1. Introduction

The terahertz (THz) frequency range (THz) lies in the gap between microwave and infrared of the electromagnetic spectrum. THz technology lags behind both microwave and infrared technologies, mainly because of the limitations in both THz generation and detection. Development of new THz sources has been recently receiving considerable interest in many applications such as security inspection, spectroscopy, medical imaging, and sensing. The application requirements for such THz sources are versatile, and it will be reasonable to classify them according to the THz sources’ compactness, frequency tunability, emission linewidth, coherence, and output power.

The most common approach used to generate THz waves is to rectify a femotosecond (fs) laser pulse using an electro-optic (EO) crystal. Efficient ultrabroad band, single-cycle THz wave generation has been realized in a few crystals such as ZnTe or GaP at a wavelength around 800 nm, under the condition of matching both optical and THz pulses’ group velocities [1]. Tradeoff for this approach is that the majority of such EO crystals have strong material dispersion, which limits the output wave’s bandwidth and power. On the other extreme, continuous-wave (cw) THz generation has been realized in free-electron lasers and quantum cascade lasers. The former offers high output power, but they are bulky and inflexible. The latter, however, provides the potential for good system compactness, high efficiency, and suitable frequency tunability, but with very limited output power availability and short wavelength coverage.

Using periodically poled nonlinear optical crystals for efficient THz generation is becoming an alternative approach. Conventional techniques such as difference frequency generation (DFG), which uses two laser sources (either nanosecond-pulsed or cw), are attractive in inducing coherent THz waves with a suitable frequency tunability. Phase matching among the three interactive waves (two optical and one THz) can be realized by artificially introducing reversed domain structures (so-called quasiphase matching (QPM) method if periodically or quasiperiodically poled [2]). Unfortunately, such techniques’ generation efficiency is low due to the strong absorption of THz waves in those commonly used nonlinear crystals such as (LN), [3].

Optical rectification of fs laser pulses using artificially poled nonlinear optical crystals is used to generate multicycle or arbitrary wave forms [4]. When an fs optical pulse propagates through a poled lithium niobate (PLN) crystal with its second-order nonlinear susceptibility reversing its sign between neighboring domains, a THz nonlinear polarization is generated via DFG or optical rectification. Due to the group velocity mismatch between optical and THz waves, the optical pulse will lead the THz by the optical pulse duration after a walkoff length . If the domain length of the poled nonlinear crystal is comparable to the walk-off length, each domain in the crystal contributes a half cycle to the radiated THz field. Similar to the above DFG approach, in this case, high material absorption to THz waves will be still the major reason for the poor generation efficiency. Apparently, either a significant improvement on the material’s transparency over those THz wavelengths or a new device design able to significantly minimize the THz absorption issue will be pressingly in demand in order to bring such devices to the more practical side of the potential THz applications.

In this article, a new device design relying on the multicladding nonlinear fiber format (MCNF) will be discussed for potential efficient THz generation application. This design has the potential to solve the nonlinear material’s absorption issue over those generated THz waves, and it maintains the high conversion efficiency that a strongly confined optical fiber may provide. To realize such new multicladding fiber designs, efficient fabrication of the poled nonlinear optical fibers will be the first and major step, and this will be discussed with details in Section 3 after introducing the device design. In Section 4, those reversed domain structures are nondestructively revealed by both crossly polarized optical microscope (CPOM) and confocal scanning optical microscope (CSOM).

2. The Multicore Nonlinear Fiber Design

Figure 1 shows the schematic of the multicladding nonlinear fiber. The main core inside the design can be a domain-inverted lithium niobate single-crystal fiber (1), while the first (2), the second (3), and the third (4) claddings can be made from the polyamide matrix materials with their refractive indices changed by certain dopants. The refractive index requirements for such designs are as follows: for the optical wave, ; and for the THz wave, and . In this case, the pumping optical wave will be confined inside the main core (1), and the generated THz wave will be coupled into and propagates in cladding 3. Geometrically, cladding 2 should be thin, which should allow the generated THz wave losslessly side-emitted into cladding 3. In the THz frequency range, LN crystal has a refractive index around 5.5, and that of those polyamide materials is normally around 2.1. For the two materials in optical frequencies, their refractive indices are around 2.2 and 1.4, respectively. Apparently, the use of such MCNF design has the potential to fully eliminate the nonlinear optical material absorption issue discussed before by separating the generated THz beam from the optical beam into a different path of propagation. Those cladding materials to be selected, such as the polyamide materials discussed here, should be highly transparent for a wide THz frequency range.

