Abstract:
In a pump/probe experiment, information about an event is detected and correlated with an accumulatable quantity representing the elapsed time interval between the arrival of the pump and probe pulses at the experiment. The accumulatable quantity is used to configure the pump and probe sources to eliminate temporal resolution problems caused by pulse timing jitter, the complexity of amplification, continuum generation, and subsequent reamplification, as well as data acquisition rate limitations. Two pulse sources serve as pump and probe pulses respectively. Each pulse is directed at the experiment. A portion of each pump pulse is diverted to a detector before it reaches the experiment, and a portion of each probe pulse is diverted to another detector. The pump and probe pulses are no-coincident in time at the experiment. Quantities related to the time difference and the information imposed on the probe pulse by the experiment are accumulated to obtain data about the temporal evolution of the event.

Description:
TECHNICAL FIELD 
     This invention relates to pump/probe experiments wherein pump pulses initiate events in experiments, while probe pulses probe the experiments from time to time in order to extract therefrom information about the experiment for observing the time-dependent behavior of said events. In particular, the present invention relates to method and apparatus for use in such experiments. 
     In this form of what we call time-resolved spectroscopy, it is of interest to observe time-dependent behavior of an event at very small time intervals and to do so with pump and probe pulses from independent sources so that the oscillating wavelength of each can be altered to fit the needs of the experiment. It is also desirable to accumulate data at rates that are higher than the present use of mechanical optical delay lines will allow, in experiments in which the pump and probe pulses originate from the same source. Moreover, for studying events that occur in extremely short time intervals of less than about 10 picoseconds, the use of independent pump and probe sources in time-resolved spectroscopy is difficult because pulse timing jitter degrades temporal resolution. And while pulse timing jitter is eliminated in pump/probe experiments when both pulses originate from the same oscillator, such systems are limited in the rate at which they can scan the time interval of interest because of their use of mechanical optical delay lines. An additional limitation of this technique is that it is difficult to obtain pump and probe pulses at a wide variety of wavelengths without resorting to the expensive process of amplification and continuum generation, and even then it is often necessary to re-amplify the desired portion of the continuum to obtain sufficient energy to initiate, or pump, an experiment. 
     Presently, the problem of prior art is to provide method and apparatus which allow observation of events that occur on picosecond and subpicosecond time scales with sources that have the flexibility to be used at differing wavelengths and energies, and can acquire data at reasonably high data acquisition rates. 
     BACKGROUND ART 
     Insofar as we are aware, the so-called ASOPS approach represents the best present state of the art of such apparatus and method. 
     Elzinga et al, for example, in an article entitled &#34;Pump/Probe Method for Fast Analysis of Visible Signatures Utilizing Asynchronous Optical Sampling&#34; (ASOPS), in Applied Optics, Vol 26, No. 19, pp 4303-4309, Oct. 1, 1987, describe monitoring subnanosecond excited-state processes by stimulating a process with pump pulses from a mode-locked laser and probing it with probe pulses from an asynchronously pumped laser. The two lasers operate at slightly different repetition rates, so a repetitive relative phase walk-out of pump and probe pulses occur. 
     In other prior pump/probe methods, the pump and probe pulses run at identical repetition rates, with an optical delay line used to control relative timing between the pulses. 
     DISCLOSURE OF THE INVENTION 
     According to the present invention, information about the event is extracted from the experiment by standard detection means, and correlated with an accumulatable quantity that represents the elapsed time interval between the arrival of the pump and probe pulses at the experiment. By means of this accumulatable quantity we can now configure our pump and probe source(s) in a multitude of ways so as to eliminate temporal resolution problems caused by pulse timing jitter in the ASOPS approach, or the complexity of amplification, continuum generation and subsequent re-amplification as well as data acquisition rate limitations imposed by mechanical optical delay lines in the &#34;single oscillator source&#34; approach. 
     One form of the preferred embodiment we refer to as time interval measurement, or TIMe, pump/probe spectroscopy. Here two pulse sources serve as sources of pump pulses and probe pulses, respectively, in time-resolved experiments. Each pulse is directed at the experiment. Conveniently, before any pump and probe pulse reaches the experiment, a portion of each pump pulse is diverted to a detector, and a portion of each probe pulse is diverted to another detector. 
