No abstract

DlFFRACTiON of light by an aperture is a well-known manifestation of the \vave nature of light. The most familiär case is that of an incident plane wave, which is diffracted into a spatial pattern that is sensitive to the properties of the aperture: the ratio of transmitted power to incident flux (the transmission cross-section σ) depends in a complicated way on the aperture area A (refs 1-3). For diffuse (that is, Isotropie rather than plane-wave) Illumination, however, the Situation is much simpler 4 : in three dimensions, σ increases with A in a series of Steps of equal height Α 2 /2ττ (where A is the wavelength of the light), and is thus independent of the detailed aperture shape. A similar simplification occurs for two-dimensional diffuse Illumination of a slit: the transmission cross-section per unit slit length increases in stepwise fashion äs a function of the slit width W, with Steps of height A/2 occurring whenever W = ηλ/2 (π = l, 2,3,...)-that is, whenever a new mode is enabled in the slit. Although the optical transmission characteristics of slits have been studied extensively for plane-wave Illumination 5 " 8 , we know of no investigation of this predicted staircase dependence for diffuse Illumination. Here we report the observation of such an effect, and suggest that it may play a part in any process of wave propagation through a constriction.The argument of ref. 4 is based on the analogy with the recently discovered quantization of conductance in ballistic electron transport 9 ' 10 . The conductance of a point contact in a two-dimensional electron gas increases in steps of 2e 2 / h äs its width is increased (e is the Charge on an electron). The origin of this effect is the quantization of the electrons' transverse momentum owing to lateral confinement within the point contact. This leads to the formation of one-dimensional sub-bands in the conduction band (analogous to the formation of transverse λ = 1.55 μιη

In a type-II GaAs-AlAs multiple quantum well three optically detected magnetic resonance lines and two level anticrossings were observed. Two of the resonance lines and the two level anticrossings are in agreement with the electronic level scheme of the heavy-hole exciton. The third resonance line is in accordance with a magnetic spin resonance of an unbound electron. These optically detected magnetic resonance measurements open up the possibility to obtain detailed information about the excitons in and the band structure of type-II quantum wells.