Typical band structure of one period of a quantum cascade detector. The optical quantum well absorbs the photon, while adjacent wells extract the photoexcited electron and cascade it into the next period.^[1]

A quantum cascade detector (QCD) is an infrared photodetector based on engineered multiple quantum wells. It detects incident infrared radiation through intersubband electronic transitions in a semiconductor heterostructure.^[2]

The word cascade refers to the path followed by photoexcited electrons through a sequence of coupled quantum wells. After absorbing a photon, an electron is promoted to a higher confined state and then transported through an extraction region into the next period of the structure.^[2]

QCDs are fabricated by stacking thin semiconductor layers on lattice-matched substrates using epitaxial growth techniques such as molecular-beam epitaxy and metal-organic vapor-phase epitaxy.^[1] By changing the thickness and composition of the wells and barriers, the detector wavelength can be tuned from short-wave infrared to long-wave infrared and terahertz regimes.^[3]^[4]^[5]^[6]

Unlike many photoconductive detectors, QCDs operate in photovoltaic mode and do not require an external bias to generate a photoresponse. In this sense they can be viewed as the photovoltaic counterpart of quantum well infrared photodetectors (QWIPs).^[7]

Applications

Because many molecular vibrational modes lie in the mid-infrared, QCDs are useful for infrared sensing and spectroscopy. They have been investigated for gas sensing, high-resolution spectroscopy, dual-comb systems and free-space optical communication.^[1]^[8]^[9]^[10]^[11]^[12]

History

In 2002, Daniel Hofstetter, Mattias Beck and Jérôme Faist reported the use of an InGaAs/InAlAs quantum-cascade-laser structure as a room-temperature photodetector. Its specific detectivity was comparable to established infrared detectors such as QWIPs and HgCdTe detectors.^[13]

This result led to research on bi-functional quantum cascade devices combining lasing and detection in a single photonic architecture.^[14]^[15]^[16]^[17]

The term quantum cascade detector was introduced in 2004 when L. Gendron and V. Berger demonstrated a cascade device dedicated to photodetection using a GaAs/AlGaAs heterostructure.^[2] Later work extended QCDs to material systems such as II-VI ZnCdSe/ZnCdMgSe, GaN/AlGaN and ZnO/MgZnO, enabling broader wavelength coverage and room-temperature operation.^[5]^[6]^[18]^[19]

Working principle

A QCD is a unipolar device: only one type of charge carrier, usually electrons, contributes to the photocurrent. Each period contains an optical quantum well and an extraction region. The optical well absorbs radiation by promoting an electron between confined subbands. The extraction region then transfers the electron through a cascade of states until it reaches the next period.^[1]

The electronic levels are controlled by band-gap engineering. In a simplified infinite-well model, the confined-state energy is

E_{n} (𝐤) = \frac{ℏ^{2} (k_{x}^{2} + k_{y}^{2})}{2 m_{e}^{*}} + \frac{π^{2} ℏ^{2}}{2 m_{e}^{*} W^{2}} n^{2},

where $ℏ$ is the reduced Planck constant, $𝐤$ is the in-plane wave vector, $m_{e}^{*}$ is the electron effective mass, $W$ is the well thickness and $n$ labels the confined state.^[21]

After photoexcitation, quantum tunnelling and scattering with longitudinal optical phonons transfer the electron through adjacent wells. Efficient extraction occurs when the energy spacing between states matches the optical phonon energy.^[22] Because longitudinal optical phonon scattering occurs on picosecond time scales, QCDs can have cut-off frequencies in the 100 GHz range.^[1]

Figures of merit

The responsivity of a quantum photodetector is

R (λ) = \frac{η λ e}{h c},

where $η$ is the external quantum efficiency, $λ$ is the wavelength, $e$ is the elementary charge, $h$ is Planck's constant and $c$ is the speed of light.^[23]

For a QCD,

R (λ) = η_{a b s} \frac{p_{e}}{N} \frac{λ e}{h c},

where $η_{a b s}$ is the absorption efficiency, $p_{e}$ is the extraction probability and $N$ is the number of active periods.^[24]

The specific detectivity $D^{*}$ is used to compare detectors with different area and bandwidth. When Johnson noise dominates, it can be estimated as

D^{*} = R_{p} \sqrt{\frac{R_{0} A}{4 K_{b} T}},

where $R_{p}$ is peak responsivity, $R_{0}$ is the zero-bias resistance, $A$ is detector area, $K_{b}$ is Boltzmann's constant and $T$ is temperature.^[2]^[22]^[24]

Optical coupling

As intersubband devices, QCDs absorb only TM-polarized light. This follows from intersubband selection rules, which require an electric-field component perpendicular to the quantum-well planes.^[26] Several coupling geometries are therefore used to direct suitable optical modes into the active region.^[1]

45° wedge configuration

In the 45° wedge configuration, incident light is reflected from a polished facet so that part of the radiation becomes TM-polarized inside the active region. It is simple to fabricate and is widely used for characterization, although only part of the input optical power is efficiently coupled.^[1]^[13]^[22]^[24]^[25]

Brewster-angle configuration

At the Brewster angle, p-polarized light is transmitted through an interface with minimal reflection. For a semiconductor of refractive index $n_{s}$ , the angle is

θ_{B} = a r c t a n (\frac{1}{n_{s}}) .

This method avoids tilted facets but generally couples only a small fraction of the total incident power into the detector.^[27]^[1]

Diffraction grating couplers

Metallic diffraction gratings can couple incident light to surface plasmon polaritons at the metal-semiconductor interface. These modes are TM-polarized and therefore compatible with intersubband absorption.^[28]^[29]

Waveguide coupling

Planar or ridge waveguides can confine the optical mode in the active region of a QCD. This approach is especially useful in integrated photonic devices and bi-functional quantum cascade laser/detector systems.^[1]^[15]^[16]^[30]

Physics:Quantum Cascade Detector

Applications

History

Working principle

Figures of merit

Optical coupling

45° wedge configuration

Brewster-angle configuration

Diffraction grating couplers

Waveguide coupling

See also

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