Q-band EPR cryoprobe

Following the success of cryogenic EPR signal preamplification at X-band, we present a Q-band EPR cryoprobe compatible with a standard EPR resonator. The probehead is equipped with a cryogenic ultra low-noise microwave amplifier and its protection circuit that are placed close to the sample in the same cryostat. Our cryoprobe maintains the same functionality and compatibility as the commercial instrument allowing high-power pulsed EPR experiments of typical samples. The performance of our setup is benchmarked using pulsed EPR and ENDOR experiments revealing a significant sensitivity improvement, which reduces the measurement time by a factor of about 40 × at low temperature


Introduction
[3][4] The thermal noise in these probeheads is significantly suppressed by simultaneous cooling of the LNA and NMR coil, independently of the sample temperature.Despite this widespread success in NMR, a signal preamplification with cryogenic LNAs is not yet widely used in EPR spectroscopy.][7][8][9][10][11] However, these setups are poorly compatible with the commercial resonators and spectrometers, typical samples or high-power pulsed EPR experiments.
Recently, we modified a commercial X-band EPR probehead by equipping it with a cryogenic LNA and its protection circuit placed close to the sample in the same cryostat, while simultaneously satisfying all aforementioned expectations of the EPR community. 12r pulsed EPR experiments, our X-band cryoprobe provided a significant improvement of the voltage signal-to-noise ratio (SNR) by a factor close to 10× below 10 K, which gradually decreased to about 2× at room temperature.Such a high sensitivity enhancement has 2 https://doi.org/10.26434/chemrxiv-2023-mghr2ORCID: https://orcid.org/0000-0002-2733-2270Content not peer-reviewed by ChemRxiv.[15] In a subsequent paper, 16 we placed the LNA and its protection circuit in a separate cryostat, which provided EPR sensitivity improvement independent of sample temperature, while also enabling an easy way to combine cryogenic preamplification with more bulky ENDOR and Q-band probeheads.However, due to the room-temperature microwave paths joining the sample and LNA cryostats, the SNR improvement was only moderate and approached a factor of 4× at X-band, which is substantially lower compared to the performance of the first design at low temperature. 12The sensitivity enhancement for the Q-band setup was even smaller reaching only a factor of 2× and thus diverting attention back to the single-cryostat approach, which is explored in this work.
Here, we report design and performance of such a Q-band EPR cryoprobe equipped with a cryogenic LNA and its protection circuit, which are connected to a commercial Qband resonator placed in a standard cryostat.Our probehead maintains ordinary sample access and resonator coupling, and is fully compatible with the high-power pulsed EPR experiments.We demonstrate the performance of our setup by performing pulsed EPR and ENDOR experiments of ordinary samples.The probehead provides a significant voltage SNR improvement by a factor of about 6.5× at 6 K, which gradually approaches 1.5× at room temperature.

Probehead design
Our cryoprobe is based on a commercial Bruker EN5107D2 Q-band microwave resonator compatible with 1.6 mm outer diameter EPR tubes.It is designed to handle high power microwave pulses from a traveling-wave tube (TWT) amplifier, as well as to retain the conventional sample access and resonator coupling capabilities.Similarly to our previously reported X-band setup, 12 the microwave circuit of our Q-band cryoprobe is based on a 10 dB https://doi.org/10.26434/chemrxiv-2023-mghr2ORCID: https://orcid.org/0000-0002-2733-2270Content not peer-reviewed by ChemRxiv.License: CC BY-NC 4.0 directional coupler (Pasternack PE2CP1126-10), which is used to guide the microwave signals (Figure 1).The incoming microwave pulses reach the resonator via the coupled port of the coupler, which also causes a partial suppression of the input thermal noise (see below) at the expense of pulse power. 12,16The reflected microwave pulses and spin echoes coming from the resonator are guided by the same directional coupler to a Pasternack PE80L2002 (20 W peak power, 63 mW flat leakage, 10 ns recovery time, 0.1% duty cycle) limiter, which is used to protect a Low Noise Factory LNF-LNC23_42B cryogenic LNA (28 dB gain, 8 K noise temperature at 4 K and 34 GHz).The LNA is thermalized to the temperature of the sample via a C110 copper bracket extending below the resonator to the bottom of the cryostat.The amplified signal leaves the probehead via a second output microwave line connected to the EPR bridge.To save otherwise severely limited space in the cryostat (Oxford CF935), we have chosen microwave components with the 2.92 mm type coaxial connectors instead of more bulky waveguides.The resonator, however, is connected to the directional coupler using a short WR-28 waveguide section with an attached waveguide-to-coax adapter.Employment of more lossy coaxial cables is acceptable, as the dominant cable length is either prior the directional coupler or after the LNA.A photograph of our probehead is given in Figure S1.
We also note that the cryoprobe has input and output ports that must be connected to the EPR bridge, while typical EPR spectrometers are designed to operate only in the reflection mode and thus have only one microwave port.As in our previous works, 12,16 we solve this problem by bypassing the internal circulator in the microwave bridge, which is a simple modification.

