Quantum Cascade Detector Research Articles

Unipolar quantum optoelectronics for high speed direct modulation and transmission in 8–14 µm atmospheric window

The large mid-infrared (MIR) spectral region, ranging from 2.5 µm to 25 µm, has remained under-exploited in the electromagnetic spectrum, primarily due to the absence of viable transceiver technologies. Notably, the 8–14 µm long-wave infrared (LWIR) atmospheric transmission window is particularly suitable for free-space optical (FSO) communication, owing to its combination of low atmospheric propagation loss and relatively high resilience to turbulence and other atmospheric disturbances. Here, we demonstrate a direct modulation and direct detection LWIR FSO communication system at 9.1 µm wavelength based on unipolar quantum optoelectronic devices with a unprecedented net bitrate exceeding 55 Gbit s−1. A directly modulated distributed feedback quantum cascade laser (DFB-QCL) with high modulation efficiency and improved RF-design was used as a transmitter while two high speed detectors utilizing meta-materials to enhance their responsivity are employed as receivers; a quantum cascade detector (QCD) and a quantum-well infrared photodetector (QWIP). We investigate system tradeoffs and constraints, and indicate pathways forward for this technology beyond 100 Gbit s−1 communication.

Multi-Gb/s free-space laser communication at 4.6-μm wavelength using a high-speed, room-temperature, resonant-cavity infrared detector (RCID) and a quantum-cascade laser.

Research has shown that free-space laser communication systems may experience fewer outages due to atmospheric impairments such as haze, fog, clouds, and turbulence by operating at a longer wavelength in the mid-wave or long-wave infrared, if disadvantages such as lower-performance transceiver components may be overcome. Here we report a resonant cavity infrared detector (RCID) with 4.6-µm resonance wavelength that enables 20-dB larger link budget than has been reported previously for ∼ 5 Gb/s operation. The device combines high responsivity, 1.97 A/W, with a low noise equivalent power (NEP) of 0.7 pW/H z at room temperature, and a high bandwidth of 6.7 GHz at 3-dB. The relatively large surface-normal-incidence device with 30-µm diameter simplifies the coupling relative to intra-subband quantum cascade detectors. Although the RCID NEP is expected to increase with frequency to ∼ 1.5 pW/H z, we estimate that the total equivalent noise power in a 2.5-GHz bandwidth is less than 200 nW. When combined with a relatively high power (∼100-mW) distributed-feedback quantum cascade laser, the difference of > 50 dB between modulated laser power and RCID noise significantly outpaces that of existing devices.

Measurement of sub-Poissonian shot noise in a quantum cascade detector

In a Quantum Cascade Detector, photocurrent is generated by the absorption of infrared and terahertz radiation in the quantum-well-based modules arranged in series. Consequently, the current responsivity is by construction inversely proportional to the number of cascading modules. Upon absorption of a photon, the electron travels through only a single period of the detector, with a mean free path corresponding to the period length. Therefore, the shot noise power density is expected to decrease by the same factor under sufficiently high illumination, reflecting the same inverse relationship with the number of cascading modules. This phenomenon leads to sub-Poissonian noise characteristics. We experimentally observe this effect in a 90-period Quantum Cascade Detector operating at 4.5 μm, confirming a reduction in the shot noise contribution by the anticipated Fano factor of 1/90. This measurement underscores the suitability of these detectors for coherent detection scenarios, particularly where shot noise dominates.

Design and performance of GaSb-based quantum cascade detectors.

