Growth and characterization of L -phenylalanine doped KDP crystals

Accepted Manuscript Title: Growth and characterization of L-phenylalanine doped KDP crystals Authors: Guanggang Zhou, Gang Li, Yadong Lu, ¨ Yue Ma, Xiaoliang Sun, Xuejian Deng, Peng Zhang, Guiwu Lu, Xinqiang Wang PII: DOI: Reference:

S0025-5408(18)32384-5 https://doi.org/10.1016/j.materresbull.2019.02.001 MRB 10373

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Received date: Revised date: Accepted date:

28 July 2018 25 November 2018 3 February 2019

Please cite this article as: Zhou G, Li G, Lu¨ Y, Ma Y, Sun X, Deng X, Zhang P, Lu G, Wang X, Growth and characterization of Lphenylalanine doped KDP crystals, Materials Research Bulletin (2019), https://doi.org/10.1016/j.materresbull.2019.02.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Growth and characterization of L-phenylalanine doped KDP crystals

Guanggang Zhoua§, Gang Lia§, Yadong Lüb, Yue Maa, Xiaoliang Suna, Xuejian Denga, Peng

a

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Zhanga, Guiwu Lua*, Xinqiang Wangb*

College of Science, China University of Petroleum (Beijing), Beijing 102249, P. R. China State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong

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b

Corresponging Authors, E-mail addresses: [email protected] (Guiwu Lu), [email protected] (Xinqiang Wang)

authors contributed equally to this work and should be considered co-first authors

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§ These

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*

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University, Jinan 250100, P. R. China

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Graphical Abstract

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Highlights  

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

An appropriate amount of L-phenylalanine doping can modify the growth habit and morphology of KDP crystals. The 1 mol% L-phenylalanine dopant can enhance the light transmission intensity of KDP. L-phenylalanine is incorporated into KDP crystals in the form of interstitial doping or inclusions.

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Abstract: Potassium dihydrogen phosphate (KDP) was grown by the slow cooling method together with seed rotation, and the effect of different concentrations of

L-phenylalanine (L-Phe) doping on the optical and thermal stabilities of KDP was

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also investigated. The results of the study showed that: (1) L-phenylalanine doping

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had no significant effect on the microstructure of the KDP crystals, but an appropriate

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amount of doping could accelerate the growth of the (100) crystal surface, modifying

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the growth manner and morphology of the KDP crystals. (2) The optimal amount of L-phenylalanine doping (1 mol%)increased the light transmission intensity of KDP

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but had no significant effect on the thermal stability and laser damage threshold. (3) L-phenylalanine can only be incorporated into KDP crystals in the form of interstitial

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doping or inclusions, affecting the thermal and optical properties of crystals through

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the regulation of the growth manner.

Keywords: KDP crystal; L-phenylalanine; transmittance; thermal analysis; damage threshold

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1. Introduction The rapid development of optical communication systems has facilitated more extensive research on nonlinear optical (NLO) materials. Materials with NLO properties play a role in modern optoelectronics similar to that of the conventional electronic circuit components, and the typical function of these components is to 2

modulate the carrier, amplify and rectify the signal, and act as fast switches[1, 2]. Potassium dihydrogen phosphate (KDP) not only has excellent piezoelectric, ferroelectric and electro-optical properties but also is a large-sized NLO crystal that can be grown rapidly[3]. Because KDP crystals have the advantages of a high NLO conversion efficiency, a wide optical transmission range, a low cutoff wavelength and a high laser damage threshold[4], it is currently the only NLO material that is used in

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the inertial confinement fusion (ICF) systems. The demand for KDP single crystals

has increased dramatically. Investigations of the growth rate and growth quality by

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changing the growth conditions and adding suitable additives have been a hot topic in this field[5-18].

KDP crystals are usually grown in an aqueous solution using the temperature

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reduction method. In the past decades, techniques for adding various types of organic

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or inorganic agents into the growth solution have been widely used. However, these

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additives only involve weak van der Waals forces, so that the prepared crystals show poor optical quality, low laser damage threshold and low mechanical hardness, and it

groups

through

their

extended

-electrons

and

exhibit

large

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acceptor

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is difficult to grow the crystals to the required size. Amino acids can link donor and

hyperpolarizability. Therefore, in recent years, many researchers have attempted to improve the performance of KDP crystals by amino acids doing. Kumaresan et al.[19, reported the thermal and dielectric properties of the KDP crystals doped with

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20]

amino acids such as L-glutamic acid, L-histidine and L-proline. They found that the

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crystal structure, linear-optical, NLO mechanical and electrical properties of KDP crystals were changed somewhat by the doping. Parikh[21] found that L-arginine doping improved the second harmonic generation (SHG) efficiency of KDP crystals.

