Abstract-Phase-locked terahertz plasmonic laser array with 2 W output power in a single spectral mode

Yuan Jin, John L. Reno, Sushil Kumar

. Longitudinal phase-locking scheme for subwavelength metallic cavities. (a) The scheme that allows for phase-locked operation of multiple parallel-plate subwavelength metallic cavities. The objective of the scheme is to enhance radiation from a long ridge cavity by splitting it into several shorter microcavities, which increases the number of radiating end facets when the microcavities are under phase-locked operation. A specific periodic arrangement of the microcavities and slit-like apertures in the top metal layer of the cavities establishes single-sided SPPs in the surrounding medium of the cavities, which leads to phase-locked operation of the microcavities. The enhanced radiation in the surface normal direction is primarily due to a larger number of radiating end facets for the microcavity array. A secondary contribution of radiation is the slit-like apertures within the microcavities. (b) A specific design of the multicavity QCL array for phase-locked operation. The distance between neighboring microcavities is equal to the wavelength of the single-sided SPPs (𝜆SPP${\lambda }_{\mathrm{S}\mathrm{P}\mathrm{P}}$) that are established in the surrounding medium. Each microcavity is of length 3×𝜆SPP$3\times {\lambda }_{\mathrm{S}\mathrm{P}\mathrm{P}}$ and has two slit-like apertures in its top metal layer with an inter-aperture spacing of 𝜆SPP${\lambda }_{\mathrm{S}\mathrm{P}\mathrm{P}}$. An illustration of the standing wave of the electric field corresponding to the lowest-loss resonant mode under phase-locked operation is given for both the vertical (𝐸𝑧$E_z$) and in-plane (𝐸𝑥$E_x$) components of the field. The radiating sites in the microcavity array include the end facets of the microcavities as well as the slit-like apertures, each of which has the same phase for the 𝐸𝑥$E_x$ field, which leads to radiation in the surface normal direction.

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-6-708

Plasmonic lasers suffer from low output power and divergent beams due to their subwavelength metallic cavities. We developed a phase-locking scheme for such lasers to significantly enhance their radiative efficiency and beam quality. An array of metallic microcavities is longitudinally coupled through traveling plasmon waves, which leads to radiation in a single spectral mode and a diffraction limited single-lobed beam in the surface normal direction. We implemented our scheme for terahertz plasmonic quantum-cascade lasers (QCLs) and measured peak output power in excess of 2W$2\mathrm{W}$ for a single-mode 3.3THz$3.3\mathrm{T}\mathrm{H}\mathrm{z}$ QCL radiating in a narrow single-lobed beam, when operated at 58K$58\mathrm{K}$ in a compact Stirling cooler. We thereby demonstrated an order of magnitude increase in power and thirty-times higher average intensity for monolithic single-mode terahertz QCLs compared to prior work. The number of photons radiated from the cavity outnumber those absorbed within its claddings and semiconductor medium, which constitutes >50%$>50\mathrm{\%}$ radiative efficiency and is significantly greater than that achieved for previous single-mode mid-infrared or terahertz QCLs.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

A breakthrough in developing multi-watt terahertz lasers

A phase-locking scheme for plasmonic lasers is developed in which traveling surface-waves longitudinally couple several metallic microcavities in a surface-emitting laser array. Multi-watt emission is demonstrated for single-mode terahertz lasers in which more photons are radiated from the laser array than those absorbed within the array as optical losses. Credit: Yuan Jin, Lehigh University

https://phys.org/news/2020-06-breakthrough-multi-watt-terahertz-lasers.html

Terahertz lasers could soon have their moment. Emitting radiation that sits somewhere between microwaves and infrared light along the electromagnetic spectrum, terahertz lasers have been the focus of intense study due to their ability to penetrate common packaging materials such as plastics, fabrics, and cardboard and be used for identification and detection of various chemicals and biomolecular species, and even for imaging of some types of biological tissue without causing damage. Fulfilling terahertz lasers' potential for us hinges on improving their intensity and brightness, achieved by enhancing power output and beam quality.

