arXiv subjects

Jeong Hyun Shim

Publications and source records attributed to Jeong Hyun Shim.

10 recordsLinked to original sources

arXiv2025

Microwave power and chamber pressure studies for single-crystalline diamond film growth using microwave plasma CVD

Single-crystalline diamond (SCD) films possess exceptional thermal, chemical, and optical properties, making them ideal for advanced applications. However, achieving uniform film quality via microwave plasma chemical vapor deposition (MPCVD) remains challenging due to spatial variations in plasma characteristics. This study systematically examines the influence of microwave power and chamber pressure on the growth of SCD films using CH4/H2 gas mixtures. Under optimized conditions (3,900 W, 120 Torr), the films exhibit low surface roughness (~2.0 nm), a sharp sp3 Raman peak at 1,332.2 cm-1, and no detectable C-H related features, indicating high crystalline purity. Cross-sectional TEM analysis confirms a uniform (100)-oriented single-crystal structure across the entire sample. These findings advance the understanding of the interplay between deposition parameters and film quality, and establish a more robust foundation for optimizing MPCVD processes in large-area, high-purity diamond fabrication.

cond-mat.mtrl-sci↗

arXiv2025

A fiber integrated N-V diamond magnetometer compatible with commercial endoscopic systems

Nitrogen-vacancy (N-V) center in diamond provides a robust, solid-state platform for magnetic field measurements at room temperature. To harness its potential in inspecting inaccessible regions, here we present a compact endoscopic configuration of an N-V diamond-based magnetometer. The endoscopic magnetometer was developed by integrating a large-core optical fiber with a bulk N-V diamond for laser excitation and photoluminescence (PL) collection. The diamond and fiber were specially shaped to enhance PL collection through the fiber. Additionally, a 3D-printed endoscope head was employed to facilitate alignment of the bias magnetic field along the N-V axis. A magnetic field sensitivity of approximately 3 nT/Hz$^{1/2}$ was achieved by using cw-magnetometry measurements. The endoscope diameter was restricted to 10 mm to match the dimensions of most commercial endoscopes. The magnetic field non-uniformity caused by the small separation between the diamond and the magnet in the endoscope head limited the overall sensitivity. It could be further improved to 0.85 nT/Hz$^{1/2}$ by using a magnet placed at a sufficient distance outside the endoscope head. Our endoscopic design is mechanically stable and provides additional opportunities for integrating other functionalities into the probe head as needed.

physics.ins-det↗

arXiv2024

Achieving 5 % $^{13}$C nuclear spin hyperpolarization in high-purity diamond at room temperature and low field

Optically polarizable nitrogen-vacancy (NV) center in diamond enables the hyperpolarization of $^{13}$C nuclear spins at low magnetic field and room temperature. However, achieving a high level of polarization comparable to conventional dynamic nuclear polarization has remained challenging. Here we demonstrate that, at below 10 mT, a $^{13}$C polarization of 5 % can be obtained, equivalent to an enhancement ratio over $7 \times 10^6$. We used high-purity diamond with a low initial nitrogen concentration ($<$ 1 ppm), which also results in a long storage time exceeding 100 minutes. By aligning the magnetic field along [100], the number of NV spins participating in polarization transfer increases fourfold. We conducted a comprehensive optimization of field intensity and microwave (MW) frequency-sweep parameters for this field orientation. The optimum MW sweep width suggests that polarization transfer occurs primarily to bulk $^{13}$C spins through the integrated solid effect followed by nuclear spin diffusion.

quant-ph↗

arXiv2023

Frequency limits of sequential readout for sensing AC magnetic fields using nitrogen-vacancy centers in diamond

The nitrogen-vacancy (NV) centers in diamond have ability to sense alternating-current (AC) magnetic fields with high spatial resolution. However, the frequency range of AC sensing protocols based on dynamical decoupling (DD) sequences has not been thoroughly explored experimentally. In this work, we aimed to determine the sensitivity of ac magnetic field as a function of frequency using sequential readout method. The upper limit at high frequency is clearly determined by Rabi frequency, in line with the expected effect of finite DD-pulse width. In contrast, the lower frequency limit is primarily governed by the duration of optical repolarization rather than the decoherence time (T$_2$) of NV spins. This becomes particularly crucial when the repetition (dwell) time of the sequential readout is fixed to maintain the acquisition bandwidth. The equation we provide successfully describes the tendency in the frequency dependence. In addition, at the near-optimal frequency of 1 MHz, we reached a maximum sensitivity of 229 pT/$\sqrt{\mathrm{Hz}}$ by employing the XY4-(4) DD sequence.

quant-ph↗

arXiv2023

Quantum diamond microscopy with optimized magnetic field sensitivity and sub-ms temporal resolution

Quantum diamond magnetometers using lock-in detection have successfully detected weak bio-magnetic fields from neurons, a live mammalian muscle, and a live mouse heart. This opens up the possibility of quantum diamond magnetometers visualizing microscopic distributions of the bio-magnetic fields. Here, we demonstrate a lock-in-based wide-field quantum diamond microscopy, achieving a mean volume-normalized per pixel sensitivity of 43.9 $\mathrm{nTμm^{1.5}/Hz^{0.5}}$. We optimize the sensitivity by implementing a double resonance with hyperfine driving and magnetic field alignment along the $<$001$>$ orientation of the diamond. Additionally, we show that sub-ms temporal resolution ($\sim$ 0.4 ms) can be achieved while keeping the per-pixel sensitivity at a few tens of nanotesla per second using quantum diamond microscopy. This lock-in-based diamond quantum microscopy could be a step forward in mapping functional activity in neuronal networks in micrometer spatial resolution.

