Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds

At a Glance

Metadata	Details
Publication Date	2021-12-29
Journal	Photonics
Authors	V. V. Kononenko, М. С. Комленок, P. A. Chizhov, V. V. Bukin, V. V. Bulgakova
Institutions	Institute of Radio-Engineering and Electronics, Prokhorov General Physics Institute
Citations	7
Analysis	Full AI Review Included

Executive Summary

This research investigates the performance of nitrogen-doped diamond substrates as large-aperture photoconductive antennas (PCAs) for efficient Terahertz (THz) generation, pumped by a 400 nm femtosecond laser.

Core Value Proposition: Nitrogen doping (N_s) in diamond acts as an effective defect level, enabling efficient single-photon excitation into the conduction band using readily available 400 nm pump lasers, circumventing the need for complex deep UV sources.
Efficiency Control: The optical absorption and subsequent THz generation efficiency are tightly correlated with the substitutional nitrogen concentration (N_s), which varied across three orders of magnitude (0.1 to 100 ppm).
Performance Metric: The key performance indicator, saturation fluence (F_sat), ranged dramatically from a highly efficient ~40 µJ/cm² (high-doped HPHT diamond) to ~12,000 µJ/cm² (low-doped HPHT diamond).
Maximum Output: A maximum THz pulse energy of ~200 pJ was measured at a relatively low bias field of 25 kV/cm, corresponding to a conversion efficiency of ~0.002%.
Future Potential: Leveraging diamond’s record dielectric strength (10 MV/cm), the conversion efficiency is theoretically projected to scale significantly, potentially exceeding 3% by increasing the applied bias field (e.g., up to 1 MV/cm).
Material Quality Impact: Polycrystalline CVD diamonds exhibited significantly higher optical losses due to Rayleigh scattering, reducing THz output compared to monocrystalline HPHT and CVD samples, even at similar N_s levels.

Technical Specifications

Parameter	Value	Unit	Context
Pump Wavelength	400	nm	Second Harmonic Generation (SHG)
Pump Pulse Duration	150	fs	Ti:sapphire laser system
Laser Repetition Rate	1	kHz
Maximum Pump Fluence	~7200	µJ/cm²	Unfocused beam center
Nitrogen Concentration (N_s) Range	0.1 to 100	ppm	Tested diamond substrates (HPHT, CVD)
Diamond Breakdown Field	10	MV/cm	Highest reported dielectric strength
Lowest Saturation Fluence (F_sat)	~40	µJ/cm²	Achieved with high-doped HPHT diamond
Highest Saturation Fluence (F_sat)	~12,000	µJ/cm²	Achieved with minimum N_s HPHT diamond
Maximum THz Energy (Measured)	~200	pJ	At 25 kV/cm bias field
Measured Conversion Efficiency (η)	~0.002	%	At 25 kV/cm bias field
Projected Conversion Efficiency (η)	>3	%	Projected at 1 MV/cm bias field
THz Emission Peak Frequency	0.2-0.3	THz	Low-frequency maximum of the spectrum
Diamond Bandgap	5.46	eV	Wide-bandgap semiconductor
Record Carrier Mobility (Diamond)	~4500	cm² V^-1 s^-1	At room temperature
Nitrogen Absorption Peak	4.6	eV	Used for N_s calculation (270 nm)

Key Methodologies

The experimental procedure focused on characterizing the material properties and measuring the THz output under pulsed bias and 400 nm excitation.

Material Selection: Nineteen diamond substrates were tested, including monocrystalline HPHT, monocrystalline CVD, and polycrystalline CVD, covering a substitutional nitrogen (N_s) range of 0.1 to 100 ppm.
Antenna Assembly: Samples were prepared as PCAs by gluing aluminum foil electrodes to the edges, creating a gap ranging from 0.5 to 2 mm.
Optical Pumping: A femtosecond Ti:sapphire laser (800 nm fundamental) was frequency-doubled using a BBO crystal to produce the 400 nm pump beam (1 mJ max pulse energy, 150 fs duration).
Bias Application: A pulsed bias voltage (up to 3 kV, 10 ns duration) was synchronized with the 1 kHz laser repetition rate. Static high voltage yielded no THz signal.
Material Characterization (N_s): Nitrogen concentration was determined primarily by UV-visible absorption spectroscopy, focusing on the 4.6 eV (270 nm) absorption peak, calibrated using Electron Spin Resonance (ESR) data.
Material Characterization (Scattering): Rayleigh scattering factors (S) were calculated from spectral data, confirming that scattering losses dominated in polycrystalline CVD diamonds.
THz Detection: THz radiation was collected using a telescope of two polytetrafluoroethylene (PTFE) lenses and measured using a Golay cell detector modulated at 15 Hz.
Performance Modeling: Experimental THz energy versus pump fluence data was fitted using a saturation model to extract the saturation fluence (F_sat), which was then correlated with N_s and sample thickness.

Commercial Applications

This technology is critical for developing robust, high-power THz sources, leveraging the superior thermal and electrical properties of diamond.

High-Power THz Spectroscopy: Creation of intense, quasi-half-cycle THz pulses required for nonlinear THz optics and high-field material studies.
Advanced THz Imaging Systems: Development of high-resolution, high-penetration imaging for non-destructive testing (NDT) in manufacturing and quality control.
Security and Screening: Potential use in active THz scanners for concealed object detection, benefiting from the high reliability and power handling of diamond substrates.
High-Bandwidth Communications: Enabling high-speed, short-range THz communication links where high peak power is advantageous.
Diamond THz Optics: The high transparency of diamond in the THz region, coupled with its excellent thermal conductivity, makes it ideal for high-power THz windows, lenses, and diffractive elements.
Wide-Bandgap Semiconductor Devices: The methodology for controlled doping and defect engineering in diamond is directly applicable to other high-power electronic and quantum sensing applications.

View Original Abstract

The efficiency of the generation of terahertz radiation from nitrogen-doped (∼0.1-100 ppm) diamonds was investigated. The synthetic polycrystalline and monocrystalline diamond substrates were pumped by a 400 nm femtosecond laser and tested for the photoconductive emitter operation. The dependency of the emitted THz power on the intensity of the optical excitation was measured. The nitrogen concentrations of the diamonds involved were measured from the optical absorbance, which was found to crucially depend on the synthesis technique. The observed correlation between the doping level and the level of the performance of diamond-based antennas demonstrates the prospects of doped diamond as a material for highly efficient large-aperture photoconductive antennas.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics9010018

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