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A photoconductive terahertz emitter based on nanoscale contact electrode gratings is presented and experimentally demonstrated. The nanoscale grating enables ultrafast and high quantum efficiency device operation simultaneously, by reducing the photo-generated carrier transport path to photoconductor contact electrodes. The presented photoconductor eliminates the need for a short-carrier lifetime semiconductor, which limits the efficiency of conventional photoconductive terahertz emitters. A photoconductive terahertz emitter based on the presented scheme is implemented and integrated with 0.5 THz dipole antenna arrays. Emitted radiation is characterized in a terahertz time-domain spectroscopy setup, measuring a full width at half maximum response time of 670 fs.

We show that using plasmonic contact electrodes significantly enhances the quantum efficiency of ultrafast photoconductors and demonstrate 50-times higher optical-to-terahertz conversion efficiencies from a plasmonic photoconductive terahertz emitter in comparison with a conventional photoconductive emitter.

A photoconductive terahertz emitter based on a long-carrier lifetime semiconductor is demonstrated experimentally. Using plasmonic grating contact electrodes enables achieving high quantum efficiency and ultrafast operation simultaneously, eliminating the need for a short-carrier lifetime substrate.

We present a comprehensive analysis of terahertz radiation from large area plasmonic photoconductive emitters in relation with characteristics of device substrate. Specifically, we investigate the radiation properties of large area plasmonic photoconductive emitters fabricated on GaAs substrates that exhibit short carrier lifetimes through low-temperature substrate growth and through epitaxially embedded rare-earth arsenide (ErAs and LuAs) nanoparticles in superlattice structures. Our analysis indicates that the utilized substrate composition and growth process for achieving short carrier lifetimes are crucial in determining substrate resistivity, carrier drift velocity, and carrier lifetime, which directly impact optical-to-terahertz conversion efficiency, radiation power, radiation bandwidth, and reliability of large area plasmonic photoconductive emitters.

We demonstrate a compact, robust, and stable terahertz source based on a novel two section digital distributed feedback laser diode and plasmonic photomixer. Terahertz wave generation is achieved through difference frequency generation by pumping the plasmonic photomixer with two output optical beams of the two section digital distributed feedback laser diode. The laser is designed to offer an adjustable terahertz frequency difference between the emitted wavelengths by varying the applied currents to the laser sections. The plasmonic photomixer is comprised of an ultrafast photoconductor with plasmonic contact electrodes integrated with a logarithmic spiral antenna. We demonstrate terahertz wave generation with 0.15-3 THz frequency tunability, 2 MHz linewidth, and less than 5 MHz frequency stability over 1 minute, at useful power levels for practical imaging and sensing applications.

We present a comprehensive analysis of spectral characteristics of terahertz radiation from plasmonic photomixers. We fabricate plasmonic photomixer prototypes with plasmonic contact electrode gratings on a low temperature grown GaAs substrate and characterize the spectral properties of the generated terahertz radiation by use of a heterodyne detection scheme. Our analysis shows that linewidth, stability, and frequency tuning range of the generated terahertz radiation are directly determined by linewidth, stability, and wavelength tuning range of optical pump beam and not affected by device geometry, substrate properties, optical pump power level and other operational settings. Our study indicates the crucial role of optical sources in realizing high performance terahertz spectroscopy and wireless communication systems based on plasmonic photomixers.

We present the design of a periodic array of metallic nano-slits that supports subwavelength TEM guided modes at terahertz and infrared frequencies, enabling efficient interaction of terahertz and infrared waves at nano-scale dimensions.

We experimentally demonstrate simultaneous continuous-wave high-harmonic generation and Raman lasing in a periodically-poled lithium niobate whispering-gallery resonator, allowing spectral lines which span a 2-octave frequency band at a record-low pump power of 200mW.

We present the design of a periodic array of metallic nano-slits that supports subwavelength TEM guided modes at terahertz and optical frequencies, enabling simultaneous focusing of TM-polarized terahertz and optical waves with more than 6-octave frequency span at nano-scale dimensions.

We present an ultrafast photoconductor that offers significantly higher quantum efficiencies compared to conventional photoconductors. The ultrafast response is achieved by excitation of surface plasmon waves, localizing a major portion of the incident light near photoconductor contact electrodes.

