A sensitive detection method for magnetization-induced second-harmonic generation under an externally applied field

Abstract

We describe the instrumentation related to the first observation of magnetization-induced enhancement of surface second-harmonic generation (SHG) from the paramagnetic Si(111)-7×7 surfaces. A judicious choice of polarization and sample orientation enabled us to isolate the magnetic-field-dependent tensor element of the nonlinear susceptibility. A conductive liquid-nitrogen system, coupled to an ultrahigh-vacuum system that is immune to the high magnetic field of 10 T, cooled the sample to about 120 K. A high extinction that is necessary to detect the magnetization-induced SHG (MSHG) was accomplished by minimizing stray optical effects such as Faraday rotation, photoluminescence and thermal birefringence in all optical components, with extra care taken for the UHV window. Consistent and stable operation of this sensitive measurement system permitted experiments involving MSHG at multiple wavelengths and temperatures. Probing the magnetization-induced optical nonlinearity was validated by quenching the surface states by oxidation of the surface layers. From MSHG measurements at two wavelengths resonant with different surface-state transitions, we were able to suggest that the observed MSHG is proportional to the number of dangling bond states of Si(111)-7×7.