Magnetically tunable telecom emission from Er3+ ions in layered WS2

This study demonstrates that magnetic fields can effectively tune the intensity, lifetime, and polarization of Er3+ telecom emission in layered WS2 by inducing Zeeman mixing of crystal-field sublevels and leveraging the host's anisotropic photonic environment.

The Gist

The Big Picture: Tuning a Radio with a Magnet

Imagine you have a tiny, super-precise radio station inside a piece of material. This station broadcasts a signal at a very specific color of light (infrared, which we can't see but is used for internet cables). This "radio station" is actually a single atom of Erbium (a rare-earth metal) stuck inside a flake of Tungsten Disulfide (WS2), which is a material as thin as a sheet of paper.

Usually, once you build a radio, you can't easily change how loud it is or which direction it points without rebuilding the whole thing. But in this study, scientists discovered something magical: They can change the volume and the direction of this light signal just by waving a magnet near it.

The Cast of Characters

1. The Erbium Ion (The Singer): Think of the Erbium atom as a singer holding a note. It naturally sings a note at 1.54 micrometers (the "C-band"). This is the perfect frequency for sending data through fiber-optic cables because it travels the furthest without getting lost.
2. The WS2 Flake (The Stage): The singer is standing on a stage made of WS2. This stage is special because it's incredibly thin (2D material) and has a very quiet environment (few noisy atoms), which helps the singer stay in tune.
3. The Magnet (The Conductor): The scientists use a small magnet to "conduct" the singer.

What Happened in the Experiment?

The scientists shined a laser on the Erbium atoms to make them sing (emit light). Then, they applied a magnetic field. Here is what they found:

• The "Dimming" Effect: When they held the magnet pointing straight down at the flake (like a lighthouse beam hitting the stage), the singer suddenly got much quieter. The light intensity dropped by more than half.
• The "Slow Motion" Effect: While the singer got quieter, they also started holding their note for much longer. The light didn't fade away as fast; the "lifetime" of the glow stretched out.
• The "Spinning" Effect: The direction the light was pointing changed. Imagine a flashlight beam that suddenly swivels to point in a completely different direction just because a magnet is nearby.

Crucially: If they held the magnet sideways (parallel to the flake), nothing happened. The effect only worked when the magnet had a "vertical" component.

Why Does This Happen? (The Analogy)

To understand the "why," imagine the Erbium atom isn't just a simple ball; it's a complex spinning top with two different ways it can spin (let's call them "Spin A" and "Spin B").

1. The Tug-of-War: Normally, these two spins are distinct. But in this specific material, they are very close to each other in energy, like two runners standing almost side-by-side at the starting line.
2. The Magnetic Nudge: When the magnet comes in (especially from the top), it acts like a gentle nudge. Because the runners are so close, the nudge causes them to mix. They start to blur together.
3. The Result: When they mix, the "rules" of how they emit light change.
   • Volume: The new mixed state is less efficient at shooting out light, so it gets dimmer.
   • Direction: The mix changes the shape of the "antenna" inside the atom, causing the light to point in a new direction.
   • Time: Because the atom is now "hesitating" between states, it takes longer to release the energy, making the glow last longer.

The Role of the Thin Stage (The Photonic Part)

The paper also found that the thickness of the WS2 flake matters.

• Thin Flakes (200 nm): The "stage" is so thin that it acts like a mirror box. The light bounces around inside the thin layer. When the magnet changes the direction of the light beam (the antenna), the way it bounces inside this thin box changes dramatically. This amplifies the effect.
• Thick Flakes (1 micron): If the stage is too thick, it acts like a normal block of glass. The light doesn't bounce around as much, and the magnet's ability to change the volume and direction is much weaker.

Why Should We Care?

This discovery is a big deal for the future of technology:

1. Quantum Internet: We need ways to control single particles of light (photons) to send secret messages. Being able to turn a light source on/off or change its direction with a simple magnet is a huge step toward building quantum computers and secure networks.
2. No Moving Parts: Usually, to change light direction, you need mechanical mirrors that move. Here, a magnet does it instantly with no moving parts.
3. Magnetic Sensors: Since the light changes based on the magnetic field, we could use these materials as super-sensitive "eyes" to detect magnetic fields in other materials, essentially creating a new type of magnetometer.

Summary

Scientists found a way to make a tiny light source inside a super-thin material dance to the tune of a magnet. By waving a magnet over it, they can make the light dimmer, longer-lasting, and change its direction. This happens because the magnet mixes the internal "spins" of the atom, and the thinness of the material amplifies the effect. It's a new tool for building the quantum internet of the future.

Technical Summary

1. Problem Statement

Erbium ions (Er³⁺) are critical for quantum communication and spin-photon interfaces due to their optical transition at 1.54 μm (telecom C-band), which aligns with the low-loss window of silica fibers. However, optimizing Er³⁺ for quantum applications faces two main challenges:

1. Host Material Selection: Finding a host that offers low nuclear spin noise (to preserve coherence) while supporting high-quality emission.
2. Integration: Decoupling emitter placement, host optimization, and photonic integration.

While bulk hosts (like garnets) offer stability, they lack the ability to tightly couple emitters to engineered photonic structures. Conversely, 2D materials like Tungsten Disulfide (WS₂) offer a low nuclear-spin environment and intimate near-field coupling but present a highly anisotropic optical environment. The specific mechanism by which magnetic fields affect Er³⁺ emission in these anisotropic 2D hosts, and how this can be leveraged for control, was previously unexplored.

