Terahertz Photoconductive Antennas Frequently Asked Questions


Q: What is Terahertz?

A: The ‘terahertz gap’ – where until recently bright sources of light and sensitive means of detection did not exist – encompasses frequencies invisible to the naked eye in the electromagnetic spectrum, lying between microwave and infrared in the range from 0.1 THz to 10 THz. Terahertz radiation, also known as T-rays, has wavelength of 3 mm-30 μm.

Terahertz technology has entered into an era with ever-growing applications in material spectroscopy and sensing, monitoring and spectroscopy in pharmaceutical industry and science, food industry, security, aerospace, oil industry, medical imaging, biology and medicine, and high-data-rate communications. Recent advances in semiconductor, laser, and optical technologies have made terahertz spectrum accessible to many technologists and scientists in diverse areas, ranging from biology and medicine to chemical, pharmaceutical, and environmental sciences to revisit their problems under the light of terahertz waves, thanks to the substantial growth of adjacent telecom, semiconductor, and laser markets in the recent years.

Q: What is a Terahertz Photoconductive Antenna (THz-PCA)?

A: Terahertz photoconductive antennas (THz-PCAs) are among the most promising devices that are used to harness the unique properties of terahertz waves for variety of applications, such as security, biology and medicine, medical imaging, material spectroscopy and sensing, and monitoring and spectroscopy in pharmaceutical industry. Since their demonstration as practical THz sources and detectors, THz-PCAs have been the subject of a vast amount of scientific and industrial reports investigating their applications as terahertz wave transmitters and receivers. Using THz-PCAs for generation and detection of THz signals, one can achieve relatively high signal-to-noise ratio and perform fast scan for imaging and spectroscopy applications. The possibility of fabricating THz-PCAs on photoconductive materials with the band gap energy equal to the energy of the photons at the telecommunication wavelengths and using them in fiber coupled schemes make THz-PCAs attractive for several real-world applications.

Q: What is the difference between Air-coupled and Fiber-coupled Terahertz Photoconductive Antennas?

A: The air-coupled THz-PCAs are excited by a free-space optical laser beam, while the fiber-coupled THz-PCAs are excited through optical fibers.

Q: What is the difference between Wide Frequency Bandwidth and High Frequency Resolution THz-PCAs?

A: The wide frequency bandwidth THz-PCAs are excited by femtosecond optical pulses to generate and detect terahertz pulses with bandwidth of up to 4 THz. The high frequency resolution THz-PCAs are excited by pair of continuous wave optical lasers with their frequency difference in the THz frequency range to generate and detect continuous wave (CW) terahertz signals. The frequency of the generated THz CW signal can be precisely tuned by adjusting the wavelength difference of the two exciting single-mode optical lasers.

Q: What kind of Lasers can we use to drive the Terahertz THz-PCAs?

A: For the wide frequency bandwidth devices, the femtosecond ultrafast lasers are used to excite the sensors. The laser generates optical pulses with pulse width less than 100 fs and enough output power to drive a pair of the transmitter and receiver modules. For the high frequency resolution devices, two single-mode CW optical lasers with frequency difference in the THz range are employed to drive the THz-PCAs. The line width of the exciting lasers should be in MHz range.

Q: What is the ultra high vacuum compatibility of the T-Era THz-PCAs?

A: The T-Era THz-PCAs have been used by our customers in ultra high vacuum chamber.

Q: How much is the diameter of the terahertz collimated beam? How much is the focal distance?

A: It depends on the terahertz optics that are used. One can use off-axis parabolic mirrors or a collimating terahertz lens to collimate the diverging beam from the transmitter emanating from the silicon hyper-hemispherical lens. The beam coming from the transmitter is diverging with an angle of 17 degrees. For the off-axis parabolic mirrors that are used in a setup with focal length of 101.6 mm, the collimated terahertz beam diameter is 50.8 mm.

Q: What is the Polarization of the generated terahertz beam?

A: The generated terahertz beam is linearly polarized.

Q: Is there a possibility to align the silicon lens on the back side of the devices?

A: Yes, the devices are shipped with the collimating silicon lenses already aligned and packaged on the back side of the devices. The silicon lens can be re-aligned after changing the chips using a silicon lens setting fixtures.

Q: Is it easy to change the sensor chip dye?

A: Yes, the devices are packaged in a modular format so that it is easy to change the chips inside the enclosures. The terahertz chip dye is mounted on a PCB board, which is connected to the SMA connectors with wires. The user can change the chip dyes and re-align the silicon lens for the best performance at a fraction of cost.