A compact and stable incidence-plane-rotating second harmonics detector S. H. Kim,1 S. Jung,1,2 B. Seok,1,2 Y. S. Kim,1,2 H. Park,2 T. Otsu,3 Y. Kobayashi,3 C. Kim,1,2 and Y. Ishida1,3, a) 1)Center for Correlated Electron Systems, Institute for Basic Science, Seoul 08826, Republic of Korea 2)Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea 3)ISSP, The University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa, Chiba 277-8581, Japan (Dated: 23 April 2021) We describe a compact and stable setup for detecting the optical second harmonics, in which the incident plane rotates with respect to the sample. The setup is composed of rotating Fresnel-rhomb optics and a femtosecond ytterbium-doped fiber-laser source operating at the repetition frequency of 10 MHz. The setup including the laser source occupies an area of 1 m2 and is stable so that the intensity fluctuation of the laser harmonics can be less than 0.2 % for 4 h. We present the isotropic harmonic signal of a gold mirror of 0.5 pW and demonstrate the integrity and sensitivity of the setup. We also show the polarization-dependent six-fold pattern of the harmonics of a few-layer WSe2, from which we infer the degree of local-field effects. Finally, we describe the extensibility of the setup to investigate the samples in various conditions such as cryogenic, strained, ultrafast non-equilibrium, and high magnetic fields. I. INTRODUCTION rotated. Since the scattering plane is fixed with respect to the sample, this method enables access to only a limited Optical second harmonics can be generated when number of the tensor components of the nonlinear optical 18–21 the medium is shaken with intense optical field of fre- polarizability. In the second method, the sample is quency ω, E(ω):1 The octave vibrionic component of the rotated while the beam and detector are spatially fixed medium, namely the induced polarization P(2ω), gener- and investigations into all the tensor components become ates the second harmonics. Second harmonics generation possible. However, its application to the samples in non- (SHG) can be manifested profoundly when the medium ambient conditions such as cryogenic, high magnetic and 18,19 is subjected to broken inversion symmetry.2 Thus, SHG ultra-high vacuum is limited because it is generally has been widely used to investigate the crystallographic difficult to rotate the sample precisely therein. Finally, symmetry and underlying electronic structural phase re- one can use the incidence-plane rotation method (Fig. 1), lated to non-centrosymmetry. The SHG technique is also in which the sample is fixed while the incident beam and sensitive to surface, edges, hinges and corners of the crys- detector rotate together. The method requires a tech- tal,3 where the inversion symmetry is intrinsically broken nique to fix the incident beam spot on the sample and and polarization charge remains macroscopic. This as- direct the SHG signal to the detector while the optical pect makes the SHG technique also useful and attractive system revolves. These requirements make this method to investigate the surface and interface phenomena4,5 as challenging and available only recently with some variety 22–25 well as the edge modes of topological materials6,7 includ- in the setups. ing those of higher-order topological insulators.8,9 An- other recent trend is to artificially enhance the nonlinear response in meta-materials and meta-surfaces.10–12 2ω z From the beginning of the technique, the SHG mea- ē P 2ω surements had been performed mostly in the normal in- ē S ω cidence configuration. This configuration is enough to ē p Incidence ω arXiv:2104.10855v1 [physics.ins-det] 22 Apr 2021 figure out the SHG signal generated by Px and Py but plane ē s not by the out-of-plane polarization Pz. Investigation θ of the latter requires us to illuminate the sample with Sample oblique (off-normal) incidence. φ − φ There are three ways to attempt rotations in the 0 y oblique incidence configuration: (1) polarization, (2) x sample and (3) incidence-plane rotation methods. In the 13–17 polarization rotation method, the beam and detector FIG. 1. Incidence-plane-rotating SHG detection. The inci- are spatially fixed and the incident beam polarization is dent beam and the SHG signal detector rotate about the sur- face normal of the sample. The incident beam has to keep the p or s polarization during the rotation. a)[email protected] The incidence-plane rotation method realized in the 2 Hsieh group in 2014 was operated with a diffractive- surface (Fig. 1). To this end, it is convenient to inject optic based stop-start scheme.22 The incidence was made the circularly polarized beam into the