     The operating parameters of the sources may be chosen to be such that, in general, pump pulses will be non-coincident in time with the probe pulses at the experiment. 
     The portions of the probe and pump pulses directed at the experiment have interacted with material or device therein. The pump pulse initiates an event in the material or device therein. For example, the event can be a chemical reaction from which a transient species evolves in response to the pump pulse, in which case the probe pulse would interact with such species such as to be affected thereby. 
     In other words, after interacting with the event, the probe pulse has had imposed thereon an information signal relating to the temporal evolution of the event. 
     Suitable hardware, including the detectors, provides a quantity which is a measure of the aforesaid time interval, and also a second quantity which is a measure of the information signal imposed on the probe beam at the end of that time interval. 
     These two quantities are accumulated, as a pair of corresponding associated numbers, by a suitable data acquisition system. The pump/probe process is repeated many times over in order to obtain sets of event information signals and time intervals which may be averaged, and/or otherwise manipulated in order to obtain data about the temporal evolution of an event over the experimental time regime of interest to the experimenter. 
     In the preferred embodiment of the present invention, the TIMe technique can also provide the time delay of prior art pump/probe systems by employing the concept of the accumulatable quantity, thereby obviating the prior art&#39;s need to maintain precise synchrony (or asynchrony) between pump and probe sources, or to reduce pulse timing jitter below the desired temporal resolution of the experiment, or to provide a scanning optical delay between the beams. It is to be noted, however, that the TIMe technique can be used to provide synchrony (or asynchrony) of pump and probe sources, that is, it is not dependent on the manner in which time delay between beams is provided. 
     In addition, higher pulse energy and a large range of wavelengths are available from the broad range of mode-locked laser oscillators now employable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a TIMe apparatus according to the invention. 
     FIG. 2 shows a quantity accumulation and pulse detection scheme which may form part of the apparatus of FIG. 1. 
     FIG. 3 shows apparatus which may be used with the quantity accumulation scheme of FIG. 2, for providing inputs to a data acquisition system. 
     FIG. 4 shows a second form of quantity accumulation scheme for TIMe apparatus. 
     FIG. 5 shows a unitary form of pulse detection scheme. 
     FIG. 6 shows a variation of a TIMe apparatus according to FIG. 1. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred form of apparatus according to the invention, as shown in FIG. 1, comprises sources 1 and 2, beam splitters 3 and 4, optics 5 and 6, pulse detectors 7 and 8, experiment 9, and signal detector 10. Sources 1 and 2 respectively provide pump pulses and probe pulses. 
     Beam splitter 3 and optics 5 function as pulse directing means for directing each pump pulse to the pulse detector 7 and to the experiment 9. Likewise, beam splitter 4 and optics 6 function as pulse directing means for directing each probe pulse to pulse detector 8, and to the experiment 9. 
     The time intervals result from arranging the system elements such that the trains of pulses emerging from the sources are on the whole non-coincident. While such arrangement can be achieved in a number of ways, in the present example of the invention, this is achieved by providing the sources 1 and 2 which function independently of one another. For example, each could be designed to have the same nominal pulse repetition rate as the other, yet while there would be instances of pulse coincidence, there would also be instances of non-coincidence of pulses. We prefer, however, that the pulses from pump laser 1 be 10 nanoseconds apart, and that the pulses from the probe laser 2, be 10.01 nanoseconds apart. Assuming that the lasers are ideal oscillators, then pulse coincidence can occur every 10 microseconds, so most of the time there will be non-coincidence. 
     According to the invention, the event information is provided to a suitable data acquisition system (DAS) by transforming event information to accumulatable quantities, as, for example, FIG. 2 illustrates. 
     In FIG. 2, detectors 7 and 8 are photoconductors which, when they receive pulses from optics 5 and 6, respectively, become highly conductive, but are at other times, non-conductive. 
     Thus, when the detector 7 receives a pump pulse, charge from power supply 12 (here shown as a battery for simplicity) flows through a diode 13 to capacitor 11. Accordingly, the capacitor 11 accumulates a quantity of electrical charge in proportion to the time interval between that pump pulse and such time as the photoconductor becomes non-conductive again. 
