Substrate-transferred crystalline coatings have emerged as a groundbreaking new concept in optical interference coatings. Building upon our initial demonstration of this technology in 2013, we have recently realized significant improvements in the optical performance of these novel single-crystal GaAs/AlGaAs multilayers. In the near-infrared, for center wavelengths spanning 1064 to 1560 nm, we have reduced the excess optical losses (scatter + absorption) to less than 5 ppm, with the direct measurement of sub-ppm optical absorption in these films, enabling the realization of a cavity finesse exceeding 600,000 at the telecom-relevant wavelength range near 1550 nm. In this presentation we outline preliminary measurements of the laser-induced damage threshold (LIDT) of these novel semiconductor-based interference coatings. For pulsed excitation (ns pulse durations at 1064 nm), the narrow bandgap of the constituent mirror materials limits the LIDT to 3-5 J/cm2. Under these conditions, laser damage is driven by two-photon absorption (TPA) in the semiconductor multilayer, primarily the high-refractive-index GaAs films. Note that improved performance may be realized for illumination wavelengths >1740 nm, where TPA is eliminated. For continuous-wave (CW) illumination, the high thermal conductivity (~30 Wm-1K-1) and low intrinsic absorption yield the potential for excellent performance. Here we present preliminary CW damage measurements for a 10-ppm transmission quarter-wave GaAs/AlGaAs Bragg mirror transferred to super-polished fused silica, with only a 1.4 K temperature rise for an intensity of ~1.5 MW/cm2. Further efforts will continue to push the limits of the structure with the aim of determining the maximum CW intensity that such mirrors can tolerate.
Tunable vertical cavity surface emitting lasers (VCSELs) offer a potentially low cost tunable optical source in the 3-5 μm range that will enable commercial spectroscopic sensing of numerous environmentally and industrially important gases including methane, ethane, nitrous oxide, and carbon monoxide. Thus far, achieving room temperature continuous wave (RTCW) VCSEL operation at wavelengths beyond 3 μm has remained an elusive goal. In this paper, we introduce a new device structure that has enabled RTCW VCSEL operation near the methane absorption lines at 3.35 μm. This device structure employs two GaAs/AlGaAs mirrors wafer-bonded to an optically pumped active region comprising compressively strained type-I InGaAsSb quantum wells grown on a GaSb substrate. This substrate is removed in processing, as is one of the GaAs mirror substrates. The VCSEL structure is optically pumped at room temperature with a CW 1550 nm laser through the GaAs substrate, while the emitted 3.3 μm light is captured out of the top of the device. Power and spectrum shape measured as a function of pump power exhibit clear threshold behavior and robust singlemode spectra.
Substrate-transferred crystalline coatings are a groundbreaking new concept for the fabrication of ultralow-loss mirrors. The single-crystal lattice structure of these substrate-transferred GaAs/AlGaAs Bragg mirrors exhibits the lowest mechanical losses and hence unmatched Brownian noise performance, which nowadays limits the stability of precision optical interferometers. Another outstanding feature of these coatings is the wide spectral coverage of the GaAs/AlGaAs material platform. Limited by interband absorption at short wavelengths and the reststrahlen band at long wavelengths, crystalline coatings can be employed as low-loss multilayers from approximately 900 nm up to 5 μm and beyond. Excellent optical performance has been demonstrated in the near-infrared with excess optical losses (scatter + absorption) as low as 3 parts per million (ppm), enabling cavity finesse values up to 360,000 at 1.55 μm. Our first attempts at applying crystalline coatings in the mid-infrared has resulted in mirrors with excess optical losses of 159 and 242 ppm at 3.3 and 3.7 μm, respectively. Remarkably, these results are already on par with current state-of-the-art amorphous mirror coatings. Absorption measurements based on photothermal common-path interferometry (PCI) reveal that the optical losses are largely dominated by optical scatter. Via, PCI, we have confirmed absorption losses below 10 ppm at 3.7 μm, showing the enormous potential of GaAs/AlGaAs Bragg mirrors at mid-infrared wavelengths. An optimized fabrication process, which is currently under development, can efficiently suppress optical scatter due to accumulated growth defects on the surface. Ultimately, we foresee excess losses significantly less than 50 ppm in the mid-infrared spectral region.