Sushil Kumar, associate professor in Lehigh University's Department of Electrical and Computer Engineering, and his research team are working at the forefront of terahertz semiconductor 'quantum-cascade' laser (QCL) technology. In 2018, Kumar, who is also affiliated with Lehigh's Center for Photonics and Nanoelectronics (CPN) reported on a simple yet effective technique to enhance the power output of single-mode lasers based on a new type of "distributed-feedback" mechanism. The results were published in the journal Nature Communications and received a lot of attention as a major advance in terahertz QCL technology. The work was performed by graduate students, including Yuan Jin, supervised by Kumar and in collaboration with Sandia National Laboratories.

Now, Kumar, Jin and John L. Reno of Sandia are reporting another terahertz technology breakthrough: they have developed a new phase-locking technique for plasmonic lasers and, through its use, achieved a record-high power output for terahertz lasers. Their laser produced the highest radiative efficiency for any single-wavelength semiconductor quantum cascade laser. These results are explained in a paper, "Phase-locked terahertz plasmonic laser array with 2 W output power in a single spectral mode" published yesterday in Optica.

"To the best of our knowledge, the radiative efficiency of our terahertz lasers is the highest demonstrated for any single-wavelength QCL to-date and is the first report of a radiative efficiency of greater than 50% achieved in such QCLs," said Kumar. "Such a high radiative efficiency beat our expectations, and it is also one of the reasons why the output power from our laser is significantly greater than what has been achieved previously."

To enhance the optical power output and beam quality of semiconductor lasers, scientists often utilize phase-locking, an electromagnetic control system that forces an array of optical cavities to emit radiation in lock step. Terahertz QCLs, which utilize optical cavities with metal coatings (claddings) for light confinement, are a class of lasers known as plasmonic lasers that are notorious for their poor radiative properties. There are only a limited number of techniques available in prior literature, they say, that could be utilized to improve radiative efficiency and output power of such plasmonic lasers by significant margins.

"Our paper describes a new phase-locking scheme for plasmonic lasers that is distinctly different from prior research on phase-locked lasers in the vast literature on semiconductor lasers," says Jin. "The demonstrated method makes use of traveling surface waves of electromagnetic radiation as a tool for phase-locking of plasmonic optical cavities. The efficacy of the method is demonstrated by achieving record-high output power for terahertz lasers that has been increased by an order of magnitude compared to prior work."

Traveling surface waves that propagate along the metal layer of the cavities, but outside in the surrounding medium of the cavities rather than inside, is a unique method that has been developed in Kumar's group in recent years and one that continues to open new avenues for further innovation. The team expects that the output power level of their lasers could lead to collaborations between laser researchers and application scientists toward development of terahertz spectroscopy and sensing platforms based on these lasers.

This innovation in QCL technology is the result of a long term research effort by Kumar's lab at Lehigh. Kumar and Jin jointly developed the finally-implemented idea through design and experimentation over a period of approximately two years. The collaboration with Dr. Reno from the Sandia National Laboratories allowed Kumar and his team to receive semiconductor material to form the quantum cascade optical medium for these lasers.

The primary innovation in this work, according to the researchers, is in the design of the optical cavities, which is somewhat independent from the properties of the semiconductor material. The newly acquired inductively-coupled plasma (ICP) etching tool at Lehigh's CPN played a critical role in pushing the performance boundaries of these lasers, they say.

This research represents a paradigm shift in how such single-wavelength terahertz lasers with narrow beams are developed and will be developed going forward in future, says Kumar, adding: "I think the future of terahertz lasers is looking very bright."

Abstract-A Broadband Polarization-Rotating Vivaldi Antenna for Beam Focusing of Terahertz Quantum Cascade Lasers

U. Senica, E. Mavrona, T. Olariu, A. Forrer, M. Beck, J. Faist, and G. Scalari

https://www.osapublishing.org/abstract.cfm?uri=CLEO_Europe-2019-cc_2_4

Terahertz (THz) Quantum Cascade Lasers (QCLs) have been of large interest in the context of very broadband and compact frequency combs in the THz spectral range. For THz QCLs, the double metal waveguide proved to be advantageous in terms of bandwidth, compactness and low dispersion. However, as it confines the optical mode to deeply subwavelength dimensions, the output beam is highly divergent. Previous solutions addressing this issue were either impractical or not so broadband. To improve the far-field properties of the double metal waveguide across a very large frequency range, we designed, fabricated and characterized a broadband, narrow beam Vivaldi Antenna .

