Discovery of betulinic acid derivatives as gut-restricted TGR5 agonists: Balancing the potency and physicochemical properties.

The pleiotropic effects of TGR5 make it an appealing target for intervention of metabolic and inflammatory disorders, but systemic activation of TGR5 faces challenges of on-target side effects, especially gallbladder filling. Gut-restricted agonists were proved to be sufficient to circumvent these side effects, but extremely low systemic exposure may not be effective in activating TGR5 since it is located on the basolateral membrane. Herein, to balance potency and physicochemical properties, a series of gut-restricted TGR5 agonists with diversified kinetophores had been designed and synthesized. Compound 22-Na exhibited significant antidiabetic effect, and showed favorable gallbladder safety after 7 days of oral administration in humanized TGR5H88Y mice, confirming that gut-restricted agonism of TGR5 is a viable strategy to alleviate systemic target-related effects.

Inserting self-assembled InAs quantum dots into quantum cascade lasers to achieve a broadband free-running frequency comb and effective radio-frequency injection.

We present what we belive to be a new band design in which self-assembled InAs quantum dots (QD) are embedded in InGaAs quantum wells (QW) to fabricate broadband single-core quantum dot cascade lasers (QDCLs) operating as frequency combs. The hybrid active region scheme was exploited to form upper hybrid QW/QD energy states and lower pure QD energy states, which expanded the total laser bandwidth by up to 55 cm-1 due to a broad gain medium provided by the inherent spectral inhomogeneity of self-assembled QDs. The continuous-wave (CW) output power of these devices was as high as 470 mW with optical spectra centered at ∼7 µm, which allowed CW operation at temperatures up to 45 °C . Remarkably, measurement of the intermode beatnote map revealed a clear frequency comb regime extending over a continuous 200 mA current range. Moreover, the modes were self-stabilized with intermode beatnote linewidths of approximately 1.6 kHz. Furthermore, what we believe to be a novel π-shaped electrode design and coplanar waveguide transition way were used for RF signal injection. We found that RF injection modified the laser spectral bandwidth by up to 62 cm-1. The developing characteristics indicate the potential for comb operation based on QDCLs as well as the realization of ultrafast mid-infrared pulse.

High power, broad tuning quantum cascade laser at λ ∼ 8.9 µm.

Many molecules have broad fingerprint absorption spectra in mid-wave infrared range which requires broadly tunable lasers to cover the interested spectrum in one scan. We report a strain-balanced, InAlAs/InGaAs/InP quantum cascade laser structure based on diagonal transition active region with high output power and and wide tuning range at λ ∼ 8.9 µm. The maximum pulsed optical power and the wall-plug efficiency at room temperature are 4 W and 11.7%, respectively. Maximum continuous wave double-facet power is 1.2 W at 25 °C for a 4 mm by 9 µm laser mounted epi-side down on a diamond/copper composite submount. The maximum pulsed and continuous wave external-cavity tuning range are from 7.71 µm to 9.15 µm and from 8 µm to 8.9 µm, respectively. The continuous wave power of the external cavity mode exceeds 200 mW across the entire spectrum.

Above 20†GHz high frequency operation of mid-infrared doughnut-shaped microcavity quantum cascade lasers.

Microresonator-based high-speed single-mode quantum cascade lasers are ideal candidates for on-chip optical data interconnection and high sensitivity gas sensing in the mid-infrared spectral range. In this paper, we propose a high frequency operation of single-mode doughnut-shaped microcavity quantum cascade laser at ∼4.6 µm. By leveraging compact micro-ring resonators and integrating with grounded coplanar waveguide transmission lines, we have greatly reduced the parasitics originating from both the device and wire bonding. In addition, a selective heat dissipation scheme was introduced to improve the thermal characteristics of the device by semi-insulating InP infill regrowth. The highest continuous wave operating temperature of the device reaches 288 K. A maximum -3 dB bandwidth of 11 GHz and a cut-off frequency exceeding 20 GHz in a microwave rectification technique are obtained. Benefiting from the notch at the short axis of the microcavity resonator, a highly customized far-field profile with an in-plane beam divergence angle of 2.4° is achieved.

