Abstract-Implantable, Degradable, Therapeutic Terahertz Metamaterial Devices

Long Sun, Zhitao Zhou, Junjie Zhong,   Zhifeng Shi, Ying Mao,  Hua Li,  Juncheng Cao, Tiger H. Tao,

https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.202000294

Metamaterial (MM) sensors and devices, usually consisting of artificially structured composite materials with engineered responses that are mainly determined by the unit structure rather than the bulk properties or composition, offer new functionalities not readily available in nature. A set of implantable and resorbable therapeutic MM devices at terahertz (THz) frequencies are designed and fabricated by patterning magnesium split ring resonators on drug‐loaded silk protein substrates with controllable device degradation and drug release rates. To demonstrate proof‐of‐concept, a set of silk‐based, antibiotics‐loaded MM devices, which can serve as degradable antibacterial skin patches with capabilities to monitor drug‐release in real time are fabricated. The extent of drug release, which correlates with the degradation of the MM skin patch, can be monitored by analyzing the resonant responses in reflection during degradation using a portable THz camera. Animal experiments are performed to demonstrate the in vivo degradation process and the efficacy of the devices for antibacterial treatment. Thus, the implantable and resorbable therapeutic MM devices do not need to be retrieved once implanted, providing an appealing alternative for in‐vivo sensing and in situ treatment application

Abstract-Towards Compact and Real-Time Terahertz Dual-Comb Spectroscopy Employing a Self-Detection Scheme

Hua Li, ZiPing Li, Wenjian Wan, Kang Zhou, Xiaoyu Liao, Sijia Yang, Chenjie Wang, J. C. Cao, Heping Zeng

 https://pubs.acs.org/doi/abs/10.1021/acsphotonics.9b01427

Due to its fast and high resolution characteristics, dual-comb spectroscopy has attracted an increasing amount of interest since its first demonstration. In the terahertz frequency range where abundant absorption lines (finger prints) of molecules are located, multiheterodyne spectroscopy that employs the dual-comb technique shows an advantage in real-time spectral detection over the traditional Fourier transform infrared or time domain spectroscopies. Here, we demonstrate compact terahertz dual-comb spectroscopy based on quantum cascade lasers (QCLs). In our experiment, two free-running QCLs generate approximately 120 GHz wide combs centered at 4.2 THz, with slightly different repetition frequencies. We observe that $\sim$490 nW terahertz power coupling of one laser into the other suffices for laser-self-detecting the dual-comb spectrum that is registered by a microwave spectrum analyzer. Furthermore, we demonstrate practical terahertz transmission dual-comb spectroscopy with our device, by implementing a short air path at room temperature. Spectra are shown of semiconductor samples and of moist air, the latter allowing rapid monitoring of the relative humidity. Our devices should be readily extendable to perform imaging, microscopy and near-field microscopy in the terahertz regime.

Abstract-Broadband THz to NIR up-converter for photon-type THz imaging

Peng Bai, Yueheng Zhang, Tianmeng Wang, Zhanglong Fu, Dixiang Shao, Ziping Li, Wenjian Wan, Hua Li, Juncheng Cao, Xuguang Guo,  Wenzhong Shen

https://www.nature.com/articles/s41467-019-11465-6

High performance terahertz imaging devices have drawn wide attention due to their significant application in healthcare, security of food and medicine, and nondestructive inspection, as well as national security applications. Here we demonstrate a broadband terahertz photon-type up-conversion imaging device, operating around the liquid helium temperature, based on the gallium arsenide homojunction interfacial workfunction internal photoemission (HIWIP)-detector-LED up-converter and silicon CCD. Such an imaging device achieves broadband response in 4.2–20 THz and can absorb the normal incident light. The peak responsivity is 0.5 AW−1. The light emitting diode leads to a 72.5% external quantum efficiency improvement compared with the one widely used in conventional up-conversion devices. A peak up-conversion efficiency of 1.14 × 10−2 is realized and the optimal noise equivalent power is 29.1 pWHz−1/2. The up-conversion imaging for a 1000 K blackbody pin-hole is demonstrated. This work provides a different imaging scheme in the terahertz band.

