Compact solid-state sources of terahertz (THz) radiation are being sought for sensing, imaging, and spectroscopy applications across the physical and biological sciences. We demonstrate that coherent continuous-wave THz radiation of sizable power can be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi2Sr2CaCu2O8. In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to increase as the square of the number of junctions reaching values of 0.5 microwatt at frequencies up to 0.85 THz, and persists up to ∼50 kelvin. These results should stimulate the development of superconducting compact sources of THz radiation.

http://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004672.pdf

Emission of Coherent THz-Radiation from Superconductors.

For terahertz optoelectronics to find broader applications, more efficient sources and detectors are needed. Towards this end, Berry et al. demonstrate the use of plasmonic contact electrodes for both terahertz emitters and detectors, finding large enhancement over standard photoconductive devices.

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Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes

Advanced applications in optical metrology demand improved lasers with high spectral purity, in form factors that are small and insensitive to environmental perturbations. While laboratory-scale lasers with extraordinarily high stability and low noise have been reported, all-integrated chip-scale devices with sub-100 Hz linewidth have not been previously demonstrated. Lasers integrated with optical microresonators as external cavities have the potential for substantial reduction of noise. However, stability and spectral purity improvements of these lasers have only been validated with rack-mounted support equipment, assembled with fibre lasers to marginally improve their noise performance. In this work we report on a realization of a heterogeneously integrated, chip-scale semiconductor laser featuring 30-Hz integral linewidth as well as sub-Hz instantaneous linewidth. Optical metrology applications require lasers with high spectral purity but on-chip devices with sub-100 Hz linewidth are yet to be realized. Here, Liang et al.present a heterogeneously integrated, chip-scale semiconductor laser with 30 Hz integral linewidth and sub-Hz instantaneous linewidth.

/pdf/ultralow-noise-miniature-external-cavity-semiconductor-laser-4ykijcqz8t.pdf

Ultralow noise miniature external cavity semiconductor laser

Terahertz (THz) science and technology have greatly progressed over the past two decades to a point where the THz region of the electromagnetic spectrum is now a mature research area with many fundamental and practical applications. Furthermore, THz imaging is positioned to play a key role in many industrial applications, as THz technology is steadily shifting from university-grade instrumentation to commercial systems. In this context, the objective of this review is to discuss recent advances in THz imaging with an emphasis on the modalities that could enable real-time high-resolution imaging. To this end, we first discuss several key imaging modalities developed over the years: THz transmission, reflection, and conductivity imaging; THz pulsed imaging; THz computed tomography; and THz near-field imaging. Then, we discuss several enabling technologies for real-time THz imaging within the time-domain spectroscopy paradigm: fast optical delay lines, photoconductive antenna arrays, and electro-optic sampling with cameras. Next, we discuss the advances in THz cameras, particularly THz thermal cameras and THz field-effect transistor cameras. Finally, we overview the most recent techniques that enable fast THz imaging with single-pixel detectors: mechanical beam-steering, compressive sensing, spectral encoding, and fast Fourier optics. We believe that this critical and comprehensive review of enabling hardware, instrumentation, algorithms, and potential applications in real-time high-resolution THz imaging can serve a diverse community of fundamental and applied scientists.

Toward real-time terahertz imaging

In this paper, we present a novel design of large-area photoconductive emitters which incorporates plasmonic contact electrodes to offer significantly higher optical-to-terahertz conversion efficiencies compared with conventional designs. Use of plasmonic contact electrodes enables a more efficient separation and acceleration of photocarriers, enhancing the effective dipole moment induced within the device active area in response to an incident optical pump. At an optical pump power level of 240 mW, we demonstrate broadband, pulsed terahertz radiation with radiation power levels as high as 3.8 mW over the 0.1–5-THz frequency range, exhibiting an order of magnitude higher optical-to-terahertz conversion efficiency compared with conventional designs.

High-Power Terahertz Generation Using Large-Area Plasmonic Photoconductive Emitters

This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies ∼ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC roll-off, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode–based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conversion efficiencies as high as 2 × 10−3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.

