Patent ID: 12222365

DESCRIPTION OF EMBODIMENTS

First Embodiment

A first embodiment of an OPP-STM (Optical Pump-Probe Scanning Tunneling Microscopy) will be explained below with reference toFIG.1toFIG.7.

(System Configuration)

FIG.1is an overall configuration diagram of an OPP-STM1according to a first embodiment. InFIG.1, light is indicated with a solid line and an electric signal is indicated with a broken line.

The OPP-STM1is composed of an optical output system2and an STM200. The optical output system2includes a first laser11, a second laser12, an arithmetic unit13, a lock-in amplifier14, a mirror M, and a beam splitter BS.

The first laser11is a laser beam source that outputs first light L1which is a laser beam. Once a pulse signal is input from the arithmetic unit13, the first laser11outputs the first light L1in a pulse state after first delay time. The second laser12is a laser beam source that outputs second light L2which is a laser beam. Once a pulse signal is input from the arithmetic unit13, the second laser12outputs the second light L2in the pulse state after second delay time. The first delay time and the second delay time are very short periods of time which are substantially the same. The first delay time and the second delay time do not have to match each other in a strict sense and the difference between them is absorbed by adjustments described later.

The intensity of the first light L1and the intensity of the second light L2may be the same or different from each other. The first light L1and the second light L2may have the same pulse width or different pulse widths. Moreover, the pulse widths of the first light L1and the second light L2may be fixed values or be variable. If the pulse widths of the first light L1and the second light L2are variable, they may be changed by manually switching a switch included in the first laser11and the second laser12or may be changed according to the pulse width of the pulse signal which is input to the first laser11and the second laser12.

The arithmetic unit13is a pulse signal generation apparatus and is configured by including a voltage generation source and an arithmetic operation unit. The arithmetic unit13is implemented by, for example, an FPGA (Field Programmable Gate Array) which is a rewritable logic circuit. The arithmetic unit13outputs a pulse signal to the first laser11and the second laser12and outputs a reference signal to the lock-in amplifier14. A signal line connecting the arithmetic unit13and the first laser11will be referred to as S1; a signal line connecting the arithmetic unit13and the second laser12will be referred to as S2; a signal line connecting the arithmetic unit13and the lock-in amplifier14will be referred to as S3.

The lock-in amplifier14: processes, as its processing target, a signal which is output by the STM200; and detects a faint signal included in the output signal of the STM200based on the reference signal which is output by the arithmetic unit13. The detected faint signal may be saved inside the lock-in amplifier14or may be saved in a storage apparatus outside the lock-in amplifier14.

The mirror M is a planar mirror. The mirror M has a mechanism for adjusting its position and posture so that a split light L11and a split light L22which are output from the beam splitter BS can be adjusted coaxially. An operator makes the split light L11and the split light L22coaxial by adjusting the position and posture of the mirror M in advance while observing the split light L11and the split light L22. Incidentally, instead of providing the mirror M with the mechanism for adjusting the position and posture, the first laser11and the second laser12may be provided with the mechanism for adjusting the position and posture. Incidentally, the split light L11and the split light L22will be hereinafter sometimes collectively referred to as “output light.”

The beam splitter BS splits incident light into transmitted light and reflected light. Specifically speaking, the beam splitter BS splits the first light L1into the split light L11which is transmitted light, and the split light L12which is reflected light; and splits the second light L2into split light L21which is transmitted light, and split light L22which is reflected light. Incidentally, the beam splitter BS may be a so-called half mirror regarding which a ratio of the transmitted light intensity to the reflected light intensity is 1:1, and the ratio of the transmitted light intensity to the reflected light intensity may be other than the ratio mentioned above. In this embodiment, the split light L12and the split light L21are not utilized, so that they are not illustrated in the drawing.

Incidentally, the beam splitter BS may also include the mechanism for adjusting the position and posture so that the split light L11and the split light L22which are output from the beam splitter BS can be adjusted coaxially. Then, for example, the position may be adjusted by adjusting an angle of the mirror M and the angle may be adjusted by adjusting the angle of the beam splitter BS.

