3.B Picosecond Microwave Pulse-generation

Transcription

1 fluorescence output s shown wth one shot and 40 shots accumulated. A large ncrease n sgnal-to-nose rato s observed. The sngle-shot data s vrtually unntellgble, whereas the averaged data has farly good sgnal-to-nose rato. The use of a smple room-temperature DC-based photoconductve swtch has been shown to ncrease the performance of a streak camera n tmng stablty (k0.8 psec jtter), sweep speed (w12 speed of lght) and ease of operaton (no pulsed-bas, vacuum, lqud ntrogen, etc.).voltage fluctuatons (sweep speed fluctuatons) were controlled to less than t 0.1%, resultng n exceptonal stablty and relablty, and the system has no shortor long-term drft. Ths permts averagng experments to extend over long perods of tme to gan tmng accuracy nto the femtosecond doman and large sgnal-to-nose ratos for detaled fluorescence studes. Sngle-shot averagng has many advantages over repettvely scanned systems. It s emphaszed that wth ths technque n many cases the complex, unrelable, and cumbersome electroncs commonly used wth streak cameras can be replaced wth an nexpensve, smple, and hghly accurate devce capable of ncreasng the usefulness of the streak camera tremendously. The addton of a wedged short-cavty dye laser provdes short pulses of numerous wavelengths across the vsble spectrum for use n tme-resolved emsson studes. It should be mentoned that the performance of ths system n terms of tmng accuracy scales wth the laser pulsewdth used to excte the GaAs. Wth the use of a ggawatt subpcosecond source currently under constructon at LLE, the expected tmng accuracy s less than 0.1 ps. 3.B Pcosecond Mcrowave Pulse-generaton Short optcal pulses have been used n the past, n conjuncton wth bulk semconductors to swt~h~~!~~ and phase shft 57 mcrowave sgnals. Here we report the generaton of pcosecond mcrowave bursts nherently synchronzed wth an optcal trgger pulse by shock exctaton of an X-band wavegude.the exctaton s generated by a hgh-voltage photoconductve swtch drven by a pcosecond optcal pulse. Ths fast swtchng technque58 provdes a means of generatng a hgh-voltage pulse n pcosecond synchronsm wth the optcal drvng pulse. Ths hgh-voltage swtchng techn~que developed and mplemented n the laser-

2 Fgure 46 A laser-nduced photoconductve swtch s coupled to an X-band wavegude. The mcrowave burst s sent to an X-band dsh-type antenna. The sgnal reflected back from the target s fed to a crystal detector by means of a c~rculator. Power Supply DEVELOPMENT IN SUBPICOSECOND RESEARCH Pcosecond Mcrowave Pulse Semconductor...-. Generator ~ ~.~-.. X-Band Sample Crculator I Detector Mode-Locked LI Sngle Beamspltter Nd:Glass Pulse (50%) Selector - 1 Varable Optcal Delay drven fuson area l now fnds a new applcaton n pcosecond mcrowave pulse generat0n.a measured pulse duraton of 50 psec, correspondng to the optcal exctaton pulsewdth, has been obtaned. Shorter pulses can be acheved by usng pcosecond or subpcosecond optcal exctaton. The concept of producng mcrowave radaton by means of "shock" exctaton of a transmsson lne s not newe2,e3. In the past, mcrowave pulses have been created n the subnanosecond tme doman by electrcally drven spark gaps. Because ths swtchng mechansm reles on avalanche multplcaton, ths technque s lmted to lower mcrowave frequences and s affected by swtchng tme fluctuatons. In addton, laser-actvated photoconductve swtchng has been used to generate rf pulses, for example, wth the frozen-wave generatorm. Mcrowave frequences were not attaned due to the long laser pulsewdth. The present effort combnes some of the features of these prevouslyreported results. The mcrowave pulsewdth has been determned by a gatng technque based on the change of reflectvty nduced n a slab of Ge by the short optcal pulse. The mcrowave pulse duraton has been found to be less than 50 psec, n good agreement wth the laser pulsewdth.the peak mcrowave power n the burst has been estmated to be on the order of 100 MW and s suffcent to allow a hgh-resoluton radar experment to be performed. A schematc representaton of the mcrowave generator s shown n Fgure 46. A pece of sem-nsulatng Cr-doped GaAs