     If, next, the detector 8 receives a probe pulse, it becomes conductive and grounds power supply 12, as indicated at 15. The diode 13, however, prevents the off-ground side of the capacitor from being grounded through the photoconductive detector 8. 
     When the system is operating, in general each pump pulse will be followed by just one probe pulse. Consequently, capacitor 11 charges when a pump pulse is detected, and then discharges when the probe pulse immediately following the pump pulse is detected. The net charge on the capacitor 11 will therefore represent the time interval by which a probe pulse follows the next-preceding pump pulse. 
     The power supply 12 is merely illustrative of charge sources. Any DC power supply would do. Conveniently, the DAS (not shown) could both provide the charging voltage, and discharge the capacitor 11 only at such times as it is desired to have pump/probe pulses produce charge on the capacitor, and to have the DAS accept information from the experiment. 
     It is also unnecessary to attempt to probe, i.e., sample every event-initiation. It will be understood that the charge on the capacitor must be measured, and that it must be discharged to ground potential before it is next charged, hence, between each measured pump pulse to probe pulse time interval, there can be another time interval (of which the length can be predetermined by one of ordinary skill in the art,) during which one or more pump/probe pulse pairs occur without being measured. 
     In FIG. 3, the pulse detectors 7 and 8 are connected to a time to amplitude converter 16 (TAC), via constant fraction discriminators (CFD&#39;s) 17 and 18. The output of the TAC 16 is a voltage representative of the time interval between a pump pulse and a probe pulse, as more particularly described hereinbelow. 
     The TAC output voltage is connected to any suitable data acquisition system (DAS), for example, a digital oscilloscope 19 whose x-input receives that voltage, and whose y-input receives the output voltage of signal detector 10. In response, the oscilloscope produces an x vs y trace 10 of successive pairs of voltage, respectively representing pulse time intervals, and event information gathered during those time intervals. Trace 20 shows the temporal evolution of the experimental event/events. 
     The embodiment of FIG. 2 uses passive circuit components, which do not introduce &#34;jitter&#34; into the pulse information it is processing, e.g., photoconductive switches. However, reflections will be created by the transmission lines and interconnections associated with the devices, and will appear as jitter. Again, active devices may introduce too much jitter if one tried to use them to handle the very low amount of charge that would be delivered by them in one picosecond--so low that signal would be buried in ambient electrical noise. Thus, these teachings will be most useful for pump/probe experiments where the event time regime is long compared to the jitter introduced by the system hardware, etc. 
     For shorter time regimes, we increase the amount of delivered charge, whereby to improve the signal to noise ratio. In addition, we increase the time scale, i.e., multiply picoseconds time intervals to nanosecond levels, thereby allowing us to use good active components without having to worry about their jitter. 
     Thus, in FIG. 4, we provide photodetectors or photoconductor switches 70 and 80. These replace photoconductors 7 and 8, respectively, of FIG. 2, and feed the voltages of power supplies 71 and 81 to a pair of charging circuits 72 and 82 composed respectively of R1, C1 and R2, C2. The resistances and capacitances are chosen to be such that the quantity R1C1/(R1C1-R2C2) is equal to about 1000, whereby to establish a nanosecond regime. If necessary, capacitors 73 and 83 can be provided in order to assure that sufficient charge will be available for charging C 1  and C 2 . 
     When it is zero, the charge differential between the capacitors causes a comparator 84 to produce an output voltage. At the same time that C 1  is being charged through switch 70, a fast counter 85 also receives charge from switch 70, and is caused thereby to count pulses or cycles from a fast oscillator or clock 86 operating at, say, 100 MHz. When the capacitors C 1  and C 2  have the same voltage, the comparator 84 produces an output voltage which stops counter 85. Thus, the number of counts divided by 1000 is a measure of the time between pump and probe pulses. 
     The active devices in the comparator 84, the switches 70 and 80, and the passive components of charging circuits 72 and 82 are stable, although the active devices could drift with temperature, in which case, they would be temperature compensated so as to prevent such drift, as by conventional means well known in the art. 
