A Diamond Terahertz Large Aperture Photoconductive Antenna Biased by a Longitudinal Field

At a Glance

Metadata	Details
Publication Date	2023-10-20
Journal	Photonics
Authors	V. V. Kononenko, V. V. Bukin, М. С. Комленок, E.V. Zavedeev, T. V. Kononenko
Institutions	Prokhorov General Physics Institute, Moscow Institute of Physics and Technology
Citations	5
Analysis	Full AI Review Included

Executive Summary

This research reports the successful development and testing of a novel Large Aperture Photoconductive Antenna (LAPCA) configuration utilizing a longitudinal bias electric field (E-field) within a nitrogen-doped synthetic diamond substrate.

Novel Configuration: The E-field is oriented longitudinally (parallel to the substrate normal), allowing for a significantly smaller interelectrode gap (substrate thickness, 0.5 mm) compared to conventional transverse designs, resulting in a higher bias field for the same applied voltage.
Material Choice: Monocrystalline nitrogen-doped diamond (HPHT) was used due to its exceptional properties: high dielectric strength (10 MV/cm), high thermal conductivity (> 10 cm²/s), and suitability for high-power THz emission when pumped by 400 nm light.
Performance Comparison: The longitudinal LAPCA with graphite grid electrodes achieved a maximum THz yield of ≈0.62 nJ, comparable to the conventional transverse configuration (≈0.76 nJ), validating the novel design principle.
Conversion Efficiency: The optical-to-THz conversion efficiency for the longitudinal configuration (≈0.0008%) was double that of the transverse configuration (≈0.0004%), demonstrating improved performance metrics.
Electrode Success/Failure: Indium Tin Oxide (ITO) electrodes failed rapidly due to degradation under repetitive high-voltage pulses. Laser-graphitized grid electrodes proved robust, providing ohmic contact and stable performance, despite shading ≈10% of the pump beam.
Modeling Validation: Numerical simulations confirmed that the THz output of the longitudinal LAPCA follows the established surface photocurrent model, adjusted for oblique incidence (E_THz ~ sin(α) x t_p).

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	N-doped Monocrystalline Diamond	N/A	HPHT growth, 5.2 x 2.6 x 0.5 mm
Nitrogen Doping Level	~10	ppm	Required for 400 nm pumping
Dielectric Strength (Diamond)	10	MV/cm	Electrical breakdown threshold
Thermal Diffusivity (Diamond)	> 10	cm²/s	Compared to Copper (~1.1 cm²/s)
Electron Mobility (Diamond)	~4500	cm² V^-1 s^-1	At room temperature
Bandgap (Diamond)	5.46	eV	Wide bandgap semiconductor
Max Bias Field Tested	~10	kV/cm	Limited by surface breakdown
Pump Wavelength	400	nm	Second harmonic of Ti:sapphire
Pump Pulse Duration	~120	fs	Used for optical pumping
Pump Repetition Rate	1	kHz	Used for optical pumping
Absorption Depth (400 nm)	≈220	µm	1/e level in the selected crystal
Saturation Fluence (F_sat)	255 ± 10	µJ/cm²	Independent of electrode design
Max THz Yield (Longitudinal)	0.62	nJ	Graphite grid electrodes
Max THz Yield (Transverse)	0.76	nJ	Edge graphitized surface
Optical-to-THz Conversion (Longitudinal)	0.0008	%	Graphite grid configuration
Optical-to-THz Conversion (Transverse)	0.0004	%	Conventional configuration
THz Pulse Duration	~1	ps	Measured by electro-optical sampling
THz Spectrum Peak	~1	THz	Main band center
THz Spectrum Bandwidth	Up to ~3	THz	High frequency shoulder
Graphite Wire Width/Spacing	10 / 100	µm	Laser-graphitized grid electrodes
Graphite Ohmic Resistance	~1	kΩ	Expected resistance of wires

Key Methodologies

The experiment involved sequential testing of three LAPCA configurations on the same diamond substrate (5.2 x 2.6 x 0.5 mm).

