Friday, September 21, 2018

Abstract-High-average and high-peak output-power terahertz-wave generation by optical parametric down-conversion in MgO:LiNbO3

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Yoshikiyo Moriguchi,  Yu Tokizane, Yuma Takida, Kouji Nawata,   Taizo Eno, Shigenori Nagano,  Hiroaki Minamide,

Schematic of the experimental setup for THz-wave generation at a pulse repetition frequency of 100 kHz. ISO, isolator; PBS, polarization beam splitter; FR, Faraday rotator; and HWP, half-wave plate.

A widely tunable terahertz (THz)-wave generation that has a high repetition rate and a narrow line width is demonstrated in this paper by injection-seeded THz-wave parametric generation (is-TPG) in a MgO:LiNbO3 crystal. By pumping the crystal using a passively Q-switched neodymium-doped yttrium vanadate microchip laser with a time duration of 140 ps and an average power of up to 5 W, a THz-wave output with an average output power of 30 μW, a peak power of 4 W, a pulse duration of 73 ps, and a pulse-repetition frequency of 100 kHz is obtained. To prevent laser damage and photorefractive damage to the crystal, the constraints on the pumping condition of the MgO:LiNbO3 crystal are experimentally studied by changing the pumping parameters. As a result, we achieved the stable generation of the THz-wave signal in the 100 kHz regime. Moreover, we performed THz-wave imaging by using the developed is-TPG source. The obtained THz image indicated that the developed system has a good stability over long periods of time.

Abstract-Contribution of terahertz waves to near-field radiative heat transfer between graphene-based hyperbolic metamaterials


Hyperbolic metamaterials alternately stacked by graphene and silicon (Si) are proposed and theoretically studied to investigate the contribution of terahertz (THz) waves to near-field radiative transfer. The results show that the heat transfer coefficient can be enhanced several times in a certain THz frequency range compared with that between graphene-covered Si bulks because of the presence of a continuum of hyperbolic modes. Moreover, the radiative heat transfer can also be enhanced remarkably for the proposed structure even in the whole THz range. The hyperbolic dispersion of the graphene-based hyperbolic metamaterial can be tuned by varying the chemical potential or the thickness of Si, with the tunability of optical conductivity and the chemical potential of graphene fixed. We also demonstrate that the radiative heat transfer can be actively controlled in the THz frequency range.

Abstract-Active Optically Controlled Broadband Terahertz Modulator Based on Fe3O4Nanoparticles

 Lu-Yao Xiong,   Bo Zhang,   Hong-Yu Ji,  Wei Wang,   Xin Liu,  Shu-Li He, Jing-Ling Shen

We report an active broadband terahertz (THz) modulator based on an Fe 3 O 4 nanoparticle/silicon (Si) structure, where the interface effects were measured in a homemade THz time-domain spectroscopy system. An approximately 100-nm Fe 3 O 4 nanoparticle thin film on the high-resistance Si substrate was easily attained by spin-coating ferrofluids. In our experiment, a modulation depth as high as 92% was achieved at an external laser irradiance of 3.6 W/cm 2 . This result can be explained by the accumulation of carriers at the interface of the hybrid structure, which induces intense absorption of the THz transmission. In addition, the limit modulated frequency of the device is ∼12 kHz. The superior performance of this device for THz wave modulation in comparison to other nanomaterial-based THz modulators and the ease of fabrication both illustrate that this is a promising method in the modulation of THz transmission. Furthermore, this modulator could also potentially provide an essential component in a wide variety of technologies, such as THz communications, THz imaging, etc.

OT-LUNA Blog-Optical Reflectometers – How Do They Compare?

