Junqi Liu

A normal incident quantum cascade detector enhanced by surface plasmons

We demonstrate a normal incident quantum cascade detector excited by surface plasmons resonance using an Au two-dimensional hole array integrated on top of the detector absorption region. With normal incidence, the device has a maximal responsivity value of 8.4 mA/W at 160 K and can work up to room temperature with a responsivity of 1.02 mA/W. Compared with the 45° polished edge coupling device, the two-dimensional hole array can lead to quantum efficiency enhancement at most measured temperatures, and at 140 K the quantum efficiency is enhanced most strongly by 69%.

19 μm quantum cascade infrared photodetectors

Two InP based InGaAs/InAlAs photovoltaic quantum cascade detectors operating at peak wavelengths of 18 μm and 19 μm using different electronic transport mechanisms are reported. A longitudinal optical phonon extraction stair combined with energy mini-steps are employed for electron transport, which suppresses the leakage current and results in high device resistance. Altogether, this quantum design leads to 15 K peak responsivity of 2.34 mA/W and Johnson noise limited detectivity of 1 × 1011 Jones at 18 μm.

Surface emitting quantum cascade lasers operating in continuous-wave mode above 70 °C at λ~ 4.6 μm

We demonstrate surface emitting distributed feedback (DFB) quantum cascade lasers for very high temperature continuous-wave (cw) operation at λ ∼ 4.6 μm. A second-order DFB grating beneath the waveguide provides efficient vertical outcoupling. cw operation is reported up to a temperature of 75 °C. Total output power of 105 mW is obtained with a record low threshold current density of 0.85 kA/cm2 at 10 °C. Single-lobed far-field radiation pattern with a low divergence angle of about 0.17° × 18.7° is achieved. Robust single-mode emission with a side-mode suppression ratio about 30 dB is continuously tunable by the heat sink temperature and injection current.

Room temperature operation of photonic-crystal distributed-feedback quantum cascade lasers with single longitudinal and lateral mode performance

We demonstrate room temperature operation of photonic-crystal distributed-feedback quantum cascade lasers emitting at 4.7 μm. A rectangular photonic crystal lattice perpendicular to the cleaved facet was defined using holographic lithography. The anticrossing of the index- and Bragg-guided dispersions of rectangular lattice forms the band-edge mode with extended mode volume and reduced group velocity. Utilizing this coupling mechanism, single mode operation with a near-diffractive-limited divergence angle of 12° is obtained for 33 μm wide devices in a temperature range of 85–300 K. The reduced threshold current densities and improved heat dissipation management contribute to the realization of devices’ room temperature operation.

Low-Threshold Continuous-Wave Operation of Distributed-Feedback Quantum Cascade Laser at

Room-temperature continuous-wave (CW) operation of distributed-feedback quantum cascade laser (DFB-QCL) emitting at 4.6 μm is presented. Strain-compensation combined with two-phonon resonance in active region design promises low-threshold current density. A record low-threshold current density of 1 kA/cm2 for DFB-QCL operated in CW mode at 30°C is realized without a high-reflectivity coating on the rear facet. Reliable dynamic single-mode emission with a sidemode suppression ratio of about 30 dB is achieved from 10 to 50°C.

\lambda \sim 4.6\ \mu

We demonstrate the fabrication and characterization of photonic-crystal distributed-feedback quantum cascade laser emitting at 4.7 microm. The tilted rectangular-lattice PCDFB structure was defined using a multi-exposure of two-beam holographic lithography. The devices exhibit the near-diffraction-limited beam emission with the full width at half maximum of the far-field divergence angles about 4.5 degrees and 2.5 degrees for stripe widths of 55 microm and 95 microm, respectively. Single-mode emission with a side mode suppression ratio of approximately 20 dB is achieved in the temperature range (80-210 K). The single-facet output power is above 1 W for a 95 microm x 2.5 mm laser bar at 85 K in pulsed operation.

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A 10.7μm quantum cascade detector based on lattice matched InGaAs/InAlAs/InP is demonstrated and characterized in terms of responsivity, resistivity and detectivity. The device operates in the 8–14 μm atmospheric window up to 140K and shows a peak reponsivity of 14.4mA/W at 78K. With a resistance-area product value of 159Ωcm2, the Johnson noise limited detectivity D*J is 2.8 × 109 Jones (cmHz1/2 W−1) at 78K.

Holographic fabricated photonic-crystal distributed-feedback quantum cascade laser with near-diffraction-limited beam quality

A coupled ridge waveguide quantum cascade laser (QCL) array consisting of fifteen elements with parallel integration was presented. In-phase fundamental mode operation in each element is secured by both the index-guided nature of the ridge and delicate loss management by properly designed geometries of the ridges and interspaces. Single-lobe lateral far-field with a nearly diffraction limited beam pattern was obtained. By incorporating a one-dimensional buried distributed feedback grating, the in-phase-operating coupled ridge waveguide QCL design provides an efficient solution to obtaining high output power and stable single longitudinal mode emission. The simplicity of this structure and fabrication process makes this approach attractive to many practical applications.

A 10.7 μm InGaAs/InAlAs Quantum Cascade Detector

High-power broad area substrate-emitting distributed feedback (DFB) quantum cascade laser emitting at λ ~ 4.6 μm is reported. Proper second-order DFB grating coupling, low waveguide loss mechanism, and epilayer-down mounting bring about a high vertical radiative optical power. The maximum peak power is as high as 1.82 W at 20 ^°C and still 1.22 W at 50 ^°C operated in pulsed mode with a pulsed width of 1 μs and duty cycle of 0.2%. A single-lobed radiation far-field with a divergence angle of 7.9 ^° in the ridge-width direction and two-lobed radiation far-field with a divergence angle of 0.61 ^° in the cavity-length direction are achieved by depositing the high-reflection coatings on both facets. Robust single-mode emission is continuously tunable by the heat sink temperature up to 115 ^°C.

Coupled ridge waveguide distributed feedback quantum cascade laser arrays

We demonstrate an InAs quantum dot quantum cascade infrared photodetector operating at room temperature with a peak detection wavelength of 4.3 μm. The detector shows sensitive photoresponse for normal-incidence light, which is attributed to an intraband transition of the quantum dots and the following transfer of excited electrons on a cascade of quantum levels. The InAs quantum dots for the infrared absorption were formed by making use of self-assembled quantum dots in the Stranski–Krastanov growth mode and two-step strain-compensation design based on InAs/GaAs/InGaAs/InAlAs heterostructure, while the following extraction quantum stairs formed by LO-phonon energy are based on a strain-compensated InGaAs/InAlAs chirped superlattice. Johnson noise limited detectivities of 3.64 × 1011 and 4.83 × 106 Jones at zero bias were obtained at 80 K and room temperature, respectively. Due to the low dark current and distinct photoresponse up to room temperature, this device can form high temperature imaging.