The STM200is a Scanning Tunneling Microscopy (STM) that uses the laser beams, which are output from the first laser11and the second laser12, as pump light and probe light. The configuration of the STM200will be described later in detail, but the STM200outputs the detected signal to the lock-in amplifier14.

The laser beam L1which is output from the first laser11is split by the beam splitter BS into the split light L11and the split light L12. The split light L11which has transmitted through the beam splitter BS enters the STM200. The laser beam L2which is output from the second laser12is reflected by a plurality of mirrors M and is split by the beam splitter BS into the split light L21and the split light L22. The split light L22which has been reflected by the beam splitter BS enters the STM200. Incidentally, the split light L11will be also hereinafter sometimes referred to as “pump light” L11and the split light L22will be also hereinafter sometimes referred to as “probe light” L22.

(Configuration of STM)

FIG.2is a schematic diagram of the STM200. The STM200includes a probe210and a tunnelling current detection unit220. Incidentally,FIG.2also illustrates the lock-in amplifier14, but it is not included in the configuration of the STM200and thereby indicated with a broken line. A sample900is set in the STM200. The split light L11and the split light L22which are emitted from outside the STM200are emitted onto a surface of the sample900including a top end211of the probe210. As the pump light L11is emitted onto the sample900, the sample900is excited; and the probe light L22is emitted mainly while the sample900is excited.

The time differences to output the split light L11and the split light L22are set in a plurality of ways as described earlier; and, therefore, the probe light L22may be emitted during the sample900is excited at least one time difference. The number of photocarriers excited by the probe light L22changes depending on the delay time which is the difference between the timings when the two split light beams are emitted onto the sample900; and a tunnelling current which flows between a probe tip51aand the sample900changes and is detected by the tunnelling current detection unit220. The tunnelling current detection unit220outputs a signal of the detected current to the lock-in amplifier14.

(Functional Configuration of Arithmetic Unit)

FIG.3is a functional block diagram illustrating the respective functions of the arithmetic unit13as functional blocks. The arithmetic unit13is implemented by an FPGA as explained earlier. When the FPGA is activated, the FPGA reads logic circuit information from an ROM (which is not illustrated in the drawing) and writes it into the FPGA. As a result of this writing of the information, a signal generation unit132, a delay decision unit133, a reference signal output unit134, an input unit135, and a storage unit136are formed. The delay decision unit133decides the value of a variable delay value138described later. The signal generation unit132includes a delay time count unit and a voltage generation source capable of generating a voltage in a pulse state. The reference signal output unit134includes a voltage generation source capable of generating a voltage signal.

The input unit135includes a physical interface such as a button for accepting the user's command, or an electric interface for accepting the user's command via an electric signal. The storage unit136stores a fixed delay value137and a variable delay value138. However, values of the fixed delay value137and the variable delay value138do not have to be stored in the ROM which is not illustrated in the drawing; and in that case, every time the arithmetic unit13is activated, the user inputs the values of the fixed delay value137and the variable delay value138via the input unit135.

The signal generation unit132outputs a pulse signal to the first laser11and the second laser12. The delay decision unit133decides the variable delay value138. The input unit135writes the value of the fixed delay value137to the storage unit136on the basis of the input by the user. A delay value, that is, a time interval between the output of the pulse signal from the signal generation unit132to the first laser11and the output of the pulse signal to the second laser12is the sum of the fixed delay value137and the variable delay value138. The fixed delay value137is to cancel any influences of optical path lengths, signal cable lengths, and individual differences between the first laser11and the second laser12; and by setting an appropriate value as the fixed delay value137, the timings for the pump light L11and the probe light L22to reach the sample900can be made simultaneous when the variable delay value is zero. The variable delay value138is a value for setting the difference between the timings for the pump light L11and the probe light L22to reach the sample900. Incidentally, the variable delay value138will be hereinafter also referred to as a “time difference Δt.”

(Example of Output Light)

FIG.4is a diagram illustrating an example of temporal changes of light intensity of the split light L11and the split light L22at the position of the sample900. Referring toFIG.4, the time elapses from the left side to the right side of the drawing; and in the example illustrated inFIG.4, the first light L11reached the sample900first and then the second light L22reached the sample900. In this embodiment, it is defined as indicated inFIG.4that the time difference Δt is a positive value when the first light L11reaches the sample900first.