3 nterrupts the center conductor of a coaxal lne and s based by a hgh-voltage DC power ~upply5~. The swtchng acton s ntated by laser-nduced photoconductvty n the semconductor crystal. The optcal drvng pulse s generated by a Nd+3: YAG laser, actvely and passvely mode locked. The laser wavelength s centered around 1.064pm and the pulse duraton s psec. For a bas voltage of a few hundred volts, 10 pj of absorbed optcal energy s requred to acheve good swtchng effcency. In a wdeband geometry the electrcal pulse exhbts a rsetme dctated by the laser pulse wdth,.e psec.thefali tme s determned by charge lne length and by the carrer recovery tme, whch s Cr-concentraton dependent. The swtch drves an X-band coaxal lne-to-wavegude transton. The transton s modfed by removal of the rearwali.ths nearlyelmnates the mcrowave reflecton from the end of the wavegude and ads n reducng the mcrowave pulsewdth. The transton s then no longer matched, n the conventonal sense, to the coaxal nput. Upon laser acton, the charge stored n the charge lne s dumped nto the transton, resultng n an RF emsson. The mcrowave burst s guded va WR-90 wavegude to a 20 db broadband ferrte crculator and then to an X-band dsh-type antenna. The sgnal reflected from a target s receved by the same antenna and crculated back to a standard mcrowave crystal detector located on the thrd port of the crculator. The crystal detector has a response tme FWH M of 500 psec.the target s a flat alumnum plate located about 4m from the antenna. Fgure 47 shows two echoes separated by about 500 psec obtaned for two target postons dfferng by8 cm.thespatal resoluton s ultmately lmted bythe osclloscope trace wdth and corresponds here to a few mllmeters. 8 cm (Target Dsplacement) Fgure 47 Osclloscope dsplay showng the detector output for two target postons 8 cm apart from a target located 4m away from the mcrowave dsh. 500 psec

4 , To Hgh Voltage ;[.c Power Supply DEVELOPMENT IN SUBPICOSECOND RESEARCH Fgure 48 The mcrowave burst generated by a GaAs swtch actvated by a 30 psec, 7.06prn optcal pulse comng from Nd: YA G mode locked laser, s gated by laser-nduced reflectvty n a Ge wafer. The mcrowave reflectvty of the Ge s measured as a functon of the tme delay between the mcrowave and the optcal bursts. The carrer recovery tme n Ge s several mcroseconds so the tme ntegral of the mcrowave burst can be generated H Tunable - Short Mcrowave % % ~ ~ % P U ~ S ~ CHARACTERISTICS Optcally synchronzed pcosecond mcrowave burst 100 GHz obtanable wth subpcosecond optcal pulse The electronc system, consstng of the osclloscope and detector, has a combned bandwdth on the order of 1 GHz and s nadequate to tme resolve the mcrowave burst. Ths dffculty was overcome by usng a gatng technque llustrated n Fgure 48 and takng advantage of the synchronsm between the mcrowave and optcal pulses. The mcrowave burst s reflected by a laser-nduced electron-hole plasma n a thn wafer of ntrnsc germanum mounted across the wavegude. (The wafer s made thn (50pm) to reduce mcrowave reflectons due to ntrnsc carrers and delectrc msmatch but s stll substantally thcker than the optcal penetraton depth of about 2pm n order to maxmze optcal carrer generaton and provde mechancal support.) The tme delay between the llumnaton of the Ge sample and the mcrowave burst can be vared over several hundreds of pcoseconds by means of an adjustable optcal delay lne. In the absence of llumnaton, the Ge s nearly transparent to mcrowaves and reflects only 1% of the ncdent wave. The transmtted pulse s absorbed n a matched load. If the mcrowave burst arrves at the semconductor sample shortly after llumnaton, the mcrowave pulse s readly reflected by the laser nduced electron hole plasma. For an ncdent optcal energy of 500pJ absorbed n the thn slab and spread over 1 cm2, roughly 30% of the ncdent mcrowave power s reflected. The reflected sgnal s ntegrated by the detecton system (detectorosclloscope). The detector output pulse heght, as observed on the osclloscope, s plotted as a functon of the tme delay between the mcrowave and optcal bursts and s llustrated n Fgure 49. Due to the relatvely long carrer lfetme n Ge (on the order of mcroseconds) the plasma reflectvty s sustaned long after the llumnaton ceases. Therefore the resultng mcrowave reflecton s a mcrowave burst truncated by the step functon of

5 the Ge reflectvty and represents the tme ntegral of the sgnal. From a knowledge of the laser pulse duraton, a FWHM of less than 50 psec can be determned for the mcrowave burst.ths result s n good agreement wth the laser pulsewdth and the wavegude cut-off frequency of 6.56 GHz whch establsh, respectvely, the upper and lower frequency boundares of the mcrowave spectrum generated. We haveshown that mcrowave bursts n pcosecond synchronsm wth an optcal pulse may be generated usng a laser-actvated photoconductve swtch coupled to an X-band wavegude. A 50 psec mcrowave pulse was generated usng a 35 f 5 psec optcal trgger pulse. Further efforts to determne the mcrowave power as well as the spectral content are underway. Current efforts also nvolve extenson of ths technque to the mllmeter wave range by usng pcosecond or subpcosecond optcal pulses. An applcaton of short mcrowave pulses s llustrated n ths secton wth a hgh-resoluton radar experment. Potental applcatons of these bursts are many and nclude studes pertanng to electronhole plasma knetcs, plasmas assocated wth laser-drven fuson, temporal nvestgaton of the surface propertes of semconductors undergong laser annealng, and lght-matter nteractons such as n bologcal materals or photographc emulsons. Fgure 49 The dervatve of the sgnal unfolded wth the laser pulsewdth leads to an upper l~mt of the mcrowave pulse of 50 psec. W -I Intrnsc Germanum (50 pm)!? Irradaton: 1.06 pm z 0 a t- Y I TIME (psec) I