Sub-micron spin-based magnetic field imaging with an organic light emitting diode

Abstract

Quantum sensing and imaging of magnetic fields has attracted broad interests due to its potential for high sensitivity and spatial resolution. Common systems used for quantum sensing require either optical excitation (e.g., nitrogen-vacancy centres in diamond, atomic vapor magnetometers), or cryogenic temperatures (e.g., SQUIDs, superconducting qubits), which pose challenges for chip-scale integration and commercial scalability. Here, we demonstrate an integrated organic light emitting diode (OLED) based solid-state sensor for magnetic field imaging, which employs spatially resolved magnetic resonance to provide a robust mapping of magnetic fields. By considering the monolithic OLED as an array of individual virtual sensors, we achieve sub-micron magnetic field mapping with field sensitivity of ~160 µT Hz^-1/2 µm^-2. Our work demonstrates a chip-scale OLED-based laser free magnetic field sensor and an approach to magnetic field mapping built on a commercially relevant and manufacturable technology.

Fig. 1. Device structure, experimental set-up, and EDMR characterization.

a Photograph of the integrated microwave resonator where an omega-shape resonator is integrated on the prepatterned ITO/glass substrate. The active area in the middle has a diameter of 80 µm, which is defined through photolithography and insulating layer deposition. The inset shows the photograph of an integrated OLED at current of I = 500 nA (corresponding current density of ~10 mA/cm²). b Sketch of the integrated device structure and the experimental measurement configuration, employed with an AC magnetic field B₁ created by the microwave resonator and a static magnetic field B₀ generated by an external electromagnet. c A conventional EDMR spectrum where the static magnetic field B₀ is swept with a fixed microwave frequency of 710 MHz. The spectrum is well described by the sum (black) of two Gaussian functions (red, blue), corresponding to the two hyperfine-field distributions (${\sigma }_1$ = 0.18(2), ${\sigma }_2$ = 0.94(2)) experienced by the electron and hole spins, respectively. σ₁ and σ₂ represent the standard deviation of the two Gaussian functions. d A frequency-swept EDMR spectrum where the microwave frequency is swept with a fixed magnetic field $B_0$ ≈ 25.2(5) mT via fixing the current in the electromagnet. The spectrum can be well fitted using two Gaussian functions with standard deviation of ${\sigma }_1$= 6.15(1) and ${\sigma }_2$ = 31.23(0), respectively. We note that the background noise caused by the frequency sweep is removed from the plots in d. More details are discussed in Supplementary Method 2. e Plot of the maximum-peak value of the magnetic field B₀ in the EDMR spectrums as a function of the applied microwave frequency. A linear fit (red line) of the data yields a gyromagnetic ratio $\gamma$ = 28.03 (±0.0024) GHz/T and a corresponding $g$-factor $g$ = 2.0026 (±0.00017).

Fig. 2. EDMR-based magnetic field sensing.

a Sketch of the experimental set-up (not to scale). A cylindrical magnet is located next to the device with the cylindrical axis of the resulting magnetic field aligned in the plane of the device substrate. 2D simulation of the spatial distribution of the decaying magnetic field strength generated by the cylindrical magnet in a region of 14.0 × 36.0 mm in the $x-y$ plane with a distance of $d$ = 10.0 mm from the magnet. The distance d corresponds to the half size of the device substrate width as the OLED is located at the center of the rectangular glass substrate (see Supplementary Fig. 7). In actual experiments, we initially set a tiny gap ($x_0$) between the substrate edge and the magnet at the starting position to avoid possible physical contact between them during the movement. The total distance between the OLED (yellow dot) and the magnet is $d$ +  $x_0$. The x and y coordinates represent the horizontal and vertical movement directions in the laboratory frame, respectively. The OLED here works as a point detector to measure the magnetic field strength generated by the magnet, and x₀ represents the starting position of the measurement. b Magnetic field detection as the device is stepped along the $x$-direction. The magnetic field strength is measured via the frequency-swept EDMR spectrum at each position, and the solid curve is the simulation with an estimated starting position of $x_0$~0.20 mm. c Magnetic field detection as the device is stepped along the y-direction with an estimated starting position of $x_0$~0.40 mm.

Fig. 3. Spatially resolved ODMR-based magnetic field mapping.

a Sketch of the set-up for spatially resolved ODMR. The inset shows the image of EL intensity captured by the sCMOS camera. The B field arrow represents the magnetic field gradient across the OLED along x-direction in the horizontal $x-y$ plane. b Scheme of pixel binning where $n$ × $n$ adjacent camera pixels are merged into one combined pixel called “super-pixel” via pixel binning process. The optical signal (EL intensity) of each super-pixel is the average of the signals of all the $n$ × $n$ individual camera pixels. c Double Gaussian fits of ODMR spectrums of two super-pixels with binning size $n$ = 3. Super-pixel 1 and super-pixel 2 corresponds to the super-pixel at position of (−63.4 µm, 0.0 µm) and (52.4 µm, 0.0 µm) in d, respectively. The solid circle dots label out the resonant peak position in the fit curves. d 2D spatial map of the resonance frequency ($f_{\mathrm{ODMR}}$) of the ODMR spectrums of 166 × 166 super-pixels with binning size $n$ = 3. The entire region contains 500 × 500 camera pixels, and the super-pixel size is about 0.91(5) × 0.91(5) µm ($n$ = 3). Weak EL signal is also observed outside the defined area of the OLED due to the high hole conductivity of the PEDOT:PSS thin film. This provides the ODMR spectrums across the entire region. e The most left figure ($n$ = 3) shows a zoom-in view of a sub-region (10 × 10 super-pixels) of the 2D map in d, which is marked by the yellow dash square in d. The $x-y$ coordinates in e are consistent with that in d. The rest four figures in e show the spatial map of the magnetic field in the same sub-region but with different binning sizes.

Fig. 4. Magnetic field gradient sensitivity.

The inset shows two virtual point sensors with given size of $w\times w$ and gap distance of $△x\left(△x\ge w\right)$. The dots at the left end of each curve represent the starting position where $△x=w$, indicating the spatial resolution limit of the field gradient. The figure is in log–log scale plot.