Diamond Photoconductive Antenna for Terahertz Generation Equipped with Buried Graphite Electrodes

At a Glance

Metadata	Details
Publication Date	2023-01-09
Journal	Photonics
Authors	T. V. Kononenko, K. K. Ashikkalieva, V. V. Kononenko, E.V. Zavedeev, Margarita A. Dezhkina
Institutions	Prokhorov General Physics Institute
Citations	4
Analysis	Full AI Review Included

Executive Summary

This research details the fabrication and testing of a novel diamond Photoconductive Antenna (PCA) designed for high-intensity Terahertz (THz) generation, utilizing deep-buried graphite electrodes.

Core Innovation: Graphite electrodes were laser-microstructured deep inside a nitrogen-doped HPHT diamond crystal to create a homogeneous, high-strength electric bias field across the entire photo-excited volume.
Material and Excitation: The PCA uses Ib-type nitrogen-doped diamond (~20 ppm N) excited by 400 nm laser pulses (second harmonic of a Ti:sapphire laser).
Electrode Design: Electrodes consisted of arrays of buried graphite pillars connected by surface graphite stripes, allowing interelectrode distances to be reduced from 3.5 mm (surface) down to 0.5 mm (buried).
Performance Metrics: The THz fluence exhibited a square-law dependence on the bias field strength, with the saturation fluence (F_sat) consistently measured between 1200 and 1600 µJ/cm².
Maximum Output: A maximum measured THz fluence of 0.04 µJ/cm² was achieved at a field strength of 30 kV/cm (0.5 mm gap).
Potential Enhancement: Estimates suggest that fully burying all electrode components could increase the DC breakdown voltage by a factor of 3, potentially raising the THz fluence to 0.36 µJ/cm² at 90 kV/cm, significantly exceeding previous diamond PCA results.
Fabrication Comparison: Nanosecond laser pulses produced buried pillars with significantly higher conductivity (9 kΩ) compared to femtosecond pulses (800 kΩ).

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	HPHT Diamond (Ib type)	N/A	Nitrogen-doped PCA base
Nitrogen Concentration	~20	ppm	Substitutional N source
Crystal Dimensions	3.4 x 3.5 x 0.66	mm³	Crystal size
Excitation Wavelength	400	nm	Second harmonic pump
Absorption Depth (400 nm)	>0.25	mm	Effective THz generation layer
Saturation Fluence (F_sat)	1200-1600	µJ/cm²	Range observed across all PCAs
Max Measured THz Fluence	0.04	µJ/cm²	PCA #5 (0.5 mm gap) at 30 kV/cm
Estimated Max THz Fluence	0.36	µJ/cm²	Potential performance at 90 kV/cm
DC Bias Voltage (Limit)	2.2	kV	Limited by surface breakdown
Pulsed Bias Voltage (Max)	4	kV	10 ns FWHM pulse duration
Interelectrode Distance (Min)	0.5	mm	PCA #5 (buried electrodes)
fs-Pillar Resistance	800	kΩ	660 µm long pillar (330 fs pulse)
ns-Pillar Resistance	9	kΩ	660 µm long pillar (8 ns pulse)
Surface Stripe Resistance	~900	Ω	2.6 mm long, 50 µm wide stripe
Diamond Dielectric Strength	2000	kV/cm	Theoretical record-high limit

Key Methodologies

The fabrication process involved precise laser microstructuring of the diamond bulk to create conductive graphite electrodes, followed by THz emission testing under various bias conditions.

