Methane-based laser source creates 120 distinct THz frequencies
What you will learn:
- How lasers are used as terahertz frequency sources.
- The impact of the transition from nitrous oxide to methyl fluoride as a laser gas.
- Results obtained by the researchers.
Interest in the terahertz band – likely the next region for wireless spectrum opportunity as well as specialized sensing – continues, with significant research at the university level. However, despite decades of research, no frequency-tunable source covers the terahertz gap between 0.3 and 3 THz.
But there is real progress: a team at Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS)working in collaboration with the DEVCOM Army Research Laboratory and DRS daylight solutions, demonstrated a continuous wave laser with over 120 discrete transitions spanning the 0.25 to 1.3 THz range. The work builds on the team’s previous prototype which proved that terahertz frequency sources could be made compact, room temperature, and widely tunable by combining a quantum cascade laser pump with a molecular nitrous oxide laser ( laughing gas).1
However, this new research more than triples the tuning range of this prototype. Among its many advances, the laser replaces nitrous oxide with methyl fluoride (CH3F), a molecule that reacts strongly with optical fields.
This laser already has the potential to be one of the most compact terahertz lasers ever designed, and researchers are aiming to make it even more compact. “A device under one cubic foot will allow us to target this frequency range for even more applications in short-range communications, short-range radar, biomedicine and imaging,” said Paul Chevalier, associate researcher at SEAS. and principal investigator of the team.
“This compound [methyl fluoride] is really good at absorbing infrared and emitting terahertz,” explained Arman Amirzhan, graduate student at SEAS and first author of the paper. “By using methyl fluoride, which is non-toxic, we have increased the laser’s efficiency and tuning range.”
How it works?
The molecular interaction, quantum-level physics and associated analysis are daunting and intense, and fully explained in the published article; the hardware itself is complicated too (Fig.1). The THz cavity is a 50cm copper pipe with an internal diameter of 4.8mm. A flat mirror with a centered pinhole of 1 mm in diameter was used as the output coupler. The resonant frequency of the cavity was tuned by changing the position of the tuning mirror to adjust the length of the cavity.
In operation, the pressure in the laser cell is controlled by cryogenic pumping, starting with the evacuation of the cell to a very high vacuum (less than 10-5 mTorr). Gas phase methyl fluoride is contained in a small 10ml metal bottle connected to the system by a needle valve. This cylinder is cooled by liquid nitrogen which causes the gas to solidify.
After this initial cooling period, the vacuum pump valve is closed and the cylinder valve is opened. The liquid nitrogen is then removed and the cylinder allowed to warm up, causing the solid methyl fluoride to sublimate, converting directly to vapor with no intermediate liquid phase, and filling the rest of the system. The cylinder valve is closed when the correct pressure is reached.
To obtain a laser emission, the laser cavity is first filled with CH3F to the desired pressure. Next, the quantum cascade laser (QCL) is tuned to a desired gas absorption line by setting the correct drive current, grating position, and laser temperature in the QCL controller. Although complicated, the result is spectacular.
Evaluation and performance
It is one thing to establish this kind of terahertz source, but it is quite another to evaluate its performance. Emission frequencies were measured with one of several heterodyne mixers and detectors depending on the place in the frequency band.
Schottky diode detectors were used between 0.25 and 1.1 THz, a Golay cell was used above 1.1 THz (a Golay cell is a type of opto-acoustic detector mainly used for infrared spectroscopy2) and heterodyne receivers were used to measure spectra between 0.3 and 1.1 THz (Fig.2). The local oscillator is provided by a tunable frequency generator while the intermediate frequency is measured on a spectrum analyzer (all test equipment vendors and specific units are identified in their article).
Laser linewidth can be measured experimentally, using the same heterodyne receiver that was used to retrieve each line frequency. In practice, the linewidth stability seemed to be limited by the mechanical stability of the cavity. The smallest measured linewidth was recovered for one of the lowest measured frequency laser lines, at approximately 402.9 GHz, with a recovered linewidth of less than 1 kHz (Fig.3).
Many variables are responsible for baseline performance and efficiency. The output power of the terahertz laser emission depends on a combination of several factors, including the choice of gain medium, the IR pumping efficiency, and the design of the laser cavity. The choice of the gain medium is also motivated by a series of factors, notably its IR absorption coefficient and its saturation.
The performance of the laser cavity is determined by its geometry, the quality factor of the cavity, the pump power loss factor inside the cavity and the terahertz losses in the output coupler. Optimal cavity dimensions, operating pressure and more accurate output powers are calculated by performing extensive molecular dynamics simulations.
The work is detailed in their seven-page paper »A 1 THz tunable quantum cascade laser pumped molecular laser” Posted in APL photonicsaccompanied by a 16-page dossier Additional material case.
1. Electronic design“Strange electromagnetic sources: laughing gas for THz waves, peeling tape for X-rays”
2. Wikipedia, “Golay cell”