Intersubband transitions in n-doped multi-quantum-well semiconductor heterostructures make it possible to engineer one of the largest known nonlinear optical responses in condensed matter systems--but this nonlinear response is limited to light with electric field polarized normal to the semiconductor layers. In a different context, plasmonic metasurfaces (thin conductor-dielectric composite materials) have been proposed as a way of strongly enhancing light-matter interaction and realizing ultrathin planarized devices with exotic wave properties. Here we propose and experimentally realize metasurfaces with a record-high nonlinear response based on the coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered electronic intersubband transitions in semiconductor heterostructures. We show that it is possible to engineer almost any element of the nonlinear susceptibility tensor of these structures, and we experimentally verify this concept by realizing a 400-nm-thick metasurface with nonlinear susceptibility of greater than 5 × 10(4) picometres per volt for second harmonic generation at a wavelength of about 8 micrometres under normal incidence. This susceptibility is many orders of magnitude larger than any second-order nonlinear response in optical metasurfaces measured so far. The proposed structures can act as ultrathin highly nonlinear optical elements that enable efficient frequency mixing with relaxed phase-matching conditions, ideal for realizing broadband frequency up- and down-conversions, phase conjugation and all-optical control and tunability over a surface.

Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions

Thanks to the recent progresses in epitaxy techniques, it is now possible to grow alternating layers of semiconductors with different band gap energies with a thickness control down to one atomic layer. It is now well established that these systems behave, as far as the electron motion is concerned, as a succession of potential barriers and wells. If the width of the potential well is less than the deBroglie wavelength of the electrons in the material (e.g. less than ≈15 nm in GaAs), the motion of the electrons may be considered, at sufficiently low temperature, as quantized in the direction normal to the growth axis1. The electrons are quantized into subbands where their wavefunctions in the growth direction have the form of envelope functions with an extension equal to the well width, i.e. in the few nanometer range. Electromagnetic waves may induce electronic transitions between these subbands. The dipole matrix elements associated to these intersubband transitions (ISBT) have the same order of magnitude as the quantum well width leading to extremely large absorption. These large absorptions have been observed for the first time in GaAs/AlGaAs multiquantum wells (MQW) by West and Eglash2.

Model system for optical nonlinearities: Asymmetric quantum wells.

Second-order optical nonlinearities in materials are of paramount importance for optical wavelength conversion techniques, which are the basis of new high-resolution spectroscopic tools. Semiconductor technology now makes it possible to design and fabricate artificially asymmetric quantum structures in which optical nonlinearities can be calculated and optimized from first principles. Extremely large second-order susceptibilities can be obtained in these asymmetric quantum wells. Moreover, properties such as double resonance enhancement or electric field control will open the way to new devices, such as fully solid-state optical parametric oscillators.

https://science.sciencemag.org/content/sci/271/5246/168.full.pdf

Quantum Engineering of Optical Nonlinearities

We study the effects of excitons in nonlinear optical rectification in one-dimensional semiparabolic quantum dots. We consider the cases that the electron and the hole are confined in semiparabolic potentials (i) with the same oscillator frequency and (ii) with the same width. In the first case we present approximate analytical results in the strong confinement regime and find a closed-form relation between the nonlinear optical rectification coefficient of an exciton and the nonlinear optical rectification coefficient when only one electron exists in the structure. In the second case we use the Hartree-Fock approximation and the potential morphing method and present results for the nonlinear optical rectification coefficient in all confinement regimes.

Effects of excitons in nonlinear optical rectification in semiparabolic quantum dots

A photovoltaic intersubband detector based on electron transfer on a cascade of quantum levels is presented: A quantum cascade detector (QCD). The highest photoresponse of intersubband transition-based photovoltaic detectors is demonstrated: 35mA∕W at null bias. The deduced absorption is of the same order of magnitude as that of a classical quantum-well infrared photodetector, i.e., 20%. Because they work with no dark current, QCDs are very promising for small-pixel large focal plane array applications.

Quantum cascade photodetector

We report the first experimental evidence of a nonlinear optical effect due to intersubband transitions in compositionally asymmetrical multiquantum wells. The effect is detected as an optical rectification signal appearing at the structure terminals when irradiated by a continuous 10.6 μm CO2 laser. The net electro‐optical coefficient of the structure is found to be 7.2 nm/V which is more than three orders of magnitude higher than for bulk GaAs. The results are in good agreement with theoretical predictions.

Observation of nonlinear optical rectification at 10.6 μm in compositionally asymmetrical AlGaAs multiquantum wells

We report the first experimental evidence of second harmonic generation due to inter-subband transitions in compositionally asymmetrical multiquantum wells using a continuous CO2 laser. The extremely large value of the second-order susceptibility (χ(2)2ω ~ 7.6×10−7m/V) is ingood agreement with theoretical predictions.

Second harmonic generation by intersub-band transitions in compositionally asymmetrical MQWs

The role of the contact electrodes on the detection mechanisms in quantum‐well photoconductors is usually overlooked. We show that these contacts may be blocking in a large temperature range. Upon illumination, the electric field is enhanced at the cathode electrode so that the injected current meets the quantum‐well requirement. This mechanism is demonstrated in a single quantum‐well photoconductor by the detailed study of the photocurrent over dark current ratios and by photocapacitance measurements.

Injection mechanism at contacts in a quantum‐well intersubband infrared detector

For the first time, an electrically tunable infrared photoconductor is made with GaAs/AlGaAs asymmetric step multiquantum wells, using the linear Stark shift of the intersubband transition. An applied electric field excursion of ±40 kV/cm is sufficient to shift the peak responsivity wavelength from 8.5 to 13.5 μm. The photoresponse tunability is studied by comparing photocurrent and absorption spectra for different applied electric fields.

Electrical tunability of infrared detectors using compositionally asymmetric GaAs/AlGaAs multiquantum wells

Electrical switching of a bispectral infrared photoconductor is demonstrated with GaAs/AlGaAs asymmetric step multiquantum wells, presenting bound‐to‐bound (tunable 8.5–9.0 μm) and bound‐to‐extended (≊5.5 μm) intersubband transitions of similar oscillator strengths. The bound‐to‐bound photoresponse is switched on by applying an electric field of sufficient magnitude to permit the collection of the photoexcited bound electrons by tunneling.

S. Delaitre

Papers

Observation of nonlinear optical rectification at 10.6 μm in compositionally asymmetrical AlGaAs multiquantum wells

Second harmonic generation by intersub-band transitions in compositionally asymmetrical MQWs

Injection mechanism at contacts in a quantum‐well intersubband infrared detector

Electrical tunability of infrared detectors using compositionally asymmetric GaAs/AlGaAs multiquantum wells

Switchable bicolor (5.5–9.0 μm) infrared detector using asymmetric GaAs/AlGaAs multiquantum well