(Measured Signal)

The outline of a measured signal of the STM200will be explained in order to help the understanding of actions of the arithmetic unit13before explaining the actions of the arithmetic unit13. However, the following explanation will be provided to explain representative measurement results when the STM200performs measurement by using a certain sample as a measurement target; and it is not presupposed as a prerequisite for the OPP-STM1that it has a similar tendency regardless of the type of the sample.

FIG.5is a diagram illustrating the relationship between an output signal of the STM200and the time difference Δt. The sample900is excited by the emission of the pump light L11and the excitation decays along the passage of time. For example, if there is a relationship of t1<t2<t3, measured values when the time differences Δt are t1, t2, t3are s13, s12, and s11and are in the relationship of s13>s12>s11. As the time difference Δt becomes larger, the measured signal decreases; and if the time difference Δt is equal to or larger than a certain value, for example, if the time difference Δt is equal to or larger than t6in the example illustrated inFIG.5, the measured signal converges to s0which is the minimum value in this measurement.

However, the difference in signal level between s0and s13is smaller than the minimum value as the measurement signal, that is, the difference in signal level between “0” on the vertical axis ofFIG.5and the minimum measurement value s0. Specifically, since the SN ratio in this measurement is very small, the normal measurement is difficult, therefore, the lock-in amplifier14is used. The problem of the low SN ratio is solved by periodically switching the delay time Δt and using the lock-in amplifier14synchronously with that period, thereby evaluating the measured values such as s13, s12, and s11not in comparison with zero, but in comparison with s0.

Incidentally, the example illustrated inFIG.5is merely an outline and the values of t6and s0are actually not clear before the measurement, so that, for example, a sufficiently long period of time such as the amount of time several times as long as an estimated decay time is often widely used instead of t6. In this embodiment, the sufficiently long period of time will be explained as t9. Furthermore, in this embodiment, the values such as s0and s11as indicated inFIG.5cannot be measured directly and a value relative to s0as a reference, that is, a value of “s13−s0” and a value of “s12−s0” are obtained.

(Measured Signal)

FIG.6is a diagram illustrating the actions of the arithmetic unit13in order to obtain s13which is referenced to s0, that is, “s13−s0.” Referring toFIG.6, time passes from the left side to the right side of the drawing and the vertical axis indicates the time difference Δt. Arrows Flip indicated in a lower part ofFIG.6indicate timings for the reference signal, which is output by the arithmetic unit13to the lock-in amplifier14, to invert. In the example illustrated inFIG.6, assuming that a modulation period is L, the arithmetic unit13switches the time difference Δt between t1and t9every specified period of time L/2 which is a half of L. During the specified period of time L/2, the pump light L11and the probe light L22are output multiple times, for example, a few dozen times or hundreds of times.

The reason why the difference between the timings for the pump light L11and the probe light L22to reach the sample900becomes equal to the variable delay value138, that is, the time difference Δt, will be explained below. The signal generation unit132firstly outputs a pulse signal to the first laser11and then outputs a pulse signal to the second laser12after a time period of sum of the fixed delay value137and the variable delay value138has elapsed from the output to the first laser11. Since the difference between duration from when the first laser11and the second laser12receive the pulse signals to when they output light pulses, and the time difference the light pass through optical path from the first laser11and the second laser12to the sample900are equal to the fixed delay value137, the difference between the timings for the pump light L11and the probe light L22to reach the sample900becomes exactly equal to the variable delay value138, that is, the time difference Δt. Incidentally, the signal generation unit132measures the elapse of time by counting the number of oscillations of, for example, a built-in oscillator. In other words, in this embodiment, the fixed delay value137and the variable delay value138can be controlled based on oscillation periods of the oscillator as units.

