Midinfrared Ultrastrong Light–Matter Coupling for THz Thermal Emission
Résumé
In condensed matter, THz optical resonances (1−5 THz) are very sensitive to thermally activated phenomena that can either equate the populations between the levels or irremediably broaden the transition to a point where it disappears. It is therefore very difficult to exploit THz electronic transitions for thermal emission. To bypass this problem, we have used a transition in the mid-infrared (10−50 THz), ultrastrongly coupled to a resonant mode of a highly subwavelength microcavity. The coupling strength in our system reaches 90% of the energy of the matter resonance and becomes so high that the lower-polariton state lies in the THz region. This mixed light−matter resonance is therefore issued from an optical transition that is much less sensitive to thermal effects, as we have experimentally demonstrated. Our system has been optimized by tailoring the cavity thickness and engineering the matter resonance, which arises from a set of heavily doped quantum wells, in order to increase the THz emissivity. By injecting a lateral current in the quantum wells that raises the electronic temperature, we have observed THz emission up to room temperature. U ltrastrong light−matter coupling is a regime of quantum electrodynamics that occurs when the light−matter coupling energy E R is of the same order of magnitude as the matter excitation energy. 1 Similarly to the case of strong coupling regime, the eigenstates of the system, the polaritons, are mixed states involving a matter and a photonic part. 2 In the ultrastrong configuration, however, a wide gap opens up in the polaritonic dispersion 3 that compresses the lower polariton branch at very low energy, as it is illustrated in Figure 1a. The ultrastrong coupling allows us therefore to take advantage of a robust matter resonance in the mid-infrared spectrum (100− 200 meV) to generate a polaritonic mode in the THz region (15−25 meV), almost 1 order of magnitude lower. Note that, in the limit of E R equal to the matter excitation, the lower polariton (LP) vanishes and only the upper polariton (UP) survives. 4 In this work we have exploited this property to demonstrate a quasi-monochromatic THz incandescent source. Our device does not need to be cooled, as opposed to usual THz electroluminescent sources. The resonance at THz frequencies arises from the ultrastrong coupling of a mid-infrared collective excitation, 4 in a doped semiconductor layer, known as Berreman mode, 5 with a plasmonic microcavity mode. The incandescent emission is excited by a current flowing in the semiconductor layer and then extracted from the top surface by a metallic grating. Our device architecture presents three main advantages with respect to THz incandescent devices existing in the literature. First of all, only the absorbing medium is heated, 6,7 thus, allowing a faster frequency modulation of the incandescence compared to systems in which the entire structure is heated. 8−10 The second advantage resides in the fact that our device is based on a robust mid-infrared collective mode, instead of a single particle THz excitation. 11 This has an important consequence on the robustness in temperature of the material resonance. Indeed a collective mode has a harmonic oscillator spectrum, which cannot be saturated when increasing the temperature, as opposed to the case of a two level system. Furthermore, the collective mode is issued from the excitation of an electron gas with Fermi energy much larger than k B T: the electronic distribution is thus almost unaffected by the temperature. The third advantage of our device is the fact that the photonic structure operates as a microcavity and also as an antenna, thus ensuring a good coupling of the polariton modes with the free space. 12,13 A sketch of our device is illustrated in Figure 1b. In Figure 1c, one can see a scanning electron microscope (SEM) picture of a