Conventional Transverse LAPCA Assembly:
- Graphite electrodes were written onto the diamond surface using direct laser writing (KrF excimer laser, 248 nm).
- The diamond was glued to a Printed Circuit Board (PCB) with conductive paste, placing the substrate over a gap cut in the PCB.
Longitudinal LAPCA Configuration 1 (ITO Electrodes):
- The diamond substrate was sandwiched between two transparent plastic films coated with Indium Tin Oxide (ITO).
- The assembly was encapsulated in a 3D-printed thermoplastic polyester package.
- Bias was applied using a pulsed voltage (up to 3 kV, ~10 ns duration) synchronized with the 1 kHz optical pump.
Longitudinal LAPCA Configuration 2 (Graphite Grid Electrodes):
- Conductive graphite grids (10 µm wires, 100 µm spacing) were fabricated directly onto both sides of the diamond substrate via laser-stimulated surface graphitization.
- High-voltage copper strips were bonded to the graphite buses using conductive paste, ensuring ohmic contact.
- The antenna was mounted in a 3D-printed plastic housing designed for oblique optical pumping (45°-50° incidence angle).
THz Generation and Measurement:
- A femtosecond Ti:sapphire laser system (800 nm, 1 mJ, 1 kHz) was used, with the second harmonic (400 nm, ≈100 µJ) serving as the pump source.
- The THz radiation was focused by a PTFE spherical lens.
- Total THz power was measured using a Golay cell (Tydex GC-1P).
- THz pulse waveform was characterized using electro-optical sampling with a ZnTe crystal.
Numerical Simulation:
- COMSOL Multiphysics (Electromagnetic Waves, Transient package) was used to model the 2D diamond slab (5 x 0.5 mm).
- Optical pumping was simulated as an electrical conductivity wave propagating obliquely (0° to 80° tilt) with a 120 fs rise time.

Commercial Applications

The development of high-power, robust THz emitters based on diamond is critical for several advanced photonics applications:

High-Power THz Sources: Diamond’s superior thermal conductivity and high breakdown voltage (10 MV/cm) make it the most promising material for next-generation high-intensity and high-average-power THz emitters, overcoming thermal limitations common in GaAs or ZnSe PCAs.
Spectroscopically Resolved Imaging: Used in life sciences and materials analysis where broadband, intense THz radiation is required for high-resolution imaging and chemical identification.
Security and Communications: Robust THz sources are essential for security screening (non-ionizing, penetrating materials) and high-speed, short-range terahertz communications (6G and beyond).
Non-Destructive Testing (NDT): High-intensity THz pulses can be used for quality control and defect detection in materials, particularly those transparent in the THz range.
Diamond Material Technology: This work leverages and validates advanced diamond processing techniques:
- HPHT Monocrystalline Diamond: The base material provides the necessary purity and crystal quality.
- Laser-Stimulated Surface Graphitization: This technique is proven effective for creating robust, low-resistance ohmic contacts and microstructures (like the grid electrodes) directly on the diamond surface, crucial for integrated device fabrication.

View Original Abstract

The novel design of a terahertz large aperture photoconductive antenna (LAPCA) is reported. It features a longitudinal orientation of the bias electric field within the photoconductive substrate, and has the advantage of a small interelectrode gap, resulting in a higher field for the same applied voltage. The proposed LAPCA configuration has been tested with a nitrogen-doped (∼10 ppm) synthetic monocrystalline diamond, which is a promising material for high-intensity and high-power terahertz sources. Two antennas with different high-voltage electrode realizations were assembled, pumped by a 400 nm femtosecond laser, and tested for THz emitter function. The experimental data are found to be in good correlation with the numerical simulation results. The performance of antennas with the conventional transverse E-field configuration and the novel longitudinal configuration is compared and discussed.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics10101169

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