Diode Laser-Based Optical Flux Monitor for MBE Control

A diode laser method for the real-time measurement of absolute flux concentrations and spatial profiles of atomic beams used in MBE will be tested. The proposed method uses the doubled output of a near-infrared diode laser to access atomic absorption lines of species such as Ga, Al and In. The detection method will combine high frequency wavelength modulation and noise canceler approaches in order to achieve absorbance sensitivity below 1 x 10-5 at 1 Hz (equivalent to 0.1% resolution of a monolayer/sec flux rate). In addition to the line of sight absorbance measurements, phase sensitive atomic emission measurements made using the modulate diode laser light source will provide spatial resolution of the atomic beam. Compared with other optical methods, the proposed technique will result in more than a 100 times increase in sensitivity and will eliminate absorbance selectivity problems caused by window deposits and particle scattering. During Phase 1, the feasibility of this approach will be established by measuring Ga atoms using a doubled 834 nm diode laser. The Phase 1 results will lead to the development of diode laser monitors for Ga, A1 and In, in Phase 2.