Laser-induced voltage of table salt for deep ultraviolet pulsed laser detection

Summary To meet the requirements of fast response and simple process of deep ultraviolet (UV) pulsed laser detector, table salt (TS) was used as laser detection material in combination with a variable resistor to achieve single-pulse laser detection. Under the irradiation of a KrF excimer laser, the laser-induced voltage (LIV) of TS was influenced by the dynamic process of laser-induced plasma, and the whole process was well fitted with the sum of the three exponential functions. As the applied bias voltage (Vb) and incident laser energy (Ein) increased, the LIV amplitude (Vp) increased and the response time decreased. When the variable resistor (R) was reduced to 14.7 Ω, the response time of LIV decreased from ∼300 μs to ∼20 ns, which is the same as the duration of laser pulse. This research provided a simple, low-cost, and fast method for the detection of UV single-pulse laser based on the laser-TS interaction.


INTRODUCTION
][8][9][10] It is crucial to measure the characteristics of the pulsed laser, especially for the real-time detection of single-pulse laser energy.With the development of laser measurement process, thermopile energy system, and wide-gap semiconductor detection materials are widely applied. 11,12In recent years, photodetectors have attracted extensive attention for ultraviolet (UV) laser detection due to its good detection performance. 13However, the high-energy UV pulse laser causes a challenge to traditional laser energy detection system due to its low thermal effect and narrow pulse width.Because the characteristics of LIP are affected by single-pulse laser energy, LIP is expected to be a sharp edge for high single-pulse laser detection. 14However, the plasma characteristics are also influenced by many factors such as the laser wavelength, the pulse duration, the target characteristics. 15It is more challenging to achieve rapid single-pulse laser signal acquisition and energy extraction based on laser-material interactions.
Because the plasma generated by laser irradiation can be used as a signal source and the electrical signal of the plasma can be collected by an external circuit, it is receiving more and more attention. 16,17When the intensity of the pulsed laser is above the plasma formation threshold, the plasma is generated in the irradiated sample and can be detected by an external circuit.The positive and negative ions in the plasma move directly under the external electric field and are collected and transmitted to an external circuit.Thus, a voltage signal is detected by the receiver of the measurement circuit and is termed as laser-induced voltage (LIV), which may be an effective means to characterize the plasma change process.][24][25][26] Table salt (TS) as a white single crystal is found in almost every household throughout the world.The main component of TS is NaCl, and it has the advantage of cheap, universally available.NaCl, only known stable compound in the Na-Cl system under ambient conditions, 27 has many natural defects in TS such as vacancies, steps, and lattice distortions. 28As we know, NaCl is wide bandgap, ionic materials.It is well characterized by the interaction with ultraviolet (UV) photons because of defects. 291][32] For this purpose, investigation of LIV from NaCl is of significance, especially for TS which is rich in defects.
4][35][36] Due to a high energy fluence the KrF pulsed laser generates a large amount of plasma when it is focused on the target.Therefore, KrF laser was selected as the irradiation laser source in this work.Based on the interaction between laser and TS, the LIV response reflects the plasma characteristics and motion processes.In the meantime, a function was established between the LIV amplitude (V p ) and laser energy through the characteristics of LIP.Here, we also focus on the impedance effect of the LIV response, where the LIV response time was significantly reduced as the external resistance decreased.The research results showed that with a small external resistance, ln(V p ) is linearly correlated with the incident laser energy and the LIV response time matches the laser pulse duration, so TS is expected to be a fast, easy-to-acquire, and low-cost material for detecting UV single-pulse laser.