Pulsed EPR
Pulsed EPR experiments using low-power microwave pulses were performed using an IF-Q option of a Bruker ELEXSYS E580/IF-Q spectrometer equipped with a 10 W AmpQ-10 solid-state amplifier (SSA).For these experiments, we placed a small amount of the Bruker DEER test sample in a 1.6 mm outer diameter EPR tube.The SNR improvement was characterized using a Hahn echo pulse sequence (π/2 − τ − π − τ − echo) with two-step phase cycling.Due to the 10 dB directional coupler, the pulse power reaching the sample was significantly reduced resulting in the shortest achievable π-pulse duration of about 200 ns in the overcoupled resonator.
A much shorter π-pulse of 40 ns was obtained on a homebuilt Q-band spectrometer equipped with a 150 W TWT amplifier.For these experiments, we measured a Cu(II) signal of a Cu(II)-nitroxide molecular ruler (Figure S2) sample (200 µM ruler concentration in 1:1 17 placed in a 1.6 mm outer diameter EPR tube.
To avoid saturation of the digitizers, the interpulse delay τ was adjusted to produce a sufficiently weak echo signal.Depending on the sample temperature, the shot repetition time was chosen to be sufficiently long to allow full recovery of the signal.
The SNR and its uncertainty were determined using 10 separate measurements of the Hahn echo.The traces were corrected by subtracting constant backgrounds, which proved to be almost negligible.The intensity of the spin signal was taken as a maximum of the echo obtained by fitting a Gaussian peak function, while noise was calculated as the standard deviation of the signal far away from the echo (at least 500 data points were used for noise calculation).
The SNR improvement provided by our cryoprobe was benchmarked against standard reflection setups obtained by reattaching the resonator to the ordinary Bruker Q-band probehead.The resonator coupling arm was tightly fixed to avoid potential variations during switching between both setups.All parameters, except for the microwave power and negligible changes in the microwave frequency and magnetic field, were kept constant in both measurements.The microwave power was adjusted to yield the same duration of the π-pulse, and the field position was verified by the echo-detected field sweep (EDFS) experiments obtained using the same Hahn echo pulse sequence.

Pulsed ENDOR
We performed pulsed 1 H ENDOR experiments at 10 K using the Bruker ELEXSYS E580/IF-Q spectrometer equipped with a Bruker DICE ENDOR system and a 150 W radiofrequency amplifier.The measurements were performed using the same Bruker DEER test sample placed in a 1.6 mm outer diameter EPR tube, which was inserted into the overcoupled EPR resonator.The Mims pulse sequence 18 was used with the microwave π/2-pulse length of 100 ns and the radiofrequency π-pulse of 8 µs.The interpulse delay τ between the microwave pulses was set to 3 µs to yield a sufficiently weak echo signal.Analogous to the pulsed EPR case, the SNR improvement of ENDOR experiments was benchmarked against a standard 6 https://doi.org/10.26434/chemrxiv-2023-mghr2ORCID: https://orcid.org/0000-0002-2733-2270Content not peer-reviewed by ChemRxiv.License: CC BY-NC 4.0 setup measured in reflection.The power of the radiofrequency pulse was carefully adjusted to yield the same ENDOR efficiency in both cases.For SNR improvement calculation, the ENDOR spectra were baseline corrected and noise was determined by calculating the standard deviation of signal far away from the ENDOR lines.