Substrate Selection: A high-pressure high-temperature (HPHT) Ib-type diamond single crystal (3.4 x 3.5 x 0.66 mm³) with a substitutional nitrogen concentration of ~20 ppm was used.
Surface Electrode Fabrication:
- Surface graphite stripes (50 µm wide) were formed using a KrF excimer laser (λ = 248 nm, τ = 20 ns).
- Processing parameters included a fluence of 30 J/cm² and a repetition rate of 50 Hz, moving the 50 x 50 µm spot at 50 µm/s.
Buried Pillar Fabrication (Femtosecond):
- Deep buried pillars were created using a femtosecond fiber laser (λ = 1035 nm, τ = 330 fs).
- Process utilized 1.6 µJ pulse energy at 3 kHz, moving the crystal at 100 µm/s. Pillars were grown from the surface stripe, stopping ~50 µm before the front face.
Buried Pillar Fabrication (Nanosecond):
- Two electrodes (f3 and r1) used pillars formed by a nanosecond laser (λ = 1064 nm, τ = 8 ns).
- Process utilized 10 µJ pulse energy, moving the crystal at 0.5 µm/s, resulting in higher conductivity (9 kΩ) but pronounced microcracks.
PCA Assembly and Biasing:
- Seven PCAs were configured by combining different pairs of electrodes (f1-f3 and r1-r4) with interspaces ranging from 0.5 mm to 3.5 mm.
- Bias voltage was applied using thin copper wires and conductive paste in two modes: (1) DC voltage (up to 2.2 kV) and (2) High-voltage pulses (up to 4 kV, ~10 ns FWHM).
THz Emission Measurement:
- Optical pumping was performed through the crystal face using 400 nm radiation (150 fs pulsewidth, 0.5 mJ).
- Emitted THz radiation was focused by a PTFE spherical lens and measured using a Golay cell (Tydex GC-1P) modulated at 10 Hz.

Commercial Applications

The development of high-performance, diamond-based THz PCAs with buried electrodes is critical for applications requiring robust, high-field THz sources.

High-Speed Wireless Communication: Enabling next-generation (6G and beyond) sub-THz communication systems by providing compact, high-efficiency emitters capable of generating subcycle pulses with high peak powers.
Non-Destructive Evaluation (NDE): Utilizing intense THz pulses for high-resolution imaging and spectroscopy in industrial quality control, material characterization, and defect detection.
Security and Screening: Developing advanced THz imaging systems for airport security, detecting concealed weapons, explosives, and drugs due to the unique spectral signatures in the THz range.
Biomedicine and Diagnostics: Providing compact THz sources for medical imaging (e.g., skin cancer detection) and spectroscopy, leveraging the non-ionizing nature of THz radiation.
Advanced Diamond Detectors: The laser microstructuring technique for creating buried, highly conductive graphite pathways is directly applicable to fabricating three-dimensional diamond detectors for ionizing radiation (e.g., neutron or particle detection).

View Original Abstract

It has been shown recently that a photoconductive antenna (PCA) based on a nitrogen-doped diamond can be effectively excited by the second harmonic of a Ti:sapphire laser (λ = 400 nm). The THz emission performance of the PCA can be significantly increased if a much stronger electric field is created between the close-located electrodes. To produce a homogeneous electric field over the entire excited diamond volume, the laser fabrication of deep-buried graphite electrodes inside the diamond crystal was proposed. Several electrodes consisting of the arrays of buried pillars connected by the surface graphite stripes were produced inside an HPHT diamond crystal using femtosecond and nanosecond laser pulses. Combining different pairs of the electrodes, a series of PCAs with various electrode interspaces was formed. The THz emission of the PCAs equipped with the buried electrodes was measured at different values of excitation fluence and bias voltage (DC and pulsed) and compared with the emission of the same diamond crystal when the bias voltage was applied to the surface electrodes on the opposite faces. All examined PCAs have demonstrated the square-law dependencies of the THz fluence on the field strength, while the saturation fluence fluctuated in the range of 1200-1600 µJ/cm2. The THz emission performance was found to be approximately the same for the PCAs with the surface electrodes and with the buried electrodes spaced at a distance of 1.4-3.5 mm. However, it noticeably decreased when the distance between the buried electrodes was reduced to 0.5 mm.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics10010075

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