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Muley et al.[22] and Kumar et al.[23]found that the addition of L-arginine and L-alanine improved the transparency, thermal stability and NLO efficiency of KDP crystals. Boopathi et al.[24] reported the effect of glycine on the optical and dielectric properties of KDP crystals, indicating that the SHG conversion efficiency of glycine-doped KDP was 1.4 times higher than that of pure KDP. Meena et al.[25]found that L-arginine had a significant effect on the electrical properties of KDP single crystals. The dielectric 3

constant, electrical conductivity and dielectric loss factor of KDP-doped crystals decreased with the increase in the L-arginine concentration. Govani et al.[26]provided a detailed characterization of the mechanism of the incorporation of L-arginine into the structure of KDP crystal using infrared absorption and Raman spectroscopy measurements. They showed that the N-H, C-H, and C-N bonds of L-arginine bind successfully to KDP crystals.

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According to the existing literature, to date, there has been no systematic report

on the growth and characterization of L-phenylalanine (L-Phe)-doped KDP single

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crystals. Moreover, the mechanism of the amino acid doping of KDP crystals is

unclear. There is still controversy whether the doping mechanism is based on substitution, gap doping, or the entry of the amino acid into the KDP crystal in the

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inclusion mode. In this paper, high-optical quality pure KDP crystals and KDP

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crystals doped with different concentrations of L-phenylalanine were grown by

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conventional solution cooling and seed rotation techniques. The crystals were characterized by X-ray diffraction (XRD), Fourier Transform infrared (FT-IR) X-ray

photoelectron

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spectroscopy,

spectroscopy

(TG/DTA)and

UV-Vis laser

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spectroscopy, thermogravimetry/differential thermal analysis

(XPS),

damage threshold experiments. The influence of L-phenylalanine doping on the performance of KDP crystals was analyzed, and the doping mechanism of

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L-phenylalanine was discussed.

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2. Experimental procedure

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2.1.Solubility and metastable zone width The solubility was measured in a constant temperature water bath (accuracy

±0.1°C). Potassium dihydrogen phosphate and L-phenylalanine were purchased from Tianjin Guangfu Science and Technology Development Co., Ltd. and Tianjin Guangfu Fine Chemicals Institute. The electrical resistivity of deionized water used in the experiments was 18.2 MΩ  cm. In the temperature range of 35-60°C, a weighing method was used to determine the solubility at different temperatures. The solubility 4

curves of KDP in solutions with different concentrations of L-phenylalanine are shown in Fig. 1a, and the widths of the metastable regions measured using the cooling methods are shown in Fig. 1b. The results presented in Fig. 1 show that the addition of L-phenylalanine increased the solubility of KDP but significantly decreased the width of the metastable region. Spontaneous nucleation in solution requires the crossing of a critical barrier given by:

16 3 2 3

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GC 

(1)

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In the above formula,  is the molecular volume of KDP,  is the surface free energy, and  is the difference between the chemical potential of the solution and the crystal. Therefore, doping with L-phenylalanine may change the surface free

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energy and reduce the critical potential energy for spontaneous nucleation of the

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solution, enabling easy spontaneous nucleation and leading to a reduction in the width

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45

35

30

40

45

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35

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40

50

a

22

Pure KDP KDP+1mol % L-Phenylalanine KDP+2mol % L-Phenylalanine KDP+4mol % L-Phenylalanine

20 Metastable width ß7 C

Pure KDP KDP+1mol % L-Phenylalanine KDP+2mol % L-Phenylalanine KDP+4mol % L-Phenylalanine

50 Solubility (g/100ml H2O)

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of the metastable region of the solution.

18 16 14 12 10 8 6

b

4 55

60

35

Temperature (C)

40

45

50

55

60

Temperature(C)

Fig.1. Solubility curves(a) and metastable zone width curves(b) for KDP doped with

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L- phenylalanine at different concentrations.

