Abstract
Abstract Efficient terahertz generation and detection are a key prerequisite for high performance terahertz systems. Major advancements in realizing efficient terahertz emitters and detectors were enabled through photonics-driven semiconductor devices, thanks to the extremely wide bandwidth available at optical frequencies. Through the efficient generation and ultrafast transport of charge carriers within a photo-absorbing semiconductor material, terahertz frequency components are created from the mixing products of the optical frequency components that drive the terahertz device – a process usually referred to as photomixing. The created terahertz frequency components, which are in the physical form of oscillating carrier concentrations, can feed a terahertz antenna and get radiated in case of a terahertz emitter, or mix with an incoming terahertz wave to down-convert to DC or to a low frequency photocurrent in case of a terahertz detector. Realizing terahertz photoconductors typically relies on short-carrier-lifetime semiconductors as the photo-absorbing material, where photocarriers are quickly trapped within one picosecond or less after generation, leading to ultrafast carrier dynamics that facilitates high-frequency device operation. However, while enabling broadband operation, a sub-picosecond lifetime of the photocarriers results in a substantial loss of photoconductive gain and optical responsivity. In addition, growth of short-carrier-lifetime semiconductors in many cases relies on the use of rare elements and non-standard processes with limited accessibility. Therefore, there is a strong motivation to explore and develop alternative techniques for realizing terahertz photomixers that do not rely on these defect-introduced short-carrier-lifetime semiconductors. This review will provide an overview of several promising approaches to realize terahertz emitters and detectors without short-carrier-lifetime semiconductors. These novel approaches utilize p-i-n diode junctions, plasmonic nanostructures, ultrafast spintronics, and low-dimensional materials to offer ultrafast carrier response. These innovative directions have great potentials for extending the applicability and accessibility of the terahertz spectrum for a wide range of applications.
Highlights
The terahertz frequency range is loosely defined as the part of the electromagnetic spectrum between 100 GHz and 10 THz, which is above microwave and below infrared frequencies
The created terahertz frequency components, which are in the physical form of oscillating carrier concentrations, can feed a terahertz antenna and get radiated in case of a terahertz emitter, or mix with an incoming terahertz wave to down-convert to DC or to a low frequency photocurrent in case of a terahertz detector
Photoconductors and photomixers with embedded plasmonic nanostructures – Enable broadband terahertz generation and detection under high photoconductive gain. – Enable record-high-power pulsed terahertz generation compared to the state-of-the-art photoconductive sources [101, 115]. – Enable record-high-sensitivity pulsed terahertz detection at very low optical pump powers compared to the state-of-the-art photoconductive detectors [48, 49]. – Are widely used for both pulsed and CW operation at different optical excitation wavelengths, through a wide range of semiconductor substrates without requiring defect engineering. – Require more demanding nanofabrication processes to be fabricated
Summary
The terahertz frequency range is loosely defined as the part of the electromagnetic spectrum between 100 GHz and 10 THz, which is above microwave and below infrared frequencies. Short-carrier-lifetime semiconductors with high photoabsorption at telecommunication wavelengths (∼1550 nm) are of strong interest This is because the combination of terahertz photoconductors with small-footprint together with highly reliable fiber lasers and fiber-optic components results in low-cost and compact terahertz systems. While some growth methods for short-carrier-lifetime semiconductors like LT-GaAs are wellestablished processes performed by many groups, most growth methods for short-carrier-lifetime semiconductors utilize non-standard processes and, in some cases rare dopant elements, that are not readily available in most molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) facilities, limiting their accessibility and widespread usage These methods have to be adapted for each specific semiconductor, limiting the materials and optical wavelengths that can be used for realizing terahertz photoconductors.
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