JP3527460B2 - Ultrashort pulse X-ray pulse waveform measurement method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a pulse waveform of ultrashort pulse X-ray with high time resolution.

[0002]

2. Description of the Related Art In recent years, a light source for generating ultrashort pulse X-rays has been developed by utilizing high-power ultrashort pulse laser light. With these ultrashort pulse X-ray light sources, X-ray pulses of about 100 ps (picoseconds) to 100 fs (femtoseconds) have been obtained. Conventionally, in order to measure these X-ray pulse waveforms, an X-ray streak camera, which is an X-ray measuring instrument having high time resolution, has been widely used.

However, since the time resolution of the X-ray streak camera is about ps, there is a problem that ultrashort pulse X-rays having a time width of ps or less cannot be measured. Moreover, the time resolution of the X-ray streak camera is determined by the sweep speed of the voltage applied to the streak tube, the initial velocity distribution of photoelectrons, the spatial resolution of the electronic system, etc., and it is theoretically possible to realize sub-ps time resolution. Extremely difficult.

Therefore, a method of utilizing the ultrafast ionization of the rare gas caused by irradiating the rare gas with the ultrashort pulsed laser light as an ultrafast shutter, that is,
By irradiating the rare gas with ultrashort pulsed laser light,
An attempt was made to measure the pulse waveform of ultrashort pulsed X-rays by extinguishing neutral atoms and measuring the rapid change in X-ray absorption characteristic of neutral atoms (MHSher, U. Mohideen, HWK).
Tom, ORWood II, GDAumiller, and RRFreeman, Opt.L
ett. 18,646 (1993)). However, in this method, since it takes a long time for the neutral atoms that have been extinguished at an ultra high speed to reappear, it does not act as a gate having a narrow time width. It is necessary to differentiate the measured data. Therefore, there is a problem that the error becomes large and the reliability is low.

[0005]

As described above, the sub-p
It was not possible with an X-ray streak camera to measure the pulse waveform of an ultrashort pulse X-ray having a time width of s.
Further, in the method of utilizing the disappearance of neutral atoms accompanying the ultrafast ionization of the rare gas caused by irradiating the rare gas with the ultrashort pulsed laser as the ultrafast shutter,
There was a problem of low reliability.

The present invention has been made to solve the above problems, and its purpose is to provide an ultrashort pulse X which enables highly reliable measurement of the pulse waveform of an ultrashort pulse X-ray having a sub-ps time width. It is to provide a method for measuring a pulse waveform of a line.

[0007]

In order to achieve such an object, the present invention provides a pulse waveform measuring method for ultrashort pulse X-rays, which uses a change in X-ray absorption depending on a state of a specific valence of an ionized gas. , Irradiating the ionized gas with two laser pulses consisting of a first laser pulse and a second laser pulse to bring the ionized gas into the state of the specific valence by the first laser pulse, and the second laser pulse The ionized gas is put into a state of another valence, and the first
The duration of the state of the specific valence is controlled by changing the time interval between the laser pulse and the second laser pulse,
The feature is that the X-ray pulse waveform is measured with arbitrary time resolution.

That is, the laser pulse for ionizing the gas is characterized by using two ultrashort pulsed laser beams (first ionization laser pulse, second ionization laser pulse) with a time difference. In addition, by using two ultra-short pulse laser beams, it is possible to control the duration of a specific ion species (ions with a specific valence), and by using it as an ultra-fast gate, ultra-short pulse X-rays can be obtained. The pulse waveform of is measured.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an explanatory diagram showing the configuration of an ultrashort pulse X-ray pulse waveform measuring method according to an embodiment of the present invention. An ultrashort pulse laser beam 2 emitted from a high power ultrashort pulse laser device 1 is shown in FIG. The first beam splitter 3 splits the laser beam 4 into an X-ray generating laser beam 4 and an ionizing laser beam 5. The X-ray generation laser light 4 is applied to the X-ray generation target 8 arranged in the X-ray generation unit 7, and ultrashort pulse X-rays 9 are generated.

The ionizing laser light 5 passes through a first delay circuit 11 which controls the time interval between the ultrashort pulse X-ray 9 and the ionizing laser light 5. Next, the ionizing laser beam 5
Is transmitted by the second beam splitter 12 to the first ionization laser pulse 13 and the second ionization laser pulse 1
It is divided into four. Second ionization laser pulse 14
Is a second delay circuit 1 for controlling the time interval between the first ionization laser pulse 13 and the second ionization laser pulse 14.
5, and the third beam splitter 16 takes the same optical path as that of the first ionization laser pulse 13 again,
The gas 19 is irradiated.

The gas 19 supplied through the pipe 20 into the interaction section 18 is ionized into monovalent ions by the first laser pulse 13 for ionization. Further, the generated monovalent ions are ionized into divalent ions by the second ionization laser pulse 14 after the time interval Δτ _P.
Ultrashort pulse X-rays 9 focused by the X-ray focusing element 10 are irradiated into the gas thus ionized, and the X-ray transmission spectrum by the gas ions is measured by the X-ray spectroscope 21 and the X-ray detector 22. To do. It should be noted that reference numerals 6 and 17 denote laser light focusing elements (lenses, reflecting mirrors, etc.).

Next, how the pulse waveform of the ultrashort pulse X-ray is measured in the configuration of the X-ray pulse waveform measuring method as described above will be described.

FIG. 2 is a schematic diagram for explaining the principle of the X-ray pulse waveform measuring method. FIG. 2 (a) shows a gas by two pulses of a first ionization laser pulse 13 and a second ionization laser pulse 14. It is a schematic diagram which showed how the ionization of 19 advances.

