Light is a powerful tool for sensing our environment. It is described by an electromagnetic spectrum. The most widely used part of this electromagnetic spectrum, visible light, is seen by our eyes. But other parts of the spectrum are also widely used in industry and science to detect phenomena not observable with visible light. This project involved so-called terahertz (THz) light, which has a much lower frequency and much longer wavelength than visible light. THz light can be transmitted through various materials that are opaque to our eyes, allowing to “see through” objects. This property is of great value in the non-destructive testing, since the it allows to infer the thickness of films or other properties without taking apart the sample or using ionizing radiation. Such measurements are widely used in industry to check that products confirm to their specifications. Similarly, THz light can be used for spectroscopic investigations, where a target (for example a material or gas cell) absorbs certain spectral components of the THz light. Measurement of this frequency dependent absorption allows to infer properties about the target.
Efficient generation and detection of THz light has been a long-standing scientific challenge, and one of the key factors determining the rate of adoption of this technology outside of scientific labs. THz light is often generated by intense and extremely short laser pulses. Specifically, infrared laser pulses with around 100 femtoseconds (10-15 s) duration are used. One pulse is focused onto a specialized antenna, which generates a corresponding THz pulse. Then, the THz pulse and another infrared pulse are sent to a second antenna. By varying the optical delay between the two infrared pulses, it is possible to recover the electric field profile of the THz pulse, which in turn contains information about any materials the THz pulse has passed through.
Most commonly, the optical delay is obtained by a mechanical delay line, which is a mirror that is moved back and forth very precisely over a range of several centimeters or more. The project involved replacing the femtosecond laser source and the mechanical delay line with a new and cost-effective approach that enables much faster scans. For this we use dual-comb modelocking technology. This is a new type of laser in which a pair of femtosecond lasers with slightly different pulse repetition rate are generated inside one optical resonator. The different repetition rates of these two pulse trains provides a replacement to traditional mechanical delay lines, without requiring any moving parts. Generating both combs in one resonator serves two main purposes: it keeps the arrival time of the pulses highly stable, and it reduces the complexity of the system. The purpose of the project was to demonstrate and advance terahertz time-domain spectroscopy via our dual-comb modelocked lasers.
The project successfully met its goals, leading to an enhanced readiness level of the technology. Early in the project we established connections with experts in the terahertz field to help guide our efforts and obtain state of the art components. We also worked on IP development during the course of the project. Building on earlier scientific investigations of dual-comb modelocking, we studied these lasers in more detail and demonstrated new regimes of operation. This work included: a novel approach to quantify the pulse timing noise of these lasers; development of an industrial prototype laser system; and extension of dual-comb modelocking to gigahertz pulse repetition rates suitable for high-speed measurements.
Through these advances in laser technology combined with data processing and analysis tools which we developed, we performed precision measurement applications including pump-probe sampling of thin-film semiconductor materials, high-precision laser ranging, trace-gas detection via spectroscopy, and terahertz time-domain spectroscopy. Our “proof of concept” terahertz results compare favorably to the start of the art, which represents a key prerequisite to commercializing the technology directly enabled by the project. Tests on representative samples validated our measurements and the suitability of the prototype system we developed for measuring industrial samples. The results yielded several scientific publications with more in preparation, numerous presentations at international conferences, and attendance at multiple trade shows. Our work is an important step forwards in practical deployment of optical technologies for sensing and testing techniques, and we are well positioned to deploy these developments in the near future. More generally, the growing adoption of these techniques will lead to improved products and more efficient production methods in many industries.
