Finding water on the moon could be easier with Goddard’s technology, which uses an effect called quantum tunneling to create a high-powered terahertz laser, filling a gap in existing laser technology. Locating water and other resources is a priority for NASA, which is key to the exploration of Earth’s natural satellite and other objects in the solar system and beyond. Previous experiments inferred and subsequently confirmed the existence of a small amount of water on the Moon. However, most technologies do not distinguish between water, free hydrogen ions and hydroxyl because the broadband detectors used cannot distinguish between the different volatiles.
Goddard engineer Dr. Berhanu Bulcha said a type of instrument called a heterodyne spectrometer could zoom in on specific frequencies to definitively identify and locate water sources on the moon. It would need a stable, high-power terahertz laser that was prototyped in collaboration with Longwave Photonics through NASA’s Small Business Innovation Research (SBIR) program. “This laser allows us to open a new window for studying this frequency spectrum,” he said. “Other missions have found hydration on the moon, but that could indicate hydroxyl or water.” If it is water, where did it come from? Did it originate from the formation of the Moon, or did it appear later in cometary impacts? How much water is there? We need to answer these questions because water is essential for survival and can be used as fuel for further exploration.
As the name suggests, spectrometers detect the spectra, or wavelengths, of light to reveal the chemical properties of the matter that the light has touched. Most spectrometers tend to work in broad parts of the spectrum. Heterodyne devices connect to very specific light frequencies such as infrared or terahertz. Hydrogen-containing compounds such as water emit photons in the terahertz frequency range 2 trillion to 10 trillion cycles per second between microwaves and infrared.
As a microscope for subtle differences in bandwidth such as terahertz, heterodyne spectrometers combine a local laser source with incoming light. Measuring the difference between the laser source and the combined wavelength provides accurate readings between sub-bandwidths. Traditional lasers generate light by exciting an electron in the outer shell of an atom, which then emits a single photon when it transitions or returns to its resting energy level.
Different atoms produce different frequencies of light based on the fixed amount of energy required to excite one electron. However, lasers fall short in a certain part of the spectrum between infrared and microwave radiation known as the terahertz gap.”The problem with current laser technology,” said Dr. Bulcha, “is that no material has the right properties to create a terahertz wave.”
Electromagnetic oscillators, such as those that generate radio or microwave frequencies, produce low-power terahertz pulses using a series of amplifiers and frequency multipliers to extend the signal into the terahertz range. However, this process consumes a lot of voltage, and the materials used to amplify and multiply the pulse have limited efficiency. This means they lose power as they approach terahertz frequencies.
From the other side of the terahertz gap, optical lasers pump energy into the gas to generate photons. However, high-power lasers in the terahertz band are large, power-hungry, and not suitable for space exploration purposes where weight and power are limited, especially handheld or small satellites. Pulse power also decreases as optical lasers push toward terahertz bandwidths.
To fill this gap, the team of Dr. Bulchy is developing quantum cascade lasers that produce photons from each electron transition event using the unique physics of quantum-scale materials layered just a few atoms thick.In these materials, the laser emits photons at a specific frequency determined by the thickness of the alternating semiconductor layers rather than the elements in the material. In quantum physics, thin layers increase the chance that a photon will penetrate the next layer instead of bouncing off the barrier. Once there, it excites more photons. Using a generator material with 80 to 100 layers less than 10 to 15 microns thick, the team’s source creates a cascade of terahertz-powered photons.
This cascade consumes less voltage to create a stable, high-power light. One disadvantage of this technology is that the beam propagates at a large angle and disperses quickly over short distances. Dr. Using innovative technology supported by Goddard’s Internal Research and Development (IRAD) funding, Bulcha and his team integrated the laser into a waveguide with a thin optical antenna to tighten the beam.
An integrated laser and waveguide unit reduces this dispersion by 50% in a package smaller than a quarter. He hopes to continue working on a flight-ready laser for NASA’s Artemis program. The laser’s small size and power consumption allow it to fit into a 1U CubeSat, about the size of a teapot, along with the spectrometer hardware, processor and power supply. It could also power handheld devices for use by future explorers on the moon, Mars and beyond.