Source: Science Magazine.

Mark Lee and Michael C. Wanke

    A revolutionary advance in THz solidstate source technology came in 2002 when Kohler et al. reported successful operation of a quantum cascade laser (QCL) at THz frequencies (5). The QCL evades semiconductor band-gap limitations on photonic devices by using sophisticated semiconductor heterostructure engineering and fabrication methods to create synthetic electron energy gaps at frequencies much smaller than those that nature provides. Since 2002, THz QCLs have progressed rapidly in frequency coverage, increased power output, and increased operating temperature. Currently, they are the only solid-state source capable of generating >10 mW of coherent average power above 1 THz, with record continuous wave power of 138 mW near 4.4 THz and an operation temperature of 10 K (6, 7). The output power of QCLs drops as the temperature increases, but milliwatts can still be obtained at liquid nitrogen temperature, and submilliwatt laser operation has been achieved up to 164 K. To date, THz QCLs have spanned the frequency range between 1.5 and 4.5 THz.

    Competing all-solid-state THz sources cannot currently meet the several milliwatt average power threshold but may exhibit other useful features. Frequency multipliers for use with high-power microwave sources are a mature and compact technology that operates at room temperature and can be easily tuned over wide frequency ranges from roughly 0.1 to 1 THz. However, intrinsic conversion losses for large frequency multiplication factors and difficulties in handling large input powers cause a multiplier¡¯s power output to drop rapidly with increasing output frequencies, so that only about 10 muW are available near 1 THz (8). Similarly, obtaining lower frequencies by mixing two solidstate near-infrared lasers on a photoconductive semiconductor switch at room temperature produces very broadly tunable power up to a few THz but currently exhibits only microwatts or less of average power above 1 THz (9). Recently, a record peak pulse power of 100 W was demonstrated in a p-doped germanium (p-Ge) laser near 2.7 THz. Unfortunately, current p-Ge lasers typically require large magnetic fields, temperatures below 15 K, and a very low duty cycle to operate (10).

    Current research in solid-state THz technology emphasizes individual component development. The present focus is on improving detector sensitivity, source power, and operating temperature with microelectronic materials and methods that are ultimately amenable to largescale production. Should this work progress at its current pace, this part of the spectrum will become as useful as the microwave and infrared frequency bands are today.