High performance distributed feedback quantum cascade laser emitting at λ∼6.12um.

Distributed feedback quantum cascade lasers emitting at a wavelength of 6.12 µm are reported. Benefitted from the optimized materials epitaxy and the modified bound to continuum transition active region design along with three pairs of phonon scattering, high device performance is achieved. For a 2-mm-long, 8.4-µm-wide device, the threshold current is as low as 130 mA, the corresponding threshold current density is only 0.77 kA/cm2, and the optical output power is 69 mW at 20 °C in continuous wave mode. The temperature of continuous wave operation can reach 100 °C, where the optical output power is still more than 8 mW. In addition, it maintains a stable single mode operation from 20 to 100 °C without mode hopping, corresponding to a total wavelength shift of 41 nm. Such low-threshold quantum cascade lasers are highly beneficial to portable and highly integrated system sensor applications.

Continuous-wave microcavity quantum cascade lasers in whispering-gallery modes up to 50 °C.

Micro-resonator-based lasers are well suited for high-density optoelectronic integration because of their small volumes and low thresholds. However, microcavity quantum cascade lasers for on-chip sensing have high thermal loads that make continuous-wave operation challenging. In this work, we designed an selective thermal dissipation scheme for the selective electrical isolation process to improve the thermal conductivity of the devices. The lasers operated at 50 °C, with 4.7-µm emission. They were fabricated as a notched elliptical resonator, resulting in a highly unidirectional far-field profile with an in-plane beam divergence of 1.9°. Overall, these directional-emission quantum cascade lasers pave the way for portable and highly integrated sensing applications.

Phase-locked terahertz quantum cascade laser array integrated with a Talbot cavity.

Increasing the power of a quantum cascade laser by widening laser ridges will lead to the degradation of the beam quality because of the operation of high-order transverse modes. We report on a phase-locked array scheme of terahertz quantum cascade laser (THz QCL) utilizing Talbot effect. By adjusting the absorbing boundary width of each ridge in the array, stable operation of the fundamental supermode is realized. A five-element array shows 4 times power amplification than that of a single ridge device. Due to the large power amplification efficiency, stable mode selection, and simple fabricating process, the phase-locked array scheme is very promising to further improve the performance of THz QCL.

Monolithically integrated mid-infrared sensor with a millimeter-scale sensing range.

On-chip sensors based on quantum cascade laser technology are attracting broad attention because of their extreme compactness and abundant absorption fingerprints in the mid-infrared wavelength range. Recent continuous wave operation microcavity quantum cascade lasers are well suited for high-density optoelectronic integration because their volumes are small and thresholds are low. In this experimental work, we demonstrate a monolithically integrated sensor comprising a notched elliptical resonator as transmitter, a quantum cascade detector as receiver, and a surface plasmon structure as light-sensing waveguide. The sensor structure is designed to exploit the highly unidirectional lasing properties of the notched elliptical resonator to increase the optical absorption path length. Combined with the evanescent nature of the dielectric loaded surface plasmon polariton waveguides, the structure also ensures a strong light-matter interactions. The sensing transmission distance obtained is approximately 1.16 mm, which is about one order of magnitude improvement over the traditional Fabry-Perot waveguide. This sensor opens new opportunities for long-range and high-sensitivity on-chip gas sensing and spectroscopy.

High-performance quantum cascade lasers at λ ∼ 9†µm grown by MOCVD.

