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 deepsubwavelength 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 λ 0 = 4.45 μm) compared to devices without antennas (<2 mV/W). KEYWORDS: Graphene, optical antennas, photodetectors, high speed, mid-infrared T here has been significant interest in developing detectors in graphene due to its broad absorption from the ultraviolet (UV) to the far-infrared (FIR) 1 and ultrashort response time. Ultrafast graphene photodetectors in the nearinfrared (near-IR) have been demonstrated with a bandwidth of over 40 GHz.2,3 However, these detectors suffer from low responsivity (∼5 mA/W), mainly due to the small optical absorption (∼2.3%) and the short lifetime of photocarriers (∼1 ps) 4−6 in graphene. Recent research has focused on enhancing optical absorption 7−12 and photocarrier multiplication 13−15 in graphene, while the low photocarrier collection efficiency, as a result of the short carrier lifetime, remains a limiting factor for high responsivity graphene detectors. One way to improve the responsivity is to increase the carrier lifetime by introducing carrier trapping mechanisms, which however also increases the detector response time to a few milliseconds or seconds. 16−18The optimal strategy to achieve high responsivity without sacrificing the detector response time is to improve the photocarrier collection efficiency while maintaining the short carrier lifetime. Here we present an antenna-assisted graphene detector design, where optical antennas are used as both lightharvesting components and electrodes to simultaneously enhance light absorption and carrier collection efficiency. We have experimentally demonstrated mid-IR graphene detectors with more than 200 times enhancement of responsivity at room temperature, which are highly desirable for applications in mid-IR spectroscopy and imaging, 19,20 biochemical sensing, 21 environmental monitoring, and health diagnostics. 22Design of Antenna-Assisted Graphene Detectors. The antenna-assisted graphene detectors are composed of ...

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 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.

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 °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.

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 distributedfeedback 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.