2.2. Crystal Growth KDP crystals doped with different molar percentages (0, 1, 2, and 4mol%) Lphenylalanine were grown by solution cooling and rotating the seed crystals. The growth device used a water bath heating device that was forced to convection by 5

continuously stirring the solution to maintain a homogeneous crystal growth environment. The device also included a seed rotation controller and a temperature controller. The rotation controller can keep the seed crystals in the crystal grower rotating in both directions (forward and reverse) so that the seeds could maintain uniform rotation to prevent stagnation or recirculation. Otherwise, occlusion was formed in the crystal due to uneven supersaturation in the solution. For growth of pure

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KDP crystals, we used a 1000-ml wide-mouth flask to prepare a saturated KDP solution with a total volume of approximately 800 ml at 50°C. The solution was

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filtered with a 0.22 μm filter to serve as the growth mother liquor. The mother liquor

was maintained in a constant temperature water bath and superheated to 70°C for 24 h. The seed crystal was fixed on the seed rod and in the middle of the crystal grower.

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The seed rod was connected to the rotating device and it was ensured that the seed rod

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rotated vertically. An external water bath was used to control the crystallizer

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temperature and the temperature fluctuation was controlled at ±0.1°C. Starting from the saturation point (50°C), the temperature was lowered at a rate of 0.4°C per day.

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After 10 days of growth, good quality crystals were harvested and the dimensions of

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the grown crystals were 38×18×15 mm3. For L-phenylalanine-doped KDP crystals, following the above experimental procedure, crystals with the dimensions of 33×23×20 mm3 were harvested after 10 days of growth. The KDP crystals with

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different L-phenylalanine doping concentrations are shown in Figs. 2(a-d).

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a

b

c

Fig. 2.Different concentrations of L-phenylalanine-doped KDP crystals. (a) 0 mol%, (b) 1 mol%, (c) 2 mol%, (d) 4 mol%.

3. Results and Discussion 6

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3.1. Powder XRD Analysis Powder XRD was used to confirm the identity of the solid materials and determine crystallinity and phase purity. The crystals were pulverized using an agate mortar, and XRD analysis was performed using a D8 Focus X-ray diffractometer manufactured by Bruker AXS. Table 1 lists the lattice parameters of these crystals. An

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examination of the data presented in the table shows that there was no significant change in the lattice parameters upon L-phenylalanine doping, indicating that the

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crystals still maintained a single-phase structure within the concentration range of the

doping. Fig. 3 shows the X-ray powder diffraction patterns of pure and doped L-phenylalanine KDP crystals. The prominent peaks observed for the pure and doped

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KDP were (101), (200), (211), (112), (220), (301) (312) and (420) [8, 27].No new peaks appeared, and no angular shifts of the peaks were observed, indicating that the doped

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KDP crystal structure was completely ordered. Fig. 3 shows that most of the

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diffraction peaks had the highest intensity at 1 mol% doping, and the peak intensity

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tended to decrease slightly with increasing doping concentration. However, the intensity of the peak corresponding to the (200) crystal surface always increased with

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doping concentration, reaching the maximum value at 4 mol% doping. The increase in the intensity of this peak was attributed to the increase in the interplanar spacing of

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the (100) KDP crystals. This is because the (100) plane consists of alternating positive and negative ions (K+ and H2PO4-). Since the O atom plane is slightly higher than the

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P atom and the K atom, the (100) surface eventually shows a negative charge, easily absorbing cations, and blocking the growth of the (100) plane. Similar to the EDTA complexing agent in solution, the amino acid coordinates with the cations in the

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solution, reduces the cation content, accelerates the growth of the (100) plane and thus increases the corresponding interplanar spacing[28, 29]. An examination of the crystals’ geometry (see Fig. 2) also showed that the crystals became wide after doping with phenylalanine, which was also caused by the accelerated growth of the (100) plane. This was also consistent with the previous report on the L-threonine doped KDP crystals[30]. 7

(424)

(404)

(521)

(501)

(420) (204)

(312)

(220) (301)

(211) (112)

(200)

60

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40

20

80

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N

0

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Intensity (a.u)

(101)

Pure KDP KDP+1mol % L-Phenylalanine KDP+2mol % L-Phenylalanine KDP+4mol % L-Phenylalanine

Fig. 3. Powder X-ray diffraction patterns of the pureand L-phenylalanine-doped KDP

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crystals

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3.2. FT-IR Spectral Analysis

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The effect of the doped L-phenylalanine on the vibrational frequencies of the functional groups of the KDP crystal was identified by FT-IR spectroscopy. Pure and