Substrate-transferred crystalline coatings have recently emerged as a groundbreaking new concept in optical
interference coatings. Building upon our initial demonstration of this technology, we have recently realized significant
improvements in the limiting optical performance of these novel single-crystal GaAs/AlGaAs multilayers. In the nearinfrared
(NIR), for center wavelengths spanning 1064 to 1560 nm, we have reduced the excess optical losses (scatter +
absorption) to less than 5 ppm, enabling the realization of a cavity finesse exceeding 300,000 at the telecom-relevant
wavelength range near 1550 nm. Moreover, we demonstrate the direct measurement of sub-ppm optical absorption at
1064 nm. Concurrently, we investigate the mid-IR (MIR) properties of these coatings and observe exceptional
performance for first attempts in this important wavelength region. Specifically, we verify excess losses at the hundred
ppm level for wavelengths of 3300 and 3700 nm. Taken together, our NIR optical losses are now fully competitive with
ion beam sputtered films, while our first prototype MIR optics have already reached state-of-the-art performance levels
for reflectors covering the important fingerprint region for optical gas sensing. Thus, mirrors fabricated via this
technique exhibit the lowest mechanical loss (and thus Brownian noise), the highest thermal conductivity, and,
potentially, the widest spectral coverage of any “supermirror” technology, owing to state-of-the art levels of scatter and
absorption losses in both the near and mid IR, all in a single material platform. Looking ahead, we see a bright future for
crystalline coatings in applications requiring the ultimate levels of optical, thermal, and optomechanical performance.
We present recent work towards the realization of a nanowire-based terahertz quantum cascade laser. Nanowires offer an additional quantum mechanical confinement of electrons in the plane of a two-dimensional quantum cascade structure. The additional quantization can greatly increase the lifetimes of intersubband transitions and therefore increase the optical gain and also the maximum operating temperature of terahertz quantum cascade lasers. We outline a fabrication process that is fully scalable from nanowire to micropillar devices and present measurements of micropillar arrays in a double metal waveguide. The results are very promising and also show the main technological challenges for realizing nanowire-based devices.
We present the realization of active photonic crystal terahertz lasers operating in higher photonic bands. The
resonator consists of an array of isolated pillars which are embedded in a metallic waveguide. These devices
reduce the overlap with gain region and increase the effect of the surrounding medium. Thereby, it is either
possible to directly manipulate the lasing mode or to sense variations in the environment.
We have developed and fabricated a novel surface-emitting waveguide for terahertz quantum cascade lasers. The
successfully employed double metal waveguide for such devices lacks of good far field pattern and low beam
divergence. We have overcome these drawbacks by combining a second-order grating for surface emission with a ring
waveguide geometry. Stable single mode emission has been observed over various operating conditions. We have
measured circular beam profiles with a FWHM of 15° and achieved a grating-induced tuning range of about 300 GHz.
By breaking the circular symmetry with two opposite π phase shifts in the grating, the far field pattern has changed into a
tight center lobe with a FWHM of 5° and a preferred polarization direction is defined.
We present a new waveguide concept for terahertz quantum-cascade laser. The double-metal waveguide confines the active region between two metallic layers. Thereby, a modal confinement of almost 100 % is achieved. However, these metal layers are also one of the dominating loss mechanisms. Replacing the conventional metal with a superconductor helps to reduce the total losses. A surface plasmon is formed at the interface between the superconductor and the semiconductor. It can be maintained even for photon energies above the superconducting band gap. In this work we use niobium with a band gap of 2.8 meV to confine the active region of a THz-QCL emitting at 9 meV.
We describe the design, simulation, fabrication and operation of ring cavity surface emitting lasers (RCSEL) based on
quantum cascade structures for the midinfrared (MIR) and terahertz (THz) spectral range. MIR RCSELs facilitate an
enhancement of optical power and a reduction in threshold current density, as compared to Fabry-Perot (FP) lasers. In
continuous wave operation the maximum temperature of ring based devices is 50 K higher than in FP emitters. Also in
THz QCLs a twofold increase in radiation efficiency is observed when compared to FP lasers. The emitters exhibit a
robust single-mode operation around 8 μm and 3.2 THz, with a side mode suppression ratio of 30 dB. The ring-shaped
resonator forms symmetric far-field profiles, represented by a lobe separation of ~1.5° and ~15° for MIR and THz lasers,