Patent Number: 
Section: claims

1. A probe apparatus for measuring an electron state on a sample surface, comprising:a cantilever provided with a probe opposing the sample surface with a gap, a charge transfer force being applied between the probe and the sample surface;a vibrating unit configured to support and vibrate the cantilever;a first light emitting module configured to emit a first light beam onto the cantilever;a sensor configured to sense the first light beam reflected from the cantilever to generate a sensor signal;a feedback circuit configured to drive the vibrating unit in response to the sensor signal so as to cause the cantilever to be self-excited and vibrated at a resonance frequency which is fluctuated due to a change of the charge transfer force;a second light emitting module configured to emit a second light beam to irradiate the sample surface, intermittently, the irradiation of the second light beam changing the charge transfer force; anda controller configured to control the second light emitting module in response to the sensor signal from the sensor, the controller detecting a current vibrating period of the probe vibrated at the resonance frequency from the sensor signal and setting an irradiation timing of the second light beam based on the current vibrating period. 2. The probe apparatus according to claim 1, further comprising:a measuring unit configured to measure a displacement of the resonance frequency from the sensor signal; anda generating unit configured to generate an image signal relating to the charge transfer force from the displacement of the resonance frequency. 3. The probe apparatus according to claim 1, further comprising:a voltage source configured to apply a voltage to one of the cantilever and the sample. 4. The probe apparatus according to claim 1, further comprising:a scanner, on which the sample is mounted, configured to move the sample in a direction perpendicular to the sample surface; anda driver configured to drive the scanner to maintain the gap between the probe and the sample surface in a predetermined range. 5. The probe apparatus according to claim 1, wherein the sample surface is irradiated with the second light beam at a predetermined irradiation timing after the probe is moved to the closest position to the sample surface. 6. The probe apparatus according to claim 1, wherein the controller sets the irradiation timing of the second light beam in a vibrating period following the current vibrating period, and the probe is moved to a position closest to the sample surface after the irradiation timing. 7. The probe apparatus according to claim 1, wherein the feedback circuit includes a phase shifter which compensates a delay of the sensor signal to output a compensated sensor signal, and a wave converter configured to convert the compensated sensor signal to a rectangular wave signal and supply the rectangular wave signal to the vibrating unit to drive the vibrating unit.