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We show that electrons on liquid helium display intrinsic bistability of resonant inter-subband absorption. The bistability occurs for comparatively weak microwave power. The underlying giant nonlinearity of the many-electron response results from the interplay of the strong short-range electron correlations, the long relaxation time, and the multi-subband character of the electron energy spectrum. 73.63.Hs, 03.67.Lx, 42.65.Pc Electrons on liquid helium provide a unique tool for studying correlation effects in two-dimensions (2D). The ratio of the characteristic Coulomb energy to the in-plane kinetic energy, the plasma parameter Γ = e 2 (πn s ) 1/2 /k B T (n s is the electron surface density), can vary by orders of magnitude, from Γ 1 where the electron system is a weakly-interacting gas to Γ > 130 where it is a Wigner crystal [1,2]. In the broad range 1 < Γ < 130 the electron system is a correlated 2D electron liquid, with unusual and sometimes counter-intuitive properties of classical and quantum 2D magneto-transport [3] and activated and tunneling escape from the surface [4].The effect of correlations in an electron liquid was also seen in weak-field spectroscopy of transitions to excited subbands of motion along the helium surface [5]. In a sense, it is a counter-part, for a strongly correlated system, of the depolarization effect in inter-subband absorption in semiconductor heterostructures with high electron densities [6]. For semiconductors, of much interest are both correlation transport effects [7] and nonlinear optical effects related to radiation-induced population of excited subbands [8]. Long sought has been optical bistability in intersubband absorption [9].For electrons on helium, optical nonlinearity should be strong, since the electron relaxation time is unusually long, reaching ∼ 10 −7 s for T > ∼ 0.1 K, and saturation of resonant inter-subband absorption has been indeed seen [10]. The interplay of the long relaxation time and strong spatial correlations provides a qualitatively new nonlinearity mechanism and should lead to new resonant effects. Such effects are indeed found in this paper.Our central result is the first, to the best of our knowledge, direct experimental observation and a theory of the bistability of resonant inter-subband absorption in a correlated electron system. The bistability is due to the correlation-induced strong dependence of the intersubband transition frequencies of an electron on the state of other electrons. An additional interest in this dependence comes from the proposals of quantum computing with electrons on helium [11], as it provides a mechanism of two-qubit gate operations.We study electrons on liquid 3 He. The experimental setup is similar to that previously described [12]. Electrons are confined on a helium surface between two circular parallel electrodes in an asymmetric potential formed by the barrier on the surface, the image force, and an electric field E ⊥ normal to the surface from the voltage on the electrodes [13]. The quantized energies of 1D ...

Measurements of the dc resistivity of surface-state electrons on liquid helium exposed to microwave radiation are reported. It is shown that the resonant microwave excitation of surface-state electrons is accompanied by a strong increase in their resistivity, which is opposite to the result expected from the previously used two-level model. We show that even a very small fraction of electrons excited to the first excited state and decaying back due to vapor-atom scattering strongly heat the electron system, causing a population of higher subbands. The calculated resistivity change is in good agreement with the observed data.