For example, the arithmetic unit13: sets the time difference Δt as t9and outputs the pump light L11and the probe light L22one hundred times in time periods of L/2, and then sets the time difference Δt as t1and outputs the output light one hundred times in time periods of L/2. The arithmetic unit13implements cyclic changes of Δt as illustrated inFIG.6by repeating the above-described actions a plurality of number of times. For example, the arithmetic unit outputs −1V as the reference signal for the time periods of L/2 when setting Δt as t9and outputs 1V as the reference signal for the time periods of L/2 when setting Δt as t1, thereby causing the lock-in amplifier14to perform lock-in detection in synchronization with the delay time changes. The output from the STM200oscillates between S0and S13with period L in accordance with the changes in the delay time. “S13-S0” which is the difference between S0and S13can be measured at a high SN ratio from the output of the lock-in amplifier which operates in synchronization with the reference signal.

(Flowchart)

FIG.7is a flowchart illustrating actions of the arithmetic unit13. Incidentally, the arithmetic unit13is implemented by the FPGA in this embodiment, so that the flowchart illustrated inFIG.7does not necessarily precisely illustrate the actions of hardware. For example, practically, a counter circuit which counts the number of pulses, a circuit which generates electric pulses when the counter is a specific value, and so on always operate independently and in parallel to each other and their inputs and outputs are connected to each other. In this embodiment, the actions of the arithmetic unit13will be explained by using the flowchart for the sake of convenience.

Regarding the respective steps explained below, S313is executed by the reference signal output unit134, S315is executed by the signal generation unit132, and other steps are executed by the delay decision unit133. In S311, the arithmetic unit13firstly decides two variable delay values138which change in a cyclic manner. These variable delay values138may be stored in the storage unit136in advance or the user may input the variable delay value(s)138from the input unit135every time the processing is executed. In the subsequent step S312, the arithmetic unit13sets either one of the two kinds of the variable delay value138, which were set in S311, as the variable delay value138to be used and then proceeds to S313.

In S313, the arithmetic unit13inverts the reference signal which is output to the lock-in amplifier14. For example, if the reference signal immediately before the execution of S313was “−1V,” the reference signal is changed to “+1V” in S313. In the next step S314, the arithmetic unit13initializes a pulse counter which counts the number of pulses to zero. In the subsequent step S315, the arithmetic unit13causes the signal generation unit132to output a pulse signal to the first laser11and the second laser12by using the variable delay value138which is currently set. In the subsequent step S316, the arithmetic unit13increases the number of counts of the pulse counter by one and judges whether the number of counts of the pulse counter has reached a specified control value, for example, 100 or not. If the arithmetic unit13determines that the number of counts of the pulse counter has reached the specified count, in other words, if the arithmetic unit13determines that the time L/2 has elapsed since the execution of S314, the processing proceeds to S317; and if the arithmetic unit13determines that the number of counts of the pulse counter has reached not the specified number of counts, the processing returns to S315.

In S317, the arithmetic unit13changes the variable delay value138to the other value, which is not the currently set value, among the two types of the variable delay values138decided in S311, and then the processing returns to S313. Incidentally, in S317, the arithmetic unit13further judges whether the number of times of processing has reached a specified number of times or not; and if the arithmetic unit13determines that the number of times of processing has reached the specified number of times, the processing may return to S311and continue measuring two types of different variable delay values or terminate the processing illustrated inFIG.7.

The following operational advantages can be obtained according to the above-described first embodiment.

(1) The optical output system2includes: the first laser11that outputs the first light L1, which is a pulse laser, according to an input signal; the second laser12that outputs the second light L2, which is a pulse laser, according to an input signal; and the arithmetic unit13which inputs a signal to the first laser11and the second laser12. The arithmetic unit13switches the variable delay value138, which is the difference between the timing to input the signal to the first laser11and the timing to input the signal to the second laser12, in a plurality of ways. Therefore, it is possible to switch between the timings for the first laser11and the second laser12to output the pulse laser by means of a simple configuration.

Meanwhile, a means of changing the optical path length in the conventional technologies requires mechanical actions such as movements of a stage or driving the mirror, which causes problems such that the occurrence of vibrations cannot be avoided and a switching speed has physical limits. There is a known configuration that utilizes a Pockels cells in order to improve these problems. However, although the Pockels cells can improve the above-described two problems, problems remain such that the light cannot be completely blocked and the apparatus configuration becomes large.