rotary optics along oblique by diffracting the original ray with a phase mask the rotation axis; see the right side of the schematic and refocusing the offset ray on the sample surface with a of Fig. 2(a). The convenience is due to the rotational reflective objective. The SHG signal was recorded while symmetry of the circular polarization; the polarization rotating the detector and the phase mask step by step. (Jones) vector for the circular polarization is stationary The system was renovated to solve the power-fluctuation seen from both the laboratory frame and the frame fixed induced low-frequency-noise issue, by using high-speed on the rotary optics. As a result, whatever the rota- (∼4 Hz) rotating phase mask based optics and position tion angle of the rotary optics may be, the polarization sensitive photodetector.23 Subsequently, the Torchinsky vector of the circularly-polarized injection is stationary group24 developed reflective optics based rotating in- when seen from the frame fixed to the rotary optics. cident beam generator for the wavelength independent measurement. However, this high speed rotation design (a) por s Stationary polarization still requires a precise alignment process to make sure the seen from Focus Polarization co-rotating frame high speed rotating optics is appropriately positioned. optics controller Offset circular optics Here, we present another design to perform the p& s por s incidence-plane rotation SHG measurement. The under- Coll. Polarization Sample Detector lying design concept is to have a compact, stable and optics selector versatile system. Our setup is based on Fresnel rhomb Rotary optics optics, a PMT detector, and a femtosecond laser source composed of optical fibers doped with ytterbium (Yb). (b) The Fresnel rhomb optics is compact, wavelength in- Lens por s linear dependent, and misalignment-tolerant in principle. Al- circular FM though we applied the stop-start-type stepping-motor Pol λ /2 method, the low frequency noise is under control with DM Pol CF Fresnel Sample the hour-long stability of the compact fiber-laser source. PMT rhomb This paper is organized as follows. After the present Rotary optics introduction (Sec. I), we describe the optics to rotate the CCD To lock-in amplifier incidence plane in Sec. II. In Sec. III, we sketch the home- made fiber laser source and its stability. In Sec. IV, we (c) describe the assembly and alignment of the entire system. Lens Pol λ In Sec. V, we show the SHG signal data of a gold mirror /2/2 Frensnel and check the operation of the SHG optics (Sec. V A), Sample rhomb RS1 and also present the SHG pattern of a few-layer WSe2, from which we infer to the crystal orientation and degree CCD of local effects (Sec. VB). The summary and outlook are RS2 provided in Sec. VI. In Appendixes A and B, we estimate DM + Pol +PMT the intensity of the faint SHG signal of gold, the order of which corresponds to 0.1 photon per pulse (Appendix A), and subsequently describe the SHG tensor analysis at the FIG. 2. Rotary optics. (a) Essential features in the rotary op- oblique incidence (Appendix B). tics. (b) Optics adopted in the present setup. (c) A snapshot of the rotary optics (Multimedia view). II. THE ROTARY OPTICS The essential features in the rotary optics are illus- trated in the dashed rectangular region in Fig. 2(a). The In this section, we first sketch the essential features to injected beam is first shifted from the roto-axis by the off- rotate the incidence plane and subsequently describe the set optics. Subsequently, the polarization is set to either specific optics that we have adopted. p or s by the polarization controller. Finally, through In the incidence-plane rotation method, there are some the focus optics, the beam is directed to the sample optical components that rotate about the axis normal to surface at an oblique angle. The SHG signal from the the sample surface [Fig. 2(a)]. The role of the rotary surface is directed to the detector through the collimat- optics is to (1) rotate the incidence plane, (2) set the po- ing optics. Before reaching the detector, the signal goes larization of the incident beam, (3) focus the beam on the through the polarization selector that filtrates the p- or sample, (4) filtrate the SHG signal and its polarization s-polarization. and (5) direct the filtered signal to the detector. Figure 2(b) shows the optics adopted in our setup. During the turn of the rotary optics, the direction of For the sake of compactness, we used a Fresnel rhomb the incident polarization also has to follow the turn and (FR600QM, Thorlabs) at the entrance of the rotary op- maintain the p- or s-polarized incidence on the sample tics.