RESULTS AND DISCUSSION
If plasma is emitted from the TS surface, the LIV signal detected with the copper electrodes.Figure 1A shows the typical LIV signal detected using the oscilloscope when the KrF laser irradiated on the TS at V b = 200 V, E in z 20 mJ, and R = N.The LIV response shows three variation processes: quick increase before 33 ns, a peak at $ 65 ms, and a gradual decay lasting $560 ms (Figure 1B).The time of plasma flight can often be modeled by a delta function source of effusing particles with a Maxwell-Boltzmann velocity distribution. 37Here, the data were well fitted by the sum of the three exponential functions with the decay times t 1 $ 12.5 ns, t 2 $ 30 ms, and t 3 $ 210 ms, LIV (t)/E in = -A 1 e -t/t1 -A 2 e -t/t2 + A 3 e -t/t3 . 38The close correspondence of measured (solid blue lines) and the calculated traces (dashed orange line) indicated that the model is suited to describe the temporal development of the LIV response, where A 1 = 5.97 V/J, A 2 = 13.43V/J, and A 3 = 18.66 V/J (Figure 1A).This LIV response is divided into three parts: expansion, mobilization, and compounding processes of the plasma.A 3 represents the weaker process of the plasma, so it has a positive parameter which is different from A 1 and A 2 .
For nanosecond (ns)-pulsed laser irradiation of the TS powder, if the laser intensity is sufficiently large, the laser irradiation could form a plasma plume at the surface of the TS, whose generation and motion can greatly affect the LIV response.Thus, by measuring the induced voltage, we can understand more information about the produced plasma situation.The plasma consists of two main groups: a ''fast'' one composed of ionized species and a ''slow'' one which is mainly due to the contribution of neutral particles. 39The laser-induced plasma has the shape of semi-spherical three-dimensional symmetries under the ionization/recombination and excitation/de-excitation mechanism. 40At the initial stage of laser irradiation on the salt, due to shielding effects, main part of the laser energy E in passes through inverse bremsstrahlung and partial reflection of the laser beam to heat the electrons instead of coupling directly to the sample. 41During laser irradiation of 20 ns the plasma is in thermal equilibrium.But after pulse termination, the cloud enters a non-equilibrium state and expands to the ambient gas at pressure P c (P c > P 0 , P 0 is the natural environmental pressure). 42Here, the plasma is ejected in the very strong field left by the laser excitation through a Coulomb process in a very short timescale by laser excitation.In this initial stage plasmas are very hot and dense, which can be approximated by a free expansion.The plasma expanding to the vicinity of the electrodes is captured by an oscilloscope and converted to a LIV signal (Figure 2A).
The subsequent slow-rising process needs more time to establish, because the neutral particles in the subsequent thermal process are influenced by the V b . 43In this work, plasma occurs the ambipolar diffusion in the cuvette due to the electric field.The electrons have higher mobility and early spread to the electrodes.The positive charge leaving behind will retard the motion of electrons and speed up the ions. 44he plasma expansion can be weakened by plume confinement, collisions between plasmas and scattering effects in atmospheric environments.The applied electric field V b can accelerate the process of ambipolar diffusion and reduce the loss of plasma in motion (Figure 2B).When the plasma reaches gas-dynamic equilibrium, the quartz cuvette still contains electrons number density of 10 16 -10 19 cm À3 . 45The plasma moved to the electrodes under the force of external electric field and then converted into an electrical signal.In addition, the existence time of this process is also affected by the collisions between different types of plasma particles, and the complex space distribution of the electrostatic retarding field and the shielding effect.The LIV signal decreases sharply with laser shot following the gradual waning of the plasma, and eventually returns to its initial state (Figure 2C).
To illustrate the effect of laser energy on the LIV traces characteristic, the change of the LIV signals with the laser energies are shown in Figure 3A.In order to further determine the relationship between the LIV signal and the laser energy E in , we extracted the values of a maximum voltage V p , an l0%-90% rise time t r and a full width at half-maximum (FWHM) of LIV traces.The results are shown in Figure 3B, where V p /E in increases with increasing E in , while FWHM and t r decrease almost linearly.This LIV signal is believed to originate from the combined action of the plasma generated by laser irradiation and the electric lines of forces distributed in the space around the electrode due to the applied V b .At high E in the number of plasmas increase, thereby increasing the LIV response.In the same way, when the laser energy increases, the sample surface creates a larger lateral pressure gradient and temperature gradient, accelerating the sprayed particles and ''pushing'' them to electrodes.Thus, the oscilloscope detects the plasma earlier and the FWHM and t r decrease with increasing E in .The slope of FWHM and rise time t r against E in is À10.38 and À4.48 ms/mJ.Here, the responsivity R* is defined as R* = V p /E in .Therefore, the experimental setup has an R* of $5.22 V/J when E in < 19.1 mJ and a maximum R* of 8.07 V/J at E in = 22.3 mJ.
In Figure 4A, with the supplied voltage V b is increased further, the V p increases proportionally under a laser energy E in of 18.2 mJ with R = N.The V p increases from 43.5 to 125 mV with V b from 110 to 200 V, and the relationship between V p and V b is described as V p = ae bVb with a = 9.3 mV and b = 12.8 V À1 .It can be seen in Figures 4B and 4C  It is shown that the V p of the LIV response characterizes the plasma density.The pulse width of KrF laser is 20 ns, but the response time of LIV is on the ms scale.In addition to characterize the dynamical processes of the plasma, we consider that the LIV signal response is affected by the impedance of the test circuit.Figure 5A displays the LIV waveforms with different external resistors R at V b = 200 V and E in = 14.5 mJ.The LIV signal amplitude V p decreased from 3.58 mV to 0.1 mV when the R changed from 1 kU to 5 U.These waveforms indicate that the LIV response process is determined by line resistance.In order to analyze the LIV response characteristics, Figure 5B provides an insight study of the FWHM and t r at different R. The responsivity R* as a function of R is shown in Figure 5B and a simple form of R*, FWHM and rising time t r is given for R < 100 U, where R*f R, FWHM f R and t r f R. A careful simulation conforms to the form of ln (R*) = kln(R), where k z 0.49 ln(V/J)/ ln(U).Both the R*, FWHM and rise time r t show high values of $3.59 V/J, 100 ns, and 40 ns at R = 1 kU and decrease gradually to $0.13 V/J, 16 ns and 18 ns at R = 6.7 U.The ln (FWHM) and ln (t r ) against lnR display approximate slopes of $0.34 and 0.16 ln(ns)/ln(U) (insets of Figure 5B).Here, the LIV response is mainly influenced by impedance effects.