The problem of incapability to block the light completely means that weak light leaks out at a timing when the light should be blocked. In other words, the weak light will be always emitted, which will cause unintended influences on measured values. The problem of the apparatus configuration required to be on the extensive scale means that since delay time resolving power which can be set solely by the Pockels cell is limited by repeated periods (up to 10 ns) of the source laser, and if the delay time resolving power which is lower than that is required, a mechanism for controlling the pulse output timings of the two lasers is separately required. If the control becomes cumbersome and complicated, the scale of the apparatus becomes extensive and the installment area and required cost of the apparatus also become large.

However, the optical output system2can freely set the delay time by controlling the timing to input the electric signal to the first laser11and the second laser12, so that problems like those of the means for changing the optical path length or the means for using the Pockels cell will not occur. Specifically speaking, the optical output system2has advantages of no vibrations, no limits on the switching speed, no emission of unnecessary faint light pulses between the pulses, and the simple apparatus configuration and the small installment area.

(2) The arithmetic unit13includes the reference signal output unit134that changes the variable delay value138in two ways in the specified period L and outputs the reference signal indicating the timings to change the variable delay value. Consequently, on the premise of the use of the lock-in amplifier14, it is possible to acquire necessary information from the faint signal which is output from the STM200.

(3) The optical output system2includes the lock-in amplifier14that processes, as a processing target, the output of the STM200using the first light L1and the second light L2and performs the lock-in detection based on the reference signal. Consequently, it is possible to acquire necessary information from the faint signal which is output from the STM200.

(4) The optical output system2includes the beam splitter BS that coaxially outputs at least part of the first light L1and at least part of the second light L2as output light. Regarding the outputs of the optical output system2, if the first light L1and the second light L2had different optical axes, these beams of light would have to be made coaxial depending on the measurement system to be used; however, both the beams of light which are the outputs of the optical output system2are coaxial, so that they have the advantage of easy usability.

(5) The optical pump-probe scanning tunneling microscope system1includes: the optical output system2; and the STM200that utilizes part of the first light L1and part of the second light L2, which are output from the optical output system2, as the pump light L11and the probe light L22. Consequently, the STM having the time resolution function with the simple configuration can be used.

(Variation 1)

In the aforementioned first embodiment, the arithmetic unit13is implemented by the FPGA. However, at least part of the arithmetic unit13may be implemented by a combination of a CPU which is a central processing unit instead of the FPGA, a ROM which is a read-only storage area, and a RAM which is a writable/readable storage area. In this case, a program(s) which is stored in the ROM is extracted to the RAM and executed. Moreover, the arithmetic unit13may be implemented by an ASIC (Application Specific Integrated Circuit), which is an application specific integrated circuit, instead of the FPGA. Furthermore, the arithmetic unit13may be implemented by a combination of different configurations, for example, a combination of the CPU, the ROM, the RAM, and the FPGA.

In other words, a program described below is also included within the scope of the present invention.

(6) The program which is executed by the arithmetic unit13which is used together with the first laser11and the second laser12executes the following: to input the signal to the first laser11and the second laser12; to switch the variable delay value138, which is the difference between the timing to input the signal to the first laser11and the timing to input the signal to the second laser12, in a plurality of ways; and to output the reference signal which indicates the timing to change the variable delay value138.

Furthermore, the present invention also includes a program for the FPGA circuit which enables the same actions as those of the above-mentioned program.

(Variation 2)

The optical output system2does not have to include the lock-in amplifier14. In this case, the arithmetic unit13does not output a pulse signal to the lock-in amplifier14.

(Variation 3)

The lock-in amplifier14may exist outside the optical output system2. In this case, the arithmetic unit13outputs a pulse signal to the lock-in amplifier14which exists outside the optical output system2.

(Variation 4)

The measurement apparatus which is used in combination with the optical output system2is not limited to the STM200. Any measurement apparatus may be used as long as it performs pump-probe measurements.

(Variation 5)

The configuration of the optical output system2illustrated inFIG.1includes only one mirror M. However, the optical output system2may include a plurality of mirrors M in order to enhance the degree of freedom such as arrangement and adjustments. Furthermore, in this case, the first laser11, the beam splitter BS, the second laser12, and the plurality of mirrors M may be located so that the optical path length from the first laser11to the beam splitter BS and the optical path length from the second laser12to the beam splitter BS can be adjusted.

