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high energy radiation can be detected directly by converting photons into electrical signals , or indirectly using scintilators and photodiodes . direct conversion detectors are expected to have higher signal - to - noise ration , higher spatial and temporal resolution , even on a single photon counting level . this could lead to spectrally resolved computed tomography and advanced medical x - ray imaging  @xcite . an efficient detector optimization is a challenging technological task . the single crystal growth is one of them , high mobility - relaxation time product ( @xmath1 ) is another one and least but not the last one is different mobility and trapping cross - section of photo - generated holes . the latter one contributes to significant degradation of temporal resolution . a wide range of materials  @xcite provide good sensitivity , but poor temporal performance is achieved . the low mobility of holes , compared to electron mobility , have been solved by coplanar grid ( cpg ) detection  @xcite in cdznte detectors , semiconductor analogue of frisch grids in gas ion chambers . as the technology of majority of proposed materials needs yet to mature , the choice of gaas , crystalline cdte or cdznte is mandatory for counting and high energy resolution applications . the problem with different electron and hole mobilities is overcome using the coplanar grid electrodes and it has been proven also to be efficient way to increase specral resolution  @xcite . the coplanar - grid as well as other electron - only detection techniques  @xcite are effective in overcoming some of the material problems of cdznte and , consequently , have led to efficient gamma - ray detectors with good energy resolution while operating at room temperature  @xcite . the main limitations yet remain to be high quality crystalline material and polarization effects . their reduction has been addressed both experimentally and theoretically . atomistic simulations  @xcite are a powerful method for exploring crystalline defects at a resolution unattainable by experimental techniques  @xcite . determination of electric field shape precisely due to relatively complicated electrodes structure and application of the bias and intergrid voltage simultaneously is experimentally also difficult task . though exact solutions are often tedious , simplified models have been also proposed . a small part of pixel detector has been considered  @xcite and spectral response modeled . polarization and carrier trapping was solved by simplified set of kinetic equations  @xcite in cdznte detectors , the effect of transport parameters inhomogenities ( charge density , mobility , potential fluctuations ) and hot carriers were discussed  @xcite and plasmons in solid state radiation detectors were studied  @xcite . the laser induced transient current technique ( l  tct ) provides information about charge collection dynamics and detector polarization  @xcite . it has been measured on pixel  @xcite and coplanar grid detectors  @xcite . though it is important for further detector optimization , the theoretical treatment is usually given only in simplified experimental geometries and basic formulation of schockley - ramo theorem  @xcite . the coplanar grid detectors have been modeled also only in simplified geometry and spectral response has been calculated only  @xcite . sparse realistic detector geometries were considered for x - ray spectra simulations  @xcite , however , no temporal response has been calculated . we provide here an insight on carrier transport in a radiation coplanar grid detector based on cdznte , valid also for other semiconductors used in direct conversion detectors . we determine influence of bulk semiconductor parameters on a temporal and spectral resolution in real detector geometry . the requirement on efficient charge collection is discussed with respect to effects of depletion width , diffusion and detector shielding . tct waveforms compared to numerical model allow evaluation of the electric field profile inside coplanar grid detector with high spatial resolution unattainable experimentally . the spectral resolution improvement is quantitatively determined in a coplanar grid detector and the differential current sensing  @xcite is shown to overcome problems of low hole mobility , high hole trapping cross - section and random nature of x-/gamma ray photon absorption within a detector . we also aim to show simplified analytically based considerations to understand detector operation , which is useful for qualitative optimization without need of elaborate numerical calculations . the l  tct response has been modeled by drift transport mechanism of photo - generated carriers . the above band gap photo - generation is assumed . holes have been neglected for their small mobility @xmath2  @xmath3v@xmath4s@xmath4 and short life time  @xcite . for this reason we assume electrons only  @xcite and collection times much shorter than electron life time . in l tct , holes are swept to cathode in subnanosecond times , hence , their influence