Optical–Microwave Pump–Probe Studies of Electronic Properties in Novel Materials
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-10-09 |
| Journal | physica status solidi (b) |
| Authors | Sándor Kollarics, András Bojtor, Kristóf Koltai, Bence G. Márkus, K. Holczer |
| Institutions | University of California, Los Angeles, HUN-REN Centre for Energy Research |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”The research details the development and characterization of two novel, broadband pump-probe spectrometers utilizing Coplanar Waveguides (CPW) instead of traditional microwave resonators, offering significant engineering advantages.
- Broadband Operation: CPWs enable frequency-swept experiments and temperature-independent operation, overcoming the narrow bandwidth (typically 100 kHz-1 MHz) and frequency drift issues associated with high-Q resonators.
- Enhanced Optical Access: The CPW design facilitates easy optical illumination and collection, crucial for Optically Detected Magnetic Resonance (ODMR) and time-resolved Photoconductivity (µ-PCD) measurements, which is difficult with enclosed cavities.
- High Efficiency: The CPW demonstrated an excellent microwave power to magnetic field conversion ratio (0.88 * 10-8 T2/W), performing equivalently to a classical cavity with a Quality factor (Q) of 4000.
- Dual Spectrometer Development: Two instruments were successfully built: a broadband ODMR spectrometer (tested on NV centers in diamond) and a fast µ-PCD decay instrument (tested on P-doped silicon).
- Cryogenic Capability: The CPW structure was successfully integrated into a closed-cycle cryostat, enabling measurements down to 77 K (liquid nitrogen temperature) without complex frequency tuning.
- Material Dynamics Study: The µ-PCD setup successfully measured transient charge carrier decay in silicon, showing a decrease in carrier lifetime upon cooling, consistent with Shockley-Read-Hall theory.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| CPW Power Conversion | 0.88 * 10-8 | T2/W | Microwave power to magnetic field conversion factor |
| Equivalent Q-factor (CPW) | 4000 | Dimensionless | Performance equivalent to a classical resonator |
| Cylindrical Cavity Conversion | 2.17 * 10-12 | T2/W | Comparison to a standard TE011 type cavity |
| ODMR CW Laser Wavelength | 532 | nm | Coherent Verdi 5G (Nd:YAG) |
| ODMR MW Amplifier Gain | 40 | dB | Kuhne KuPa 270330-10A |
| Maximum External B-Field | 1.2 | T | Electromagnet limit for ODMR |
| µ-PCD Pulsed Laser Wavelength | 527 | nm | Coherent Evolution-15 (Nd:YLF) |
| µ-PCD Pulse Energy | 150 | µJ | Excitation energy |
| µ-PCD Pulse Width | 1.7 | µs | Excitation pulse duration |
| µ-PCD Repetition Rate | 1 | kHz | Laser repetition rate |
| µ-PCD Time Resolution Limit | 100 kHz - 1 MHz | Bandwidth | Limit imposed by resonator bandwidth (CPW overcomes this) |
| Cryostat Vacuum Level | 10-6 | mbar | Achievable vacuum for cryogenic measurements |
| Lowest Measurement Temperature | 77 | K | Liquid nitrogen temperature (LN2) |
| Silicon Sample Resistivity | 0.528 | Ω-cm | Phosphorus-doped silicon wafer |
| Silicon Sample Thickness | 200 | µm | Thickness of the wafer used for µ-PCD |
Key Methodologies
Section titled “Key Methodologies”The experimental approach centered on integrating broadband CPWs into two distinct pump-probe setups, enabling high-resolution measurements of spin and charge dynamics.
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Coplanar Waveguide (CPW) Design:
- CPWs consist of metallic lines on a high dielectric constant substrate, confining the microwave fields in a quasi-TEM mode.
- The specific CPW used had gaps of 250 µm, separated by 1400 µm, with a total width of 6 mm.
- The CPW was terminated with a 50 Ω load in the ODMR setup.