The response time of LIV signal can be reduced to $20 ns, which is equal to the pulse width of the KrF pulsed laser, by adjusting the load resistance R in the circuit.R rapid discharge leads to LIV response enhancement, but with loss of plasma dynamics process information.The results in Figure 5 display that the FWHM $21 ns and the rise time t r $ 22 ns of LIV signal when R = 14.7 U.The response characteristics are similar to the KrF pulsed laser.Figure 6A shows the typical LIV transients to pulsed laser irradiation with different E in under V b = 200 V and R = 14.7 U.These LIV responses show similarly triangular and symmetrical traces, and the time constant is completely limited by the laser pulse duration.Consistent with the findings in Figure 3B, the LIV signal peak V p is elevated with increasing laser energy E in as shown in Figure 6B, satisfying the logarithmic relationship of ln(V p ) f E in with a slop of $0.17 ln(mV)/mJ, slightly V b dependent.Meanwhile, this result provides a possible way for real-time detection of laser pulse characteristics.
The oscilloscope can be equated to a resistor and capacitor in parallel, so the response time of the circuit is affected by the resistor-capacitance circuit constant, and a lower R will result in a much shorter decay time of the LIV response.The periodic oscillation of $20 ns at the end of LIV trace, which may be due to an impedance mismatch in the circuit caused by the mismatch between the co-axis cable and the 1 MU oscilloscope.The impedance change detected by the circuit throughout the detection process is composed of the structural impedance and the plasma impedance generated by the sample (Figure 7A).Considering the test circuit as a voltage divider, the real signal U ts generated by TS under the laser irradiation is calculated from the oscilloscope readout voltage U o by U ts = U o (1+R ts /R i ), where R ts is the sample impedance and R i = R//R 0 z R is the impedance of the oscilloscope input channel.Here the LIV signal amplitude is mainly dependent on the circuit impedance, so the V p increases monotonically with the impedance R.
The plasma is emitted from the TS because of laser irradiation.At the same time, heating, melting, boiling, evaporation, and phase explosion occur on the surface of TS. 46,47 In fact, laser induced plasma contains many charged particles including electrons and ions, which can be driven by an electric field.Here, the laser induced plasmas can be considered as an equivalent circuit as shown in Figure Due to the generalized flow is driven by different kinds of generalized forces existing in the system, the plasma is driven by density gradient and temperature gradient to the vicinity of the electrodes for rapid expansion.At this moment, the plasma density and temperature are highest in the center of pulsed laser irradiation.The temperature of the plasma increases after absorbing laser energy and gradually decreases after expanding outward.The supplied electric field cannot penetrate the region of high density plasma aggregation, meaning that the process is not affected by V b .The electrons in TS can be easily excited to air due to the low electron affinity of NaCl.Since the mass of positive ions are much larger than electrons, the speed of electrons is much larger than that of ions.Therefore, throughout the diffusion process, electrons will collide with the electrodes earlier than the ions and accumulate near the electrodes.
As shown in Figure 7B, electrons and positive ions are collected near the electrodes on both sides by diffusion and external electric field.Here, we focus on the process of electron movement.The electrons will prevent electrons from moving near the electrode after reaching a  certain amount.Simultaneously, some positive ions move toward the electrode where the electrons accumulate, and neutralize some electrons under the effect of density gradient, resulting that the ability to stop electrons is weakened.Finally, the two kinds of particles achieve dynamic equilibrium near the electrodes, and this small area is defined as the sheath.Thus, the rising part of LIV signal is divided into two phases due to the change of the plasma dynamics process.And because of the sheath, the LIV signal is influenced by the incident laser energy and bias voltage.
In this work, the laser energy changes are reflected by the kinetic characteristics LIP, unlike conventional photoelectric effect.We investigated the characteristics of TS on deep UV single-pulsed laser detection through LIV signals.The LIV responses are positively correlated with the laser energy.The response time of this measurement system is reduced to $20 ns by changing small external resistor, which provides a prerequisite for the application of single-pulse laser energy detection.The direct detection of LIP kinetic processes in real time is an important practical problem.The LIV method opens up new possibilities for kinetic research on laser-induced plasma, exhibiting a performance level and application promise.We accurately monitored three processes in the LIP of TS.The LIV method holds potential to improve the LIP process monitoring and, combining with micro-drilling machining, will improve the efficiencies of process.
Laser-induced plasma is a very complex and various phenomenon as it depends on laser pulse characteristics, as well as on the state of the irradiated sample and its chemical and physical properties.The dynamics of laser-induced plasma are highly dependent on the environment where plasma generated, such as vacuum, atmospheric, and gaseous background, meanwhile, the energy exchange at the contact wall is formed, which plays a crucial role on the expansion dynamics. 48The background environment of present research for LIV was atmospheric, and the LIV signal of TS under laser irradiation with different pulsed energy E in differs significantly because of the motion properties of the plasma.The corresponding relation between laser parameters and electrical signals was found by analyzing the characteristics of the LIV.The effect of electrode geometry on spark ignition has been investigated in recent years, but most of the researches have focused on the theoretical modeling stage.It would be very meaningful and interesting to observe the plasma dynamics when the pressure in the quartz cuvette is

