Picosecond Streak Camera Fluorometry-A Review
Total Page：16
File Type：pdf, Size：1020Kb
IEEE JOURNALOF QUANTUM ELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983 585 Picosecond Streak Camera Fluorometry-A Review ANTHONY J. CAMPILLO AND STANLEY L. SHAPIRO (Invited Paper) Abstract-A general tutorial survey is presented describing the use of Several excellent reviews [ 1] - [4] have appeared describing the ultrafast streak cameras in picosecond fluorometry.Current instruments operation and history of the streak camera. This present paper exhibit time resolutions of 1-10 ps with detection sensitivitiesof a few goes beyond the instrumental aspects, and reviews the applica- photoelectrons. When linear photoelectric recording is employed,a real-time direct display of optical transients is provided. Representa- tion of laser fluorescence spectroscopy in some detail. We hope tive examples from the literature inphysics, chemistry, and biology this paper satisfies what appears to be a deficiency in this re- are given, as well as an extensive bibliography. gard in the open literature. We have attempted to present a reasonably self-contained primer in streak camera operation, including examples from the literature in physics, chemistry, INTRODUCTION and biology. We include an extensive bibliography, which is HE ultrafast streak camera is one of the most versatile representative of the field as a whole. Due to the continued Tinstruments used today in picosecondspectroscopy. Its growth of this subject, we are not able to include a complete usefulness arises from its excellent time resolution, its ability list of references here. to determine optical temporal profiles directly and in a single The key principles of electron optical streak camera opera- shot, if need be, and its straightforward integration into re- tion were established during the period from the late 1940’s markablysimple experimental configurations. Although the through the early 1960’s. In 1949, Courtney-Pratt employed a device itself is electronicallysophisticated, it resembles a deflectable image converter [5],[6] to measuretemporal/ photomultiplier/oscilloscope combination in convenience and spatial characteristics of light given off during explosive burn- operation. This is not coincidental, since the streak camera is ing in lead azide. This early device resembled currently used a photoelectric image converter camera possessing a time-base systems in most essential details, and exhibited a subnanosec- deflection system similar in principle to those used in cathode- ondresolution. In the mid-l950’s, Zavoiskii and Fanchenko ray oscilloscopes. However, the streak camera possesses two [7] , [8] calculated the conditions under which picosecond or significantadvantages over theoscilloscope. First, it yields subpicosecondresolutions might be obtainedfrom these thetemporal history of one spatialdimension. Second, devices, andenumerated the fundamental limitations. They demonstrated bandwidths far exceed that of any present or pointedout that picosecond resolution required the use of projected oscilloscope. high electric fields (tens of kV/cm) to extract photoelectrons The streak camera historically was first used as a diagnostic with sufficientlyuniform velocity from thecathode. These tool to characterize the output of mode-locked lasers, and for authors also coined the term “electron-optical chronography” monitoring laser fusionimplosion experiments, principally [9] for the new science that would employ these devices to atgovernment laboratories. However, the exceptional sensi- study rapidly varying luminous phenomena. However, experi- tivity ofthese devices makesthem especially attractive for mental progress was stalledbecause ofthe lack of suitable monitoringfluorescence. Recently, an explosive growth has opticaltransients to testpractical designs. Thesituation occurredin streak camera fluorometry, and this instrument changed dramatically in 1966 with the invention of the mode- is already regarded by many as a required laboratory device in locked Nd :glasslaser, which provided thenecessary short this application. That this is true, considering the significant optical pulse source. Because conventional detectors of light, expense associated with such systems, is fair testimony to the such as photodiodes, areincapable of measuring suchbrief power and reliability of the technique. pulses, these sources were first characterized with the aid of It seemed appropriate to includein this special issue devoted autocorrelationtechniques such as secondharmonic genera- to picosecond phenomena a general tutorial and review paper tion [lo] or two- [ll] orthree-photon fluorescence. How- describing the use of streak camerasin picosecond fluorometry. ever, these techniques do not unambiguously characterize the pulse shape. Streak