/pdf/tunable-continuous-wave-terahertz-photomixer-sources-and-rulvqaamg0.pdf

Tunable, continuous-wave Terahertz photomixer sources and applications

A micromagnetic model based on a regular mesh is built to simulate CoX/Pt magnetic recording media containing a polycrystalline structure model using the Voronoi tessellation method. The crystal grains have to be segregated in media; thus, the two phases in crystalline grain and at grain boundary are treated separately. The demagnetizing field in a cell is calculated by the fast Fourier transform method, even if the $M_{s}$ distribution is highly nonuniform: in cells at grain boundary, $M^{\prime } _{s}$ is much smaller. The $M$ – $H$ loops are simulated with different media parameters, such as interfacial anisotropy field $H_{k}$ , grain boundary parameters, and magnetoelastic field in plane. The orientation distribution of $H_{k}$ has to be controlled to keep high perpendicular magnetization before flipping.

Micromagnetics Studies of CoX/Pt Media With Interfacial Anisotropy Based on Polycrystalline Structure Model Using Voronoi Tessellation Method

The signal-to-noise ratio (SNR) gain of microwave-assisted magnetic recording (MAMR) with a spin-transfer oscillator is analyzed by a micromagnetic model simulating the write process on a CoX/Pt media, by including the Voronoi polycrystalline microstructure built upon a regular mesh of micromagnetic cells. With a fixed head of 40 nm-width main pole and a 10.5 kOe maximum perpendicular field, a 2 dB SNR gain is found for low-coercivity media (0 K $H_{c} kOe) and a 4 dB gain is obtained for high-coercivity media (0 K $H_{c}>14$ kOe). Besides, the MAMR performance considering the finite rise time of single-pole-type head field in a bit is also calculated. It is found that the MAMR SNR gain is obtained in the media with lower anisotropy because of the weak writing capacity during the rise time.

Signal-to-Noise Ratio Improvement of MAMR on CoX/Pt Media

The optimum multilayered anisotropy distribution in [CoX/Pt]4 media is studied for microwave-assisted magnetic recording by micromagnetic simulations. The model of the magnetic CoX layer was built upon a regular mesh, including a Voronoi-tessellation polycrystalline structure. If each CoX layer has the same anisotropy, it is found that the moments of grains at the same in-plane position but in different layers may be in the opposite directions, which damages the signal-to-noise ratio (SNR) performance and the thermal stability of recording media. In the optimum media design, the anisotropy field constant $K_{n}$ decreases from the top layer to the bottom layer, where the magnetic moments of grains in each bits are switched nearly uniformly. A medium SNR gain of $1.5\sim 6.8$ dB assisted by spin-transfer oscillator field is confirmed for media with average $\bar H_{k}$ in a range of 2–2.4 T. A best SNR performance of 15.8 dB is found in media with an average $\bar H_{k} =2$ T, with ( $K_{1}-K_{4})$ of $2\times 10^{6}$ erg/cc and an interlayer exchange field constant of 2 kOe.

http://ieeexplore.ieee.org/iel7/20/7498694/07372455.pdf

Optimum Multilayer Anisotropy Distribution for Microwave-Assisted Magnetic Recording [CoX/Pt] n Media

A two-particle model is utilized to study the switching phase diagram of ECC media in the microwave-assisted magnetic recording, by applying an in-plane AC magnetic field for microwave and a DC magnetic field for write head field. An interesting result is found that, by introducing two mixed linearly polarized AC fields when the ratio of frequencies f2/f1 is in a certain range, the minimum switching DC field can be almost zero when Hac is 20% of Hk1 of hard magnetic layer. This phase diagram is also checked by a 3D micromagnetic model of ECC media with polycrystalline structure included; with two linearly polarized Hac applied, the coercivity Hc can be reduced to almost zero when Hac = 2.0 Hk1, and Hc still decreases from 10.5 kOe to 7.7 kOe when Hac = 0.02 Hk1.

Longze Wang

Papers

Tunable, continuous-wave Terahertz photomixer sources and applications

Micromagnetics Studies of CoX/Pt Media With Interfacial Anisotropy Based on Polycrystalline Structure Model Using Voronoi Tessellation Method

Signal-to-Noise Ratio Improvement of MAMR on CoX/Pt Media

Optimum Multilayer Anisotropy Distribution for Microwave-Assisted Magnetic Recording [CoX/Pt] n Media

Switching Phase Diagram of Two Frequencies MAMR for ECC Media