Since the days of Hertz, radio transmitters have evolved from rudimentary circuits emitting around 50 MHz to modern ubiquitous Wi-Fi devices operating at gigahertz radio bands. As wireless data traffic continues to increase there is a need for new communication technologies capable of high-frequency operation for high-speed data transfer. Here we give a proof of concept of a new compact radio frequency transmitter based on a semiconductor laser frequency comb. In this laser, the beating among the coherent modes oscillating inside the cavity generates a radio frequency current, which couples to the electrodes of the device. We show that redesigning the top contact of the laser allows one to exploit the internal oscillatory current to drive an integrated dipole antenna, which radiates into free space. In addition, direct modulation of the laser current permits encoding a signal in the radiated radio frequency carrier. Working in the opposite direction, the antenna can receive an external radio frequency signal, couple it to the active region and injection lock the laser. These results pave the way to new applications and functionality in optical frequency combs, such as wireless radio communication and wireless synchronization to a reference source.

Since the days of Hertz, radio transmitters have evolved from rudimentary circuits emitting around 50 MHz to modern ubiquitous Wi-Fi devices operating at gigahertz radio bands. As wireless data traffic continues to increase, there is a need for new communication technologies capable of high-frequency operation for high-speed data transfer. Here, we give a proof of concept of a compact radio frequency transmitter based on a semiconductor laser frequency comb. In this laser, the beating among the coherent modes oscillating inside the cavity generates a radio frequency current, which couples to the electrodes of the device. We show that redesigning the top contact of the laser allows one to exploit the internal oscillatory current to drive a dipole antenna, which radiates into free space. In addition, direct modulation of the laser current permits encoding a signal in the radiated radio frequency carrier. Working in the opposite direction, the antenna can receive an external radio frequency signal, couple it to the active region, and injection lock the laser. These results pave the way for applications and functionality in optical frequency combs, such as wireless radio communication and wireless synchronization to a reference source.

Bi-functional active regions, capable of light generation and detection at the same wavelength, allow a straightforward implementation of the mid-infrared quantum cascade technology for integrated photonics. Different parts of the chip can be used for laser and for photodetectors. Potential applications are on-chip integrated sensors or lasers with integrated power monitoring capabilities. In the first bi-functional designs, wavelength matching was achieved using thicker barriers and reduced energy splitings between the extraction levels, but with the drawback of a reduced laser performance. The following introduction of the horizontal-vertical extraction scheme was a significant step towards high performance laser operation. In this work, we combine our design experience with optimized, laterally overgrown waveguides and refined bandstructure modelling. The device was designed for emission at 8 μm to show that wavelength matching can also be achieved at longer wavelengths, where it becomes increasingly difficult due to the smaller ratio between photon energy and LO-phonon energy. Graded interfaces were used in the bandstructure design to consider the behaviour of the MOVPE growth. In pulsed mode a threshold current density of 1.3 kA/cm 2 and a total wallplug efficiency of over 10% was achieved. In continuous-wave operation, the device emits 80mW output power in an episide-up configuration. A much higher performance can be expected after episide-down mounting on AlN substrates. In detector operation the device has a responsivity at the emission wavelength of about 20mA/W.

Bifunctional active regions, capable of light generation and detection at the same wavelength, allow a straightforward realization of the integrated mid-infrared photonics for sensing applications. Here, we present a high performance bifunctional device for 8 μm capable of 1 W single facet continuous wave emission at 15 °C. Apart from the general performance benefits, this enables sensing techniques which rely on continuous wave operation, for example, heterodyne detection, to be realized within a monolithic platform and demonstrates that bifunctional operation can be realized at longer wavelength, where wavelength matching becomes increasingly difficult and that the price to be paid in terms of performance is negligible. In laser operation, the device has the same or higher efficiency compared to the best lattice-matched QCLs without same wavelength detection capability, which is only 30% below the record achieved with strained material at this wavelength.

The quality of epitaxial layers in quantum cascade lasers (QCLs) has a primary impact on QCL performance, and establishing correlations between epitaxial growth and materials properties is of critical importance for continuing improvements. We present an overview of the growth challenges of these complex QCL structures; describe the metalorganic vapor phase epitaxy growth of AlInAs/GaInAs/InP QCL materials; discuss materials properties that impact QCL performance; and investigate various QCL structure modifications and their effects on QCL performance. We demonstrate uncoated buried-heterostructure 9.3-mu m QCLs with 1.32-W continuous-wave output power and maximum wall plug efficiency (WPE) of 6.8%. This WPE is more than 50% greater than previously reportedWPEs for unstrained QCLs emitting at 8.9 mu m and only 30% below strained QCLs emitting around 9.2 mu m.

The measured emission wavelengths of AlInAs/GaInAs/InP quantum cascade lasers (QCLs) grown by metal organic vapor phase epitaxy (MOVPE) have been reported to be ~ 0.5–1µm longer than the designed QCL wavelength. This work clarifies the origin of the red-shifted wavelength. It was found that AlInAs/GaInAs heterointerfaces are compositionally graded over ~ 2.5–4.5nm, and indium accumulates at the AlInAs-to-GaInAs interface. Thus, the as-grown QCLs are far from the ideal abrupt interfaces used in QCL modeling. When graded layers are incorporated in QCL band structure and wavefunction calculations, the emission wavelengths are red shifted. Furthermore, we demonstrate that QCLs with graded interfaces can be designed without compromising performance and show greatly improved correlation between designed and measured emission wavelength. QCLs were designed for emission between 7.5 and 8.5µm. These structures were grown and wet-etched ridge devices were fabricated. The QCLs exhibit room temperature peak powers exceeding 900mW and pulsed efficiencies of ~8 to 10%.

We report on a study of the effects of intentional thickness and doping variations on QCL performance. The measured QCL data had very similar trends to those predicted by an in-house QCL model. It was found that absolute changes to the QCL period had a very small effect on emission wavelength (wavelength/period change <10nm/Å), whereas the complementary thickness changes between the wells and barriers had a large effect (wavelength/thickness change=550nm/Å). The threshold voltage also changed with these variations and generally agreed well with the model. We show through modeling and experiments that intentional structure variations can have largely different magnitudes of effect on QCL performance.

AlInAs/GaInAs superlattices (SLs) with barrier and well layers of various thicknesses were grown by organometallic vapor phase epitaxy to optimize growth of quantum cascade lasers (QCLs). High-resolution x-ray diffraction data of nominally lattice-matched SLs show a systematic shift toward more compressively strained SLs as the barrier/well layer thicknesses are decreased below about 10nm. This shift is attributed to In surface segregation in both AlInAs and GaInAs. This shift is compensated for in the growth of ultra-thin layers in QCL structures. QCLs with tapered gain regions and emitting at 9.6μm are demonstrated with peak power as high as 5.3W from one facet at 20°C.