Calculation of sensitivity improvement
The SNR improvement was calculated using the approach developed in our previous work, 16 which is based on the effective noise temperature formalism. 7We define the sensitivity improvement provided by the cryoprobe as the output voltage SNR ratio between the cryoprobe (C) and unmodified (U) setups: Here, F and T in denote the noise factor of the microwave circuit and the noise temperature at its input, respectively.The noise factor can be calculated from the total effective noise temperature T e as where T e can be obtained using the Friis equation. 16,19 our calculations, we assume T U in = 294 K independent of the sample temperature, since in a standard setup the sample is not isolated from room temperature thermal noise.
In contrast, as demonstrated in our previous works, 12,16 additional attenuation on the cold input line of the EPR cryoprobe provides this isolation, and thus T C in may be significantly lower than 294 K. Our current setup is equipped with a 10 dB directional coupler, which provides 10× reduction in the input thermal noise power resulting in T C in = 30 K for sample temperature T S lower than 30 K. For T S > 30 K, we set T C in = T S .
To calculate the noise factor of the cryoprobe and Bruker setups, we measured their microwave losses using a vector network analyzer (VNA).The measurement results are sumhttps://doi.org/10.26434/chemrxiv-2023-mghr2ORCID: https://orcid.org/0000-0002-2733-2270Content not peer-reviewed by ChemRxiv.License: CC BY-NC 4.0 marized in Figure S3.The gain and noise temperature of the LNAs were taken from the manufacturer specifications.

Results and discussion
First, we investigated the performance of our Q-band cryoprobe by measuring the Hahn echo of a Bruker DEER test sample at 10 K and comparing it to the unmodified Bruker setup.A comparison of the obtained echoes are presented in Figure 2a showing that our cryoprobe provides a highly significant voltage SNR improvement by a factor of about 6×, which translates to the measurement time reduction close to 35× at 10 K.The obtained sensitivity gain at this temperature is about 3× higher compared to our previously reported external cryoprobe case. 16A significantly better performance of our current setup mainly originates from the suppression of the input thermal noise by the 10 dB directional coupler and the absence of the room-temperature microwave paths prior the cryogenic LNA.The suppression of the input thermal noise occurs at the expense of pulse power limiting the π-pulse duration to 200 ns with a 10 W SSA. To assess the compatibility of our probehead with high-power pulses, we switched to a homebuilt Q-band spectrometer equipped with a 150 W TWT amplifier.For this purpose, we measured a Hahn echo of a Cu(II) signal from a Cu(II)-nitroxide molecular ruler sample (see experimental details) at 10 K temperature.A comparison of the echoes obtained using our cryoprobe and unmodified setups is presented in Figure 2b indicating a sensitivity improvement of 5.3×, which is slightly lower compared to the low-power measurement.This small discrepancy can be attributed to slightly lower losses (by about 0.5 dB) of the homebuilt spectrometer compared to the Bruker setup.The measurements with the TWT show a full compatibility of our Q-band cryoprobe with highbandwidth pulsed EPR experiments.Even shorter π-pulse duration might be achieved by reducing the losses of the input microwave path or using a more powerful TWT amplifier, provided the pulse power does not exceed the peak power (20 W) of the limiter.
We also measured the EDFS spectra of the same samples with the cryoprobe and unmodified setups (Figure 3).The obtained spectra reveal similar SNR improvements as determined from the Hahn echo experiments.These experiments also demonstrate that the cryoprobe does not affect the lineshapes indicating no saturation effects for signals of moderate intensity.
The temperature dependence of the SNR improvement obtained from the Hahn echo experiments is presented in Figure 4 revealing a gradual decrease of the sensitivity gain from 6.5× at 6 K to about 1.5× at room temperature.A remaining small sensitivity improvement at room temperature indicates that our cryoprobe setup has lower microwave losses prior the LNA compared to the Bruker setup.A similar behaviour was also observed in our previous designs of the X-band cryoprobe. 12,16 used a VNA to determine the microwave losses of our cryoprobe and unmodified setups (see Figure S3) allowing us to compare the measured sensitivity improvement with our theoretical model given by Eq. 1.The calculated temperature dependence of the SNR improvement is also presented in Figure 4 revealing a good agreement with the experimental Our model also allows to predict the sensitivity gain for highly attenuated input line, which in practise could be achieved using a 20-30 dB directional coupler.In this case, T C in = T S in the whole range of accessible sample temperature T S .For such a setup, the SNR enhancement would approach a factor of 10× at 4 K providing an impressive 100fold reduction in the measurement time (Figure 4).Such a high input attenuation may still be exploited for high-spin systems (e.g.Mn(II) or Gd(III)), which require significantly lower power for excitation.Note that above 30 K, the additional attenuation does not provide higher SNR improvement compared to the 10 dB case, as 10 dB input attenuation corresponds to T C in ≥ 30 K.
We also calculated the lower bound of the sensitivity enhancement provided by our cryoprobe by assuming no suppression of the input thermal noise (Figure 4).In such a case, the SNR improvement still approaches a substantial factor of about 2.6× at low temperature.However, implementation of such a setup is challenging, as it requires a ferrite circulator to be placed in the magnetic field at low temperature in the vicinity of the sample instead of a simple directional coupler.
Our probehead design also allows us to perform ENDOR experiments.We benchmarked the ENDOR sensitivity gain at 10 K using the Mims ENDOR pulse sequence and Bruker DEER test sample.The 1 H ENDOR spectra obtained using the cryoprobe and unmodified setups are presented in Figure 5

