Direct VUV Comb Spectroscopy of a 29th Nuclear Transition
Abstract
We are pursuing an imminent experiment on direct laser excitation and spectroscopy of the first nuclear excited state of the thorium isomer Th-229, paving the way for a new generation of time-keeping devices: nuclear optical clocks. With all the necessary tools assembled in our lab, including the worldÕs most precise clock, the most stable laser, the highest power XUV frequency comb, we are optimally positioned to make a breakthrough in this field. The two most useful aspects of nuclear clocks would be their portability and their high sensitivity to new physics such as potential time-dependent variations of fundamental constants. The largest uncertainty in the optical lattice clocks in our lab arises from the blackbody radiation. An optical clock based on a nuclear transition would benefit from insensitivity to such perturbations. Most of the nuclear transitions lie in an energy range that is not accessible with current laser systems. The thorium isomer Th-229 however represents a unique isotope due to the exceptionally low excitation energy of its first excited state, located in the deep ultraviolet spectral region. This spectral region is readily accessible with our VUV frequency comb generated via high harmonics of an infrared laser source. The excited state of Th-229 is metastable and its long lifetime result in a correspondingly narrow linewidth, making it an excellent candidate for nuclear clock experiments. Recently, the excitation energy of Th-229 was constrained to near 8.3 eV, corresponding to a wavelength near 149.7 nm. The isomeric energy is well matched to existing frequency comb technology in the vacuum- and extreme-ultraviolet (VUV/XUV) spectral region. We therefore propose a direct laser spectroscopy and laser excitation of Th-229 based on the frequency comb source operational at JILA. The laser source is based on an ytterbium-doped fiber-based frequency comb with a center wavelength around 1070 nm. The infrared frequency comb is coupled into an enhancement cavity to reach high average powers up to 10 kW. An intracavity focus and helium-xenon gas-jet allow for the generation of high harmonics with up to mW-level power in a single harmonic. The generated 7th harmonic (?8.1 eV) matches well with the Th-229 energy and will be used for direct frequency comb spectroscopy. One comb mode will be tuned to resonance with the nuclear transition. The decay of the excited nuclei by internal conversion is then detected by guiding the emitted electrons to a low-energy electron detector, which will provide a signal for detecting the nuclear resonance. This experimental scheme will provide a new estimation of the excitation energy of Th-229 with unprecedented precision. To maximize the detection signal of the nuclear resonance, we will use a non-collinear geometry for the generation of the high-order harmonics. In this geometry, the harmonics and the fundamental wave propagate along different directions, allowing a simple geometric outcoupling of the VUV/XUV light and giving access to a large fraction of the generated power. The wavelength of the generated frequency comb needs to be tunable to search for the nuclear resonance across the uncertainty range of 0.17 eV. We are planning to construct a new mode-locked Yb:fiber laser system that will satisfy this requirement. The experimental setup for direct frequency comb spectroscopy of Th-229 consists of three parts: (1) a wavelength-tunable VUV frequency comb source, (2) a Th-229 target, and (3) a low-energy electron detection system. The VUV frequency comb has a mode spacing of 75 MHz and spans an energy range of 0.038 eV (9.2 THz). Each comb component has a linewidth of about 1 kHz. The IR frequency comb is phase-locked to a cw laser referenced to our Sr optical clock, providing an absolute frequency reference. The comb modes are scanned over the mode spacing by changing the offset frequency of the phase-locked-loop. Thus, the full bandwidth (0.038 eV) of the comb is used
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 09, 2020
- Source ID
- W911NF2010182
Entities
People
- Jun Ye
Organizations
- Army Contracting Command
- United States Army
- University of Colorado Boulder