quant-ph↗

arXiv2021

Multiplexed sensing of magnetic field and temperature in real time using a nitrogen vacancy spin ensemble in diamond

Nitrogen-Vacancy (NV) spin in diamond is a versatile quantum sensor, being able to measure physical quantities such as magnetic field, electric field, temperature, and pressure. In the present work, we demonstrate a multiplexed sensing of magnetic field and temperature. The dual frequency driving technique we employ here is based on frequency-division multiplexing, which enables sensing both measurables in real time. The pair of NV resonance frequencies for dual frequency driving must be selected to avoid coherent population trapping of NV spin states. With an enhanced optical collection efficiency higher than 50 $\%$ and a type 1b diamond crystal with natural abundance $^{13}$C spins, we achieve sensitivities of about 70 pT/$\sqrt{\mathrm{Hz}}$ and 25 $μ$K/$\sqrt{\mathrm{Hz}}$ simultaneously. A high isolation factor of 34 dB in NV thermometry signal against magnetic field was obtained, and we provide a theoretical description for the isolation factor. This work paves the way for extending the application of NV quantum diamond sensors into more demanding conditions.

quant-ph↗

arXiv2021

Optical dynamic nuclear polarization of $^{13}$C spins in diamond at a low field with multi-tone microwave irradiation

Most of dynamic nuclear polarization (DNP) has been requiring helium cryogenics and strong magnetic fields for a high degree of polarization. In this work, we instead demonstrate an optical hyperpolarization of naturally abundant $^{13}$C nuclei in a diamond crystal at a low magnetic field and an ambient temperature. It exploits continuous irradiations of pump laser for polarizing electron spins of nitrogen vacancy centers and microwave for transferring the induced polarization to $^{13}$C nuclear spins. Triplet structures corresponding to $^{14}$N hyperfine splitting were clearly observed in the spectrum of $^{13}$C polarization. The powers of microwave irradiation and pump laser were optimized. By simultaneously irradiating three microwave frequencies matching to the peaks of the triplet, we achieved a $^{13}$C bulk polarization of 0.113 %, leading to an enhancement of about a factor of 90,000 over the thermal polarization at 17.6 mT. We believe that the multi-tone irradiation can be universally adopted to further enhance the $^{13}$C polarization at a low magnetic field.

quant-ph↗

arXiv2019

Dynamic nuclear polarisation of liquids at one microtesla using circularly polarised RF with application to millimetre resolution MRI

Magnetic resonance imaging in ultra-low fields is often limited by mediocre signal-to-noise ratio hindering a higher resolution. Overhauser dynamic nuclear polarisation (O-DNP) using nitroxide radicals has been an efficient solution for enhancing the thermal nuclear polarisation. However, the concurrence of positive and negative polarisation enhancements arises in ultra-low fields resulting in a significantly reduced net enhancement, making O-DNP far less attractive. Here, we address this issue by applying circularly polarised RF. O-DNP with circularly polarised RF renders a considerably improved enhancement factor of around 150,000 at 1.2 microtesla. A birdcage coil was adopted into a ultra-low field MRI system to generate the circularly polarised RF field homogeneously over a large volume. We acquired an MR image of a nitroxide radical solution with an average in-plane resolution of 1 mm. De-noising through compressive sensing further improved the image quality.

physics.app-ph↗

arXiv2013

Experimental implementation of assisted quantum adiabatic passage in a single spin

Quantum adiabatic passages can be greatly accelerated by a suitable control field, called a counter-diabatic field, which varies during the scan through resonance. Here, we implement this technique on the electron spin of a single nitrogen-vacancy center in diamond. We demonstrate two versions of this scheme. The first follows closely the procedure originally proposed by Demirplak and Rice (J. Phys. Chem. A 107, 9937 (2003)). In the second scheme, we use a control field whose amplitude is constant, but its phase varies with time. This version, which we call the rapid-scan approach, allows an even faster passage through resonance and therefore makes it applicable also for systems with shorter decoherence times.

quant-ph↗

arXiv2013

High precision nano scale temperature sensing using single defects in diamond

Measuring local temperature with a spatial resolution on the order of a few nanometers has a wide range of applications from semiconductor industry over material to life sciences. When combined with precision temperature measurement it promises to give excess to small temperature changes caused e.g. by chemical reactions or biochemical processes. However, nanoscale temperature measurements and precision have excluded each other so far owing to the physical processes used for temperature measurement of limited stability of nanoscale probes. Here we experimentally demonstrate a novel nanoscale temperature sensing technique based on single atomic defects in diamonds. Sensor sizes range from millimeter down to a few tens of nanometers. Utilizing the sensitivity of the optically accessible electron spin level structure to temperature changes we achieve a temperature noise floor of 5 mK Hz$^{-1/2}$ for single defects in bulk sensors. Using doped nanodiamonds as sensors yields temperature measurement with 130 mK Hz$^{-1/2}$ noise floor and accuracies down to 1 mK at length scales of a few ten nanometers. The high sensitivity to temperature changes together with excellent spatial resolution combined with outstanding sensor stability allows for nanoscale precision temperature determination enough to measure chemical processes of few or single molecules by their reaction heat even in heterogeneous environments like cells.

quant-ph↗