Laser Pulses for Studying Photoactive Spin Centers with EPR

At a Glance

Metadata	Details
Publication Date	2025-03-28
Journal	Micromachines
Authors	G. V. Mamin, Ekaterina Dmitrieva, Fadis F. Murzakhanov, Margarita A. Sadovnikova, S. S. Nagalyuk
Institutions	Kazan Federal University, Ioffe Institute
Citations	1
Analysis	Full AI Review Included

Executive Summary

This research details the development and validation of a low-cost, high-precision pulse sequence programmer for studying photoactive solid-state spin centers (potential qubits) using Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) spectroscopy.

Core Achievement: A custom pulse sequence programmer, built around a 32-bit STM32F373 microcontroller, enables synchronized, nanosecond-scale control of optical, microwave (MW), and radio frequency (RF) pulses.
Value Proposition: The system facilitates high-fidelity spin initialization by converting continuous laser radiation (405-1064 nm) into pulsed mode, eliminating detrimental effects of continuous wave (CW) excitation.
Performance Improvement (SiC NV Centers): Switching from CW to pulsed laser operation increased the phase coherence time (T₂) of NV centers in 6H-SiC by 38.4% at 50 K (from 53.4 µs to 73.9 µs).
Decoherence Mitigation: Pulsed operation avoids local sample heating (heating reduced from 40 K in CW mode to less than 3 K in pulsed mode) and suppresses charge noise fluctuations that degrade quantum coherence.
Materials Validated: The methodology was successfully tested on key wide-band-gap semiconductor matrices: isotopically enriched 6H-²⁸SiC, diamond (NV centers), and hexagonal boron nitride (hBN, V_B^- centers).
ENDOR Fidelity: Pulsed laser detection successfully preserved weak electron-nuclear coupling signals (Hyperfine Interaction, HFI) in hBN, which were completely suppressed under CW optical irradiation.

Technical Specifications

Parameter	Value	Unit	Context
Microcontroller Core	STM32F373 (ARM Cortex-M3)	N/A	Programmer core operating at 72 MHz.
EPR Frequency	94 (W-band)	GHz	Bruker Elexsys E680 spectrometer operation frequency.
MW Pi/2 Pulse Duration	40	ns	Standard pulse duration for ESE sequence.
MW Pulse Excitation Spectrum	~0.9	mT	Achieved using short MW pulses (36-44 ns).
ENDOR RF Power	150	W	Power of the radio frequency generator.
ENDOR RF Pulse Width (Pi)	18	µs	Duration of the RF pulse (Mims sequence).
Laser Wavelength Range	405 to 1064	nm	Range of semiconductor and solid-state lasers tested.
SiC T₂ (50 K, CW Laser)	53.4 ± 0.2	µs	Phase coherence time under continuous irradiation.
SiC T₂ (50 K, Pulsed Laser)	73.9 ± 0.4	µs	Phase coherence time under pulsed irradiation.
SiC T₂ Improvement	38.4	%	Improvement achieved by using pulsed mode at 50 K.
SiC Local Heating (CW Mode)	~40	K	Estimated local heating of the crystal at 500 mW CW power.
SiC Local Heating (Pulsed Mode)	<3	K	Estimated heating during 9 ms pulse duration (low duty cycle).
Programmer Max Repetition Time	2.3 x 10⁹	ms	Maximum Sequence Repetition Time (SRT).
Programmer Max Pulse Duration	2.3 x 10⁹	µs	Maximum duration for P1 and P2 pulses.
CNI 1064 nm Laser Delay	9000 ± 10	µs	Turn-On Delay Time for the 2 W 1064 nm laser.

Key Methodologies

The experimental approach combines custom electronics for precise pulse timing with high-frequency magnetic resonance spectroscopy (EPR/ENDOR) to characterize spin defects.

