古新安，朱韋臻，羅志偉，ANDREEV Y M，LANSKII G V，SHAIDUKO A V，IZAAK T I，SVETLICHNYI V A，VAYTULEVICH E A，ZUEV V V

(1.國立交通大學(xué)，新竹30010，中國 臺灣; 2.俄羅斯科學(xué)院西伯利亞分院氣候與生態(tài)系統(tǒng)監(jiān)測研究所，托木斯克634055，俄羅斯; 3.俄羅斯托木斯克洲立大學(xué)西伯利亞物理技術(shù)研究所，托木斯克634050，俄羅斯; 4.中國科學(xué)院長春光學(xué)精密機械與物理研究所，吉林長春130033)

1 Introduction

Due to a set of extreme optical properties，GaSe has been successfully employed to generate coherent radiation in the mid-infrared and down to the terahertz(THz) frequency range［1-2］. On the other hand，low hardness and easy cleaving lead to low optical properties that hamper the applications of large-area crystals.Fortunately，GaSe is a good matrix material for doping with various impurities.An original ε-polytypic structure of GaSe is strengthened by doping impurities and other physical properties being responsible for the frequency conversion efficiency and exploitation parameters are also significantly modified［3-9］.Nevertheless，while THz generation by optical rectification and down-conversion in GaSe has been studied in detail［2，10-11］，there are very few tasks devoted to the experimental study of THz range optical properties and generation in doped GaSe.Optical properties and efficiency of THz generation is studied experimentally in S-doped GaSe(GaSe∶S)［12-17］，GaSe∶In［15-16，18］，GaSe∶Er［19-20］，GaSe∶Al［21］and GaSe∶Te［15-16，18，22-23］.The studies of the physical properties and THz generation in the intricate crystals grown from the melt GaSe∶AgGaS2(identified as double doped GaSe∶S∶Ag)and GaSe∶AgGaSe2(identified as GaSe∶Ag) are reported［21，24］，relatively.

Beyond mid-IR，the THz absorption of a nonlinear crystal places a practical limit on frequency range of optical rectification and down conversion.The THz absorption of a crystal is usually attributed to infrared-active phonon modes or their combination modes.The impact of the phonon modes including rigid layer modeE'(2)centered at 0.586 THz on refractive indices and THz generation efficiency in GaSe∶Er was studied experimentally in Ref.［20］.Second order phonon modes were identified in centimeter-sized high optical quality GaSe∶S crystals in Ref.［12-13］.No rigid mode transformation with doping were ever been reported but rising up of the rigid modeE'(2)at 1.78 THz in intensity［14］，so as degeneracy of the rigid layer modeE'(2)in low doped GaSe∶S［14，17］and GaSe∶Te［25］.Thus，the available data on optical properties in centimetersized doped GaSe in the THz region are still inconsistent.Besides，no optimal doping by an impurity in GaSe has ever been reported.

This work reports the growth of hexagonal structure GaSe doped with 0.05%，0.1%，0.5%，1%，2%，5% and 10%(mass percent)Te or GaSe∶Te(0.05%，0.1%，0.5%，1%，2%，5% and 10%(mass percent))in the charge composition and for the fist time in our knowledge experimental study of the rigid modes transformation with doping is carried out.Criterion for determination of the optimal Te-doping is proposed and checked in an optical rectification experiment.

2 Crystal growth and characterization

2.1 Crystal Growth

Creation of GaSe1－xTexcrystals involves two principle steps.Initially synthesis polycrystalline material with the mass of 120－150 g has been produced in a two-zone horizontal furnace by using high purity(99.999 9%)gallium(Ga)，selenium(Se)，and 99.9% tellurium(Te).A weighted charge of Ga and Se were placed in the boats located at hot and cold ends of the ampoule.Chemical reaction of the reagents up to GaSe formation has been produced on the first stage by sublimation of Se at 690℃and interaction of the vapor with Ga melt at 970℃，that is，the GaSe compound is synthesized under selenium vapor pressure in the reaction ampoule.The second stage provides the melt homogenization at 1 000℃ due to diffusion，whereas on the third stage the melt is cooled for 36 h and the homogeneous largeblock GaSe ingots are grown.The process of GaSe synthesis is performed by three sequential stages at different temperature profiles over the ampoules as it is described in details elsewhere［26-32］.Doping by Te atoms was developed by 0.05%，0.1%，0.5%，1%，2%，5% and 10%(mass percent)into the boat with gallium during synthesis of the compound.The temperature gradient at the crystallization front was 10℃/cm and crystal pulling rate was 10 mm/d.The samples were prepared by cleaving an asgrown ingot parallel to thec-plane layer and used without any additional treatment.The lengths of the specimens prepared and studied were 1.14，1.0，1.0，0.98 and 0.86 mm，respectively.