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L-phenylalanine-doped KDP crystals were ground with KBr to form a powder compact. The FT-IR spectra of the samples were measured using a RT-DLATGS spectrometer in the wavelength range of 400-4000 cm-1. Fig. 4 shows the FT-IR

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spectra of the pure and L-phenylalanine-doped KDP samples. The peak at 920 cm-1 was due to the P-OH stretching vibration, the strong peak at 1100 cm-1 was due to the symmetrical stretching vibration of PO2, and the peak at 1299 cm-1 was due to the anti-symmetric stretching vibration of PO2. In addition, the peak at 1638 cm-1 was due to the O=P-OH stretching vibration, the peak at 540 cm-1 was due to the vibration of HO-P-OH, and the peak at 3447 cm-1 was due to the stretching vibration of O-H. The 8

weak peak at 1381 cm-1 in Fig. 4 is assigned to the COO- antisymmetric stretching vibration of the L-phenylalanine molecule. The small peaks at 2846 cm-1 and 2924 cm-1 were assigned to the symmetrical and antisymmetric vibrations of the saturated hydrocarbon CH2. The peak at 2347 cm-1 was considered to be due to the vibration of NH2+ or NH3+ in the amino acid[27,

31, 32]

. These results indicated that the

L-phenylalanine molecules in the solution were successfully doped into KDP crystals

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during crystal growth. Considering that there was no change in the characteristic

vibration mode of the crystal after doping, it is likely that the amino acid molecules

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N

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enter the KDP crystal in the form of interstitial doping.

500

1000 1500 2000 2500 3000 3500 4000

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0

Fig. 4. FT-IR spectra of the pure and L-phenylalanine doped KDP crystals

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3.3. XPS analysis

An ESCALAB 250 X-ray photoelectron spectrometer (XPS) manufactured by ThermoFisher SCIENTIFIC was used to obtained the XPS spectra of the pure and-doped KDP crystals. The crystals were sputter-scan measured on the (100) face with an Al Kα laser source. The XPSPEAK41 software was used for the peak fitting 9

of the experimental results. Fig. 5 shows the full XPS scan spectra of the pure and-doped KDP crystals. It is observed from the figure that the major elements on the surface of the pure KDP crystals were P, K, and O (H cannot be characterized by XPS). The crystals doped with phenylalanine were also scanned for C and N elements. In addition, the Na element signal was observed on the surfaces of the crystals, indicating that potassium

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dihydrogen phosphate or L-phenylalanine drugs contained a small amount of Na

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N

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impurities.

Fig. 5. XPS spectra of the pure and L-phenylalanine doped KDP crystals

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To further prove that L-phenylalanine was incorporated into the KDP crystal, a

fine scan of the C1s and N1s for the doped crystals was performed, and XPSPEAK41 software was applied to peak fitting of the fine spectra of the samples. The peak

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fitting of the C1s fine spectrum is shown in Figs. 6 (a-c). It is observed from the figures that the C1s spectrum had spectral peaks at the energies of C-H and C-C, C-OH, and C=O. The experimental data for the KDP crystals-doped at different concentrations are listed in Table 2. The carbon atom with signals close to 284.6 eV were only connected to carbon atoms and hydrogen atoms, corresponding to the C-H and C-C bonds. The signal in the vicinity of 285.6 eV was mainly due to the 10

connection of the carbon and hydroxyl groups, i.e., C-OH, because -OH is polar and leads to the larger electronegativity, increasing the electron binding energy accordingly. The binding energy in the 287.8-288.5 eV range corresponds to a C=O linkages found in amino acid COOH and NH-C=O, where NH-C=O was produced by the dehydration condensation of the amino acids. In addition, the relative contents of C-H and C-C functional groups reached 78%-82% for the doping concentrations of 1

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and 2mol%, while the C=O functional group content only accounted for 6%. However, when the doping concentration reached 4 mol%, the relative contents of crystalline

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C-H and C-C functional groups dropped to 50%, while the C=O functional group

content increased to 12%. The C-OH content also increased by a factor of three. This observation showed that only C-C and C-H were incorporated into the crystal at low

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concentrations, while at high concentrations, the carboxyl groups in the amino acid

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also entered the KDP crystal. Fig. 6(d) showed the fine scan of N1s. As shown in the

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figure, the binding energy of L-phenylalanine-doped ammonium ions was observed at 399.5 eV on the surface of the doped crystal. No signal was observed at this energy in

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the pure KDP crystals. Based on this peak, it was concluded that L-phenylalanine was

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incorporated into KDP crystals. Since C and N elements did not show any signals at the binding energies corresponding to bonding with either K atoms or P, and O atoms, L-phenylalanine was believed to be only incorporated into the KDP crystals in the

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form of interstitial doping or inclusions.