We analyze the interaction of electromagnetic waves with double-layered subwavelength metallic slits on a dielectric substrate. This structure allows efficient transmission of an incident TM-polarized electromagnetic wave into the dielectric substrate, due to the presence of surface modes which couple the incident wave to the TEM waveguide modes supported by the subwavelength metallic slits. Our study shows that electromagnetic transmission through double-layered subwavelength metallic slits is strongly geometry dependent. Based on this observation, a terahertz modulation scheme is presented which, compared to existing terahertz modulator solutions, has the promise of significant enhancement in modulation index over a broad range of terahertz frequencies.

We experimentally demonstrate generation of continuous-wave UV, visible, and near-IR waves in a periodically-poled lithium niobate whispering-gallery resonator pumped with a telecommunication-compatible IR source, at a record-low pump power of 200mW.

Broadband modulation of propagating terahertz waves at room temperature is demonstrated using subwavelength metallic slit arrays. Our designs enable the modulation of terahertz transmission by at least 55% at their modulation frequencies.

A high quality tunable filter at terahertz frequencies has been designed using subwavelength metallic slit arrays. The bandpass center frequency is tunable over the range of 0.592THz to 0.994THz. Tunning is achieved by relative vertical movement of the subwavelength metallic slit layers.

A new scheme for broadband terahertz modulation based on periodic arrays of subwavelength metallic slits is presented. By using this scheme, we demonstrate low loss terahertz modulation with 100% modulation index over a broad modulation bandwidth of more than 0.2THz.

Broadband modulation of freely propagating terahertz waves at room temperature is demonstrated using subwavelength metallic slit arrays. These devices enable the control of the extraordinary terahertz transmission through the metallic films by changing the geometrical parameters of the arrays. Our designs achieve an intensity modulation depth of 65% and modulation bandwidth of 2 THz. Implementation of these devices with micro-electro-mechanical fabrication techniques can led to very high modulation speeds.

We present a terahertz modulation scheme which employs reconfigurable metallic slits to achieve higher modulation depth and higher modulation bandwidth compared to the state-of-the art. The initial theoretical and experimental results promise feasibility of a terahertz modulation depth of more than 4.5 over a broad modulation bandwidth of 2THz.

We present plasmonically-enhanced photoconductive antenna arrays and experimentally demonstrate enhanced light localization into device dimensions less than one-tenth of the wavelength. We present the fabrication and characterization of the device.

There has been a lot of interest in developing high-performance terahertz imaging systems which have valuable applications in surveillance, security screening, medical tomography and nondestructive inspection of various pharmaceutical, agricultural, and industrial products [1, 2]. Conventional terahertz imaging systems based on mechanical scanning of the imaging beam, object or the detector suffer from significantly low acquisition speeds [3–5]. Additionally, realtime terahertz imaging systems based on two dimensional arrays of THz sources and detectors tend to have lower sensitivity, higher complexity and higher operational cost [6–8].

A new scheme for a compact, continuously tunable terahertz source based on photoconductive antenna arrays is presented. Narrowband terahertz radiation is generated by superposition of time-delayed wideband radiations from an array of sub-picosecond photoconductive antennas. We demonstrate an 8 μW average power, radiation bandwidth of 0.05 THz, and a tuning range of 0.65–1.4 THz, through a 2.5×10 mm2 photoconductive antenna array prototype.

We report the design and monolithic fabrication of phase modulator and photodetectors for RF photonics. We demonstrate phase modulation efficiency of 270°V-1mm-1, and a bandwidth of 18 GHz. Photodetectors response time is measured to be 8ps.

An analog-to-digital converter (ADC) architecture that extracts the required quantization energy directly from the input analog signal and sampling clock through a spatial quantization scheme is presented. We experimentally demonstrate 8-level quantization consuming only 7.2 pJ per quantization operation with 18 GHz bandwidth. Measured 8 ps full-width half-maximum photodetector output voltages, promises the potential of realizing a 3 bit 125 GS/s ADC through this system.

We present an optical spatial quantized analog-to-digital converter (ADC) and experimentally demonstrate 8-level quantization consuming only 7.2 pJ per quantization operation. Measured 8 ps full-width half-maximum photodetector outputs, promises the potential of realizing a 3 bit 125 GS/s ADC.

