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-Graphene‐Coupled Terahertz Semiconductor Lasers for Enhanced Passive Frequency Comb Operation

Ming Yan. Wenjian Wan. Tao Zhou, Kang Zhou, Ziping Li, Juncheng Cao, Qiang Yu, Kai Zhang, Min Li, Junyi Nan, Boqu He. Heping Zeng

https://onlinelibrary.wiley.com/doi/10.1002/advs.201900460

Optical frequency combs, consisting of well‐controlled equidistant frequency lines, have been widely used in precision spectroscopy and metrology. Terahertz combs have been realized in quantum cascade lasers (QCLs) by employing either an active mode‐locking or phase seeding technique, or a dispersion compensator mirror. However, it remains a challenge to achieve the passive comb formation in terahertz semiconductor lasers due to the insufficient nonlinearities of conventional saturable absorbers. Here, a passive terahertz frequency comb is demonstrated by coupling a multilayer graphene sample into a QCL compound cavity. The terahertz modes are self‐stabilized with intermode beat note linewidths down to a record of 700 Hz and the comb operation of graphene‐coupled QCLs is validated by on‐chip dual‐comb measurements. Furthermore, the optical pulse emitted from the graphene‐coupled QCL is directly measured employing a terahertz pump–probe technique. The enhanced passive frequency comb operation is attributed to the saturable absorption behavior of the graphene‐integrated saturable absorber mirror, as well as the dispersion compensation introduced by the graphene sample. The results provide a conceptually different graphene‐based approach for passive comb formation in terahertz QCLs, opening up intriguing opportunities for fast and high‐precision terahertz spectroscopy and nonlinear photonics.

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-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