EXPERIMENTAL MODEL AND STUDY PARTICIPANT DETAILS
Our study does not use experimental models typical in the life sciences.

METHOD DETAILS
Figure S1 shows the measurement system employed in this work.A KrF excimer laser COMPexPro50 from Coherent Inc. was used as the source at room temperature in air, operating at a wavelength of 248 nm (5 eV photon energy) with 20 ns duration at a 1 Hz repetition rate in this experiment.The maximum pulse energy of laser is 150 mJ with the maximum pulse frequency of 50 Hz and the average power of 7 W.The beam dimensions and the beam divergence are 14 3 7.5 mm 2 and 2 3 1 mrad, 2 respectively.At the distance of 90 mm from the sample, the laser irradiated vertically onto the sample surface through a 4 mm 3 7.5 mm diaphragm.The pulse energy E in on sample was varied between 13 and 25 mJ.The sample was filled into a quartz cuvette with geometry of 20 mm 3 45 mm and a thickness of 10 mm.Four parallel copper electrodes with 2 mm intervals were fixed in the cuvette.In the above configuration, the two electrodes near the side wall of the cuvette were supplied with a DC voltage V b .Here, the power supply was derived from the Keithley 2400 source meter.The other two adjacent electrodes were connected to an adjustable resistor R and an oscilloscope.In order to reduce the response time of the detected signal, R was changed from 1 kU to 5 U.The LIV trace was measured through the Tektronix DPO4032 sampling oscilloscope (350 MHz bandwidth terminated into R 0 = 1 MU and 2.5 GS/s sampling rate) with the LIV signal as an input to signal port of the oscilloscope, and the oscilloscope is grounding during signal detection.TS was filled into the cuvette at a high of 21.5 mm.The air pressure in the cuvette is 101 kPa (P 0 ) which corresponds to the outside environment.
TS in present experiment was from China National Salt Industry Group Co., Ltd.As-supplied TS powder without any processing has been directly placed on the sieve.TS particles were fractionated by manual shaking for approximately three minutes using four sieves with mesh sizes of 550, 380, 250 and 180 mm, thus particle size classes were indicated with the corresponding size range of > 550 mm, 380-550 mm, 250-380 mm, 180-250 mm and < 180 mm.The samples were screened for five times to ensure the size distribution of 7.1%, 21.8%, 46%, 15% and 10.1% particles are < 180 mm, 180-250 mm, 250-380 mm, 380-550 mm and > 550 mm, respectively.In order to measure accurately, the data was the mean value of the five pulse tests.In this study, the component information of the TS was examined with X-ray diffraction (XRD).As shown in Figure S2, the XRD peaks of (111), ( 200), (220), ( 222), (400), (420), and (422) indicated that the main component of TS was cubic phase NaCl.

