Two Schottky contacts permit the effective application of an external bias to the hetero- structure. b Close-up of a single 1D ribbon of the device. By applying on it an external bias, the amplitude of the reflected beam is modulated. The response bandwidth has been measured at room-temperature, showing a −3 dB cutoff at 750 MHz and the modulation of a mid-IR laser beam up to 1.5 GHz has also been reported.Ī Sketch of the modulator geometry: the active region is embedded in a metal–metal structure. We demonstrate a clear modulation of the strong coupling condition after bias application. The system is designed to operate in reflectance: it is in strong coupling when no external bias is applied. It’s based on a GaAs/AlGaAs heterostructure, embedded in a metal–metal optical resonator (scheme in Fig. In this article we follow the latter strategy by proposing a standalone device capable of modulating a mid-IR beam at room-temperature up to 1.5 GHz. This approach does not require a specific integration of the source and can in principle be applied to laser sources beyond QCLs. An alternative is to develop modulators that can apply an ultra-fast RF modulation to a propagating beam, either in reflection or in transmission. In both cases the QCL source must be properly integrated in the system. It can rely on SiGe/Si photonic platforms 18, or on the more natural InGaAs/AlInAs-on-InP platform 19. One way forward is photonic integration: scaling to the mid-IR the approach already developed for silicon photonics. On the other hand, high biases can be a significant problem when targeting fast and/or ultra-fast performances. Higher doping is necessary to achieve the same Rabi splitting: this means higher biases to get a frequency tunability comparable to that obtained in systems based on charge modulation. Exploiting the Stark effect in ISB-based systems can effectively lead to impressive performances, but it can suffer of an intrinsic drawback: the diagonal transition has lower oscillator strength with respect to a vertical one.
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In 14, 15, 16, 17 a giant confined Stark effect in a coupled QW system embedded in a metallic resonator, designed to be in the strong coupling regime between light and matter, was exploited. An alternative approach is to frequency shift the ISB absorption, instead of modulating its intensity. These devices operate on the principle that, at a given wavelength, a change in absorption translates in a modulation of the transmitted power. Recently, different approaches have been proposed to actively tune the reflectance/transmission of mid-IR and/or THz beams: phase transition in materials like VO 2, liquid crystals orientation, carrier density control in metal–insulator-semiconductor junctions 13. Operation at room-temperature was obtained in ref. In both cases, the application of an external bias depletes or populates the ground state of the QW at cryogenic temperatures (from 4 K up to 130 K) thus inducing a modulation of the ISB absorption 11. The first attempts based on the Stark shift were then followed by a number of works exploiting coupled QWs 9, 10. Since the 80s, proposals have been put forward to exploit intersubband (ISB) absorption in semiconductor quantum well (QW) systems to modulate mid-IR radiation. Holmström 8 shows numerical performances up to 190 GHz with step QWs in a waveguide geometry at λ = 6.6 μm, but no experimental data are provided.
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To date, standalone, efficient and broadband amplitude/phase modulators are missing from present mid-IR photonics tools. The latter ones can operate up to modulation speeds of 20 GHz, but their efficiency is very low: <0.1% sideband/carrier ratio (see “Methods” for definition). For modulators, this leads to advantages in terms of RF power, laser linewidth and flatness of the modulation bandwidth.Ĭommercially available mid-IR modulators are either acousto-optic devices with narrow modulation bandwidth, or very narrow band ( ~100 kHz) electro-optic modulators based on GaAs or CdTe 7. Interestingly, in the visible/near-IR spectral ranges the preferred solution is to separate the functionalities: independent modulators, filters, interferometers are employed that are physically separated from the source. The fastest modulation speeds, 20–30 GHz, have been obtained with the direct modulation of mid-IR quantum cascade lasers (QCLs), but this requires specially designed devices and elevated injected RF (radiofrequency) powers 4, 5, 6. However, the fast and ultra-fast (1–40 GHz) modulation of mid-IR radiation is a largely under-developed functionality. Fast amplitude and phase modulation are essential for a plethora of applications in mid-infrared (IR) photonics, including laser amplitude/frequency stabilization 1, coherent detection, FM (frequency modulation) and AM (amplitude modulation) spectroscopy and sensing, mode-locking, and optical communications 2, 3.