is negligible . the current response caused by moving charge between electrodes have been calculated using shockley - ramo theorem . the foundation of shockley - ramo theorem is based on known electrostatic potential in two cases . the first one consists of real electrostatic potential distribution within a detector . the second is a potential distribution of a weighting potential . here , the electrode of interest is biased to 1  v and all other electrodes are at 0  v. the corresponding electric field determines the charge response on a given electrode and the time derivative determines the current response  @xcite . the full real potential and corresponding electric field determines the trajectory of photo - generated carriers . we assume only small enough concentration of photo - generated carriers that they do not spread by diffusion or electrostatic repulsion on the length scale of detector size before they are all swept by anode . this is clearly well fulfilled since the diffusion time broadening is about 15  ns and carriers move about 1  @xmath5s from cathode to anode . the limitations of this assumption are discussed comparing the model and experimental data . tct current response is modeled by assuming 1.4  mm wide laser spot size on partially transparent cathode . we trace 300 electron trajectories equally spaced on a cathode center . the motion of each electron ( electron packet ) has been calculated by integrating their drift velocity . the current response is then calculated for each of 300 trajectories and summed up . the resulting summed current response has been compared to experimental data . we also assume positively charged depletion width below cathode and negatively charged space region below anode , as commonly observed in cpg . these space charge regions give rise to current decrease ( increase ) at the time of 0 ( 1  @xmath5s ) . the experiments done by hard x - ray and @xmath6-ray transient current technique ( x  tct ) has been modeled by solution of poisson equation and , in the second computation step , electron trajectories have been obtained by integrating their velocities ( as in l  tct curves ) . however , here , the initial position of photo - generated carriers have been determined in a monte - carlo loop . random position of photo - generated carriers is a model of large absorption length of x / gamma - ray radiation , hence the model holds for more than 100 s  ev energetic photons . here , we have not summed all trajectories with a random initial position . instead , the current response has been calculated for each trajectory separately and the total collected charge has been calculated by integrating current response in time . the electron current response has been considered only since holes mobility is @xmath7 lower  @xcite and holes have higher trapping cross - section . we have traced 1500 trajectories and total collected charge from electron motion has been statistically analyzed . the electrostatic potential has been calculated by solving poisson s equation @xmath8 by finite element method . the detector geometry has been chosen to properly model a redlen inc . state - of - the - art cdznte cpg detector . the mesh triangulation has been used to properly describe size effects of rectangular metal anode grid . the anode behaves as a set of parallel charged wires when charge is far from the anode compared to anode characteristic size ( anode strip width ) . and , the anode behaves like infinitely large planar electrode if the charge is close enough to anode compared to characteristic anode size . we consider the latter case as a measure of sufficiently dense triangulation . this regime can be analytically treated as well , and pronounces itself as a constant electric field and linear - in - distance electrostatic potential . the strong electric field at both electrodes causes velocity saturation . the field maximal values are @xmath9  v / cm , giving drift speed up to @xmath10  cm / s assuming electron mobility is @xmath11@xmath12v@xmath4s@xmath4 . the saturation velocity in cdznte is about @xmath13  cm / s  @xcite therefore the velocity field dependence is expected to show non - linear behavior . the velocity saturation  @xcite is taken into account by @xmath14 . the modeled l  tct waveforms have been compared to experimental data measured on coplanar grid cdznte detector . the detector , purchased from redlen inc . , has dimensions @xmath15  mm@xmath16 . the details of our ltct setup  @xcite and data proccessing are descibed elswhere  @xcite . the detector cathode was irradiated by optical pulses ( @xmath17  ns as fwhm , 200  ps rise and fall time ) using the laser diode ( 660  nm , 300  mw ) that is powered by an ultra - fast pulse generator . pulse output energy was 0.4  nj and repetition rate was 100  hz . a collimation and imaging optics and a translation stage is used in the setup to adjust the laser diode beam onto the examined spot of @xmath17  mm@xmath18 on the cathode . neutral density disc filter is placed in the beam line for optimal intensity attenuation of the laser pulse . negative bias @xmath19 is applied to the cpg detector cathode ( up to 1700  v ) and supplementary intergrid voltage @xmath20 between the collecting and non  collecting anode grid is introduced by an additional adjustable low noise voltage supply ( @xmath21  v ) .    , 4.0 and 16.0 @xmath22  @xmath23 are distinguished by red , orange and violet color . solid ( dashed ) curves in ( a , c , e ) show numerical simulation for intergrid bias @xmath24  v ( 150  v ) . solid ( dashed ) curves in ( b , d , f ) show numerical simulation for negative cathode bias @xmath25  v ( 1000  v ) . the insets ( e , f ) depict detail of the current response around @xmath26s . the space charge width below cathode is @xmath27 . the simulation is shown for ( a , b , e , f ) collecting grid and ( c , d ) non - collecting grid . the i@xmath28 trend ( a , c , e ) is studied for negative v@xmath29=1700  v and the v@xmath29 trend ( b , d , f ) is studied for v@xmath30=150  v.,width=340 ] the effect of doping is shown in fig . [ fig1 ] for positive space charge @xmath31 , 4.0 and 16@xmath22  @xmath23 . the current response in l tct is modeled for collecting grid ( cg ) in fig . [ fig1 ]  ( a , b , e , f ) and for non - collecting grid ( ncg ) in fig .  [ fig1 ]  ( c , d ) . the current response can be split into three characteristic intervals . the first one is given by charge deceleration in a positive space charge below cathode . the second regime is at intermediate times when the charge moves through the part of the detector without any fixed space charge . the third regime is a time of charge collection at the anode . it can be seen from numerical simulation that the peak current response at the collection time increases with decreasing doping . the initial current peak is decreasing with decreasing doping and ideally disappears . the initial current peak can give rise to a spurious signal contributing to a low energy spectral shoulder in @xmath6-ray spectra . the anode intergrid bias is shown for two values 0 and 150 v in fig . [ fig1 ]  ( a ) and ( c ) . we note that the intergrid bias does not influence the initial current response , as is shown in the inset fig . [ fig1 ]  ( e ) , in contrast to cathode bias , inset ( f ) . this is caused by the local effects of intergrid bias on a small area around anode . the cathode bias @xmath19 ( cathode to ncg bias ) influences mean field within a detector , thus changing current response for all times from the photo - carrier generation to their complete collection on cg . the black , red and green curves correspond to depletion width 0.64 , 0.80 and 0.85  dw , respectively . the numerical simulation is shown for ( a ) collecting and ( b ) non - collecting grid . the inset in ( a ) depicts detail of current evolution at @xmath26s . the intergrid bias @xmath32  v and cathode bias negative @xmath33  v.,width=340 ]    the cathode depletion width dependence of the l  tct waveform is shown in fig .  [ fig2 ] . the cathode is a planar electrode and the constant current is expected for uncharged bulk semiconductor . the space charge ( fixed or mobile ) causes deceleration of carriers hence decreasing current . the equilibrium depletion width ( @xmath34 ) is given by @xmath35 , where @xmath36 is doping density ( or fixed space charge ) , @xmath37 is a voltage drop within a depletion width and @xmath38 is permitivity . the width of the depletion region varies from this simple model when carrier trapping and de - trapping is assumed , or more than one trapping level is located in the vicinity of the fermi level . it can be seen from fig . [ fig2 ] that the peak current increases with decreasing depletion width . this effect is due to the reduced total voltage drop in a depletion region and consequently higher voltage drop at the anode . the latter causing higher electric field and higher current . we note here , that this is in contrast to doping density . the current response of a ncg is shown in fig . ( 2 )  ( b ) . the current response is , similarly to cg , stronger for thinner depletion width . @xmath23 ) . diffusion peak broadening @xmath39  0 , 10 , 20 and 30  ns has been assumed . the effect of diffusion is simulated for ( a ) collecting and ( b ) non - collecting grid . the intergrid bias @xmath32  v and cathode bias @xmath33  v.,width=340 ]    the effect of diffusion is shown in fig .  [ fig3 ] . diffusion takes important role for long enough transient times @xmath40 when @xmath41 t is comparable or larger than drift current temporal width . for this reason we show only the current peak modification at the charge collection on anode . the temporal broadening can be estimated from @xmath42 , where @xmath43 is electron diffusion constant related to electron mobility @xmath44 , temperature @xmath45 and mean drift velocity @xmath46 , which can be estimated from the transient time @xmath40 and detector height @xmath47 ; @xmath48 . the current response for cg and ncg is shown for four diffusion broadening times ( 0 , 10 , 20 , 30  ns ) . the current response is calculated for @xmath49  @xmath23 and space charge width 0.64 of the equilibrium depletion width . we study here the effect of electrostatic shielding on inner potential distribution within a detector . the detector shielding has been assumed to cover 2/3 of detector height . this coverage maximizes the shielding effect yet reduces a risk of material and surrounding air dielectric breakdown caused by strong electric field between cathode ( top shielding edge ) and anode . the critical electric field @xmath50  kv / mm in air provides possibility to apply @xmath51  kv between shielded cathode and anode ( shielding - anode gap 3  mm for 2/3 of detector height coverage ) . we show three trial electron trajectories in fig . [ fig9 ] for ( a ) unshielded and ( b ) shielded detector . the electron trajectories are almost parallel for unshielded detector and they tend to get localized to one trajectory in the shielded case .        the current response is shown in fig . the initial current peak decreases to about 40% of the unshielded value and the collection peak current rises by @xmath52 . both effects improve detector functionality . the first peak reduction will lead to lower spurious signal and the stronger collection peak to better signal - to - noise ratio . the measured l  tct waveforms in a coplanar grid detector on a cg and ncg anode are shown in fig . the data show good qualitative agreement , see fig . [ fig8 ] , and in many aspects also quantitative comparison with simulated data can be made , especially concerning relative intensities of current peaks at the beginning and at the end of charge collection . the initially decreasing current is caused by decelerating electrons in a positive space charge below the cathode . the second collection current peak is stronger than the first one especially due to the strong electric field between cg and ncg . this current originates also in a negative space charge below anode . the current peak at the time of charge collection @xmath53  ns is broadened by electron diffusion . the amount of broadening estimated from the numerical simulation is @xmath5410  ns . v and three inter - grid biases @xmath24  v , 20  v , 100  v depicted by black , red and green curves , respectively.,width=264 ] here we study importance of the measurement technique to detect photo - generated carriers . an example of three current waveforms detected on cg , ncg and differentially measured signal between cg and ncg is shown in fig . the advantage of differential sensing is lack of any signal before charge reaches coplanar grid anode . there is no influence of the positive space charge at cathode , since such a current response is equal for cg and ncg . we note that the differential sensing does not bring significant improvement for l  tct , where the differential current waveform can be formed by data post - processing , as has been done in numerical simulation in fig . [ fig5 ] to illustrate the method . the advantage is more pronounced when @xmath6-ray photons are detected . here , the absorption events occur randomly within a detector volume and they contribute to signal broadening . the @xmath6-ray photon detection is modeled in fig .  [ fig6 ] . voltage drop at the cathode ( positive space charge @xmath49 @xmath23 ) and @xmath55 voltage drop at the anode ( negative space charge @xmath56  @xmath23 ) . the current response measured at the collecting grid ( solid blue curve ) , non - collecting grid ( dashed red curve ) and differential signal ( solid black curve ) . the applied biases are @xmath32  v and negative @xmath25  v.,width=226 ]    we have traced 1500 trajectories of randomly generated photo - excited electrons and the total electron - induced charge has been calculated . an example of 50 random trajectories is shown in fig . [ fig6 ]  ( d ) . we note , that if holes are considered the total collected charge is @xmath57 , however , we assume low hole mobility and high trapping cross - section for holes . this leads to much weaker current response , or , the current response on very long time scales in comparison to electron induced tct waveforms . for this reason , electron traced from below cathode gives larger induced charge than electrons traced from the anode vicinity . the statistics of all monte - carlo simulated absorption events are shown in fig . [ fig6 ]  ( a , b , c ) for cg , ncg and for differential sensing , respectively . the cg and ncg current waveforms have rectangular like shape caused by equal distribution of absorption events within a detector . the mean collected charge on ncg is shifted towards zero because ideally no charge is collected by ncg . we consider here negative space charge at the anode . this space charge screens the electric field of neutral bulk detector ( no charge collected by ncg ) and it allows certain electron trajectories to be collected by ncg .    -ray photon absorption events . doping density , depletion widths and applied biases are the same as in fig . the absorption probability is assumed to be equal throughout the detector volume . currents induced by holes are neglected . the histograms ( a , b , c ) depict statistics of collected charge for all 1500 absorption events . the statistics is shown for the case of current measured on ( a ) collecting , ( b ) non - collecting grid and ( c ) differential signal between collecting and non - collecting grid . equipotential lines ( blue to red contour lines ) , detector area ( solid black lines ) and 50 selected random absorption events are shown in ( d ) . , width=340 ]    in contrast to cg current response , the differential sensing , see fig . [ fig6 ]  ( c ) shows the same mean charge collection , however , the full width at half maximum is 10@xmath0 smaller . this is caused by the signal insensitivity to the position of random - in - nature absorption events of @xmath6-ray photons . we also note that there is no spurious signal at 0  ns , fig . [ fig5 ] , caused by positive space charge hence reduced low energy spectral shoulder  @xcite is expected beside the compton scattering contribution . the effect of holes is also reduced regardless of assumptions made in our simulations . holes are attracted towards cathode giving even weaker response on a cg anode . general considerations to understand current waveforms follow from shochley - ramo theorem @xmath58 . decreasing / increasing current shows on decreasing / increasing electric field in a positive / negative space charge at cathode / anode . the space charge at anode forms weak current response at @xmath59  0v and it causes current shoulder for early time of charge collection . the dominant signal at the charge collection time for @xmath60  0 is caused by accelerated electrons in a spatially modulated electric field by inter - grid bias @xmath20 . more specifically , we have shown that the cathode bias increases the peak current response . it also increases the initial signal caused by positive space charge at the cathode  [ fig1 ]  ( b ) . this can lead to spurious signal leading to low energy spectral shoulder in x ray spectra . at the same time , since the charge is swept faster through detector bulk , there is lower probability for carrier trapping and detector polarization . the lowest doping density is still the ultimate option here because it worsens the charge collection efficiency . the effect of diffusion is twofold . first , the higher is diffusion / mobility , the faster is a response time at a given detector size . second , as absorption efficiency increases with detector size , diffusion lowers the signal . hence , for fast response and strong current signal , thinner detector is acceptable . we also point out that thinner detector volume also eliminates carrier trapping . the optimal detector thickness and coplanar grid dimensions can be established for differential signal sensing . the final peak width in the case of differential sensing is caused by space charge at the anode , grid size ( signal increases for denser anode grid ) and by carrier diffusion . low diffusion rate also leads to better signal - to - noise ratio . the diffusion length on a distance of detector size should be smaller than anode intergrid spacing . if diffusion length is comparable or larger than the anode intergrid spacing , the cg and ncg diffusion dominated current will be equal , and , differential signal will be negligible . the shielding has been shown to improve the current waveforms in a solely drift model , as shown experimentally  @xcite . it improves signal to noise ratio , deceases the initial spurious signal caused by positive space charge at the cathode and eliminates noise from the external detector environment . contrary to that , it slows the detector response time by about @xmath61 and directs carriers into one single conducting channel inside detector . the channeling effect leads to increased diffusion and electron repulsion even for intermediate times before charge collection . next , further simplifications of the proposed numerical model are discussed . it is shown that many current waveform characteristics can be explained separately one by one using simple physical considerations . these are based on electrostatic potential spatial distribution of the space charge in bulk semiconductor and assuming proper scaling of charge - to - electrode distance . if charge is close to planar cathode with respect to the cathode dimension , the electric field can be considered constant and electrostatic potential linearly scales with distance from cathode . in the presence of space charge below cathode and charge - cathode proximity , the field scales linearly and electrostatic potential quadratically with the charge - cathode distance . the schockley - ramo theorem than gives quadratic current waveform . the situation at the anode has to be split into two regimes . when electron is far from anode compared to the anode dimension , the electric field can be tought as that of charged wire . the intergrid potential difference will be negligible and the electron effectively moves in the effective electric field given by mean potential on cg and ncg anode . this mean potential influences the transient time by the order of 100d / l  % , where d is anode width and l is a detector thickness . when