(Variation 6)

The arithmetic unit13may include an input interface to enabling the user to input and adjust the fixed delay value137. For example, the arithmetic unit13may include a volume switch so that the user may increase or decrease the fixed delay value137in accordance with a turning direction and a turning amount of the volume switch.

Second Embodiment

A second embodiment of the optical pump-probe scanning tunneling microscope system will be explained with reference toFIG.8. In the following explanation, differences from the first embodiment will be mainly explained by assigning the same reference numerals to the same components as those in the first embodiment. Matters which are not particularly explained are the same as those in the first embodiment. In this embodiment, the main difference from the first embodiment is that the optical output system independently outputs two laser beams and the two laser beams are made coaxial outside the optical output system.

FIG.8is an overall configuration diagram of an optical pump-probe scanning tunneling microscope system1A according to the second embodiment. The difference from the first embodiment is that the mirror M and the beam splitter BS are not included in the optical output system2A. Other configurations are similar to those of the first embodiment.

The above-described second embodiment is useful for a case where the measurement apparatus which is combined with the optical output system2A performs pump-probe measurements based on light which is not coaxial. Furthermore, even when performing the pump-probe measurements based on coaxial light, operational advantages similar to those of the first embodiment can be obtained by using the mirror M and the beam splitter BS in the same manner as in the first embodiment.

Third Embodiment

A third embodiment of the optical pump-probe scanning tunneling microscope system will be explained with reference toFIG.9. In the following explanation, differences from the first embodiment will be mainly explained by assigning the same reference numerals to the same components as those in the first embodiment. Matters which are not particularly explained are the same as those in the first embodiment. In this embodiment, the main difference from the first embodiment is that the optical output system outputs two laser beams without making them coaxial.

FIG.9is an overall configuration diagram of an optical pump-probe scanning tunneling microscope system1B according to the third embodiment. The difference from the first embodiment is that the beam splitter and the mirror are not included. In other words, in this embodiment, the output light L1and the output light L2are directly input to the STM200.

The above-described third embodiment is useful for a case where the measurement apparatus which is combined with the optical output system2A performs pump-probe measurements based on light which is not coaxial.

Fourth Embodiment

A fourth embodiment of the optical pump-probe scanning tunneling microscope system will be explained with reference toFIG.10. In the following explanation, differences from the first embodiment will be mainly explained by assigning the same reference numerals to the same components as those in the first embodiment. Matters which are not particularly explained are the same as those in the first embodiment. In this embodiment, the main difference from the first embodiment is that the time difference Δt can be controlled with high accuracy.

In the above-described first embodiment, the duration of oscillations of the oscillator included in the arithmetic unit133is sufficiently short as compared to the required accuracy of the time measurement. However, if the condition of the sufficiently short duration of the oscillations of the oscillator included in the arithmetic unit13as compared to the required accuracy of the time measurement is not satisfied, it is effective to also use an analogue delay circuit explained below.

FIG.10is an overall configuration diagram of an optical pump-probe scanning tunneling microscope system10according to the fourth embodiment. The optical pump-probe scanning tunneling microscope system10includes an optical output system2C and the STM200. The difference between the optical output system2C and the optical output system2is that the optical output system2C further includes a delay circuit18. In this embodiment, the pulse signal from the arithmetic unit13to the second laser12is output via the delay circuit18.

Delay time of the delay circuit18can be adjusted by a control signal from the arithmetic unit13via S25. The delay circuit18can adjust the delay time, for example, on a ps basis. After the pulse signal is input via S21from the arithmetic unit13, the delay circuit18outputs the pulse signal via S22to the second laser12after being delayed by the delay time which is set according to properties of the analog circuit.

According to the above-described fourth embodiment, the time difference Δt can be controlled with higher accuracy. For example, if the frequency of the oscillator included in the arithmetic unit13is 1 GHz, one period is 1 ns and, therefore, it is impossible to implement the control on the ps basis, where ps is a period of time less than one period. However, based on the delay circuit18, the time difference Δt can be controlled with a time resolution shorter than one period of the oscillator included in the arithmetic unit13.