Figure 1: Schematic of the multicladding nonlinear fiber design. (a): the cross-section view, and (b): the axial index profile.

The energy and the momentum conversion laws for generating the THz frequencies via the DFG, for example, can be described as where E is the photon energy, K is the wave vector at each frequency, and is the grating’s reciprocal vector. The generated THz frequency can be then determined by where is the domain-poled period, and are refractive indices for optical and THz waves, respectively, and is the internal direction of the generated THz emission, which will be a key parameter when designing the device for realizing efficient coupling of the generated THz wave from the main core into cladding 3, as shown in Figure 2 before.

Figure 2: Schematic of the domain-reversed nonlinear core, the generated THz wave coupling, and both forward and backward propagation THz waves.

Figure 3 shows the calculated dependence of forward and backward THz frequencies on the poled domain periods when using lithium niobate single crystals. Listed , and are wave vectors for the pump , the signal , and the generated THz wave, respectively. Aside each curve, schematic of the corresponding wave-vector diagram is also shown. Inset inside Figure 3 shows a relationship of THz frequency versus the internal emission angle at a fixed poling period around 50 m using the crystal. Combining with the general waveguide theories of optical fibers, simulation results obtained here can be further used to design the device including both material selection and the dimensional determination of claddings and the main core. However, this is a separate research effort that will be published in somewhere else. In the following sections, we mainly report the fabrication of a periodically poled LiNbO₃ single-crystal fiber, which is the key to further make the MCNF device.

Figure 3: The generated THz frequency versus the domain period in crystal for both forward and backward propagation schemes. , are wave vectors for the pump , the signal , the generated THz wave, and the reciprocal vector of the grating, respectively. Inset shows the relationship between the generated THz frequency and the internal emission angle (this angle is to be used for further coupling the THz wave out from the main core).

3. Poling the LN Single-Crystal Fiber

The a-axis-oriented LN single-crystal fibers having cross-section dimensions ranging from 100 m to 130 m and lengths from 10 mm to 50 mm were grown using the LHPG method [5]. Normally, an a-axis LN single-crystal fiber has an elliptical cross-section with two ridges as shown in Figure 4(a). Its c-axis orientation is determined along the short axis of the ellipse and the b-axis along its long axis. This natural configuration makes it convenient to use contacting electrodes to electrically pole the fiber, simply for the reason that the applied electric field is required to be aligned parallel to the c-axis (the polarization direction). During the poling, the fiber is placed on top of a copper block and a gold grating is slightly pressed onto the fiber. The gold grating, which has the predetermined structures, was made on a " sapphire wafer using a standard photolithographic process (Figure 4(b)). Listed and inside the inset are gold electrode widths, which are changeable by designing, in order to adjust the poled domain period duty cycle. The copper block and gold grating are connected to each polarity of a high-voltage power supply. LN crystals have a coercive field approximately 22 kV/mm. For an LN fiber with a dimension of 120 m along the c-axis, the applied voltage should be close to 4 kV. In our experiments, special measures such as filling with Teflon coatings were taken to prevent the breakdown of air, simply because of the short electrode space and high voltage (this is different from puling bulk crystal wafers). Most as-grown single-crystal fibers present a single-domain structure. However, it is difficult to discriminate between the or surfaces without using destructive methods such as wet etching. In our case, simply switching the applied electrical field’s polarity and observing the sudden current change during poling can find the right polarization arrangement.