Abstract-Towards Compact and Real-Time Terahertz Dual-Comb Spectroscopy Employing a Self-Detection Scheme

Hua Li, Ziping Li, Wenjian Wan, Kang Zhou, Xiaoyu Liao, Sijia Yang, Chenjie Wang, J. C. Cao, Heping Zeng

https://arxiv.org/abs/1904.03330

Due to its fast and high resolution characteristics, dual-comb spectroscopy has attracted an increasing amount of interest since its first demonstration. In the terahertz frequency range where abundant absorption lines (finger prints) of molecules are located, multiheterodyne spectroscopy that employs the dual-comb technique shows an advantage in real-time spectral detection over the traditional Fourier transform infrared or time domain spectroscopies. Here, we demonstrate compact terahertz dual-comb spectroscopy based on quantum cascade lasers (QCLs). In our experiment, two free-running QCLs generate approximately 150 GHz wide combs centered at 4.2 THz, with slightly different repetition frequencies. We observe that ∼270 nW terahertz power coupling of one laser into the other suffices for laser-self-detecting the dual-comb spectrum that is registered by a microwave spectrum analyzer. Furthermore, we demonstrate practical terahertz transmission dual-comb spectroscopy with our device, by implementing a short air path at room temperature. Spectra are shown of semiconductor samples and of moist air, the latter allowing rapid monitoring of the relative humidity. Our devices should be readily extendable to perform imaging, microscopy and near-field microscopy in the terahertz regime.

Abstract-Sideband generation of coupled-cavity terahertz semiconductor lasers under active radio frequency modulation

Ziping Li, Hua Li, Wenjian Wan, Kang Zhou, Juncheng Cao, Gaolei Chang, and Gangyi Xu

Fig. 1 (a) Schematic of the coupled-cavity terahertz QCL. The inset is a scanning electron microscope image of the fabricated short cavity and air gap of the laser. (b) Calculated total propagation losses as a function of eigen mode frequency of the coupled-cavity with material loss (green) and without material loss (black). The scatters are the calculated results and the solid lines are for the guide of eyes. The red stars are the measured lasing frequencies obtained from (d). (c) Measured light-current-voltage (L-I-V) characteristic of the coupled-cavity laser in continuous wave (cw) mode at 20 K. (d) Normalized terahertz emission spectra as a function of drive current without RF modulation at 20 K. Each spectrum is shifted vertically for clarity. The insets of (d) are magnified spectra to clear show the spectral degeneracy.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-25-32675

The radio frequency (RF) modulation is a powerful tool, which is used for generating sidebands in semiconductor lasers for active mode-locking. The two-section coupled-cavity laser geometry shows advantages over traditional Fabry-Pérot cavities in the RF modulation efficiency, because of its reduced device capacitance of short section cavity. Further, it has been widely used for active/passive mode-locking of semiconductor diode lasers. For semiconductor-based quantum cascade lasers (QCLs) emitting in the far-infrared or terahertz frequency bands, the two-section coupled-cavity configuration can strongly prevent the laser from multimode emissions. This is because of its strong mode selection (loss modulation), which the cavity geometry introduces. Here, we experimentally demonstrate that the coupled-cavity terahertz QCL can be actively modulated to generate sidebands. The RF modulation is efficient at the frequency that equals the difference frequency between the fundamental and higher order transverse modes of the laser, and its harmonics. We show for the first time that, when the laser is modulated at the second harmonic of the difference frequency, the sideband generation in coupled-cavity terahertz QCLs and the generated sidebands are equally spaced by the injected microwave frequency. Our results, which are presented here, provide a novel approach for modulating terahertz coupled-cavity lasers for active mode-locking. The coupled-cavity geometry shows advantages in generating dense modes with short cavities for potential high-resolution spectroscopy. Furthermore, the short coupled-cavity laser consumes less electrical power than Fabry-Pérot lasers that generate a similar mode spacing.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Unambiguous real-time terahertz frequency metrology using dual 10 GHz femtosecond frequency combs

Oliver Kliebisch, Dirk C. Heinecke, Stefano Barbieri, Giorgio Santarelli, Hua Li, Carlo Sirtori, Thomas Dekorsy,

Fig. 1. (a) Schematic overview of the dual-comb sampling setup. The two femtosecond lasers have the same wavelength but are depicted in red and orange for visual clarity. The terahertz beam path is indicated in gray. The dashed black line marks the common 10 MHz reference clock shared among all synthesizers. The dashed light blue path matches the stabilization feedback loop, which is shown in more detail in Fig. 1(b). A full description is given in the text. ZnTe, 2 mm thick ZnTe crystal; QWP, quarter-wave plate; PBSC, polarizing beam-splitter cube; BP, electronic bandpass filter; (b) detailed view of the signal conditioning of the first photodiode for frequency stabilization and in-loop characterization. After the longitudinal beat-mode spectrum is amplified, and a single mode is filtered, the signal is split into a feedback loop branch and an in-loop characterization branch [not shown in Fig. 1(a)].

