The fabrication and characterization of an infrared photodetector based on GaAs droplet epitaxy quantum dots embedded in Al0.3Ga0.7As barrier is reported. The high control over dot electronic properties and the high achievable number density allowed by droplet epitaxy technique permitted us to realize a device using a single dot layer in the active region. Moreover, thanks to the independent control over dot height and width, we were able to obtain a very sharp absorption peak in the thermal infrared region (3-8 μm). Low temperature photocurrent spectrum was measured by Fourier spectroscopy, showing a narrow peak at 198 meV (∼6.3 μm) with a full width at half maximum of 25 meV. The observed absorption is in agreement with theoretical prediction based on effective mass approximation of the dot electronic transition.

We demonstrate p-type SiGe quantum well infrared photodetectors (QWIPs) on a strained-silicon-on-insulator (sSOI) substrate. The sSOI system allows strain-balancing between the QWIP heterostructure with an average composition of Si0.7Ge0.3 and the substrate, and therefore lifts restrictions to the active material thickness faced by SiGe growth on silicon or silicon-on-insulator substrates. The realized sSOI QWIPs feature a responsivity peak at detection wavelengths around 6 µm, based on a transition between heavy-hole states. The fabricated devices have been thoroughly characterized and compared to equivalent material simultaneously grown on virtual Si0.7Ge0.3 substrates based on graded SiGe buffers. Responsivities of up to 3.6 mA/W are achieved by the sSOI QWIPs at 77 K, demonstrating the large potential of sSOI-based devices as components for a group-IV optoelectronic platform in the mid-infrared spectral region.

As recently demonstrated, defect-enhanced Ge quantum dots (Ge-DEQDs) in a crystalline Si matrix can be employed as CMOS-compatible gain material in optically pumped lasers. Due to the stability of their optical properties up to temperatures beyond 300 K, the Ge-DEQD system is a highly promising candidate for the realization of an electrically pumped group-IV laser source for integration in a monolithic optoelectronic platform fit for room-temperature operation. We report on the realization of light-emitting diodes based on Ge-DEQDs operating at telecom wavelengths and above room temperature. The DEQD electroluminescence characteristics were studied spectrally resolved as a function of driving current and device temperature. The experimental results show that the excellent optical properties of Ge-DEQDs are maintained under electrical pumping at high current densities and at device temperatures of at least 100 degrees C. Furthermore, the emission intensity scales with the number of quantum dot layers embedded into the p-i-n diode structures, thus, indicating the scalability of the approach for large gain material volumes. The presented results form an essential step toward the future demonstration of a CMOS-compatible, electrically pumped room-temperature laser based on Ge-DEQDs.

Efficient coupling to integrated high-quality-factor cavities is crucial for the employment of germanium quantum dot (QD) emitters in future monolithic silicon-based optoelectronic platforms. We report on strongly enhanced emission from single Ge QDs into L3 photonic crystal resonator (PCR) modes based on precise positioning of these dots at the maximum of the respective mode field energy density. Perfect site control of Ge QDs grown on prepatterned silicon-on-insulator substrates was exploited to fabricate in one processing run almost 300 PCRs containing single QDs in systematically varying positions within the cavities. Extensive photoluminescence studies on this cavity chip enable a direct evaluation of the position-dependent coupling efficiency between single dots and selected cavity modes. The experimental results demonstrate the great potential of the approach allowing CMOS-compatible parallel fabrication of arrays of spatially matched dot/cavity systems for group-IV-based data transfer or quantum optical systems in the telecom regime.

We report on mapping of the local density of states in L3 photonic crystal resonators (PCR) via deterministically positioned single Ge quantum dots (QDs). Perfect site-control of Ge QDs on pre-patterned silicon-on-insulator substrates was exploited to fabricate in one processing run almost 300 L3 PCRs containing single QDs in systematically varying positions in the cavities. The alignment precision of the QD emitters was better than 20 nm. This type of parallel processing is essentially based on standard Si device technologies and is therefore scalable to any number and configuration of PCR structures. As a first demonstrator, we probed the coupling efficiency of a single Ge QD to the L3 cavity modes as a function of their spatial overlap. The results are in very good agreement with finite-difference time-domain simulations.