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XPS peak fit of N1s (d)

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3.4. UV-Vis Analysis

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Fig. 6. XPS peak fit of C1s. (a) 1 mol%, (b) 2 mol%, (c) 4 mol%.

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The transmission spectra of the pure and-doped crystals were measured in the

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wavelength range of 200-900 nm using a UV-2550 UV-vis spectrophotometer. The

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crystal samples with a thickness of 2 mm were used for the transmission measurements and the measured results are shown in Fig.7. The transmission spectra

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showed that the optical transparency of the doped KDP crystals was significantly higher than that of the pure KDP crystal over the entire visible region. In the UV band,

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the optical transparency of the doped KDP crystals was also slightly higher than that of the pure KDP crystal. The highest transmittance of pure KDP crystal was 81%, and

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the highest transmittance of the crystals was as high as 93% at 1 mol% doping, but as the doping concentration increased further, the transmittance of the crystal decreased. The transmittance of the crystal doping at 4 mol% was lower than that of pure KDP.

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When the doping concentration of L-phenylalanine is too high, the grown crystals will be prone to form defects such as occlusions and cracks that will result in a decreased crystal transmittance. Therefore, doping with an appropriate concentration of L-phenylalanine enhanced the optical clarity of the crystal, and we found that 1 mol% was the optimal doping concentration.

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3.5. Thermal Analyses

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Fig.7. Transmission spectra of the pure and L-phenylalanine-doped KDP crystals

TG and DTA curves of the pure and L-phenylalanine doped crystals were measured using a Diamond TG/DTA instrument manufactured by Perkin Elmer, USA.

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The sample was heated using a platinum crucible, and the temperature was raised at a rate of 10℃/min in a temperature range of 30-500℃. The entire experiment was

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performed under nitrogen flux. Figs.8 (a-d) show the TG and DTA curves of the pure and L-phenylalanine-doped KDP crystals. Based on the observed TG curves, it is clear that the pure KDP crystal was stable at the temperatures lower than 212℃, and

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then experienced significant weight loss at higher temperatures. The temperature of the initial mass loss of the doped crystal is only 1-2℃ higher than that of pure KDP. In the DTA curves, the endothermic peak of the pure KDP crystal appeared at 222℃. In the case of L- phenylalanine addition, the endothermic peaks of 1, 2 and 4mol%-doped KDP appeared at 223℃, 223℃, 224℃, respectively, indicating that the decomposition point of the KDP crystal with L- phenylalanine was only slightly 13

changed by 1-2℃. These observations are very similar to those reported for the L-tyrosine-doped KDP single crystals, and indicated that L-phenylalanine doping also had no significant effect on the thermal stability of KDP crystals[27]. 102 6

4 297

94

3

92

2

Pure KDP

1 TG DTA

263

88

0 -1

86 84

222

82 0

100

200

300

400

212

5

98

4

96 94

KDP+1mol % L-Phenylalanine

92 90 263

88

84

-3

82

100

5

98

92

KDP+2mol % L-Phenylalanine

2

262

1

TG DTA

88

0

86

300

400

-2

82 80

214

100

200

-3

300

400

500

6

4 295

94 92

3 KDP+4mol % L-Phenylalanine

90

TG DTA

264

88

2 1 0 -1

84

224

-2

82 -3 0

100

200

300

400

500

Temperature (C)

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Temperature (C)

7

5

-3

0

0

500

86

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-1

223

96

d

N

3

84

200

U

102

6

4

309

94 Weight (%)

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Weight (%)

213

98

90

100

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c

Heat Flow Endo Up (mW)

102

96

1

Temperature (C)

Temperature (C)

100

TG DTA

-2

223

0

500

2

-1

86

-2

3

295

Heat Flow Endo Up (mW)

Weight (%)

96

6

b

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98

90

100

5

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212

Weight (%)

a

Heat Flow Endo Up (mW)

100

Heat Flow Endo Up (mW)

102

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Fig. 8. TG and DTA curves of the pure and L-phenylalanine-doped KDP crystals