     While characteristically the spectrum of the pump pulse will be such as to be largely absorbed by the experiment, the spectrum of the probe pulse will be such as to be at least partially transmitted through the experiment, so as to be available for detection by signal detector 10. 
     As has been explained more fully hereinabove, the pulse detectors 7 and 8 provide for measuring time intervals, namely, the time elapsed between pairs of consecutive pump and probe pulses. Therefore, in principle, as shown in FIG. 5 should the experiment not absorb all of the pump and probe pulse, the pulse detector 7 could be located on the side of the experiment opposite to what FIG. 1 shows as the site of pulse detector 7. The signal detector 10 would then double as pulse detector 8, and the beam splitters 3 and 4 would be omitted. For those skilled in the art, other geometries are possible: e.g. all reflective schemes, etc.,as would be useful in picking up scattered light from the event. However, it is easy enough to dimension the apparatus shown in FIG. 1 so that there would be no significant discrepancies in arrival times due to splitting pulses into multiple paths. 
     Regardless of the detection arrangement, pulse detection provides for creating the quantities needed for observing event behavior at the aforesaid time intervals. 
     In ASOPS as described by Elzinga, et al, the temporal resolution of the system employed was limited by two factors, pulse width and, more significantly, pulse timing jitter or phase noise between the two pulse sources. Because pulse timing jitter was initially much larger than the pulse width from each source, the temporal resolution of their system was poor. With modification, they were successful in reducing phase noise to about the pulse width of each source--which was about 1 ps. 
     Similar problems exist with most short pulse sources. While they may be able to produce pulse widths of several tens of femtoseconds, their pulse timing jitter can be 1000 times worse, or several tens of picoseconds--making them poor candidates for experiments that require femtosecond resolution. And while phase noise can be minimized by referencing their resonator cavities to an external oscillator, residual phase noise may still be on the order of 1 ps. 
     FIG. 6 shows a unitary form of TIMe pulse detection scheme that will address this problem. This geometry can also be a means for generating a signal by which the pulse timing jitter between two sources can be minimized, or for programming-in suitable delays between pump and probe pulses. 
     We expand pump pulse 30 beam diameter as shown by the dimension L1 and direct the expanded beam onto the sheet of material 31, at the angle B1. This can be done using a suitably designed cylindrical lens beam expander. Similarly, the probe beam 32 is expanded to the dimension L2 and directed onto the material 31 at the angle B2 which may be but is not necessarily the same as the angle B1 of pump beam 30. Both angles are measured from the normal to the material 31. Material 31 is a straight strip of uniform thickness, of a so-called &#34;non-linear&#34; material, e.g. 13130, KDP, etc. 
     A detectable quantity is produced when the two pulses 30 and 32 overlap in the material 31. For example, the material could be a crystal such that the overlap of pump pulse 30 and probe pulse 32 produces a third pulse 33 at a wavelength that represents the sum or difference frequency of pump pulse 30 and probe pulse 32. Or it could be an organic molecule with a two-photon absorption at the wavelength sum frequency of 30 and 32. 
     In any case it can be seen that pump pulse 30 sweeps across the material 31 from left to right. Similarly, probe pulse 32 sweeps across material 31 from right to left. Thus it can be observed that when the path lengths between the source of the pump pulse and the source of the probe pulse are identical and the two sources emit pulses in precise synchrony, then they will overlap in the center of the material 31. At the position of overlap, the material 31 will generate another pulse 33 that is imaged by the optical system represented by 34 onto an array detector 35. 
     A change in the arrival time between the pump and probe pulsed will cause a corresponding shift in the point where pump pulse 30 and probe pulse 32 overlap in the material 31. The result will be a measurable change in the position where the pulse 33 is generated in the material 31 and detected at the array 35. This change in position of 33 is a measure of the time delay between the pump and probe pulse. Optionally, a wave-length cut-off filter 36 may be provided for preventing pulses 30 and 32 from reaching the array 35. 
     Of course, this is a limitation imposed on the range of time intervals between the arrival of the pump and probe pulse at material 31, and thus their corresponding difference in arrival times at the experiment. This restriction is a practical one associated with size. But for reasonable size and cost arrays, the range of intervals could be several hundreds of picoseconds with sub-100 femtoseconds resolution, which is more than adequate for most ultrashort experiments. Data can be acquired and processed at kHz rates with relatively inexpensive, commercially available data acquisition boards. 
