Ultrafast, High-Field Electron Spin Resonance (HF-ESR)

Over the past few years, our lab has been developing a state-of-the-art magnetic resonance facility that takes advantage of UCSB’s Free Electron Laser (FEL) to study excitations of many body systems. The FEL can produce an immense amount of microwave field strength compared to commercial sources. The higher power allows us to saturate samples with shorter pulses, giving us a window into the dynamics of short-lived excitations. While high-field ESR spectrometers based on solid state sources can measure lifetimes in microseconds, our new lab aims to measure lifetimes as short as a few nanoseconds. This increased time resolution lets us see the dynamics of many-body quantum systems that would otherwise be inaccessible. We are working towards this facility becoming fully operational in the near future.

Planned capabilities of our new facility

  • Pulsed and continuous wave ESR
  • Magnetic fields from 0 – 16 Tesla
  • Frequency Agile (170 GHz to 420 GHz)
  • Microwave power of 1kW (with the potential for 100kW for 40 ns while cavity dumping)
  • Sample temperatures as low as 1.8 K
  • Three axis rotation of single crystals
  • “Two color” pulsed experiments using two frequencies of microwave

Electron Spin Resonance

The facility relies on pulsed Electron Spin Resonance (ESR) for these measurements (sometimes called Electron Paramagnetic Resonance (EPR) when used to study unpaired electrons). ESR probes the local environment of an electron by using microwave radiation and a strong magnetic field to resonate with its magnetic moment. By sending the microwave signal in pulses, we can coherently drive the magnetic moments, allowing us to directly measure the lifetimes of quasiparticles and spin excitations.

Figure 1: Typical pulse sequence for measuring the coherence lifetime of a spin or qubit. It is a two pulse Hahn Echo Sequence, which gives rise to a resonant echo and a Free Induction Decay (FID).

More specifically, the coherence lifetime of a spin excitation can be measured using a Hahn-Echo sequence in pulsed ESR [1,2]. As seen in figure (1), one pulse is used to create spin excitations. Following this, they evolve at different rates until they are hit by a second pulse which refocuses them. The spins then reach the same point at the same time and resonate microwave radiation coherently as an “echo” [1,2]. As spins decohere, the height of the echo shrinks. Thus, by changing the delay time (t) between two pulses, we can track the height of the echo and fit the exponential decay to get a lifetime (T2T_2) [1,2].

However, there is a second competing effect that occurs within a sample: Free Induction Decay (FID). After every pulse, the sample radiates away energy as seen in Figure 1. [2,3]. The challenge is that for short lifetimes on the order of nanoseconds, the FID and the echo often overlap and both depend upon the pulse delay in a nontrivial way. This means that the signal we are interested in can be swamped by a changing background, which would make fitting the decay near impossible. The solution is to shift the relative phases between the two pulses. The echo signal depends on the relative phase, while the FID does not, allowing us to separate the two [3].

Using the Free Electron Laser for pulsed ESR

High power sources like Free Electron Lasers and Gyrotrons are not typically used for pulsed ESR. This is because our FEL produces incredibly long pulses, which on their own cannot be shaped or manipulated. This is why we have designed and built modular devices that slice and phase shift the FEL output into a desired pulse sequence.

Figure 2: Pulse slicing bar. Contains three pulse slicing and two phase shifting modules, allowing us to cut the FEL’s output into up to three arbitrary pulses and control their relative phases.

The pulse slicer works by having two channels, an additive channel and a subtractive channel. The FEL output by default stays in the subtractive channel until one of the silicon wafers in the center is activated by a green laser, at which point it reflects the microwave into the additive channel [4], as shown in Figure 3. 

Figure 3: Pulse slicing module (figure taken from [4]). Each module allows the user to cut a single pulse out of a longer output, leaving the remaining power to be used by future modules. 

References

[1] E. L. Hahn. “Spin Echoes”. en. In: Physical Review 80.4 (Nov. 1950), pp. 580–594. ISSN: 0031-899X. DOI: 10.1103/PhysRev.80.580. URL: https://link.aps.org/doi/10.1103/PhysRev.80.580.
[2] John Weil and James Bolton. Electron Paramagnetic Resonance: Elementary Theory and Practical Applications, Second Edition. John Wiley & Sons, Inc, May 2006. ISBN: 978-0-471-75496-1
[3] C. Blake Wilson et al. “Multi-step phase-cycling in a free-electron laser-powered pulsed electron paramagnetic resonance spectrometer”. en. In: Physical Chemistry Chemical Physics 20.26 (2018), pp. 18097–18109. ISSN: 1463-9076, 1463-9084. DOI: 10.1039/C8CP01876F. URL: https://xlink.rsc.org/?DOI=C8CP01876F.
[4] Brad D. Price, Antonín Sojka, Nikolay Agladze, Mark S. Sherwin. “Compact module for complementary-channel THz pulse slicing” Applied Physics Letters, 124(2), 021107 (2024).

Principal Investigators

  • Prof. Mark Sherwin (Physics, UCSB)

Graduate Students

  • Alex Giovannone (Physics, UCSB)
  • Johanna Schubert (Experimental Biophysics, UCSB)

Collborators

  • Prof. Vinnicuis Santana (CEITEC)
  • Prof. Chee Wei Wong (UCLA)

Publications

Two-fluid mobility model from coupled hydrodynamic equations for simulating laser-driven semiconductor switches
Qile Wu, Antonín Sojka, Brad D. Price, Nikolay I. Agladze, Anup Yadav, Sophie L. Pain, John D. Murphy, Tim Niewelt, Mark S. Sherwin
Phys. Rev. Applied 24, 014007 (2025)

Compact module for complementary-channel THz pulse slicing
Brad D. Price, Antonín Sojka, Nikolay Agladze, Mark S. Sherwin
Applied Physics Letters, 124(2), 021107 (2024)

Order-of-Magnitude SNR Improvement for High-Field EPR Spectrometers via 3D-Printed Quasioptical Sample Holders
Antonin Sojka, Brad D. Price, Mark S. Sherwin
Science Advances 9, eadi7412 (2023)

Scroll to Top