We present frequency-tunable, continuous-wave terahertz sources based on GaAs plasmonic photomixers, which offer high terahertz radiation power levels at 50% radiation duty cycle. The use of plasmonic contact electrodes enhances photomixer quantum efficiency while maintaining its ultrafast operation by concentrating a large number of photocarriers in close proximity to the device contact electrodes. Additionally, the relatively high thermal conductivity and high resistivity of GaAs allow operation under high optical pump power levels and long duty cycles without reaching the thermal breakdown limit of the photomixer. We experimentally demonstrate continuous-wave terahertz radiation with a radiation frequency tuning range of more than 2 THz and a record-high radiation power of 17 μW at 1 THz through plasmonic photomixers fabricated on a low temperature grown GaAs substrate at 50% radiation duty cycle.

A miniature MEMS switch is designed, fabricated, and incorporated in a reconfigurable metallic mesh filter for broadband terahertz modulation. The mechanical, electrical, and geometrical properties of the MEMS switch are set to enable broadband terahertz modulation with relatively low modulation voltage, high modulation speed, and high device reliability. The implemented miniature MEMS switch exhibits an actuation voltage of 30 V, a fundamental mechanical resonance frequency of 272 kHz, and an actuation time of 1.23 μs, enabling terahertz modulation with a record high modulation depth of more than 70% over a terahertz band of 0.1-1.5 THz, with a modulation voltage of 30 V and modulation speeds exceeding 20 kHz.

We experimentally demonstrate an order of magnitude enhancement in the quasi-continuous-wave radiated power from a photomixer with plasmonic contact electrodes in comparison with an analogous conventional photomixer without plasmonic contact electrodes in the 0.25-2.5 THz frequency range when pumped at an optical wavelength of 1550 nm. The significant efficiency enhancement results from the unique capability of the plasmonic contact electrodes to reduce the average transport path of photocarriers to the device contact electrodes, increasing the ultrafast photocurrent that drives the terahertz antenna.

We present a 1550 nm plasmonic photomixer operating under pumping duty cycles below 10%, which offers significantly higher terahertz radiation power levels compared to previously demonstrated photomixers. The record-high terahertz radiation powers are enabled by enhancing the device quantum efficiency through use of plasmonic contact electrodes, and by mitigating thermal breakdown at high optical pump power levels through use of a low duty cycle optical pump. The repetition rate of the optical pump can be specifically selected at a given pump duty cycle to control the spectral linewidth of the generated terahertz radiation. At an average optical pump power of 150 mW with a pump modulation frequency of 1 MHz and pump duty cycle of 2%, we demonstrate up to 0.8 mW radiation power at 1 THz, within each continuous wave radiation cycle.

An array of 3 × 3 plasmonic photoconductive terahertz emitters with logarithmic spiral antennas is fabricated on a low temperature (LT) grown GaAs substrate and characterized in response to a 200 fs optical pump from a Ti:sapphire mode-locked laser at 800 nm wavelength. A microlens array is used to split and focus the optical pump beam onto the active area of each plasmonic photoconductive emitter element. Pulsed terahertz radiation with record high power levels up to 1.9 mW in the 0.1–2 THz frequency range is measured at an optical pump power of 320 mW. The record high power pulsed terahertz radiation is enabled by the use of plasmonic contact electrodes, enhancing the photoconductor quantum efficiencies, and by increasing the overall device active area, mitigating the carrier screening effect and thermal breakdown at high optical pump power levels.

A photoconductive terahertz detector based on plasmonic contact electrodes is presented. The use of plasmonic electrodes mitigates the inherent tradeoff between high quantum efficiency and ultrafast operation of the employed photoconductor, enabling significantly higher detection sensitivities compared to conventional photoconductive terahertz detectors. Prototypes of comparable photoconductive detectors with and without plasmonic contact electrode gratings were fabricated and characterized in a time-domain terahertz spectroscopy setup under the same operation conditions. The experimental results show that the plasmonic photoconductive detector offers more than 30 times higher terahertz detection sensitivities compared to the comparable conventional photoconductive detector without plasmonic contact electrodes over 0.1-1.5 THz frequency band.

