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

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$2W$for a single-mode$3.3THz$QCL radiating in a narrow single-lobed beam, when operated at$58K$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$><#comment/>50%<#comment/>$radiative efficiency and is significantly greater than that achieved for previous single-mode mid-infrared or terahertz QCLs.

Authors:
; ;
Publication Date:
NSF-PAR ID:
10160787
Journal Name:
Optica
Volume:
7
Issue:
6
Page Range or eLocation-ID:
Article No. 708
ISSN:
2334-2536
Publisher:
Optical Society of America
1. Wavelength beam-combining of four terahertz (THz) distributed-feedback quantum-cascade lasers (QCLs) is demonstrated using low-cost THz components that include a lens carved out of a plastic ball and a mechanically fabricated blazed grating. Single-lobed beams from predominantly single-mode QCLs radiating peak power in the range of$50−<#comment/>170mW$are overlapped in the far field at frequencies ranging from$3.31−<#comment/>3.54THz$. Collinear propagation with a maximum angular deviation of$0.3∘<#comment/>$is realized for the four beams. The total power efficiency for the focused and beam-combined radiation is as high as$25%<#comment/>$. This result could pave the way for future commercialization of beam-combined monolithic THz QCL arrays for multi-spectral THz sensing and spectroscopy at standoff distances.
2. Electro-optic quantum coherent interfaces map the amplitude and phase of a quantum signal directly to the phase or intensity of a probe beam. At terahertz frequencies, a fundamental challenge is not only to sense such weak signals (due to a weak coupling with a probe in the near-infrared) but also to resolve them in the time domain. Cavity confinement of both light fields can increase the interaction and achieve strong coupling. Using this approach, current realizations are limited to low microwave frequencies. Alternatively, in bulk crystals, electro-optic sampling was shown to reach quantum-level sensitivity of terahertz waves. Yet, the coupling strength was extremely weak. Here, we propose an on-chip architecture that concomitantly provides subcycle temporal resolution and an extreme sensitivity to sense terahertz intracavity fields below 20 V/m. We use guided femtosecond pulses in the near-infrared and a confinement of the terahertz wave to a volume of$VTHz∼<#comment/>10−<#comment/>9(λ<#comment/>THz/2)3$in combination with ultraperformant organic molecules ($r33=170pm/V$) and accomplish a record-high single-photon electro-optic coupling rate of, 10,000 times higher than in recent reports of sensing vacuum field fluctuations in bulk media. Via homodyne detection implemented directly on chip, the interaction results into an intensity modulation of the femtosecond pulses. The single-photon cooperativity is$C0=1.6×<#comment/>10−<#comment/>8$, and the multiphoton cooperativity is$C=0.002$at room temperature. We show$><#comment/>70dB$dynamic range in intensity at 500 ms integration under irradiation with a weak coherent terahertz field. Similar devices could be employed in future measurements of quantum states in the terahertz at the standard quantum limit, or for entanglement of subsystems on subcycle temporal scales, such as terahertz and near-infrared quantum bits.
3. In terahertz (THz) photonics, there is an ongoing effort to develop thin, compact devices such as dielectric photonic crystal (PhC) slabs with desirable light–matter interactions. However, previous works in THz PhC slabs have been limited to rigid substrates with thicknesses$∼<#comment/>100s$of micrometers. Dielectric PhC slabs have been shown to possess in-plane modes that are excited by external radiation to produce sharp guided-mode resonances with minimal absorption for applications in sensors, optics, and lasers. Here we confirm the existence of guided resonances in a membrane-type THz PhC slab with subwavelength ($λ<#comment/>0/6−<#comment/>λ<#comment/>0/12$) thicknesses of flexible dielectric polyimide films. The transmittance of the guided resonances was measured for different structural parameters of the unit cell. Furthermore, we exploited the flexibility of the samples to modulate the guided modes for a bend angle of$θ<#comment/>≥<#comment/>5∘<#comment/>$, confirmed experimentally by the suppression of these modes. The mechanical flexibility of the device allows for an additional degree of freedom in system design for high-speed communications, soft wearable photonics, and implantable medical devices.
4. In this Letter, we present a high extinction ratio and compact on-chip polarization beam splitter (PBS), based on an extreme skin-depth (eskid) waveguide. Subwavelength-scale gratings form an effectively anisotropic metamaterial cladding and introduce a large birefringence. The anisotropic dielectric perturbation of the metamaterial cladding suppresses the TE polarization extinction via exceptional coupling, while the large birefringence efficiently cross-couples the TM mode, thus reducing the coupling length. We demonstrated the eskid-PBS on a silicon-on-insulator platform and achieved an ultra-high extinction ratio PBS ($≈<#comment/>60dB$for TE and$≈<#comment/>48dB$for TM) with a compact coupling length ($≈<#comment/>14.5µ<#comment/>m$). The insertion loss is also negligible ($<<#comment/>0.6dB$). The bandwidth is$><#comment/>80$(30) nm for the TE (TM) extinction ratio$><#comment/>20dB$. Our ultra-high extinction ratio PBS is crucial in implementing efficient polarization diversity circuits, especially where a high degree of polarization distinguishability is necessary, such as photonic quantum information processing.
5. We present a compact heterodyne laser interferometer developed for high-sensitivity displacement sensing applications. This interferometer consists of customized prisms and wave plates assembled as a quasi-monolithic unit to realize a miniaturized system. The interferometer design adopts a common-mode rejection scheme to provide a high rejection ratio to common environmental noise. Experimental tests in vacuum show a displacement sensitivity level of$11pm/Hz$at$100mHz$and as low as$0.6pm/Hz$above$1pm$. The prototype unit is$20mm×<#comment/>20mm×<#comment/>10mm$in size and weighs$4.5g$, allowing subsequent integration in compact systems.