cameras were subsequently used by several Manuscript received August 11, 1982; revised November 30, 1982. groups to examine individual pulses from thislaser [ 121-[ 161 , The work of S. L. Shapiro was supported in part by the U.S. Air Force allowing for the first time a direct measurement of the inten- Office of Scientific Research. sityprofile I(t). It was discovered [13]that photoelectron A. J. Campillo is with the Optical Sciences Division, U.S. Naval Re- search Laboratory, Washington, DC 20375. time dispersion could be minimized by locating a planar, fine- S. L. Shapiro was with the Molecular Spectroscopy Division, National mesh,high-potential electrode, called theextraction mesh, Bureau of Standards, Washington, DC 20234. He is now deceased. close to the tube photocathode, thereby achieving temporal U.S. Government work not protected by U.S. copyright 586 JOURNALIEEE OF QUANTUM ELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983 MODE-LOCKED LASER resolutions of approximately 5 ps. Similar results were achieved M by applying a high-voltage pulse to the shutter grid element of a standard RCA C73435 image converter tube [14]. Shortly thereafter,streak cameras were used to monitor fastlaser- induced fluorescence [ 171 - [27] . STREAKCAMERA OPERATION Fig. 1 shows, in schematic form, some of the essential ele- ments in theoperation of astreak camera fluorometer. A mode-locked laser sourceprovides the necessary short burst of light to excite the sample. A lens collects a portion of the DEFLECTION FILM resulting fluorescence and images the sample onto a slit, which PLATES is in turn imaged onto the photocathode of the streak tube. Fig. 1. Schematic of typical streak camera fluorometer. Electrons are promptly emitted by the cathode by means of the photoelectric effect, and are rapidly accelerated through a mesh and toward the anode. The extraction mesh is provided With deflection velocities approaching that of light and a spa- to minimize the velocity spread in the distribution of emitted tial resolution of about 7 lp/mm, the technical time limit is in photoelectrons.The resulting electron beam current, as a the subpicosecond range. As slower streak rates are used, this function of time, closely resembles the envelope function of quantity can become quite large (-ns). In general, the spatial the fluorescence I(t). Anaperture in the anode allows the resolution is determined by the electrostatic lensing character- electron beam to pass, ultimately impinging on a phosphores- istics of the image convertertube. During the streak, the cent screen. The focus cone provides an electrostatic lensing dynamic spatial resolution is somewhat poorer thanin the static field whichsharply images the slit onto the backphosphor mode. Equation (1) must be modified if the size of the mag- screen. Indeed, if the slit is removed, a well-formed image of nified/demagnified image of the slit on the phosphor screen is the illuminated sample cell will be projectedonto thephosphor. larger than 1/L. In this case, this quantity is substituted for By dispensing with one spatial dimension through the use of a ljt. Additional degradation of the technical time resolution slit, and by sweeping the electron beam across the phosphor may be caused by saturation effects in the streak system and screen by applying a linear voltage ramp to a set of deflection limitations due to the spatial resolution of the intensifier and/ plates, a measure of time can be achieved. This occurs because or recording medium. those electrons leaving the photocathode at earlier times will A second limiting factor is that due to photoelectron time arrive at the phosphor at one position, while those that leave dispersion AT,. This effect, which is alsocalled “chromatic at later times will arrive at a different position, and time is time dispersion,” arises because electrons are emitted from the effectivelytransformed intospatial position. The resultant cathode with a distribution of velocities, and results incathode phosphorescence emitted by the screen at any spatial position to phosphor time-of-flight differences and subsequent loss of P(x) is proportional to the electron beam current, and conse- the observed signal is near the photocathode red cutoff, but quently, also to the fluorescence intensity Z(t). Typically, the becomes progressively more severe at shorter wavelengths. resulting phosphorescent spatial streak is intensified and sub- The electron time spread is given by the following relation: sequently photographed or electronically recorded. The deflection plates are driven by a ramp generatortriggered by a portion of the original excitation pulse. Typically, the deflectionplates are biased to initially image thebeam off where p is the electron charge/mass ratio, Av is the half-width screen. As the ramp is applied, the beam