(Variation of Fourth Embodiment)

The delay circuit18may be configured by being integrated with the arithmetic unit13. For example, by using an FPGA equipped with an output delay circuit, the arithmetic unit13in which the delay circuit18is built can be implemented.

Fifth Embodiment

A fifth embodiment of the optical pump-probe scanning tunneling microscope system will be explained with reference toFIG.11. In the following explanation, differences from the first embodiment will be mainly explained by assigning the same reference numerals to the same components as those in the first embodiment. Matters which are not particularly explained are the same as those in the first embodiment. In this embodiment, the main difference from the first embodiment is that the first laser continuously outputs laser pulses in a specific period without having a signal input from outside.

FIG.11is an overall configuration diagram of an optical pump-probe scanning tunneling microscope system1D according to the fifth embodiment. Differences from the first embodiment are that: a first laser11A which operates independently instead of the first laser11is included; two beam splitters BS1and BS2for splitting the first light L1which is output from the first laser11A is included; and a photodetector15that detects the output of the first laser11A is included. Also, the actions of the arithmetic unit13are different from those of the first embodiment.

The frequency of the first laser11A to output the pulse laser is known and is, for example, 100 kHz. However, this frequency is not necessarily strict and at least complete synchronization with the oscillator which is built in the arithmetic unit13cannot be expected.

The photodetector15transforms the received light into an electric signal and outputs the electric signal. The photodetector15may be implemented by using a photo multiplier or may be implemented by using a photodiode which utilizes a p-n junction of a semiconductor. The split light L12which is part of the first light L1output by the first laser11is input to the photodetector15. The photodetector15outputs the electric signal, which has been transformed from the received light, as a synchronization signal S5to the arithmetic unit13.

The arithmetic unit13generates a clock that is several hundred to several thousand times larger than the first laser using the synchronization signal S5as the reference timing and uses it to count the variable delay138. For example, if the output of the first laser11A is 100 kHz, the arithmetic unit13generates 100-MHz clocks and counts the variable delay value138with reference to the reception time of the synchronization signal S5.

The following operational advantage can be obtained according to the abovementioned fifth embodiment.

(7) An optical output system2D includes: the first laser11A that outputs the first light L1which is light pulses in a specified period; the second laser12that outputs the second light which is a pulse laser according to an input signal; and the arithmetic unit13that inputs a signal to the second laser12with reference to the timing to output the first light L1. The arithmetic unit13switches the variable delay value138, which is the difference between the timing to output the first light L1and the timing to input the signal to the second laser12, in a plurality of ways. Consequently, operational advantages similar to those of the first embodiment can be obtained by using a laser oscillator which has been conventionally used and outputs laser beams in a specified period. Since the output of the second laser12or the like which outputs the pulse laser according to the input pulse signal is not necessarily high, the configuration of this embodiment has a further advantage of the capability to use the high-output laser oscillator.

(Variation of Fifth Embodiment)

The output of the first apparatus1011may be other than a laser. For example, the first apparatus1011may be a synchrotron and output X-ray pulses in a specified period.

Sixth Embodiment

A sixth embodiment of the optical pump-probe scanning tunneling microscope system will be explained with reference toFIG.12. In the following explanation, differences from the first embodiment will be mainly explained by assigning the same reference numerals to the same components as those in the first embodiment. Matters which are not particularly explained are the same as those in the first embodiment. In this embodiment, the main difference from the first embodiment is that the timing for the signal generation unit132to output the electric signal is specified.

A hardware configuration of the optical pump-probe scanning tunneling microscope system according to the sixth embodiment is similar to that of the first embodiment, so that an explanation about it is omitted. A functional configuration of the optical pump-probe scanning tunneling microscope system according to the sixth embodiment is similar to that of the first embodiment, except the details of the implementation of the signal generation unit132. Actions of the signal generation unit132and the arithmetic unit13including the signal generation unit132will be explained.