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-11-1431

Terahertz frequency metrology by radio frequency downconversion using femtosecond optical sampling relies on the harmonic factor retrieval between the terahertz frequency and the optical sampling rate. At typical femtosecond laser repetition rates, this imposes an ambiguity for frequency metrology. We report on a dual-comb sampling system for the unambiguous frequency measurement of terahertz quantum cascade lasers with hertz-level precision. Two Ti:sapphire oscillators with 10 GHz repetition rate are used for the electro-optic sampling of terahertz radiation at 2.5 THz emitted by actively mode-locked terahertz quantum cascade lasers with 9.7 GHz and 19.6 GHz repetition rates. By coherent downconversion, the emitted terahertz waveforms are measured in the radio frequency domain. The terahertz frequency comb is stabilized by employing a phase-locked loop on a radio frequency beat-note signal. A second infrared sampling comb is used to measure the absolute frequencies of the terahertz radiation. This method, which is based on the detuning of the sampling repetition rates, allows the direct retrieval of the quantum cascade laser’s absolute frequency in real time without using additional optical frequency references for calibration. In order to demonstrate the feasibility of the stabilization and readout technique, a high-resolution spectroscopy measurement on gaseous methanol is presented.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract-Terahertz quantum cascade laser frequency comb and fast detection

Hua Li and J. C. Cao

https://www.osapublishing.org/abstract.cfm?uri=isuptw-2018-WF3&origin=search

In this work, we report the homogeneous spectral spanning of terahertz QCL frequency comb characterized by a kHz level frequency stability. With microwave modulation, the terahertz comb can span over 300 GHz continuously which is 8% of the central frequency of the laser. Employing the homogeneous spectra, we successfully demonstrate the spectroscopic application in GaAs etalon transmission measurement and ammonia gas identification. For the fast detection, we show that the terahertz intersubband quantum well photodetectors are able to respond fast modulated terahertz light by optimizing the chip size and microwave transmission line. The terahertz quant well photodetector mixer can be also used for imaging by converting the terahertz frequency into microwave regime.

© 2018 The Author(s)

Abstract-Terahertz Nano-imaging of graphene

Jiawei Zhang, Xinzhong Chen, Scott Mills, Thomas Ciavatti, Ziheng Yao, Ryan Mescall, Hai Hu, Vyacheslav Semenenko, Zhe Fei, Hua Li, Vasili Perebeinos, Hu Tao, Qing Dai, Xu Du, Mengkun Liu,

https://pubs.acs.org/doi/abs/10.1021/acsphotonics.8b00190?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std

Accessing the non-radiative near-field electromagnetic interactions with high in-plane momentum (q) is the key to achieve super resolution imaging far beyond the diffraction limit. At far infrared and terahertz (THz) wavelengths (e.g. 300 μm = 1 terahertz = 4 meV), the study of high q response and nanoscale near-field imaging is still a nascent research field. In this work, we report on THz nanoimaging of exfoliated single and multi-layer graphene flakes by using the state-of-the-art scattering-type near-field optical microscope (s-SNOM). We experimentally demonstrated that the single layer graphene is close to a perfect near-field reflector at ambient environment, comparable to that of the noble metals at the same frequency range. Further modeling and analysis considering the nonlocal graphene conductivity indicate that the high near-field reflectivity of graphene is a rather universal behavior: graphene operates as a perfect high-q reflector at room temperature. Our work uncovers the unique high-q THz response of graphene, which is essential for future applications of graphene in nano-optics or tip-enhanced technologies.

Abstract-Active Modulation of Coupled-Cavity Terahertz Quantum Cascade Lasers for Sideband generation

Ziping Li, Hua Li, Wenjian Wan, Kang Zhou, Tao Zhou, and Juncheng Cao

https://www.osapublishing.org/abstract.cfm?uri=ISUPTW-2018-TuK29

We experimentally demonstrate that the coupled-cavity terahertz quantum cascade lasers can be actively modulated to generate sidebands under injecting frequencies that are equal to the harmonics of the difference frequency between transverse modes.

