved terahertz radiation powers as high as 1.3mW, 106μW, and 12μW at each continuous wave radiation cycle at 0.44, 1.20, and 2.85THz, respectively. We also achieved higher terahertz radiation powers at higher optical pump powers, as illustrated in Figure 2(c). Linewidth measurements for the generated signals exhibit radiation linewidths of about 2MHz, with a frequency stability of less than 5MHz over a one-minute time frame.
In summary, we have demonstrated a novel compact terahertz radiation source that is based on a two-section D-DFB laser diode and plasmonic photomixer. The high power, broad frequency tunability, compact size, and robust operation of our terahertz source means that it offers a good solution for future high-performance terahertz imaging and sensing systems. In our future work, we will investigate the inclusion of plasmonic photomixing elements inside various types of bimodal lasers to offer a single-chip terahertz radiation source.
We gratefully acknowledge financial support from the Office of Naval Research, National Science Foundation, Army Research Office, Science Foundation Ireland Irish Photonic Integration Centre program, and the European Space Agency project Far IR Local Oscillator.
Dublin City University