We demonstrate a high power InP-based quantum cascade laser (QCL) (λ ∼ 9 µm) with high characteristic temperature grown by metalorganic chemical vapor deposition (MOCVD) in this article. A 4-mm-long cavity length, 10.5-µm-wide ridge QCL with high-reflection (HR) coating demonstrates a maximum pulsed peak power of 1.55 W and continuous-wave (CW) output power of 1.02W at 293 K. The pulsed threshold current density of the device is as low as 1.52 kA/cm2. The active region adopted a dual-upper-state (DAU) and multiple-lower-state (MS) design and it shows a wide electroluminescence (EL) spectrum with 466 cm-1 wide full-width at half maximum (FWHM). In addition, the device performance is insensitive to the temperature change since the threshold-current characteristic temperature coefficient, T0, is as high as 228 K, and slope-efficiency characteristic temperature coefficient, T1, is as high as 680 K, over the heatsink-temperature range of 293 K to 353 K.

Broad tuning range, high power quantum cascade laser at λ ∼ 7.4 µm.

In this article, we report a high power quantum cascade laser (QCL) at λ∼7.4 µm with a broad tuning range. By carefully designing and optimizing the active region and waveguide structure, a continuous-wave (CW) output power up to 1.36 W and 0.5 W is achieved at 293 K and 373 K which shows the excellent temperature stability. A high wall-plug efficiency (WPE) of 8% and 13.6% in CW and pulsed mode at 293 K are demonstrated. The laser shows a characteristic temperature T0 of 224 K and T1 of 381 K over a temperature range from 283 K to 373 K. In addition, a far field of pure zero order transverse mode and a fairly wide external cavity (EC) tuning range (280 cm-1) from 6.54 µm to 8 µm are achieved in pulsed operation. In addition, an EC single mode output power of 226 mW is obtained under CW operation at 293K.

GaSb surface grating distributed feedback interband cascade laser emitting at 3.25 µm.

A second-order distributed feedback interband cascade laser emitting at 3.25 µm was designed, grown, and fabricated. By coherent epitaxy of a GaSb cap layer instead of the conventional thin InAs cap on top of the laser structure, a high-quality surface grating was made of GaSb and gold. Enough coupling strength and a significant inter-modal loss difference were predicted according to the simulation within the framework of couple-wave theory. Lasers having 2-mm-long cavities and 4.5-µm-wide ridges with high-/anti-reflection coatings were fabricated. The continuous-wave threshold current and maximum single-mode output power were 60 mA and 24 mW at 20°C, respectively. The output power of 5 mW was still kept at 55°C. Continuous tuning free from mode hopping and high single-mode suppression ratios (>20 dB) were realized at all injection currents and heat-sink temperatures, covering a spectral range of over 20 cm-1.

Dual-wavelength switchable, mid-infrared quantum cascade laser with two shallow-etched distributed Bragg reflectors.

A dual-wavelength quantum cascade laser (QCL) with two shallow-etched distributed Bragg reflectors is designed and fabricated. Based on a heterogeneous active region within a single waveguide, single-mode emission at 7.6µm and 8.2µm was achieved. The two wavelengths can be independently controlled by selective current injection on different regions of the device, which are electrically isolated. High optical powers of about 275mW and 218mW at room temperature were obtained for the single-mode emission at 7.6µm and 8.2µm, respectively. The presented design concept for high power, dual-wavelength switchable, mid-infrared QCLs is significant in developing miniaturized multi-species gas detection systems.

Mid-wave/long-wave dual-color infrared quantum cascade detector enhanced by antenna-coupled microcavity.

We have demonstrated a mid-wave/long-wave dual-color infrared quantum cascade detector enhanced by antenna-coupled microcavity. By optimizing the size of patches, the coupling wavelength of the antenna-coupled microcavity can be conveniently tuned to match the targeted intersubband transition energy. At 77 K, the peak responsivity of our detector is 4.1 mA/W for long wave (10.4 µm) and 0.6 mA/W for mid wave (5.8 µm), while the detectivity is 1.8×109 cm·Hz1/2/W (Jones) and 2.6×108 cm·Hz1/2/W (Jones), respectively. Compared with a reference device with a 45° multi-pass geometry, the responsivity of our detector has been increased by a factor of 9.1 for the long wave and 2.7 for the mid wave. Our results illustrate how to realize a dual-color infrared detector and improve the optoelectronic performance through the concept of antenna-coupled microcavity.