2. Methodology

The researchers employed a combination of experimental characterization and first-principles theoretical modeling:

• Sample Preparation: High-purity WS₂ crystals were mechanically exfoliated onto SiO₂ substrates. Erbium ions were introduced via ion implantation (75 keV, $10^{14}$ ions/cm²) followed by thermal annealing (400°C) to activate the emitters.
• Experimental Setup:
  • Optical: A custom confocal microscope operating at room temperature with 980 nm excitation (pumping the $^4I_{11/2}$ manifold) and detection of photoluminescence (PL) in the 1500–1600 nm range using a superconducting nanowire single-photon detector (SNSPD).
  • Magnetic Control: A permanent magnet provided variable magnetic fields ($B < 0.2$ T) at adjustable angles ($\theta$) relative to the flake surface normal.
  • Measurements: Time-resolved PL (lifetime), polarization-resolved emission, and spectral mapping were performed under varying field orientations and strengths.
• Theoretical Modeling:
  • DFT & Quantum Embedding: Density Functional Theory (DFT) was used to model a substitutional Er³⁺ defect (Er$_W$) in monolayer WS₂.
  • Effective Hamiltonian: A semi-empirical model was constructed using Wannierization of Er 4f states, incorporating Crystal Field (CF) splitting, Spin-Orbit Coupling (SOC), and Zeeman interactions.
  • Photonic Modeling: Numerical estimates of the Local Density of Optical States (LDOS) were calculated for WS₂ slabs of varying thickness to account for guided modes and dielectric anisotropy.

3. Key Contributions

• Discovery of Anisotropic Magneto-Photonic Response: The study demonstrates that Er³⁺ emission in WS₂ is highly sensitive to the orientation of the magnetic field, a phenomenon not observed in isotropic bulk hosts.
• Mechanism Elucidation: The paper identifies that the observed effects arise from a complex interplay between:
  1. Atomic Physics: Zeeman-induced mixing of near-degenerate Crystal Field (CF) sublevels, which alters the magnitude and orientation of Transition Dipole Moments (TDMs).
  2. Photonic Physics: The reoriented TDMs couple differently to the anisotropic LDOS of the finite-thickness WS₂ flake, modifying radiative rates.
• Quantitative Characterization: The authors provide a rigorous analysis of how magnetic fields suppress radiative rates and rotate emission dipoles, distinguishing these effects from simple spectral shifts.

4. Key Results

Experimental Observations

• Field-Dependent Dimming & Lifetime Extension:
  • Applying a magnetic field normal to the flake ($\theta = 0^\circ$) or at 45° caused a dramatic reduction in PL intensity (dimming to <33% of zero-field values) and a significant increase in excited-state lifetime (from ~4.5 ms to ~11 ms).
  • In-plane fields ($\theta = 90^\circ$) produced negligible changes.
  • The lifetime increase was attributed to a suppression of the radiative rate (by a factor of 2.7), not an increase in non-radiative decay. Quantum yield remained high (93% at zero field, dropping to ~83% at 0.2 T).
• Polarization Rotation:
  • The emission polarization axis rotated significantly under out-of-plane fields.
  • At 1520 nm, the axis rotated by ~30°.
  • At 1540 nm, the axis rotated by up to ~90°, with the direction of rotation reversing compared to the 1520 nm line.
• Thickness Dependence:
  • In thin flakes (~200 nm), the magnetic effects were pronounced.
  • In thicker flakes (~1 µm), the field-induced changes in intensity and polarization were substantially diminished. This confirms that the finite-thickness photonic environment (LDOS) plays a secondary but crucial role in amplifying the atomic effects.

Theoretical Insights

• Zeeman Mixing: The DFT-based model revealed that several Kramers doublets in the ground ($^4I_{15/2}$) and excited ($^4I_{13/2}$) manifolds are separated by energies comparable to the Zeeman splitting.
• Dipole Reorientation: Moderate magnetic fields mix these near-degenerate states. Because the states possess non-parallel Transition Dipole Moments (TDMs), this mixing effectively "rotates" the net TDM of the optical transition.
• Anisotropy: The sensitivity depends on the Magneto-Crystalline Anisotropy (MCA). Transitions involving "easy-plane" doublets are most susceptible to out-of-plane field mixing, while "easy-axis" doublets are less sensitive.
• Photonic Coupling: The model confirmed that the reoriented dipole couples differently to the guided modes of the WS₂ slab, explaining the thickness-dependent modulation of the emission rate.

5. Significance and Outlook

• New Control Paradigm: This work establishes a method to dynamically tune telecom-band emission (intensity, lifetime, and polarization) using modest magnetic fields (< 0.2 T) in a 2D material platform.
• Quantum Applications: The ability to control spin-photon interfaces and emission directionality without complex cavity engineering makes Er:WS₂ a promising platform for:
  • Tunable quantum light sources.
  • Integrated quantum memories.
  • All-optical magnetometry (using Er³⁺ as a probe for local magnetic textures in 2D magnets).
• Strain vs. Field: The authors note that strain could produce similar effects, suggesting future work should disentangle magnetic and strain engineering to further optimize 2D quantum emitters.

In summary, the paper bridges the gap between atomic-scale crystal field physics and mesoscopic photonic engineering, demonstrating that layered WS₂ provides a unique, tunable host for rare-earth ions where magnetic fields can act as a precise control knob for quantum optical properties.