The unique properties of terahertz radiation mean it is of interest for a wide range of potential applications, including non-invasive medical imaging and the detection of hazardous substances. Terahertz waves can penetrate many materials that are opaque to visible light and, unlike X-radiation, do not pose a risk of damage to biological tissue. In addition to this, many substances have a molecular fingerprint in the terahertz range, allowing them to be detected using spectroscopic methods.
Creating a frequency comb with a broadband ‘laser sandwich’
With this special feature of being able to determine the laser wavelengths themselves, several quantum cascade structures with different emission frequencies can be stacked on top of one another, with the aim of generating broadband terahertz radiation. “Heterogeneous active zones of this kind are ideally suited for implementing broadband terahertz amplifiers and generating ultrashort terahertz pulses,” explains Dominic Bachmann from the Photonics Institute. Plus, if the discrete laser lines are linked together to establish a fixed phase relationship between the laser modes, something known as a ‘frequency comb’ will be created.
Watching lasers at work
One method developed by the group led by Prof. Unterrainer makes it possible to analyse internal quantum cascade laser parameters during laser operation. This technique is based on time-resolved spectroscopy, with broadband terahertz pulses penetrating the sample to be measured. Based on femtosecond lasers, this technology can be used to collect the full information content relating to the time and frequency range with just one single measurement.
One unresolved issue with terahertz quantum cascade lasers had been the existence of laser lines with different propagation speeds. If there are laser modes with a higher lateral order, the intensity is distributed very unevenly between the laser lines, thereby reducing the usable bandwidth and preventing the generation of a frequency comb. In order to stop these modes from oscillating, the losses have to be increased to such an extent that they do not reach the laser threshold. By adding a tailored lateral absorber to the edges of the laser resonator, the researchers managed to suppress the higher lateral modes entirely, without having any relevant impact on the fundamental modes.
Contacts and sources:
Bachmann et al., “Short pulse generation and mode control of broadband terahertz quantum cascade lasers", Optica 3, 1087 (2016), DOI: 10.1364/OPTICA.3.001087.
Bachmann et al., "Dispersion in a broadband terahertz quantum cascade laser”, Appl. Phys. Lett. 109, 221107 (2016), DOI: 10.1063/1.4969065.