Watt-level, high wall plug efficiency, continuous-wave room temperature quantum cascade laser emitting at 7.7µm.

In this article, a InP based strain-balanced In0.58Ga0.42As/In0.47Al0.53As quantum cascade laser emitting at 7.7µm is reported. The active region is based on a slightly-diagonal bound to continuum design with 50 cascade stages and a low voltage defect Δinj of 96 meV. By optimizing the active region and waveguide structure, the waveguide loss αw of 1.18cm-1 are obtained, which contribute to a high wall-plug efficiency (WPE) of 9.08% and low threshold current of only 1.09 kA/cm2 in continuous-wave(CW) operation at 293K. The maximum single facet output power of 1.17W in CW operation and 2.3W in pulsed operation are measured at 293K. The narrow ridge and buried ridge structure epi-side-down-mounted on the diamond heatsink improved the heat dissipation of the device. A beam of pure zero order mode and a broad external-cavity tuning range from 7.16µm to 8.16µm are also achieved.

Ultralow power consumption of a quantum cascade laser operating in continuous-wave mode at room temperature.

We report an ultralow power consumption of a quantum cascade laser (QCL) emitting at λ ∼ 4.6 µm operating in continuous-wave mode at room temperature. The ultralow power consumption is achieved by using a high gain active region and shortening the device size. For the device with a 0.5-mm-long cavity and 3.2-µm-wide ridge, the threshold power consumption is as low as 0.26 W with an optical output power of 12.6 mW at 10 °C in continuous-wave mode, which represents the world's most advanced level. Furthermore, the threshold power consumption varies linearly with the operating temperature, where the linear change rate of 2.3 mW/K from 10 to 40 °C is low. As a result, the devices also show low threshold power consumption values of 0.33 W even at 40 °C in continuous-wave mode with an optical output power of 6.1 mW. In addition, the lasers can maintain a single-mode operation due to the short cavity length even if no distributed feedback grating is applied.

Room-temperature quantum cascade superluminescent light emitters with wide bandwidth and high temperature stability.

The realization of room-temperature (RT) mid-infrared (MIR) broadband light sources is fundamentally interesting and highly desirable for a number of applications. Recently, superluminescent light emitters (SLEs) based on quantum cascade (QC) structures have emerged as excellent candidates among mid-infrared broadband light sources. However, it is challenging to achieve RT-QCSLEs due to the very low efficiency of the spontaneous emission in the intersubband transitions. Here, we demonstrate the realization of a set of ~5 µm RT-SLEs under continuous wave (CW) or quasi-CW (10% duty circle) operation by using a two-phonon resonant QC active region and monolithic integrated waveguide structures. In addition, with the design of an inclined tapered cavity, the SLEs exhibit high milliwatt power, large spectral width of more than 200 cm-1 and good temperature characteristic. These demonstrated results are believed to be a big step forward to the applications of broadband MIR semiconductor light sources.

Mid-infrared broadband superluminescent light emitter arrays.

Mid-infrared (MIR) room-temperature (RT) and continuous-wave (CW) broadband quantum cascade superluminescent light emitters (QCSLEs) have emerged as ideal broadband light sources for a number of applications of biomedical imaging, security inspection, and gas detection. It is quite challenging to attain a RT-CW output power up to milliwatt level due to the very low efficiency of the spontaneous emission in the intersubband transitions in QCSLEs. In this work, for the first time to the best of our knowledge, a compact light emitter array is realized by integrating several single emitters exhibiting a very high RT-CW power of 2.4 mW, which is attributed to the sufficient low reflectivity provided by the waveguide structure that includes three sections with a short straight part adjacent to a tilted stripe and to a J-shaped waveguide, and the two-phonon resonance QC active structure. This advancement is certainly a big step forward to the applications of broadband light sources towards MIR photonics.