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Optical quantum information science and technologies require the capability to generate, control, and detect single or multiple quanta of light. The need to detect individual photons has motivated the development of a variety of novel and refined single-photon detectors (SPDs) with enhanced detector performance. Superconducting nanowire single-photon detectors (SNSPDs) and single-photon avalanche diodes (SPADs) are the top-performer in this field, but alternative promising and innovative devices are emerging. In this review article, we discuss the current state-of-the-art of one such alternative device capable of single-photon counting: the resonant tunneling diode (RTD) single-photon detector. Due to their peculiar photodetection mechanism and current-voltage characteristic with a region of negative differential conductance, RTD single-photon detectors provide, theoretically, several advantages over conventional SPDs, such as an inherently deadtime-free photon-number resolution at elevated temperatures, while offering low dark counts, a low timing jitter, and multiple photon detection modes. This review article brings together our previous studies and current experimental results. We focus on the current limitations of RTD-SPDs and provide detailed design and parameter variations to be potentially employed in next-generation RTD-SPD to improve the figure of merits of these alternative single-photon counting devices. The single-photon detection capability of RTDs without quantum dots is shown.

Resonant tunneling diode photodetectors appear to be promising architectures with a simple design for mid-infrared sensing operations at room temperature. We fabricated resonant tunneling devices with GaInAsSb absorbers that allow operation in the 2-4 μm range with significant electrical responsivity of 0.97 A/W at 2004 nm to optical readout. This paper characterizes the photosensor response contrasting different operational regimes and offering a comprehensive theoretical analysis of the main physical ingredients that rule the sensor functionalities and affect its performance. We demonstrate how the drift, accumulation, and escape efficiencies of photogenerated carriers influence the electrostatic modulation of the sensor's electrical response and how they allow controlling the device's sensing abilities.

We have studied the sensitivity of AlGaAs/GaAs double barrier resonant tunneling diode photodetectors with an integrated GaInNAs absorption layer for light sensing at the telecommunication wavelength of λ = 1.3 μm for illumination powers from pico- to microwatts. The sensitivity decreases nonlinearly with power. An illumination power increase of seven orders of magnitude leads to a reduction of the photocurrent sensitivity from S I  = 5.82 × 10(3) A W(-1) to 3.2 A W(-1). We attribute the nonlinear sensitivity-power dependence to an altered local electrostatic potential due to hole-accumulation that on the one hand tunes the tunneling current, but on the other hand affects the lifetime of photogenerated holes. In particular, the lifetime decreases exponentially with increasing hole population. The lifetime reduction results from an enhanced electrical field, a rise of the quasi-Fermi level, and an increased energy splitting within the triangular potential well. The non-constant sensitivity is a direct result of the non-constant lifetime. Based on these findings, we provide an expression that allows us to calculate the sensitivity as a function of illumination power and bias voltage, show a way to model the time-resolved photocurrent, and determine the critical power up to which the resonant tunneling diode photodetector sensitivity can be assumed constant.

Sensor miniaturization together with broadening temperature sensing range are fundamental challenges in nanothermometry. By exploiting a large temperature-dependent screening effect observed in a resonant tunneling diode in sequence with a GaInNAs/GaAs quantum well, we present a low dimensional, wide range, and high sensitive nanothermometer. This sensor shows a large threshold voltage shift of the bistable switching of more than 4.5 V for a temperature raise from 4.5 to 295 K, with a linear voltage-temperature response of 19.2 mV K(-1), and a temperature uncertainty in the millikelvin (mK) range. Also, when we monitor the electroluminescence emission spectrum, an optical read-out control of the thermometer is provided. The combination of electrical and optical read-outs together with the sensor architecture excel the device as a thermometer with the capability of noninvasive temperature sensing, high local resolution, and sensitivity.

Conventional semiconductor laser emission relies on stimulated emission of photons, which sets stringent requirements on the minimum amount of energy necessary for its operation. In comparison, exciton-polaritons in strongly coupled quantum well microcavities can undergo stimulated scattering that promises more energy-efficient generation of coherent light by 'polariton lasers'. Polariton laser operation has been demonstrated in optically pumped semiconductor microcavities at temperatures up to room temperature, and such lasers can outperform their weak-coupling counterparts in that they have a lower threshold density. Even though polariton diodes have been realized, electrically pumped polariton laser operation, which is essential for practical applications, has not been achieved until now. Here we present an electrically pumped polariton laser based on a microcavity containing multiple quantum wells. To prove polariton laser emission unambiguously, we apply a magnetic field and probe the hybrid light-matter nature of the polaritons. Our results represent an important step towards the practical implementation of polaritonic light sources and electrically injected condensates, and can be extended to room-temperature operation using wide-bandgap materials.

We create a large exciton-polariton condensate and employ a Michelson interferometer setup to characterize the short- and long-distance behavior of the first order spatial correlation function. Our experimental results show distinct features of both the two-dimensional and nonequilibrium characters of the condensate. We find that the gaussian short-distance decay is followed by a power-law decay at longer distances, as expected for a two-dimensional condensate. The exponent of the power law is measured in the range 0.9-1.2, larger than is possible in equilibrium. We compare the experimental results to a theoretical model to understand the features required to observe a power law and to clarify the influence of external noise on spatial coherence in nonequilibrium phase transitions. Our results indicate that Berezinskii-Kosterlitz-Thouless-like phase order survives in open-dissipative systems.

We report the fabrication of single mode quantum cascade lasers using a shallow-etched distributed Bragg reflector as frequency selective element. Quasi-continuous single mode tuning over 15 cm-1 at room temperature and 25 cm-1 via temperature tuning at Peltier temperatures is demonstrated. The behavior of both electro-optic and spectral characteristics under variation of the segment currents is analyzed, showing a maximum peak output power at room temperature of 600 mW. Thermal crosstalk between the laser segments is investigated. The spectral resolution of a gas absorption experiment is determined to be better than 0.0078 cm-1.

Chaos presents a striking and fascinating phenomenon of nonlinear systems. A common aspect of such systems is the presence of feedback that couples the output signal partially back to the input. Feedback coupling can be well controlled in optoelectronic devices such as conventional semiconductor lasers that provide bench-top platforms for the study of chaotic behaviour and high bit rate random number generation. Here we experimentally demonstrate that chaos can be observed for quantum-dot microlasers operating close to the quantum limit at nW output powers. Applying self-feedback to a quantum-dot microlaser results in a dramatic change in the photon statistics wherein strong, super-thermal photon bunching is indicative of random-intensity fluctuations associated with the spiked emission of light. Our experiments reveal that gain competition of few quantum dots in the active layer enhances the influence of self-feedback and will open up new avenues for the study of chaos in quantum systems.

Bose-Einstein condensation (BEC) is a thermodynamic phase transition of an interacting Bose gas. Its key signatures are remarkable quantum effects like superfluidity and a phonon-like Bogoliubov excitation spectrum, which have been verified for atomic BECs. In the solid state, BEC of exciton-polaritons has been reported. Polaritons are strongly coupled light-matter quasiparticles in semiconductor microcavities and composite bosons. However, they are subject to dephasing and decay and need external pumping to reach a steady state. Accordingly the polariton BEC is a nonequilibrium process of a degenerate polariton gas in self-equilibrium, but out of equilibrium with the baths it is coupled to and therefore deviates from the thermodynamic phase transition seen in atomic BECs. Here we show that key signatures of BEC can even be observed without fulfilling the self-equilibrium condition in a highly photonic quantum degenerate nonequilibrium system.