InAs/AlSb quantum cascade detectors (QCDs) grown strain-balanced on GaSb substrates are presented. This material system offers intrinsic performance-improving properties, like a low effective electron mass of the well material of 0.026 m 0, enhancing the optical transition strength, and a high conduction band offset of 2.28 eV, reducing the noise and allowing for high optical transition energies. InAs and AlSb strain balance each other on GaSb with an InAs:AlSb ratio of 0.96:1. To regain the freedom of a lattice-matched material system regarding the optimization of a QCD design, submonolayer InSb layers are introduced. With strain engineering, four different active regions between 3.65 and 5.5 µm were designed with InAs:AlSb thickness ratios of up to 2.8:1, and subsequently grown and characterized. This includes an optimized QCD design at 4.3 µm, with a room-temperature peak responsivity of 26.12 mA/W and a detectivity of 1.41 × 108 Jones. Additionally, all QCD designs exhibit higher-energy interband signals in the mid- to near-infrared, stemming from the InAs/AlSb type-II alignment and the narrow InAs band gap.

Very long wave infrared quantum cascade detector based on modular band structure

Very long wave infrared quantum cascade detector based on modular band structure

Simulations of 2-D Materials-Based Field Effect Transistors for Quantum Cascade Detectors

Simulations of 2-D Materials-Based Field Effect Transistors for Quantum Cascade Detectors

Broadband quantum cascade detectors with a cutoff wavelength of 20 μm

An InP-based photovoltaic quantum cascade detector with a cutoff wavelength of 20 μm is reported. A wave function tailored bound-to-miniband diagonal-transition active structure is proposed for the optical detection, effectively improving the photocurrent extraction efficiency and absorption strength, reducing the dark current leakage and consequently enhancing the responsivity and detectivity. The device demonstrates a broadband response of 16–20 μm over the operating temperature range. At 17.8 μm, a peak responsivity of 29.6 mA/W and a Johnson noise limited detectivity of 9 × 1010 Jones is obtained with a mesa of 200 μm × 200 μm, while maintaining a responsivity of 3.7 mA/W at 20 μm.

Electronic transport driven by collective light-matter coupled states in a quantum device

In the majority of optoelectronic devices, emission and absorption of light are considered as perturbative phenomena. Recently, a regime of highly non-perturbative interaction, ultra-strong light-matter coupling, has attracted considerable attention, as it has led to changes in the fundamental properties of materials such as electrical conductivity, rate of chemical reactions, topological order, and non-linear susceptibility. Here, we explore a quantum infrared detector operating in the ultra-strong light-matter coupling regime driven by collective electronic excitations, where the renormalized polariton states are strongly detuned from the bare electronic transitions. Our experiments are corroborated by microscopic quantum theory that solves the problem of calculating the fermionic transport in the presence of strong collective electronic effects. These findings open a new way of conceiving optoelectronic devices based on the coherent interaction between electrons and photons allowing, for example, the optimization of quantum cascade detectors operating in the regime of strongly non-perturbative coupling with light.

Influence of Fröhlich Interaction on Intersubband Transitions of InGaAs/InAlAs-Based Quantum Cascade Detector Structures Investigated by Infrared Modulated Photoluminescence

We demonstrate the use of an infrared modulated photoluminescence (PL) method based on a step-scan Fourier-transform infrared spectrometer to analyze intersubband transition (ISBT) of InGaAs/InAlAs quantum cascade detector (QCD) structures. By configuring oblique and parallel excitation geometries, high signal-to-noise ratio PL spectra in near-to-far-infrared region are measured. With support from numerical calculations based on the k ⋅ p perturbation theory, the spectra is attributed to intraband and interband transitions of InGaAs/InAlAs QCD structures. Temperature evolution results show that the k-dependent transitions caused by longitudinal optical phonon-assisted scattering (Fröhlich interaction) plays an important role in the ISBT. These results suggest that this infrared modulated-PL method has great potential in characterizing QCD devices and conducting performance diagnostics.