The progress on multi-wavelength quantum cascade laser arrays in the mid-infrared is reviewed, which are a powerful, robust and versatile source for next-generation spectroscopy and stand-off detection systems. Various approaches for the array elements are discussed, from conventional distributed-feedback lasers over master-oscillator power-amplifier devices to tapered oscillators, and the performances of the different array types are compared. The challenges associated with reliably achieving single-mode operation at deterministic wavelengths for each laser element in combination with a uniform distribution of high output power across the array are discussed. An overview of the range of applications benefiting from the quantum cascade laser approach is given. The distinct and crucial advantages of arrays over external cavity quantum cascade lasers as tunable single-mode sources in the mid-infrared are discussed. Spectroscopy and hyperspectral imaging demonstrations by quantum cascade laser arrays are reviewed.

Graphene is an attractive photoconductive material for optical detection due to its broad absorption spectrum and ultrashort response time. However, it remains a great challenge to achieve high responsivity in graphene detectors because of graphene's weak optical absorption (only 2.3% in the monolayer graphene sheet) and short photocarrier lifetime (<1 ps). Here we show that metallic antenna structures can be designed to simultaneously improve both light absorption and photocarrier collection in graphene detectors. The coupled antennas concentrate free space light into the nanoscale deep-subwavelength antenna gaps, where the graphene light interaction is greatly enhanced as a result of the ultrahigh electric field intensity inside the gap. Meanwhile, the metallic antennas are designed to serve as electrodes that collect the generated photocarriers very efficiently. We also elucidate the mechanism of photoconductive gain in the graphene detectors and demonstrate mid-infrared (mid-IR) antenna-assisted graphene detectors at room temperature with more than 200 times enhancement of responsivity (∼0.4 V/W at λ0 = 4.45 μm) compared to devices without antennas (<2 mV/W).

We demonstrate a multi-wavelength distributed feedback (DFB) quantum cascade laser (QCL) operating in a lensless external micro-cavity and achieve switchable single-mode emission at three distinct wavelengths selected by the DFB grating, each with a side-mode suppression ratio larger than 30 dB. Discrete wavelength tuning is achieved by modulating the feedback experienced by each mode of the multi-wavelength DFB QCL, resulting from a variation of the external cavity length. This method also provides a post-fabrication control of the lasing modes to correct for fabrication inhomogeneities, in particular, related to the cleaved facets position.

We demonstrate surface emission of terahertz (THz) frequency radiation from a monolithic quantum cascade laser with built-in control over the degree of circular polarization by "fishbone" gratings composed of orthogonally oriented aperture antennas. Different grating concepts for circularly polarized emission are introduced along with the presentation of simulations and experimental results. Fifth-order gratings achieve a degree of circular polarization of up to 86% within a 12°-wide core region of their emission lobes in the far field. For devices based on an alternative transverse grating design, degrees of circular polarization as high as 98% are demonstrated for selected far-field regions of the outcoupled THz radiation and within a collection half-angle of about 6°. Potential and limitations of integrated antenna gratings for polarization-controlled emission are discussed.

We demonstrate tapered quantum cascade lasers monolithically integrated with a distributed Bragg reflector acting as both a wavelength-selective back mirror and a transverse mode filter. Each of the 14 devices operates at a different wavelength between 9.2 and 9.7 μm, where nine devices feature single-mode operation at peak powers between 0.3 and 1.6 W at room temperature. High output power and excellent beam quality with peak brightness values up to 1.6 MW cm−2 sr−1 render these two-terminal devices highly suitable for stand-off spectroscopy applications.

We report on multi-wavelength arrays of master-oscillator power-amplifier quantum cascade lasers operating at wavelengths between 9.2 and 9.8 μm. All elements of the high-performance array feature longitudinal (spectral) as well as transverse single-mode emission at peak powers between 2.7 and 10 W at room temperature. The performance of two arrays that are based on different seed-section designs is thoroughly studied and compared. High output power and excellent beam quality render the arrays highly suitable for stand-off spectroscopy applications.