FIG.12is a diagram for explaining actions of the arithmetic unit13according to the sixth embodiment.FIG.12(a)is a diagram illustrating the actions of the arithmetic unit13according to the sixth embodiment;FIG.12(b)is a diagram illustrating a reference signal; andFIG.12(c)is a diagram illustrating actions of a comparative example. Incidentally, inFIG.12(a)toFIG.12(c), time axes in a horizontal direction in the drawing are synchronized with each other. In the example illustrated inFIG.12, for the sake of convenience of the illustration, each of the first light L1and the second light L2is output three times in average during a time period of L/2 which is a half of the time period of a modulation period. T101, T102, T103, and so on indicated inFIG.12are the timing when the length of L/2 is divided by N, for example, the length of L/2 is divided by three in the example illustrated inFIG.12, with reference to a starting timing of the modulation period L. Each of these timings such as T101and T102will be referred to as a “reference timing.”

In this embodiment, the signal generation unit132makes the time difference between the first light L1and the second light L2to reach the sample900to be set as the variable delay value138by shifting the first light L1ahead of each reference timing such as T101and T102by just a half of the value of the variable delay value138and shifting the second light L2later than the reference timing by just a half of the value of the variable delay value138as illustrated inFIG.12(a). For example, if the variable delay value138is t1, the signal is output to the first laser11and the second laser12so that the first light L1reaches the sample earlier than the reference timing only by t1/2and the second light L2reaches the sample later than the reference timing only by t1/2. Also, if the variable delay value138is t9, the signal is output to the first laser11and the second laser12so that L1reaches the sample earlier than the reference timing only by t9/2and L2reaches the sample later than the reference timing only by t912. On the other hand, in the comparative example illustrated inFIG.12(c), the signal generation unit132makes the time difference for the first light L1and the second light L2to reach the sample900to be set as the variable delay value138by operating so that the first light L1always reaches the sample at the reference timing and the second light L2reaches the sample later than the reference timing only by the amount of the variable delay value138.

The difference betweenFIG.12(a)andFIG.12(c)becomes significant around the time(s) of day to switch the variable delay value. InFIG.12(a), the density in a time direction of the total number of light pulses of the first light L1and the second light L2around the time of switching is substantially constant and there is no sparse or dense density in terms of time. However, in the comparative example, while the density in the time direction of the number of light pulses of the first light L1is completely constant regardless of the timings, regarding the second light L2the light emitted toward the sample900becomes sparse in terms of time around T104and the light emitted toward the sample900becomes dense in terms of time around T107. Consequently, in the comparative example, the output of the STM200may possibly include not only the influence caused by changing the variable delay value138every half period of the modulation period L, but also the influence caused by the sparse or dense density of the emitted light, which occurs every half period, in terms of time. On the other hand, in this embodiment, the density of the light emitted toward the sample900hardly becomes sparse or dense even around the time of the modulation, so that this embodiment has the advantage of the capability to easily measure the influence of the variable delay value138.

The following operational advantage can be obtained according to the abovementioned sixth embodiment.

(8) The signal generation unit132outputs the signal to the first laser11and the second laser12so that the average timing for the first light L1to reach the sample900and the timing for the second light L2to reach the sample900will match the reference timing which is referenced to the modulation timing of the reference signal. Consequently, the density of the light emitted toward the sample900hardly becomes sparse or dense even around the time of any change of the variable delay value138and the influence on the variable delay value138can be easily measured.

The aforementioned embodiments and variations may be combined with each other. Various embodiments and variations have been described above; however, the present invention is not limited to the content of these embodiments and variations. Other aspects which can be thought of within the scope of the technical idea of the present invention are also included within the scope of the present invention.

Incidentally, the disclosure content of the following basic priority application is incorporated herein by reference: Japanese Patent Application No. 2018-233878 (filed on Dec. 13, 2018).

REFERENCE SIGNS LIST

1,1A,1B,1C,1D: optical pump-probe scanning tunneling microscope system2,2A: optical output system3: arithmetic unit11: first laser12: second laser13: arithmetic unit14: lock-in amplifier15: photodetector131: adjustment unit132: signal generation unit133: delay decision unit134: reference signal output unit135: input unit136: storage unit137: fixed delay value138: variable delay value900: sampleL1: first lightL11: pump lightL2: second lightL22: probe light