     While some synchrony between the pump and probe sources may be needed to keep them within the range of time intervals detectible by our TIMe scheme, this can be done using active cavity length stabilization as known to the prior art, (e.g., as in Spence et al, Darack et al, and Rodwell, et al)*. *OPTICS LETTERS/V. 16, No. 22/Nov. 15, 1991/pp. 1762-4; OPTICS LETTERS/V. 16, No. 21/Nov. 1, 1991/pp. 1677-9; IEEE J. QUANTUM ELECTRONICS/V. 25, No. 4/April, 1989/pp. 817-27, respectively. The virtue of our approach is that the pulse timing jitter is an advantage, as it provides for some random sample of time intervals that can be used to map out the temporal evolution of the event being investigated, whereas prior art pulse timing jitter was a disadvantage because it reduced temporal resolution. Indeed, it may be desirable to use the position signal from array detector 35 to program-in various time intervals so as to obtain a representative sample over the interval range of interest, and thus &#34;induce&#34; phase noise into the system. Or, it may be desirable to use this position signal to minimize pulse timing jitter between two oscillators still further, and thus produce more perfect synchrony between two sources. 
     Only ordinary skill in the art would be required to choose suitable components for use in the present invention. Suitable choices are presently available as follows: 
     Laser 1: Model CPM-1 Colliding Pulse Mode-locked Dye laser operating at 630 nm 
     Laser 2: Model NJA-2 Self mode-locked Ti:Sapphire laser operating at 780 nm. 
     Signal Detector 10: photomultiplier, Burle 1P28. 
     CFD&#39;s 17, 18: Ortec 935 
     TAC 16: Ortec 567 
     DAS 19: Multi-channel Analyzer. 
     Pulse detectors 7 and 8: Williamson-type Interdigitated photoconductors. N.B. In particular, the 1.2 picosecond photodetector disclosed by Y. Chen et al, in an article entitled &#34;1.9 PICOSECOND OPTICAL TEMPORAL ANALYZER USING 1.2 PICOSECOND PHOTODETECTOR AND GATE, in International Electron Device Meeting, Washington, D.C. Dec. 8, 1991, page 417 PhB 3131192, possibly constructed on different substrates, like silicon or GaAs, which have different carrier lifetimes. 
     It is to be noted that all the foregoing specifications of components and circuit elements are exemplary, and will be sufficient guidance for those of ordinary skill in the art to choose equivalents, or make desirable modifications. 
     Suitable uses for the present invention include, electro optic and optical wave form sampling; experiments involving studying carrier dynamics and electromagnetic transient measurements in semi-conductors; experiments involving studying chemical reaction dynamics; and in general, making time-resolved studies of various kinds in chemistry, biology, physics, electronics, and other scientific/technological fields. 
     It is also to be noted that the use to which our invention is put will have a bearing on the nature of the pump and/or probe pulses. For instance, the foregoing description of our invention relates in great part to optical pulses. However, experiments can involve studying the properties of fast electronic devices and components, in which case the probe and pump pulses could be microwaves, as provided by microwave oscillators, e.g. masers, or photo-enhanced electron beams. We believe that the principles of our invention apply to microwaves, electron beams and electomagnetic radiation pulses in general, as well as to the above-described optical pulses. 
     In the foregoing we have described the preferred embodiments of our invention, and, as well, components, which we regard as suitable for use in our invention, and which are available commercially, or otherwise, e.g., constructible by technicians using only ordinary skill in the art of optics, electronics, and other skills of the maker of scientific and technical apparatus. Such description is intended solely as fulfillment of the requirements of Title 35, USC 112, first paragraph, and it is not to be taken as limiting the claims appended hereto. 
     Again, while we have also recited certain uses which may be made of our invention, these are merely exemplary. Such recital is not to be taken as limiting, for we believe that there likely are, or will arise, other suitable uses therefore, which we have not recited herein, but which those of ordinary skill in the art now recognize, or will in the future.