2.2 Crystal composition and structure

The composition of the GaSe∶Te crystal was provided by Z-8000 atomic-absorption spectrometry，Hitachi spectrometer(air-acetylene flame)and by inductively coupled plasma optical emission spectrometry(ICP-OES)with spectrometer iCAP 6500，Thermo Scientific after dissolution of sample weights in nitric acid.Crystals grown from the charge with 0.05%，0.1%，0.5%，1% and 2%(mass percent)of Te are identified as GaSe∶Te(0.01%，0.07%，0.38%，0.67% and 2.07%(mass percent))crystals，respectively.The ε-polytype structure of the observed specimens was identified by the proposed non linear method［33］through φ-angle dependence of a femtosecond Ti∶sapphire laser optical rectification.For all crystals，rectified signal versus φ-angle was clear six-petal-flower type which is similar to that in ε-GaSe，as it goes from the relation for efficient nonlinear susceptibility coefficientdeff=d22cosθ sin3φ for the type-I of three frequency interactions.

2.3 Optical properties

UV-visible transmission of the close length specimens observed was recorded by Cary 100 Scan(Varian，Inc.，Austria)spectrometer:wavelength range is 190 900 nm，spectral resolution is 0.2－4 nm，wavelength deviation is ±1 nm.Mid-IR transmission was recorded by FTIR Nicolet 6700(Thermo Electron Corp.)spectrometer:operation wavelength range is 11 000 － 375 cm－1，spectral resolution is 0.09 cm－1.Selected spectra are presented in Fig.1(a)and(b).

Absolute values of the attenuation coefficients at maximal mid-IR transparency range were measured at chosen points on the crystal faced with low power φ1.0 mm beam at wavelength of 9.6 μm CO2laser band to minimize the influence of the surface defects on the measurement results.

Terahertz range absorption coefficient andnospectra in the crystals were determined by a homemade THz-TDS spectrometer with 50 fs Ti∶sapphire laser system(800 nm)described elsewhere［14］.The THz beam was normally incident to the crystal face.The THz absorption and dispersion spectra are shown in Fig.1(c)and(d).

Fig.1 UV-visible(a)，mid-IR(b)transmission，THz o-wave absorption(c)and nodispersion spectra(d)in GaSe∶Te.

3 Results and discussion

Crystalsgrown from the charge with 0.05%，0.1%，0.5%，1% and 2%(mass percent)of Te are identified as，respectively，ε-GaSe∶Te(0.01% ，0.07%，0.38%，0.67% and 2.07%(mass percent))single crystals that are suitable for THz generation via optical rectification and down conversion.At maximal transparency range of mid-IR GaSe∶Te(0.01，0.07，0.38 mass%)crystals possessesowave absorption coefficient α from ～ 0.2 cm－1to 0.5 cm－1.GaSe∶Te(0.67% ，mass percent)crystals is characterized by dramatically increased α≥5 cm－1mainly due to high density of Te precipitates.Crystals grown from the charge with 5% and 10%(mass percent)Te appear as usefulness polycrystalline low-optical-quality material.

In Fig.1(a)，it is seen that the structure of phonon modes in mid-IR are independent but rigid modes in THz range transforming with Te-doping(Fig.1(c)).The rigid layer modeE'(2)at ～0.59 THz is expected to be the mode in which the layer vibration are as rigid units against each other and that there is no relative displacement of the Ga and Se atoms within a layer.For low Te-doped crystals the center of theE'(2)mode is well in coincidence with the data for pure GaSe in Ref.［34］.Measuring the phonon absorption spectra in mm-length crystals we reportE'(2)is rising up till reaching the highest value at Te-doping between 0.07% and 0.38%(mass percent).This process correlates well with the improvement in the optical quality in GaSe∶Te due to the decreasing in the number of point and layer stacking defects［35］.

The rigid layer modeE'(2)is decreasing in the intensity till degeneracy with further Te-doping.Simultaneously，the absorption peak of the rigid modeE″(2)centered at 1.78 THz is rapidly rising up in the intensity and noticeably influence thenodispersion at Te-doping≥0.67%(mass percent)(Fig.1(d)).It is the mode in which the layers vibrate a-gainst each other with the relative displacement of two Ga-Se sub-layers within a four-atom layer［36］.It was established that bothE'(2)rigid layer mode degradation andE″(2)rigid mode rising up correlate well with the decreasing in the crystal optical quality that is due to numerous reasons such as structural defects(polytypism，stacking faults，dislocations)［35，37-38］，defect complexes［9，37］，exciton phonon and exciton impurity interactions［37］，interlayer interstitials［35，39］and internal strains［40］.

No a distinct reason was formulated to explain the rising up in intensity ofE″(2)rigid mode with Tedoping.Possibly interlayer intercalation of larger size of Te atoms［9，37］leads to the formation of the local strained regions that bond hard Se and Ga layers.

4 Conclusion

The rigid mode transformation in GaSe∶Te crystal was studied for the first time.This process correlates well with the transformation of the optical quality in GaSe∶Te.Doping level that is resulting in the highest intensity of the absorption peak of rigid layer modeE'(2)is proposed as the criterion of optimal Tedoping in GaSe.

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