Use of optical polarization states to control a ponderomotive phase plate
A ponderomotive phase plate, also called a laser phase plate or standing wave optical phase plate, has a first minor and a second minor that define an optical cavity. An electron beam passes through a focal spot of the optical cavity. A laser with variable polarization angle of laser light is coupled to the optical cavity. A standing wave of polarized laser light, with an anti-node at the focal spot of the optical cavity, causes variable modulation of the electron beam. The variable modulation of the electron beam is controllable by the variable polarization angle of the laser light. In a transmission electron microscope, an image plane receives the electron beam modulated by the standing wave optical phase plate. An image formed at the image plane is based on the variable polarization angle of the polarized laser light.
1. A system, comprising:
a transmission electron microscope (TEM) having a back focal plane;
a plurality of mirrors forming an optical cavity, a focal spot of the optical cavity positioned at the back focal plane of the TEM, the optical cavity positioned to allow an electron beam provided by the TEM to pass through the focal spot of the optical cavity, and the optical cavity being operable to admit a laser beam;
a laser with variable polarization angle of laser light coupled to the optical cavity and operable to provide a laser beam of a specified wavelength and the variable polarization angle to enter the optical cavity, the laser beam being reflected from the plurality of mirrors to provide a standing wave optical phase plate focused at the back focal plane of the TEM to cause a modulation of the electron beam; and
an image plane of the TEM positioned to receive the electron beam modulated by the standing wave optical phase plate, to form an image according to the variable polarization angle.
2. The system of claim 1 , further comprising:
a half-wave plate; and
a rotator, arranged to hold and rotate the half-wave plate to provide the variable polarization angle of the laser light.
3. The system of claim 1 , further comprising:
a fiber-optic member to couple the laser to the optical cavity, wherein the fiber-optic member is bendable or rotatable to provide the variable polarization angle of the laser light.
4. The system of claim 1 , further comprising:
an electron camera or one or more sensors, positioned at the image plane and operable to analyze a Ronchigram and provide feedback for automatic control of the variable polarization angle of the laser light.
5. The system of claim 1 , wherein the polarization angle of the laser light is variable between at least a first phase plate profile having a standing wave in a Ronchigram formed at the image plane, and a second phase plate profile without a standing wave in the Ronchigram.
6. The system of claim 1 , wherein the polarization angle of the laser light has two or more presets.
7. The system of claim 1 , wherein the polarization angle of the laser light has one or more of a manual adjustment or an automatic adjustment.
8. A method, comprising:
generating an electron beam in a transmission electron microscope (TEM) having a back focal plane;
admitting the electron beam along an axis through a center of an optical cavity, the optical cavity being positioned at the back focal plane, and the optical cavity being defined by a first mirror and a second mirror;
admitting a laser beam having a variable polarization angle of laser light to the optical cavity, the laser beam being reflected from the first mirror and the second mirror to generate a standing wave optical phase plate focused at the back focal plane of the TEM to cause a modulation of the electron beam;
imaging the electron beam in an image plane of the TEM positioned to receive the electron beam modulated by the standing wave optical phase plate, to form an image; and
varying the polarization angle of the laser light.
9. The method of claim 8 , further comprising:
rotating a half-wave plate, to provide the variable polarization angle of the laser light.
10. The method of claim 8 , further comprising:
rotating or bending a fiber-optic member that couples a laser to the optical cavity, to provide the variable polarization angle of the laser light.
11. The method of claim 8 , further comprising:
analyzing, based on output of sensors or an electron camera, a Ronchigram that is formed at the image plane; and
controlling the variable polarization angle of the laser light, based on the analyzing.
12. The method of claim 8 , wherein the varying the polarization angle of the laser light is to vary contrast enhancement of the image, and wherein varying the polarization angle of the laser light comprises:
varying between a first phase plate profile having a standing wave in a Ronchigram formed at the image plane, and a second phase plate profile without a standing wave in the Ronchigram.
13. The method of claim 8 , wherein the varying the polarization angle of the laser light comprises:
determining the polarization angle of the laser light based on two or more presets.
14. The method of claim 8 , wherein the varying the polarization angle of the laser light comprises:
determining the polarization angle of the laser light based on manual adjustment or automatic adjustment.
15. A system, comprising:
a transmission electron microscope (TEM);
a plurality of mirrors forming an optical cavity, the optical cavity positioned to allow an electron beam provided by the TEM to pass through a focal spot of the optical cavity, and the optical cavity being operable to admit a laser beam;
a laser with variable polarization angle of laser light coupled to the optical cavity and operable to provide a laser beam of a specified wavelength and the variable polarization angle to enter the optical cavity, the laser beam being reflected from the plurality of mirrors to provide a standing wave optical phase plate to cause a modulation of the electron beam; and
an image plane of the TEM positioned to receive the electron beam modulated by the standing wave optical phase plate, to form an image according to the variable polarization angle.