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3.6. Laser Damage Threshold Measurement The Nd:YAG laser was used to measure the laser damage thresholds of the pure

and L-phenylalanine-doped KDP crystals, with the damage threshold W calculated

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according to:

W

E  r 2

(2)

where E is the incident energy corresponding to the surface damage,  is the pulse width, and  r 2 is the irradiated area. The laser damage thresholds of the pure KDP crystals and the KDP crystals doped with different concentration of L14

phenylalanine are listed in Table 3. The results showed that a small amount of Lphenylalanine had no effect on the laser damage threshold of the KDP crystals, whereas excessive doping (i.e., doping concentrations greater than 4 mol%) greatly reduced the damage threshold of the crystal. It should be noted that the laser damage threshold depends on the wavelength, energy, pulse duration, longitudinal and transverse mode structures, beam size and beam position. The mechanism of

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laser-induced material damage is often very complicated, involving processes such as electron avalanche and multiphoton absorption.[33] We believe that a doping

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concentration that is too high will give rise to defects inside the crystal, causing an increase in light absorption and thus reducing the damage threshold.

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4. Conclusion

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The pure KDP crystal and 1, 2,and 4 mol% L-phenylalanine-doped KDP crystals

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were successfully grown by a combination of solution cooling and a rotating seed

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crystal. It was found that the addition of L-phenylalanine increased the solubility of the KDP sample, but the width of the metastable region of the growth solution showed

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a significant decrease. XRD measurements showed that the pure and doped KDP crystals were tetragonal, and the XRD diffractograms and lattice parameters indicated

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that the crystals still maintained a single-phase structure within the doping concentration range. FT-IR spectroscopy and XPS electron spectroscopy confirmed

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the successful incorporation of L-phenylalanine into the KDP crystals, and it was concluded that L-phenylalanine was incorporated into KDP crystals as inclusions or by interstitial doping. The doped KDP crystal had a higher transmittance in the visible

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light region than the pure KDP crystal, and the KDP crystal had the highest transparency for the doping of 1 mol%. TG/DTA studies confirmed that with the increase in the L-phenylalanine concentration, the decomposition point of the KDP crystal increased slightly, indicating that the doping did not reduce the stability of the crystal. Laser damage threshold measurements showed that the damage threshold of the pure KDP crystal reached a high value of 1.35 GW/cm2, and a small amount of L15

phenylalanine had no obvious effect on the laser damage threshold of the KDP crystal. Experimental studies have shown that a significant increase in the growth size and solubility of KDP crystals is obtained for an appropriate doping of L-phenylalanine. Moreover, while not degrading the laser damage threshold and optical thermal stability, L-phenylalanine doping at the optimal concentration significantly enhances the optical transmittance of the KDP crystal. This is highly

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significant for the application of KDP crystals in optical devices, highlighting its

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prospects for use in NLO materials.

Acknowledgements

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The authors thank the National Natural Science Foundation of China (Grant

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Nos.51372140) and Shandong University Young Scholars Program(2016WLJH27).

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Table 1. Cell parameters of the pure and L-phenylalanine-doped KDP crystals Crystal

Crystal system

Space group

KDP

Tetragonal

I-42d

a=b=7.453，c=6.972

387.275

KDP+1mol% L-Phe

Tetragonal

I-42d

a=b=7.452，c=6.975

387.338

KDP+2mol% L- Phe

Tetragonal

I-42d

a=b=7.452，c=6.974

KDP+4mol% L- Phe

Tetragonal

I-42d

a=b=7.453，c=6.972

IP T

Cell parameters（Å） Volume（Å）3

387.282

SC R

387.275

Binding energy and functional group content of KDP+ L-Phe Eb/eV 1mol% ri /%

C-OH

C=O

285.58

287.85

78.44

14.92

6.64

284.58

285.70

287.83

81.70

12.19

6.11

Eb/eV

284.52

285.43

288.46

ri /%

49.85

38.11

12.04

2mol%

N

A

ED

ri /%

284.60

M

Eb/eV

C-H, C-C

PT

4mol%

U

Table 2. Relative content of the functional groups doped into KDP crystals

A

CC E

Table 3.Laser damage thresholds of the KDP crystals doped with different concentrations of L-phenylalanine

Dopant concentration of L-Phe（mol%）

0

1

2

4

Laser damage threshold （GW/cm2）

1.35

1.35

1.35

0.96

19