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Electron Paramagnetic Resonance (EPR) is a sensitive probe of the local environment around a paramagnetic species. EPR can be used to study a variety of systems in physics, material science, chemistry, and biology. In particular, EPR has proven to be exceptionally important in molecular biology where "tagging" a biomolecule with a stable paramagnetic allows site-specific measurements of structure and function of proteins and other biological systems. EPR becomes more powerful as the applied magnetic field is increased, but applications of high-field EPR are limited by to the absence of suitable high-power sources.
In the Sherwin lab, we are developing novel EPR instrumentation by using the UCSB Free Electron Lasers (FELs) to do high-powered, pulsed EPR at fields more than twice as large as previously possible. The UCSB FELs offer ~kWatts from 100 GHz to 5 THz, a range of frequencies where typically less than 1 Watt is available.
Fig. 1: LOREM IPSUM
Currently operating at 240 GHz, special switches driven by powerful Nd:YAG lasers (see Fig. 2) are used to generate up to 2 pulses with significant tuning of lengths and separation to excite spins faster. The FELEPR project has led to the operation of the highest-field, high-power pulsed EPR spectrometer as well as offering a platform for instrument and method development for high-field EPR.
Fig. 2: Nd:YAG lasers
High-power green ND:YAG lasers are used to drive the switches that generate up to two pulses from the FEL.
As a result of the extremely high power offered by the FELs, the capabilities of the FELEPR spectrometer are transformative compared to other, low-power sources operating at similar fields. By measuring the pulse length necessary to rotate the spins (Rabi oscillations), we can characterize the strength of the pulses we are applying. The measured Rabi oscillations are shown in Fig. 3 demonstrate a ~13ns 180º pulse (from the first maximum, to the second), corresponding to a 90-degree length of ~6.5ns, roughly 50-100 times faster than possible with other spectrometers at similar frequencies.
Fig 3: Rabi oscillations
The Rabi oscillations demonstrate our ability to excite spins in only 6 ns (by tipping them 90°). This is only possible due to the high power produced by the FEL, and is around 50-100 times faster than with other sources.
A further advantage of the short pulses used here is the substantial increase in excitation bandwidth achieved. This is demonstrated in Fig. 4, where the EPR signal from a nitrogen impurity in diamond (split into 3 lines by the coupling to the S=1 nitrogen nucleus) is shown with all three lines being excited. Using a longer pulse, as is necessary at lower powers, the narrow excitation bandwidth prevents observing more than one line at a time.
Fig 4: Extended excitation bandwidth
By utilizing short pulses we dramatically expand the excitation bandwidth of our pulses. This improves sensitivity and allows probing multiple populations at once, as is shown for the three hyperfine lines in a nitrogen vacancy in diamond.
Finally, by carrying out 2-pulse echo measurements, FELEPR can be shown to measure relaxation times ten times faster than is possible with low power sources, as is shown in Fig. 5, where the echo is detected within ~300 ns, half of the typical length of a single pulse on other spectrometers. Our FELEPR spectrometer, while operational, is regularly being upgraded. Ongoing work includes on recently implemented phase cycling techniques, and work towards measurements of the electron T1e.
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Researchers (UCSB)
Researchers (UCSB)
PI:
PI:
Mark Sherwin
Songi Han (Chemistry and Biochemistry)
Graduate Students
Graduate Students
Blake Wilson
Brad Price
Alex Giovannone
Collaborators:
Collaborators:
Yun Zhang (Technical University of Munich)
References:
References:
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