Measuring the return loss along a fiber optic network, or within a photonic integrated circuit, is a common and very important technique when characterizing a network’s or device’s ability to efficiently propagate optical signals. Reflectometry is a general method of measuring this return loss and consists of launching a probe signal into the device or network, measuring the reflected light and calculating the ratio between the two.
Spatially-resolved reflectometers can map the return loss along the length of the optical path, identifying and locating problems or issues in the optical path. There are three established technologies available for spatially-resolved reflectometry:
• Optical Time-Domain Reflectometry (OTDR)
• Optical Low-Coherence Reflectometry (OLCR)
• Optical Frequency-Domain Reflectometry (OFDR)
The OTDR is currently the most widely used type of reflectometer when working with optical fiber. OTDRs work by launching optical pulses into the optical fiber and measuring the travel time and strength of the reflected and backscattered light.
These measurements are used to create a trace or profile of the returned signal versus length. OTDRs are particularly useful for testing long fiber optic networks, with ranges reaching hundreds of kilometers. The spatial resolution (the smallest distance over which it can resolve two distinct reflection events) is typically in the range of 1 or 2 meters. All OTDRs, even specialized ‘high-resolution’ versions, suffer from dead zones – the distance after a reflection in which the OTDR cannot detect or measure a second reflection event. These dead zones are most prevalent at the connector to the OTDR and any other strong reflectors.
OLCR is an interferometer-based measurement that uses a wideband low-coherent light source and a tunable optical delay line to characterize optical reflections in a component. While an OLCR measurement can achieve high spatial resolution down to the tens of micrometers, the overall measurement range is limited, often to only tens of centimeters. Therefore, the usefulness of the OLCR is limited to inspecting individual components, such as fiber optic connectors.
Finally, OFDR is an interferometer-based measurement that utilizes a wavelength-swept laser source. Interference fringes generated as the laser sweeps are detected and processed using the Fourier transform, yielding a map of reflections as a function of the length. OFDR is well suited for applications that require a combination of high speed, sensitivity and resolution over short and intermediate lengths.
Luna’s Optical Backscatter Reflectometers (OBRs) are a special implementation of OFDR, adding polarization diversity and optical optimization to achieve unmatched spatial resolution. An OBR can quickly scan a 30-meter fiber with a sampling resolution of 10 micrometers or a 2-kilometer network with 1-millimeter resolution.
This graphic summarizes the landscape of these established technologies for optical reflectometry. By mapping the measurement range and spatial resolution of the most common technologies, the plot illustrates the unique application coverage of OBR.

Thursday, September 20, 2018

Abstract-Multicolor Terahertz Frequency Mixer Using Multibunching of Free-Electron Beams

Weihao Liu, Linbo Liang, Qika Jia, Lin Wang, and Yalin Lu

We propose a free-electron-beam (FEB)-driven frequency mixer for generating terahertz-wave radiation. It employs an initial steady-current FEB to drive two cascaded gratings. On the first grating, the FEB is macrobunched by interacting with the self-stimulated backward slow waves. On the second grating, which operates at high harmonics of the first grating, it is further microbunched at high frequencies, producing mixed bunching components within the FEB. The multibunched FEB then generates a series of superradiant Smith-Purcell radiations from the second grating, achieving multicolor radiations in the terahertz region. The radiation can be tuned by adjusting the beam energy, covering the frequency range from 0.8 to 1.8 THz. It is a promising broad-tunable and multicolor terahertz source.

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Abstract-Investigating the non-radially polarized component of terahertz wave emission during single-colour femtosecond laser filamentation in air

Jiayu Zhao, Hui Gao, Shichang Li, Chang Liu, Yamin Chen, Yan Peng,  Yiming Zhu

Recently, simultaneous emission of radially and non-radially polarized terahertz (THz) pulses during single-colour femtosecond laser filamentation has been reported. In this work, the latter radiation has been specifically investigated, instead of the well-studied THz radial polarization. Briefly, cut-back measurements have verified that the ellipticity of the generated THz pulse with non-radial polarization decreased (became more linearly polarized) with the increasing filament length. The underlying mechanism responsible for this phenomenon is the existence of a propagation effect of THz wave along the filament plasma channel. In this case, the resulted off-axis propagation of THz wave inside the plasma column played a dominant role on the generated non-radial THz polarization, rather than the expected on-axis THz birefringence induced by the high laser intensity. This discovery will greatly renew the understanding of THz emission from plasma sources.