This NSF Small Business Innovation Research Phase II project proposes to develop a compact THz-ABCD (air-biased coherent- detection). spectrometer based on a new technique for generating and measuring ultra-broadband THz waves utilizing a laser induced plasma in ambient air and selected gases. A focused optical pulse with >100 uJ pulse energy and <100 femtosecond pulse duration in gas creates a plasma (ionized gas molecules), which produces very intense (>300 kV/cm), highly-directional (<6 degree), and ultra-broadband (10% bandwidth from 0.1 to 10 THz) THz waves in the far field. Through the reciprocal process, air or selected gases also serve as an ultra-broadband sensor of pulsed THz waves through air-biased coherent- detection (ABCD).The region of the electromagnetic spectrum from 0.3 to 10 THz (1 mm - 30 um in wavelength) is now a frontier area for research in physics, chemistry, biology, materials science and medicine. Recently, the observations of THz wave generation and detection in the laser induced atmospheric plasma provide new method in remote sensing and spectroscopy. The use of air as THz wave emitter and sensor provides unprecedented bandwidth (spectral range of 0.1 to 30 THz), sensitivity (heterodyne method), and spectral resolution ( Recent advances in the use of air/gases to emit, control, enhance, and measure broadband THz waves open up a range of research opportunities. Applications including nondestructive testing, tomographic imaging, label-free genetic analysis, cellular level imaging, explosives detection, and chemical/biological sensing have thrust THz research, from relative obscurity, to new heights. The proposed development of a compact THz ABCD spectrometer will provide a key enabling technology for interdisciplinary research. In addition it will advance numerous sensing and imaging concepts in the THz frequency range, with an immediate impact on non-destructive spectroscopic analysis (eg: pharmaceutical R&D, materials research), a near-term application (3 to 5 years) for homeland security and a longer-term interest (5 to 10 years) in the biomedical sector. If successful the outcome of this project will make significant contributions to academic and governmental laboratory collaboration, student education, and instrumentation development. The Zomega THz-ABCD (Terahertz Air Biased Coherent Detection) Spectrometer is unique in its bandwidth and power output. We have been successful in promoting this system and supporting equipment to more than 20 organizations that want to be early adopters of this new, exciting technology. Extremely broadband THz spectrometry has been commercially available as complete integrated systems starting at the beginning of the second phase of this project. By the end of this project, we have further innovated the spectrometer as a complete system, and also developed supporting equipment for this technology which are marketed as components. These components include THz optics, ABCD detectors, control electronics, and high voltage modulators. A compact THz-ABCD Spectrometer, with the use of air plasma by mixing two laser beams (fundamental and its SHG beam) to emit intense THz waves and the biased air as heterodyne sensor to detect pulsed THz waves, will produce generation and detection of very intense, highly directional THz waves in the far field (< 6 degree), high SNR and ultra-broadband THz waves. These capabilities are unique in the THz community and valuable for applications in a wide range of scientific and industrial domains. The intense THz field (>100 kV/cm) is essential for the nonlinear THz spectroscopy; and a broad spectral range (0.1 THz to >30 THz) is critical for the spectroscopic sensing and identification to reduce the false alarm rate. These unique features are not possible with the current THz time-domain spectrometers commercially available. We also provide a method for measuring both components of the THz polarization simultaneously during a single measurement, which allows customers to measure nonlinear polarization effects on samples which morph/degrade over time. We lowered the pump laser requirements of this product to enable this technology to reach more industrial markets, but the overall signal-to-noise ratio could not be matched with that using high energy lasers. The products and technology developed and commercialized during this award are currently being used to study semiconductor, explosive, and pharmaceutical materials. With the extended THz spectral range and high power of our spectrometer, more materials can be studied, fingerprinted, and identified in a non-contact manor. This device has an immediate impact on homeland security with its ability to positively identify explosives and explosive related compounds (ERCs), but will have a long term impact in the pharmaceutical and semiconductor sectors. These products are also utilized for general material research, including meta-materials. This system advances analytical research by expanding the bandwidth, field strength, spectral resolution and signal-to-noise ratio in the THz and far-infrared band. It also allows non-linear spectroscopic measurements in the same spectral region. The new instrument is a multi-disciplinary project, it involves basic scientific concepts and advanced technology, including atomic scale tunneling ionization in air-plasma, third order optical nonlinearity in both static and dynamic scale, attosecond optical phase control and stabilization, THz Gouy phase shift, as well as heterodyne detection. The creative, original, and potentially transformative elements of this THz-ABCD system are significant in both intellectual merit and broader impact as evidenced when scientists become aware of the opportunities of using THz radiation for research progress in their fields (physics, chemistry, biology, materials science and medicine). We expect that the development of THz-ABCD will have an impact in many areas, far beyond its historical uses in the astronomy and the laser fusion communities; and that the technology it develops will be applied around the world. It has the potential to trigger transformational advances that will impact the field at every level, including nondestructive testing, pharmaceutical, homeland security, and biomedical applications.Project Report