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ODMR Spectrometer Operation (Spin Dynamics):
- Pump: Continuous Wave (CW) 532 nm laser illuminates the sample (NV centers in diamond).
- Probe/Excitation: Microwave frequency is swept (2.7-3.1 GHz range shown) and chopped by a TTL signal from the lock-in amplifier.
- Detection: Luminescence (PL) is collected (680 nm detection window) and measured by a photomultiplier tube (PMT) connected to a lock-in amplifier (AC component is the ODMR signal).
- Demonstration: ODMR maps were generated by sweeping both MW frequency and optical wavelength, resolving spin transitions and degeneracy under a 5 mT magnetic field.
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Microwave Detected Photoconductivity (µ-PCD) Operation (Charge Dynamics):
- Pump: Pulsed 527 nm laser (1.7 µs pulse width) generates non-equilibrium charge carriers in the silicon sample.
- Probe: A continuous microwave field (MW) is directed toward the CPW via a magic tee and directional coupler bridge setup.
- Detection: Changes in sample conductivity due to photo-generated carriers alter the reflected MW intensity. This reflected signal is downconverted using an IQ mixer (I and Q components) and digitized by a 200 MHz bandwidth oscilloscope, triggered by the laser pulse.
- Measurement: The transient decay of the I and Q signals is measured to determine charge carrier lifetime on the nanosecond timescale.
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Diamond Sample Preparation (NV Centers):
- Starting Material: Type-Ib HPHT diamond (3 x 3 x 0.3 mm) with less than 200 ppm substitutional nitrogen.
- Vacancy Creation: Neutron irradiation (8 hours at 100 kW power, fluence approx. 1017 1/cm2).
- NV Formation: Annealing at 800-1000 °C under dynamic vacuum (10-5 mbar) to facilitate vacancy diffusion and NV center creation.
Commercial Applications
Section titled “Commercial Applications”The developed instrumentation and methodologies are highly relevant to several high-technology sectors requiring precise control and measurement of spin and charge carrier dynamics.
- Quantum Computing and Sensing:
- ODMR on NV centers is the standard technique for characterizing solid-state spin qubits. The broadband CPW setup allows for faster, more flexible qubit control and readout.
- Semiconductor Manufacturing and Quality Control:
- µ-PCD is essential for contactless determination of impurity concentration and charge carrier lifetime in silicon wafers (e.g., P-doped silicon tested here) and other non-silicon semiconductors (e.g., CdTe).
- Photovoltaics and Energy Materials:
- The fast µ-PCD setup is ideal for studying charge carrier dynamics and recombination mechanisms in novel materials, specifically mentioned for methylammonium halide perovskites.
- RF and Microwave Device Miniaturization:
- The successful integration of CPWs demonstrates a pathway for miniaturizing complex microwave circuitry, such as millimeter-scale circulators and surface mount device (SMD) integration, for compact sensor systems.
- Spintronics Research:
- The ability to study microwave-induced spin transitions with high energy resolution and high optical efficiency supports the development of spintronic devices and materials.
View Original Abstract
Combined microwave-optical pump-probe methods are emerging to study the quantum state of spin qubit centers and the charge dynamics in semiconductors. A major hindrance is the limited bandwidth of microwave irradiation/detection circuitry which can be overcome with the use of broadband coplanar waveguides (CPWs). The development and performance characterization of two spectrometers is presented as follows: an optically detected magnetic resonance spectrometer (ODMR) and a microwave‐detected photoconductivity measurement. In the first method, light serves as detection and microwaves excite the investigated medium, whereas in the second, the roles are interchanged. The performance is demonstrated by measuring ODMR maps on the nitrogen‐vacancy (NV) center in diamond and time‐resolved photoconductivity in p ‐doped silicon. The results demonstrate both an efficient coupling of the microwave irradiation to the samples as well as an excellent sensitivity for minute changes in sample conductivity.