© 2018 OSA

Abstract-Metamaterials to see in terahertz in “colors”

Zhitao Zhou,  Hua Li,  Tao Zhou,  Zhifeng Shi,  Juncheng Cao,  Hu Tao

https://ieeexplore.ieee.org/document/8346677/

We report real-time, multi-spectrally selective terahertz (THz) imaging using multi-spectral metamaterial focal plane arrays (MM-FPAs) that are fully compatible with standard micro-electro-mechanical systems (MEMS) processes. A set of frequency switchable quantum cascade lasers (QCLs) is used as THz sources for multi-spectral illumination. Both MM-FPAs and QCLs can be readily tuned to operate at multiple discrete THz frequencies to match the “fingerprints” of the analytes. Frequency selective material identification of concealed substances are successfully demonstrated, which promises a great potential of this technique in security sensing applications.

Abstract-6.2-GHz modulated terahertz light detection using fast terahertz quantum well photodetectors

Hua Li, Wen-Jian Wan, Zhi-Yong Tan, Zhang-Long Fu, Hai-Xia Wang, Tao Zhou, Zi-Ping Li, Chang Wang, Xu-Guang Guo, Jun-Cheng Cao

(Submitted on 9 May 2017)

https://arxiv.org/abs/1705.03182

The fast detection of terahertz radiation is of great importance for various applications such as fast imaging, high speed communications, and spectroscopy. Most commercial products capable of sensitively responding the terahertz radiation are thermal detectors, i.e., pyroelectric sensors and bolometers. This class of terahertz detectors is normally characterized by low modulation frequency (dozens or hundreds of Hz). Here we demonstrate the first fast semiconductor-based terahertz quantum well photodetectors by carefully designing the device structure and microwave transmission line for high frequency signal extraction. Modulation response bandwidth of gigahertz level is obtained. As an example, the 6.2-GHz modulated terahertz light emitted from a Fabry-P\'{e}rot terahertz quantum cascade laser is successfully detected using the fast terahertz quantum well photodetector. In addition to the fast terahertz detection, the technique presented in this work can also facilitate the frequency stability or phase noise characterizations for terahertz quantum cascade lasers.

Abstract-Beat note analysis and spectral modulation of terahertz quantum cascade lasers with radio frequency injection

Yonghao Zhu, Hua Li, Wenjian Wan, Li Gu, Tao Zhou, Stefano Barbieri, and Juncheng Cao

https://www.osapublishing.org/col/abstract.cfm?uri=col-15-1-011404

We demonstrate the electrical beat note analysis and radio frequency (RF) injection locking of a continuous wave (cw) terahertz quantum cascade laser (QCL) emitting around 3 THz (∼100 μm). In free running the beat note frequency of the QCL shows a shift of ∼180 MHz with increasing drive current. The beat note, modulation response, injection pulling, and terahertz emission spectral characteristics in the different current regimes I, II, and III are investigated. The results show that in the current regime I close to the laser threshold we obtain a narrower beat note and flat response to the RF modulation at the cavity round trip frequency. The pulling effect and spectral modulation measurements verify that in the current regime I the RF injection locking is more efficient and a robust tool to modulate the mode number and mode frequency of terahertz QCLs.

© 2016 Chinese Laser Press

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Abstract-Dynamics of ultra-broadband terahertz quantum cascade lasers for comb operation.

Hua Li, Pierre Laffaille, Djamal Gacemi, Marc Apfel, Carlo Sirtori, Jeremie Leonardon, Giorgio Santarelli, Markus Rösch, Giacomo Scalari, Mattias Beck, Jerome Faist, Wolfgang Hänsel, Ronald Holzwarth, Stefano Barbieri

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http://www.pubfacts.com/detail/26831993/Dynamics-of-ultra-broadband-terahertz-quantum-cascade-lasers-for-comb-operation

We present an experimental investigation of the multimode dynamics and the coherence of terahertz quantum cascade lasers emitting over a spectral bandwidth of ~1THz. The devices are studied in free-running and under direct RF modulation. Depending on the pump current we observe different regimes of operation, where RF spectra displaying single and multiple narrow beat-note signals alternate with spectra showing a single beat-note characterized by an intense phase-noise, extending over a bandwidth up to a few GHz. We investigate the relation between this phase-noise and the dynamics of the THz modes through the electro-optic sampling of the laser emission. We find that when the phase-noise is large, the laser operates in an unstable regime where the lasing modes are incoherent. Under RF modulation of the laser current such instability can be suppressed and the modes coherence recovered, while, simultaneously, generating a strong broadening of the THz emission spectrum.