The first ionizing laser pulse 13 arriving earlier by the gas 19 causes the originally existing neutral gas density to decrease rapidly and the monovalent ion density to increase rapidly. By appropriately selecting the intensity of the first laser light pulse for ionization, all neutral rare gases can be ionized into monovalent ions. Furthermore, the time interval Δ
When the second laser pulse 14 for ionization is irradiated after τ _P , the density of monovalent ions rapidly decreases and the density of divalent ions rapidly increases. Second ionization laser pulse 14
All monovalent ions can be ionized into divalent ions by appropriately selecting the intensity of the. Ultrashort pulsed X-ray 9 in which a time interval Δt is given to the laser light 5 for ionization in the gas 19 which causes such a temporal change in density.
Through.

FIG. 2B is a schematic diagram of the X-ray transmission spectrum showing how the X-ray transmission spectrum changes by changing the time interval Δt. neutral,
Focusing on the absorption lines characteristically appearing only in the monovalent ions and the divalent ions, the amount of absorption changes according to the time interval Δt. That is, the first ionization laser beam 13
If the ultra-short pulsed X-ray 9 passes through the gas before the arrival of, only the originally neutral absorption line is visible, as in the bottom spectrum. As the time interval Δt increases, the absorption line of monovalent ions gradually becomes visible. Furthermore, the absorption line of divalent ions begins to be seen and increases, and the absorption line of monovalent ions becomes weaker.

In FIG. 2B, attention is paid to the absorption amount of the absorption line of monovalent ions, and FIG. 2C is a graph showing the absorption amount as a function of the time interval Δt. The waveform shown in FIG. 2C shows an X-ray pulse waveform having a wavelength corresponding to the absorption line of monovalent ions in the ultrashort pulse X-ray 9.

FIG. 3 is a diagram showing the result of a numerical calculation simulation of the time variation of the ion density when Kr is selected as the gas 19. Time interval Δτ
_P is 300 fs, and the intensity of the first ionization laser pulse 13 and the second ionization laser pulse 14 is 3 × 1 respectively.
0 ¹⁴ W / cm ² , 1 × 10 ¹⁵ W / cm ² , pulse width 2
It was assumed to be 00fs. Under the conditions shown in FIG. 3, an X-ray pulse waveform of an ultrashort pulse X-ray of 15.5 nm, which corresponds to the wavelength of the absorption line of Kr monovalent ions, has a time resolution of about 300 f.
It is possible to measure with s.

Further, in the present invention, the time interval Δτ _P is
By adjusting the second delay circuit 15, the duration of monovalent ions can be controlled, and the time resolution can be selected. Further, the wavelength of the X-ray to be measured can be changed by changing the ion species (valence) of interest and the type of gas used to Ar, Xe, O ₂ , N _2, or the like.

[0020]

As described above, according to the present invention, the ionized gas is irradiated with two laser pulses consisting of the first laser pulse and the second laser pulse, and the ionized gas is irradiated with the specified value by the first laser pulse. Number of states, the ionized gas is changed to another state of valence by the second laser pulse, and the first
By changing the time interval between the laser pulse and the second laser pulse, the duration of the state of the specific valence is controlled, and the X-ray pulse waveform is measured with an arbitrary time resolution.
Ionization of gas using two pulses and measurement of absorption of ultrashort pulsed X-rays by the generated ions gives pulse waveform of ultrashort pulsed X-ray with high time resolution and high reliability. It is possible to obtain an extremely excellent effect.

Further, it is effective in all ultrashort pulse X-ray light sources which are generated by using a high-power ultrashort pulse laser such as X-ray by laser plasma and X-ray by high-order harmonic light, and which can be easily synchronized with laser light. There is an extremely excellent effect.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of a pulse waveform measuring method for an ultrashort pulse X-ray that is an embodiment of the present invention.

FIG. 2 is a diagram illustrating the measurement principle of the present invention.

FIG. 3 is a diagram showing a result of numerical simulation of time variation of ion density in Kr gas.

[Explanation of symbols]

1 High-power ultrashort pulse laser system 2 Ultra short pulse laser light 3 First beam splitter 4 X-ray generation laser light 5 Laser light for ionization 7 X-ray generator 8 X-ray generation target 9 Ultra short pulse 10 X-ray condensing element 11 First Delay Circuit 12 Second beam splitter 13 First Ionization Laser Pulse 14 Second Ionization Laser Pulse 15 Second delay circuit 16 Third beam splitter 18 Interaction part 19 gas 20 piping 21 X-ray spectrometer 22 X-ray detector 6,17 Condensing element

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Naoshi Uesugi 2-3-1, Otemachi, Chiyoda-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation (56) References JP 2001-56378 (JP, A) JP Heihei 5-99740 (JP, A) JP-A-10-221173 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01T 1/00 G01J 11/00 H01S 4/00 G01N 23 / 00

Claims (2)

(57) [Claims] 1. X depending on the state of specific valence of the ionized gas
In a method for measuring a pulse waveform of an ultrashort pulse X-ray using a change in line absorption, the ionized gas is irradiated with two laser pulses including a first laser pulse and a second laser pulse, and the first laser pulse causes The ionized gas is brought to the state of the specific valence, the ionized gas is brought to another state of the valence by the second laser pulse, and the time interval between the first laser pulse and the second laser pulse is changed to obtain the above-mentioned special valence. A pulse waveform measuring method for ultrashort pulse X-rays, characterized by controlling the duration of a constant-value state and measuring the X-ray pulse waveform at an arbitrary time resolution. 2. The ultrashort pulse X-ray pulse waveform measuring method according to claim 1, wherein the duration is controlled by delay means.