Broadband Response and a Transformation between Dual‐ and Single‐Wavelength Detection in Coupled Doped‐Well Quantum Cascade Detector

AbstractAs a third‐generation infrared detector, the quantum cascade detector (QCD) exhibits an accurately adjustable wavelength, low noise, and ultrafast response characteristics. By introducing an additional doping layer, QCD also shows excellent application prospects in the broadband response. Herein, a coupled doped‐well QCD with an array structure located at very long‐wave infrared (VLWIR, ≈15 µm) is prepared. Based on the energy levels interaction and carrier distribution, the regulatory mechanism of the applied bias on the response characteristics is explored. At zero bias, the detector exhibits dual‐wavelength detection owing to the splitting of the energy levels, then transforms into single‐wavelength detection with the bias increasing. Simultaneously, the QCD device exhibits a broadband response (≈13–16 µm) from 15 to 300 K and an excellent detectivity of 1.52 × 1012 cm Hz1/2 W−1 at 15 K. A high R0A (>106 Ω cm2) and robust detectivity (>109 cm Hz1/2 W−1) are obtained at room temperature. The results of the response characteristics presented in this work provide a strategy for the flexible application of QCD in infrared detection.

High Operating Temperature Mid-Infrared InGaAs/GaAs Submonolayer Quantum Dot Quantum Cascade Detectors on Silicon

Monolithic integration of infrared photodetectors on a silicon platform is a promising solution for the development of scalable and affordable photodetectors and infrared focal plane arrays. We report on integration of submonolayer quantum dot quantum cascade detectors (SML QD QCDs) on Si substrates via direct growth. Threading dislocation density has been reduced to the level of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 10^{7}$ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-2}$ </tex-math></inline-formula> with the high-quality GaAs-on-Si virtual substrate. We also conducted a morphology analysis for the SML QD QCDs through a transmission electron microscope strain contrast image and to the best of our knowledge, high quality InGaAs/GaAs SML QDs were clearly observed on silicon for the first time. Photoluminescence decay time of the as-grown SML QD QCDs on Si was measured to be around 300 ps, which is comparable to the reference QCDs on lattice-matched GaAs substrates. With the high-quality III-V epitaxial layers and SML QDs, the quantum cascade detectors on Si achieved a normal incident photoresponse temperature up to 160 K under zero bias.

Auto-Calibrated Charge-Sensitive Infrared Phototransistor at 9.3 µm.

Charge-sensitive infrared photo-transistors (CSIP) are quantum detectors of mid-infrared radiation (λ=4 µm-14 µm) which have been reported to have outstanding figures of merit and sensitivities that allow single photon detection. The typical absorbing region of a CSIP consists of an AlxGa1-xAs quantum heterostructure, where a GaAs quantum well, where the absorption takes place, is followed by a triangular barrier with a graded x(Al) composition that connects the quantum well to a source-drain channel. Here, we report a CSIP designed to work for a 9.3 µm wavelength where the Al composition is kept constant and the triangular barrier is replaced by tunnel-coupled quantum wells. This design is thus conceptually closer to quantum cascade detectors (QCDs) which are an established technology for detection in the mid-infrared range. While previously reported structures use metal gratings in order to couple infrared radiation in the absorbing quantum well, here, we employ a 45° wedge facet coupling geometry that allows a simplified and reliable estimation of the incident photon flux Φ in the device. Remarkably, these detectors have an "auto-calibrated" nature, which enables the precise assessment of the photon flux Φ solely by measuring the electrical characteristics and from knowledge of the device geometry. We identify an operation regime where CSIP detectors can be directly compared to other unipolar quantum detectors such as quantum well infrared photodetectors (QWIPs) and QCDs and we estimate the corresponding detector figure of merit under cryogenic conditions. The maximum responsivity R = 720 A/W and a photoconductive gain G~2.7 × 104 were measured, and were an order of magnitude larger than those for QCDs and quantum well infrared photodetectors (QWIPs). We also comment on the benefit of nano-antenna concepts to increase the efficiency of CSIP in the photon-counting regime.