A photoconductive terahertz emitter based on plasmonic contact electrode gratings is presented and experimentally demonstrated. The nanoscale grating enables ultrafast and high quantum efficiency operation simultaneously, by reducing the photo-generated carrier transport path to the photoconductor contact electrodes. The presented photoconductor eliminates the need for a short-carrier lifetime semiconductor, which limits the efficiency of conventional photoconductive terahertz emitters. Additionally, the photo-absorbing active area of the plasmonic photoconductive terahertz emitter can be increased without a significant increase in the capacitive loading to the terahertz radiating antenna, enabling high quantum-efficiency operation at high pump power levels by preventing the carrier screening effect and thermal breakdown. A plasmonic photoconductive terahertz emitter prototype based on the presented scheme is implemented and integrated with dipole antenna arrays on a semi-insulating In0.53Ga0.47As substrate. Emitted terahertz radiation is characterized in a terahertz time-domain spectroscopy setup, measuring a terahertz pulse width of 590?fs full-width at half maximum in response to 150?fs pump pulses at 925?nm.

Techniques that use optical pulses extending from the picosecond to the attosecond have become an important adjunct to nanoscale science.

We experimentally demonstrate continuous-wave ultraviolet emission through forth-harmonic generation in a millimeter-scale lithium niobate whispering-gallery resonator pumped with a telecommunication-compatible infrared source. The whispering-gallery resonator provides four spectral lines at ultraviolet, visible, near-infrared and infrared, which are equally spaced in frequency via the cascaded-harmonic process and span a 2-octave frequency band. Our technique relies on a variable crystal poling and high transverse order of the modes for phase-matching and a resonator quality factor of over 10(7) to allow cascaded-harmonic generation up to the fourth-harmonic at input pump powers as low as 200 mW. The compact size of the whispering gallery resonator pumped at telecommunication-compatible infrared wavelengths and the low pump power requirement make our device a promising ultraviolet light source for information storage, microscopy, and chemical analysis.

We analyze the unique property of periodic arrays of subwavelength metallic slits to allow extraordinary electromagnetic transmission at multiple frequency bands. The diffraction limit in periodic arrays of subwavelength metallic slits is mitigated by excitation of surface waves which assist efficient coupling of a transverse magnetic–polarized incident electromagnetic wave into the TEM waveguide modes of the subwavelength slab waveguides formed by metallic slits. By investigating the geometry dependence of the electromagnetic guided modes supported by periodic arrays of subwavelength metallic slits, we present the design of a periodic array of metallic nanoslits which enables efficient interaction of terahertz and optical waves at nanoscale dimensions.

We present a high-speed optical switching scheme based on phased array optical beam steering, and analyze the trade-off between the switch power efficiency, signal-to-noise-ratio, number of output channels, and switching speed. For the proof of concept, a two-channel optical switch has been fabricated, using a high-speed GaAs∕AlGaAs multiple quantum well phase modulator. We demonstrate a beam deflection angle of 100mrad at the fastest ever reported speed of 18GHz, consuming 1.8mW. A signal-to-noise ratio of 8dB is measured at each output channel. The relatively low signal-to-noise ratio can be further improved by increasing the number of phased arrays.

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.

We experimentally demonstrate that utilizing plasmonic contact electrodes enhances the optical-to-terahertz conversion efficiency of photoconductive terahertz emitters and offers more than 50-times higher radiation powers compared to conventional photoconductive terahertz emission techniques without plasmonic electrodes.

International audience; This contribution addresses theoretically and experimentally the photomixing signal in a low-temperature-grown GaAs (LTG-GaAs) device with a planar antenna that is driven with an alternative electric field superposed on a bias electric field. The total electric field is applied on the optically-driven nonlinear photoconductance of LTG-GaAs. A theoretical framework based on unidimensional carrier transport is developed to calculate the amplitudes and the phases of different spectral components of the current density. Radiofrequency modulation of the photomixing signal in the microwave domain is experimentally demonstrated. Conversely, the photomixer is exploited as an optically-driven detector. Heterodyne detection with a photomixer is experimentally demonstrated with modulated THz-waves. The photomixing approach allows to extract THz modulation parameters by measurements with a modulated optical beat.