Figure 4: (a) The poling setup used for poling a-axis-grown lithium niobate single-crystal fiber (inset: the fiber’s elliptic cross-section image). (b) One design of the top interdigital electrode. and inside the inset are gold electrodes’ width, which are changeable by design in order to adjust the poled domain period duty cycle.

Periodic domain structure inside the LN fiber was then revealed using both a crossly polarized optical microscope (CPOM) and a confocal scanning optical microscope (CSOM) [6]. The main reason to use the two methods is because of their nature of being nondestructive to samples under the measurement. When using the CPOM, the poled LN fiber was simply put between a crossly polarized light. The method can disclose periodic domains in a relatively larger view field. Because of this, in this research the CPOM was mainly used to evaluate both completeness and uniformity of the periodic domains along the length of the fiber, characteristics that can be regarded as the poling quality at macroscale level. The CSOM, on the other hand, provides a means for measuring the electro-optic response to a small ac electrical field modulation. With the use of lock-in amplification, the sensitivity of this method can be greatly enhanced, allowing one to obtain high-contrast ferroelectric images using a relatively small ac field. The linear relationship between the electro-optic coefficient and the ferroelectric polarization allows this technique to work over a broad range of field amplitudes, frequencies, and orientations of electric field, ferroelectric axis, and light polarization. The CSOM method was used because of its capability to nondestructively study the LN single-crystal fibers and to disclose the domain structure details including domain boundary or period duty circle inside the fibers, characteristics that can be regarded as the poling quality at the microscale level.

In the CSOM, a 632.0 nm HeNe laser is used as a light source. The beam then passes a linear polarizer and half-wave plate which provides a capability to select the beam’s polarization direction. The light is focused to a diffraction-limited spot using a high NA objective. The fiber was mounted on a three-dimensional piezoelectric scanner. In the measurement, the LN fiber was tightly clamped by two silver-coated copper plates as electrodes. The soft silver coating was used for reaching better electric contact. The LN fiber was aligned with its b-axis normal to the plates. The ac voltage was applied onto the two electrodes variable from 0–100 V, and, therefore, the ac field can vary between kV/cm.

Figure 5 shows an image of the periodically poled ferroelectric domains inside the LN single-crystal fiber under a crossly polarized optical microscope. The periodicity of the structure was measured to be roughly 20 m, which is very close to the designed value for further generating 2.5 THz wavelength. A typically scanned image of linearly polarized light reflected from the LN fiber’s surface is shown in Figure 6(a). This is actually an optical image when observed using a conventional optical microscope. The stripes and dark dots are due to surface defects and possible surface contaminations. Using this operational mode, ferroelectric domains or boundaries are invisible. In Figure 5(b), a simultaneously acquired image of the ac field-modulated signal is presented. The dark and bright areas represent ferroelectric domains having and polarizations. The observed domain boundaries are thin and sharp, and are below the resolution of the CSOM. The image confirms the periodicity of and a period duty circle of about 1:1. Uniformity of the period duty cycle across the fiber length was examined to be quite good.

Figure 5: The periodic domains inside a 17 mm long LN single-crystal fiber revealed by a crossly polarized optical microscope.

Figure 6: The confocal scanning optical microscope images under both dc-mode (a) and ac-mode (b). Uniformity of the period duty cycle across the fiber length is good.

4. Conclusion

A new multicladding nonlinear fiber design was proposed to generate THz waves with the potential of reaching high efficiency. Inside the design, the main core of the fiber will be a domain-reversed nonlinear crystal such as periodically poled LN crystal and the multiple claddings are from those THz transparent materials such as polyamides. This design has the potential to reduce the common material absorption issue over the THz frequency and keeps the high efficiency when using the fiber format. As one of the major experimental efforts, periodic ferroelectric domain structures were successfully introduced into lithium niobate single-crystal fibers. The poling completeness and domain uniformity were examined by using a crossly polarized optical microscope. The periodic domains’ boundary and period duty circle were examined by a confocal scanning optical microscope.