A Quantum Cascade Detector With Enhanced Extraction Efficiency Utilizing Localized Electric Field

An InGaAs/AlInAs quantum cascade detector (QCD) with localized electric field is proposed to produce a 40% higher extraction efficiency of photon-induced electrons when comparing with a conventional QCD. The localized electric field introduced by modulation doping extends the degrees of freedom in the regulation of electronic states in quantum wells, which has significant influence on the transport of electrons. The electric field can enhance the extraction efficiency by enlarging the transition matrix element related to the extraction process. The proposed mid-IR QCD demonstrates a peak responsivity of 156.9 mA/W and a Johnson noise limited detectivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.9\times 10 ^{11}$ </tex-math></inline-formula> Jones at 80K.

High-Speed 9.6-μm Long-Wave Infrared Free-Space Transmission With a Directly-Modulated QCL and a Fully-Passive QCD

Free-space optics (FSO) in the mid-infrared (mid-IR) contains rich spectral resources for future ultrahigh-speed wireless communications yet is currently under-exploited. Two atmospheric transmission windows at the mid-IR, namely, the mid-wave IR (MWIR, 3–5 μm) and the long-wave IR (LWIR, 8–12 μm), show great potential in supporting free-space communications for both terrestrial and space application scenarios. Particularly, the LWIR signal with a longer wavelength has high intrinsic robustness against aerosols’ scattering and turbulence-induced scintillation and beam broadening effects, which are the main concerns hindering the wide deployment of practical FSO systems. In this context, high-bandwidth semiconductor-based mid-IR FSO transceivers will be desirable to meet the requirements of low energy consumption and small footprints for large-volume development and deployment. Quantum cascade devices, including quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), appear promising candidates to fulfill this role. In this work, we report a high-speed LWIR FSO transmission demonstration with a 9.6-μm directly-modulated (DM)-QCL and a fully passive QCD without any active cooling or bias voltage. Up to 8 Gb/s, 10 Gb/s, and 11 Gb/s signal transmissions are achieved when operating the DM-QCL at 10 °C, 5 °C, and 0 °C, respectively. These results indicate a significant step towards an envisioned fully-connected mid-IR FSO solution empowered by the quantum cascade semiconductor devices.

High bitrate data transmission in the 8-14 µm atmospheric window using an external Stark-effect modulator with digital equalization.

High bitrate mid-infrared links using simple (NRZ) and multi-level (PAM-4) data coding schemes have been realized in the 8 µm to 14 µm atmospheric transparency window. The free space optics system is composed of unipolar quantum optoelectronic devices, namely a continuous wave quantum cascade laser, an external Stark-effect modulator and a quantum cascade detector, all operating at room-temperature. Pre- and post-processing are implemented to get enhanced bitrates, especially for PAM-4 where inter-symbol interference and noise are particularly detrimental to symbol demodulation. By exploiting these equalization procedures, our system, with a full frequency cutoff of 2 GHz, has reached transmission bitrates of 12 Gbit/s NRZ and 11 Gbit/s PAM-4 fulfilling the 6.25 % overhead hard-decision forward error correction threshold, limited only by the low signal-to-noise ratio of our detector.

Room-Temperature Response Performance of Coupled Doped-Well Quantum Cascade Detectors with Array Structure

Energy level interaction and electron concentration are crucial aspects that affect the response performance of quantum cascade detectors (QCDs). In this work, two different-structured array QCDs are prepared, and the detectivity reaches 109 cm·Hz1/2/W at room temperature. The overlap integral (OI) and oscillator strength (OS) between different energy levels under a series of applied biases are fitted and reveal the influence of energy level interaction on the response performance. The redistribution of electrons in the cascade structure at room temperatures is established. The coupled doped-well structure shows a higher electron concentration at room temperature, which represents a high absorption efficiency in the active region. Even better responsivity and detectivity are exhibited in the coupled doped-well QCD. These results offer a novel strategy to understand the mechanisms that affect response performance and expand the application range of QCDs for long-wave infrared (LWIR) detection.

Longwave IR lattice matched L-valley Ge/GeSiSn waveguide quantum cascade detector.