© 2019 IEEE

Abstract-Parasitic transport paths in two-well scattering-assisted terahertz quantum cascade lasers

Li Wang, Tsung-Tse Lin, Ke Wang,  Hideki Hirayama,

https://iopscience.iop.org/article/10.7567/1882-0786/ab2b56

Using nonequilibrium Green's functions, possible parasitic paths are identified in two-well scattering-assisted terahertz quantum cascade lasers operating at 3.5 THz. The majority of electrons in the upper laser state can escape through these paths, causing a 66% loss of population inversion at 50 K. Three types of paths are clarified: one is responsible for non-selective injection via LO-phonon scattering, the other two lead to leakages via high-lying states to downstream periods by sequential tunneling. Finally, several ways of suppressing these paths are suggested by having small oscillator strength (<0.3), or employing asymmetric structure.

Abstract-Wideband, high-resolution terahertz spectroscopy by light-induced frequency tuning of quantum-cascade lasers

T. Alam, M. Wienold, X. Lü, K. Biermann, L. Schrottke, H. T. Grahn, and H.-W. Hübers

Fig. 1 (a) Schematics of the experimental setup. The QCL (yellow box) is mounted in a He-flow cryostat. BS - dichroic beamsplitter; OL - objective lens; Ge:Ga - photoconductive Ge:Ga detector. (b) Microscope image of the illuminated QCL facet. The excitation spot with a diameter of approximately 90 μm originates from a multimode diode laser emitting at 809 nm and exhibits essentially a flat-top profile. (c) Calculated profile of the waveguide mode in the vertical (epitaxial-growth) direction for different frequencies (the mode propagates perpendicular to y along the waveguide ridge). The active region (a. r.) has a height of 10 μm and corresponds to the QCL ridge structure in (b).

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-4-5420

Near-infrared optical excitation enables wideband frequency tuning of terahertz quantum-cascade lasers. In this work, we demonstrate the feasibility of the approach for molecular laser absorption spectroscopy. We present a physical model which explains the observed frequency tuning characteristics by the optical excitation of an electron-hole plasma. Due to an improved excitation configuration as compared to previous work, we observe a single-mode continuous-wave frequency coverage of as much as 40 GHz for a laser at 3.1 THz. This represents, for the same device, a ten-fold improvement over the usually employed tuning by current. The method can be readily applied to a large class of devices.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Basic phase-locking, noise, and modulation properties of optically mutual-injected terahertz quantum cascade lasers

Yuanyuan Li, Ning Yang, Yan Xie, Weidong Chu, Wei Zhang, Suqing Duan, and Jian Wang

Fig. 2 Time evolution of |EA|, |EB | (first column), the corresponding power spectral density (second column), and instantaneous frequency (third column) with different coupling strength κ and detuning frequency ΔΩ/2π. The coupling strength in the first three rows are set as κ = 9.87 × 10−3, which is the case of moderate coupling. (a)–(c) within the phase-locking regime, ΔΩ/2π = 0.5GHz, (d)–(f) out of the pahse-locking regime, ΔΩ/2π = 5GHz, (g)–(i) out of the pahse-locking regime, ΔΩ/2π = 0.55GHz. The fourth row is the case of strong coupling with κ = 0.247. (j)–(l) out of the pahse-locking regime, ΔΩ/2π = 0.2GHz. The effective injection current is 1.5Ith in all simulations.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-3-3146

The phase-locking, noise, and modulation properties of two face-to-face optically mutual-injected terahertz (THz) quantum cascade lasers (QCLs) are analyzed theoretically. In the phase-locking range, the two THz QCLs are in stationary states working at the same frequency. Outside the phase-locking range, the amplitude and the instantaneous frequency of the optical field oscillate with time, and the power spectrum shows a series of discrete peaks. For strong mutual injection, the optical field of the THz QCL array also exhibits oscillatory behavior. Coherent collapse or chaotic behavior is not observed within the range of the parameters used in this simulation. The spontaneous emission noise of phase-locked THz QCLs is higher than that of THz QCLs at free-running operation, and mutual injection may introduce additional modulation peaks in the noise spectrum. The modulation response of the mutual-injected THz QCLs to an individual modulation is investigated. It is found that the modulation bandwidth and the phase difference are significantly dependent on the modulation parameters. These results are helpful for further understanding the nonlinear dynamic behaviors of THz QCLs under optical injection and provide theoretical support for the development of THz QCL phase-locked arrays.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement