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In recent years, the use of ultrashort pulse lasers to excite noble gases to generate high-order harmonics provides a simple and cost-controllable extreme ultraviolet light source for time-resolved and angle-resolved photoelectron spectroscopy systems and extreme ultraviolet light coherent diffraction imaging.
The generation of higher harmonics is an extreme nonlinear optical process, the key of which is to focus intense laser pulses into controllable concentrations of noble gases.
1. Question
In the process of high-order harmonic generation, in order to reach practical application effects, the corresponding vacuum system needs to meet the following requirements:
(1) In the process of high-order harmonic generation, the change of macroscopic parameters will affect the atomic density and ionization fraction, and the corresponding macroscopic phase matching can be reached, so macroscopic parameters such as effective interaction length, laser intensity and gas can be used adaptive adjustment of the pressure to change the higher harmonic spectrum.
When changing these parameters, not only the harmonic order changes, but also the The spectral weighting also changes, which means that the gas pressure (vacuum level) inside the gas cell needs to be finely tuned and stably controlled in order to achieve the desired higher harmonic generation.
(2) Because high-order harmonics are easily absorbed by the atmosphere, it is required that the optical path of high-order harmonics must be maintained in a high vacuum state. It can be seen that the high-order harmonic generator needs to include two areas with different vacuum degrees, one is the low-vacuum area inside the gas cell with an absolute pressure range of 1~100Torr, and the other is the absolute pressure on the high-order harmonic optical path High vacuum area on the order of 0.001Pa.
This article will propose corresponding solutions for the control of vacuum degree in these two different areas, especially the stable and precise control of vacuum degree in the low vacuum area inside the gas cell is introduced in detail.
2. Solution
The vacuum chamber is divided into two areas according to the degree of vacuum, which are respectively used for gas cell and high-order harmonic transmission optical path. For vacuum control in these two areas, the following two vacuum circuits are used.
(1) High vacuum control circuit of optical path
In the high vacuum circuit of the vacuum chamber, the exhaust pipeline is directly connected to the wall of the vacuum chamber, and the gas in the vacuum chamber is pumped by a molecular pump to make it reach a high vacuum with an absolute pressure of 0.001Pa, and use a Pirani gauge to monitor changes in vacuum.
In order to take out the sample after the test is completed, it is necessary to inflate the vacuum chamber to return to the normal pressure atmosphere. An electric deflation valve is arranged on the wall of the vacuum chamber, and the opening and closing of the deflation valve can be controlled by the program.
(2) Low vacuum control circuit of gas cell The vacuum pipeline is directly connected to the gas pool, and forms a low vacuum with a high-pressure gas cylinder, an electric needle valve for adjusting the intake flow, a capacitance gauge, KaoLu’s Proportional Pressure Regulator for adjusting the exhaust flow, a dry pump, and a control loop of vacuum controller.
It is a dynamic balance control method to adjust the intake and exhaust flow to reach the vacuum degree control. The characteristic of this method is that it can reach the precise control of vacuum degree in the range of 1Pa~0.1MPa, especially the combination of 24-bit AD and 16 DA-bit dual-channel high-precision vacuum controller, the control accuracy can reach within ±1%.
For further information about KaoLu’s Proportional Pressure Regulator, please contact us for more details!
The generation of higher harmonics is an extreme nonlinear optical process, the key of which is to focus intense laser pulses into controllable concentrations of noble gases.
1. Question
In the process of high-order harmonic generation, in order to reach practical application effects, the corresponding vacuum system needs to meet the following requirements:
(1) In the process of high-order harmonic generation, the change of macroscopic parameters will affect the atomic density and ionization fraction, and the corresponding macroscopic phase matching can be reached, so macroscopic parameters such as effective interaction length, laser intensity and gas can be used adaptive adjustment of the pressure to change the higher harmonic spectrum.
When changing these parameters, not only the harmonic order changes, but also the The spectral weighting also changes, which means that the gas pressure (vacuum level) inside the gas cell needs to be finely tuned and stably controlled in order to achieve the desired higher harmonic generation.
(2) Because high-order harmonics are easily absorbed by the atmosphere, it is required that the optical path of high-order harmonics must be maintained in a high vacuum state. It can be seen that the high-order harmonic generator needs to include two areas with different vacuum degrees, one is the low-vacuum area inside the gas cell with an absolute pressure range of 1~100Torr, and the other is the absolute pressure on the high-order harmonic optical path High vacuum area on the order of 0.001Pa.
This article will propose corresponding solutions for the control of vacuum degree in these two different areas, especially the stable and precise control of vacuum degree in the low vacuum area inside the gas cell is introduced in detail.
2. Solution
The vacuum chamber is divided into two areas according to the degree of vacuum, which are respectively used for gas cell and high-order harmonic transmission optical path. For vacuum control in these two areas, the following two vacuum circuits are used.
(1) High vacuum control circuit of optical path
In the high vacuum circuit of the vacuum chamber, the exhaust pipeline is directly connected to the wall of the vacuum chamber, and the gas in the vacuum chamber is pumped by a molecular pump to make it reach a high vacuum with an absolute pressure of 0.001Pa, and use a Pirani gauge to monitor changes in vacuum.
In order to take out the sample after the test is completed, it is necessary to inflate the vacuum chamber to return to the normal pressure atmosphere. An electric deflation valve is arranged on the wall of the vacuum chamber, and the opening and closing of the deflation valve can be controlled by the program.
(2) Low vacuum control circuit of gas cell The vacuum pipeline is directly connected to the gas pool, and forms a low vacuum with a high-pressure gas cylinder, an electric needle valve for adjusting the intake flow, a capacitance gauge, KaoLu’s Proportional Pressure Regulator for adjusting the exhaust flow, a dry pump, and a control loop of vacuum controller.
It is a dynamic balance control method to adjust the intake and exhaust flow to reach the vacuum degree control. The characteristic of this method is that it can reach the precise control of vacuum degree in the range of 1Pa~0.1MPa, especially the combination of 24-bit AD and 16 DA-bit dual-channel high-precision vacuum controller, the control accuracy can reach within ±1%.
For further information about KaoLu’s Proportional Pressure Regulator, please contact us for more details!