We report on the demonstration of an array of master-oscillator power-amplifier quantum cascade lasers (QCLs) operating in single-mode at different wavelengths between 9.2 and 9.8 μm. In each device, the output of a distributed feedback QCL is injected into a tapered QCL section which acts as an amplifier while maintaining a high beam quality due to adiabatic mode spreading. All array elements feature longitudinal as well as transverse single-mode emission at peak powers between 0.8 and 3.9 W at room temperature. The high output power and excellent beam quality render the array highly suitable for stand-off spectroscopy applications.

We report the fabrication and thorough characterization of tensile strained p-type SiGe quantum well infrared photodetectors (QWIPs) grown on a Si0.74Ge0.26 pseudosubstrate. The QWIPs operate from a light-hole (LH) ground state and feature responsivity peaks in both the terahertz and mid-infrared regimes with responsivity values up to 3.7 mA/W, originating from LH–LH, LH–heavy-hole, and LH–split-off-band transitions.

We demonstrate continuous voltage control of the nonradiative transition lifetime in semiconductor heterostructures. The results were obtained by picosecond time-resolved experiments on biased SiGe valence band quantum well structures using a free electron laser. By varying the applied voltage, the intersubband hole relaxation times for quantum well structures were varied by a factor of 2 as the wave functions and their overlaps were tuned. The range of magnitudes for the lifetime indicates a possible route to silicon-based quantum cascade lasers.

In the SiGe system, freedom in the design of quantum well (QW) devices is constrained by the 4.2% lattice mismatch between silicon and germanium. The substitution of the Si substrate by a SiGe pseudosubstrate customized to the respective QW structure's requirements enables the growth of a p-type SiGe QW infrared photodetector featuring interfaces between pure Si and SiGe layers of ultrahigh Ge content for a full exploitation of the band offset between the two materials. Our presented device realizes design concepts for narrowing the spectral response and reducing the noise gain made feasible by the utilization of a Si(0.5)Ge(0.5) pseudosubstrate.

Integrated IR-absorption sensors are attractive for various industrial applications, e.g., online condition monitoring of liquids such as lubrication oil. In this contribution, we present theoretical and experimental results showing that an appropriately designed waveguide represents a suitable absorption sensor element. To this end, we utilize the evanescent field of a single-mode waveguide with high sensitivity in the mid-infrared region, which is particularly interesting for a number of applications. Grating couplers are utilized for coupling broadband IR-radiation in and out of the waveguide, where the coupling angle can be used for spectral separation. This concept enables to integrate all components of an absorption sensor into a single sensor element.

Previously we have shown that an appropriately designed waveguide can be used as an infrared absorption sensor element yielding comparable sensitivities to IR-transmission setups. We utilize the evanescent field of a monomode waveguide, which, together with a thermal IR-emitter and detector, will be integrated into a miniaturized sensor system. Besides the filter effect of the utilized grating couplers, a wavelength selective IR-detector is required to maintain the systempsilas functionality when using thermal non-polarized radiation. In this contribution, we consider the wavelength selectivity of the absorber structure of a thermal IR-detector. In contrast to previous investigations, we have expanded the assumption of a perfect back mirror towards a real metal of the non symmetric Fabry-Perot absorption structure. The presented analysis uncovers novel characteristics associated with this structure yielding the opportunity for a novel design requiring only one metal layer instead of two.

The molecular beam epitaxial (MBE) fabrication of blocked-impurity-band detectors (BIB) has been a technologically complex and delicate matter ever since its demonstration in silicon, and has not been adapted for other material systems offering detection onsets at lower terahertz frequencies. We report the fabrication and characterization of a vertical Si:B BIB, circumventing the intrinsically troublesome MBE growth of an ultrapure blocking layer by employing ion implantation. We present a thorough characterization of our device, which exhibits highly competitive figures of merits. Our results not only increase the accessibility of BIB fabrication tools for ultrasensitive terahertz detection but also open a road to other material systems.

Recently, tunable SiGe quantum cascade injector structures designed for detection in the MIR spectra region have been demonstrated. The detectivity of this type of quantum well infrared photodetectors (QWIPs) can be switched between two bands centered around 6μm and 3μm by reversing the sign of the externally applied bias voltage. In order to suppress the observed spectral overlap of these bands and to increase the device detectivity, a process for integrating the detector into an optical resonator based on a low temperature (T <250°C) etch mask deposition was developed.