Conclusions
In this work, we constructed and tested a Q-band EPR cryoprobe based on a commercial microwave resonator and a cryogenic ultra low-noise microwave preamplifier.Our cryoprobe is compatible with high power microwave pulses, while simultaneously maintaining the con- factor above 6× at 6 K, which gradually decreased to about 1.5× at room temperature.
Our calculations revealed that the sensitivity gain may be further improved by a higher suppression of the input thermal noise and reduction of the microwave losses prior the LNA.
As discussed in our previous works, cooling of the LNA and the sample should be ultimately decoupled eliminating the observed decrease of the sensitivity gain with increasing sample temperature.
Our microwave circuit and probehead design should be also compatible with other Qband microwave resonators such as broadband 3 mm tube resonators. 20A full exploitation of wide bandwidth resonators for spin-1/2 species, however, may be impeded by the increased duration of the π-pulse due to the 10 dB directional coupler.This problem can be solved by applying frequency-swept pulses 21,22 or by using higher power TWT amplifiers and lower coupling directional couplers, provided the peak power of the limiter is not exceeded.
In general, the obtained sensitivity gains can be used to reduce the spin concentration or sample volumes allowing advanced Q-band EPR experiments (e.g.dipolar spectroscopy [23][24][25][26] with greatly increased sensitivity.

Figure 1 :
Figure1: Schematic of the microwave circuit within our Q-band cryoprobe containing a cryogenic LNA.The probehead is connected to the microwave bridge using two microwave ports.In practice, all microwave components are closely packed close to the resonator.

Figure 3 :
Figure 3: EDFS of the (a) Bruker DEER sample and (b) Cu(II) signal of the Cu(II)-nitroxide molecular ruler obtained using (a) 10 W SSA and (b) 150 W TWT. The EDFSs are normalized to the signal level.Arrows mark field positions, at which other EPR experiments were performed.The asterisk in (a) indicates E ′ centers present in the clear fused quartz sample tube.Experimental parameters: (a) τ = 6 ms, 1 average, t π = 200 ns, 6 K, and (b) τ = 12 ms, 4 averages, t π = 40 ns, 10 K.

Figure 4 :
Figure 4: SNR improvement vs. sample (cryoprobe) temperature measured using Hahn echo experiments with Bruker (10 W SSA, red circles) and homebuilt (150 W TWT, blue diamond) Q-band spectrometers.Solid curve shows the calculated SNR improvement (10 dB input attenuation, T C in ≥ 30 K) based on the measured microwave losses of our cryoprobe and Bruker setups.Pink area indicates the region of theoretical SNR enhancement bounded by the dashed curves representing the cases of the full (∞ dB input attenuation, T C in = T S ) and no suppression (0 dB, T C in = T U in = 294 K) of the input thermal noise.The gray region marks SNR improvement less than one.The error bar is about the size of the data point.

Figure 5 :
Figure 5: Normalized 1 H Mims ENDOR spectrum of the Bruker DEER sample obtained at 10 K with and without the cryoprobe with the corresponding voltage SNR improvements of 5.7×.Experimental parameters: τ = 3 ms, 1 average, t π = 200 ns (10 W SSA), t rf = 8 ms.

Figure S1 :Figure S2 :Figure S3 :
Figure S1: Photo of our Q-band cryoprobe based on the Bruker EN5107D2 Q-band microwave resonator.The LNA is thermalized to the temperature of the sample via a copper bracket extending below the resonator to the bottom of the cryostat.
revealing the SNR improvement factor of 5.7×, which is in a good agreement with the Hahn echo experiments.This indicates a full compatibility of our Q-band cryoprobe with the pulsed ENDOR experiments.