We propose a lattice-matched Ge/GeSiSn quantum cascade detector (QCD) capable of operating in the longwave infrared. The optical absorption and carrier transport based on intersubband transitions all occur within the L-valley of the conduction band of the group-IV material system using N-doped quantum wells (QWs). The waveguided lattice matched structure can be deposited strain free on top of a Ge buffer grown on Si substrate, and is end-coupled to low-loss on-chip Ge waveguides. We optimized the QCD structure through the analysis of the photoresponsivity and detectivity D*. The QCD operates in photovoltaic mode with narrow spectral response that is peaked anywhere in the 9 to 16 µm range, tunable by design. This work aims to push the optical response of the photodetectors made from the SiGeSn material system to longer wavelengths. The study suggests the QCD response can indeed significantly extend the spectral range beyond that of the photodiodes and photoconductors made from the same group-IV system for a wide variety of applications in imaging, sensing, lidar, and space-and-fiber communications.

High-capacity free-space optical link in the midinfrared thermal atmospheric windows using unipolar quantum devices

Free-space optical communication is a very promising alternative to fiber communication systems, in terms of ease of deployment and costs. Midinfrared light has several features of utter relevance for free-space applications: low absorption when propagating in the atmosphere even under adverse conditions, robustness of the wavefront during long-distance propagation, and absence of regulations and restrictions for this range of wavelengths. A proof-of-concept of high-speed transmission taking advantage of intersubband devices has recently been demonstrated, but this effort was limited by the short-distance optical path (up to 1 m). In this work, we study the possibility of building a long-range link using unipolar quantum optoelectronics. Two different detectors are used: an uncooled quantum cascade detector and a nitrogen-cooled quantum well-infrared photodetector. We evaluate the maximum data rate of our link in a back-to-back configuration before adding a Herriott cell to increase the length of the light path up to 31 m. By using pulse shaping, pre- and post-processing, we reach a record bitrate of 30 Gbit s − 1 for both two-level (OOK) and four-level (PAM-4) modulation schemes for a 31-m propagation link and a bit error rate compatible with error-correction codes.

Monolithically integrated mid-infrared sensor with a millimeter-scale sensing range.

On-chip sensors based on quantum cascade laser technology are attracting broad attention because of their extreme compactness and abundant absorption fingerprints in the mid-infrared wavelength range. Recent continuous wave operation microcavity quantum cascade lasers are well suited for high-density optoelectronic integration because their volumes are small and thresholds are low. In this experimental work, we demonstrate a monolithically integrated sensor comprising a notched elliptical resonator as transmitter, a quantum cascade detector as receiver, and a surface plasmon structure as light-sensing waveguide. The sensor structure is designed to exploit the highly unidirectional lasing properties of the notched elliptical resonator to increase the optical absorption path length. Combined with the evanescent nature of the dielectric loaded surface plasmon polariton waveguides, the structure also ensures a strong light-matter interactions. The sensing transmission distance obtained is approximately 1.16 mm, which is about one order of magnitude improvement over the traditional Fabry-Perot waveguide. This sensor opens new opportunities for long-range and high-sensitivity on-chip gas sensing and spectroscopy.

High-responsivity operation of quantum cascade detectors at 9 µm.

Quantum cascade detectors (QCDs) are devices operating at zero external bias with a low dark-current. They show linear detection and high saturation intensities, making them suitable candidates for heterodyne detection in long-wave infrared (LWIR) free space optical communication systems. We present an approach to mitigate the performance limitation at long wavelengths, by a comparison of similar single and multi-period QCDs for optimizing their responsivity and noise behaviour. Our InGaAs/InAlAs/InP ridge QCDs are designed for operation at λ = 9.124 µm. Optical waveguide simulations support the accurate optical characterization. A detailed device analysis reveals room-temperature responsivities of 111 mA/W for the 15-period and 411 mA/W for the single-period device.