Pulse Sequence Programmer Implementation:
- Hardware: Based on the STM32F373 microcontroller, utilizing four independent timers (three 32-bit, one 16-bit) to generate stable pulses independent of CPU load.
- Pulse Generation: TTL pulses (3.3 V or 5 V) are shaped using x-OR and dual OR gate circuits.
- Synchronization: A synchronizing pulse is generated and shifted relative to the laser pulse trailing edge to ensure MW/RF measurements occur in the absence of light exposure.
- Control Interface: Parameters are configured manually via an encoder/OLED screen or remotely via a virtual COM port (USB interface).
Sample Preparation (Defect Creation):
- 6H-²⁸SiC: Grown by PVT, irradiated with 2 MeV electrons (4 x 10¹⁸ cm^-2), followed by annealing at 900 °C in an argon atmosphere (creating NV centers and divacancies).
- Diamond: HPHT synthesis, irradiated with 2 MeV electrons (10¹⁸ cm^-2), followed by annealing at 800 °C in a hydrogen atmosphere (creating NV centers).
- hBN: Irradiated at room temperature with 2 MeV electrons (6 x 10¹⁸ cm^-2) (creating boron vacancies V_B^-).
Spectroscopic Measurement:
- EPR: Performed at 94 GHz (W-band) using a pulsed Hahn sequence (π/2 - τ - π - τ - ESE) to measure the Electron Spin Echo (ESE) amplitude and transverse relaxation time (T₂).
- ENDOR: Performed using the Mims pulse sequence (π_MW/2 - τ - π_RF - π_MW/2 - τ - ESE) with a 150 W RF source to probe weak electron-nuclear interactions (HFI and QI).
- Pulsed Optical Excitation: Laser pulses (405-1064 nm) are synchronized with the magnetic resonance sequences, ensuring the spin initialization occurs, but the subsequent T₂ or ENDOR measurement is performed in the “dark” state to minimize decoherence.

Commercial Applications

The ability to precisely control spin initialization and measure coherence properties in wide-band-gap semiconductors is foundational for several emerging quantum technologies.

Application Area	Relevance to Research Findings
Quantum Computing (Qubits)	Spin defects (NV in diamond, divacancies in SiC, V_B^- in hBN) are leading solid-state qubit candidates. The improved T₂ times achieved via pulsed excitation are critical for reducing gate errors and increasing computational fidelity.
Quantum Memory	The successful investigation of electron-nuclear coupling (ENDOR) in pulsed mode is essential for developing quantum registers utilizing long-lived nuclear spins (nuclear qubits) for data storage.
Quantum Sensing	Spin defects are used as highly sensitive sensors for magnetic fields, temperature, and strain. Precise optical control allows for optimized initialization and readout protocols, enhancing sensor sensitivity and dynamic range.
Secure Communication	The technology supports the development of spin-based photonic quantum interfaces, crucial for linking solid-state qubits into quantum networks for secure communication protocols.
Semiconductor Manufacturing	Provides a cost-effective, high-precision tool for characterizing defects created by irradiation or implantation in SiC and hBN, aiding in quality control and optimization of material processing for quantum devices.

View Original Abstract

Quantum technologies are currently being explored for various applications, including computing, secure communication, and sensor technology. A critical aspect of achieving high-fidelity spin manipulations in quantum devices is the controlled optical initialization of electron spins. This paper introduces a low-cost programming scheme based on a 32-bit STM32F373 microcontroller, aimed at facilitating high-precision measurements of optically active solid-state spin centers within semiconductor crystals (SiC, hBN, and diamond) utilizing a multi-pulse sequence. The effective shaping of short optical pulses across semiconductor and solid-state lasers, covering the visible to near-infrared range (405-1064 nm), has been validated through photoinduced electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopies. The application of pulsed laser irradiation influences the EPR relaxation parameters associated with spin centers, which are crucial for advancements in quantum computing. The presented experimental approach facilitates the investigation of weak electron-nuclear interactions in crystals, a key factor in the development of quantum memory utilizing nuclear qubits.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi16040396

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