QUANTIFICATION AND STATISTICAL ANALYSIS
The Origin 9 package was used for statistical analysis.Data reported are expressed as the mean of five separate laser pulse experiments.SEM was used for error bars.

REAGENT or RESOURCE SOURCE IDENTIFIER
Chemicals, peptides, and recombinant proteins

Figure 1 .
Figure 1.Time evolution of a typical LIV response of TS under a selected E in of $20 mJ, V b = 200 V, and R = N (A) Linear axis, and dotted lines show the calculations with the three exponential functions.(B) Logarithmic axis.
that the FWHM and r t strongly depend on the applied voltage and decrease linearly with the V b .FWHM and t r as the function of V b are presented as FWHM = a 1 V b + b 1 and t r = a 2 V b + b 2 , where a 1 = À0.87 ms$V À1 and b 1 = 456.8ms, a 2 = À0.137ms$V À1 and b 2 = 85.8 ms.With this electrode configuration, a portion of the electric lines can interact with the plasma due to the horizontally electric field, promoting the movement of the plasma toward the electrode and resulting in a LIV signal.When the V b increases, the plasma expanding speed is faster and faster, so the plasma reaches the electrodes very early, and FWHM and t r decrease with the V b .Meanwhile, a larger V b leads to a stronger electric field effect, which increases the LIV signal amplitude.

Figure 2 .
Figure 2. Dynamics of laser-induced plasma (A) The stage of free expansion.(B) The process of ambipolar diffusion under electric field.(C) The last stage of plasma in motion.

Figure 4 .
Figure 4. LIV signal parameters of salt as function of V b when E in = 18.2 mJ (A) The relationship between V p and V b .Data are represented as mean G0.008 V. (B) The relationship between FWHM and V b .Data are represented as mean G8 ms.(C) The relationship between R t and V b .Data are represented as mean G1 ms.

Figure 3 .
Figure 3. LIV response from table salt with different E in at V b = 200 V, which were measured without load resistance (A) The LIV traces.(B) V p /E in (solid point), FWHM (half-up point), and rise time t r (open point) as a function of E in .Data are represented as mean G SEM. V p /E in 's SEM = 0.3 V/J, FWHM's SEM = 10 ms, and rise time t r 's SEM = 5 ms.

Figure 6 .
Figure 6.LIV as a function of E in under and R = 14.7 U (A) The LIV traces when V b = 200 V. (B) V p as a function of E in selected V b of 200, 170, 140 V and 100 V. Error bars represent standard deviation, with the fluctuations of G9%, G12%, G11% and G20%, respectively.

Figure 5 .
Figure 5. Temporal evolution of the LIV signals below 1 kU with a selected E in of $14 mJ and V b = 200 V (A) The LIV traces.(B) R*, FWHM and rise time t r as a function of R. Data are represented as mean G0.12 V/J, G3 ns, G3 ns, respectively.