electron approaches anode on a distance much smaller than anode width , the situation becomes equal to the one at cathode in close charge - cathode proximity . the iterative schemes can be applied to describe analytically field profiles through whole detector volume . deviations from these assumptions lead to another model parameters . we have studied negative space charge below anode as one example and discussed diffusion due to localized carriers in one narrow potential minimum when shielding is applied as a second example of these deviations . the first additional parameter is easily taken into account in our drift model , the second one exceeds our model limitations . the proper treatment of the latter case has to be solved by coupled set of drift - diffusion , continuity and poisson equation . the numerical hardness of such simulation however hinders basic physical considerations as discussed above . we have provided simple drift - based model to understand laser induced transient current waveforms . the model has been compared to experimental data where qualitatively full current waveform has been reproduced . the capability of the proposed model has been shown to provide even quantitative agreement within limitations given mainly by diffusion and electron repulsion . we have simplified the model even more based on our experience from the numerical simulations . the simplified analytical considerations have been shown to provide a fruitful insight into detector operation . the current waveforms were studied under varied doping density , positive and negative space charge , cathode and intergrid bias , diffusion length and detector shielding . the trends on how current waveforms are influenced by these parameters provide numerically insight on a detector optimization . a mutual influence of these parameters has been discussed and their possible contradiction was pointed out . finally , we have quantified increased efficiency of the commonly employed differential sensing in hard x - ray and @xmath6-ray detectors . the more elaborate extension of the work has been discussed in terms of nonlinear set of coupled drift - diffusion , continuity and poissons equation . the presented work will be useful for quick insight on a radiation detector operation and optimization for even interdisciplinary scientific community involving broader physics and medical physics . financial support from the technological agency of the czech republic under project te01020445 , and the grant agency of the czech republic under contract p102/16/23165s are gratefully acknowledged . 38ifxundefined [ 1 ] ifx#1 ifnum [ 1 ] # 1firstoftwo secondoftwo ifx [ 1 ] # 1firstoftwo secondoftwo `` `` # 1''''@noop [ 0]secondoftwosanitize@url [ 0 ] + 12$12  & 12#1212_12%12@startlink[1]@endlink[0]@bib@innerbibempty @noop * * ,   ( ) link:\doibase 10.1088/0957 - 0233/17/4/r01 [ * * ,   ( ) ] link:\doibase 10.3390/s110505112 [ * * , ( ) ] link:\doibase    { 10.1209/0295 - 5075/96/60010 } [ * * ,   ( ) ] @noop * * , ( ) @noop * * ,   ( ) @noop * * , ( ) link:\doibase    10.1063/1.2776373 [ * * ,   ( ) ] link:\doibase    10.1063/1.1502922 [ * * ,   ( ) ] link:\doibase    10.1103/physrevb.86.245203 [ * * ,   ( ) ] link:\doibase    10.1103/physrevb.85.115206 [ * * ,   ( ) ] @noop * * ,   ( ) link:\doibase 10.1088/0022 - 3727/42/17/175101 [ * * ,   ( ) ] link:\doibase 10.1103/physrevb.77.035205 [ * * ,   ( ) ] link:\doibase 10.1103/physrevb.77.195207 [ * * , ( ) ] link:\doibase 10.1209/0295 - 5075/23/2/004 [ * * ,   ( ) ] @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * , ( ) link:\doibase    10.1063/1.4963353 [ * * ,   ( ) ] @noop * * ,   ( ) @noop * * ,   ( ) link:\doibase 10.1063/1.2755931 [ * * ,   ( ) ] link:\doibase    10.1016/j.apradiso.2014.09.003 [ * * , ( ) ] link:\doibase    10.1016/j.vacuum.2015.09.014 [ * * ,   ( ) ] in @noop _ _ ,  , vol . ,  ( ,  )  pp . ,   link:\doibase 10.1016/j.nima.2006.01.072 [ * * ,   ( ) ] @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop _ _ ( ,  ) @noop * * ,   ( ) @noop * * ,   ( ) link:\doibase 10.1209/0295 - 5075/110/42002 [ * * ,   ( ) ] link:\doibase    10.1016/j.cap.2015.04.027 [ * * ,   ( ) ]
we have modeled laser - induced transient current waveforms in radiation coplanar grid detectors . 
 poisson s equation has been solved by finite element method and currents induced by photo - generated charge were obtained using shockley - ramo theorem . the spectral response on a radiation flux 
 has been modeled by monte - carlo simulations . 
 we show 10@xmath0 improved spectral resolution of coplanar grid detector using differential signal sensing . 
 we model the current waveform dependence on doping , depletion width , diffusion and detector shielding and their mutual dependence is discussed in terms of detector optimization . 
 the numerical simulations are successfully compared to experimental data and further model simplifications are proposed . the space charge below electrodes and a non - homogeneous electric field on a coplanar grid anode 
 are found to be the dominant contributions to laser - induced transient current waveforms .