We have performed measurements on terahertz (THz) apertureless near-field microscopy that show that the temporal shape of the observed near-field signals is approximately proportional to the time-integral of the incident field. Associated with this signal change is a bandwidth reduction by approximately a factor of 3 which is observed using both a near-field detection technique and a far-field detection technique. Using a dipole antenna model, it is shown how the observed effects can be explained by the signal filtering properties of the metal tips used in the experiments. Apertureless near-field scanning optical microscopy (ANSOM) is an attractive technique for obtaining subwavelength resolution in optical imaging at visible and midinfared wavelengths. [1][2][3] In this technique, light is scattered off a subwavelength-sized metal tip which is held close to a surface. The scattered light is collected in the far-field, giving subwavelength resolution in the immediate neighborhood of the tip apex. Very recently, there have been reports showing how ANSOM can be adapted for the terahertz (THz) frequency domain, by combining it with THz time-domain spectroscopy. [4][5][6][7] This is an exciting development, as it is becoming increasingly clear that many systems of interest, including biological molecules and a variety of artificial nanostructures, have absorption features in the THz frequency range, but are far smaller than the wavelength of the THz radiation. [8][9][10][11] These developments therefore offer great promise for THz microscopy. In the THz ANSOM experiments, two different detection techniques are currently employed: THz electric fields are either detected in the nearfield of the tip using electro-optic sampling, or in the far-field of the tip with a receiver. In the latter case, near-field information from the tip apex is obtained by vibrating the metal tip at a high frequency, followed by phase-sensitive detection at this frequency. However, an important aspect of both experiments remains unexplained: The measured temporal profile of the near-field signals is very different from that of the incident THz pulses. Associated with this shape change is a bandwidth reduction of about a factor of 3, which could limit the utility of the technique in spectroscopic measurements. For practical applications, such as THz microscopy, it is essential that the origin of this bandwidth reduction is understood.Here, we present measurements and calculations which demonstrate that the observed bandwidth reduction is a consequence of the antenna properties of the metal tips used in the experiments. We demonstrate that, regardless of whether the near-field signal is observed directly under the tip or in the far-field, it is approximately proportional to the timeintegral of the incident THz signal. Near-field calculations based on a dipole antenna model qualitatively reproduce all the essential features observed in the measurements and provide insight into ANSOM experiments in general.The two THz ANSOM configurations, used ...

We report on a method to obtain a subwavelength resolution in terahertz time-domain imaging. In our method, a sharp copper tip is used to locally distort and concentrate the THz electric field. The distorted electric field, present mainly in the near field of the tip, is electro-optically measured in an ͑100͒ oriented GaP crystal. By raster scanning the tip along the surface of the crystal, we find the smallest THz spot size of 18 m for frequencies from 0.1 to 2.5 THz. For our peak frequency of 0.15 THz, this corresponds to a resolution of /110. Our setup has the potential to reach a resolution down to a few m.

We present a new method to measure the polarization state of a terahertz pulse by using a modified electrooptic sampling setup. To illustrate the power of this method, we show two examples in which the knowledge of the polarization of the terahertz pulse is essential for interpreting the results: spectroscopy measurements on polystyrene foam and terahertz images of a plastic coin. Both measurements show a sampleinduced rotation of the terahertz electric field vector, which is surprisingly large and is a strong function of frequency. A promising aspect of our setup is the possibility of simultaneously measuring both transversal electric field components. © single-shot imaging, 4 and near-field imaging. 5 A characteristic of these techniques is that only one component of the electric field vector is measured. This makes the images obtained with these methods sometimes difficult to interpret. A decrease in the amplitude of the measured field, for instance, is commonly interpreted as being caused by absorption or scattering. However, such a decrease could also be caused by a rotation of the electric field vector induced by a birefringence present in the sample. Besides birefringence, there are various other effects that can change the direction of a terahertz electric field, such as not-normalincidence reflection and multiple scattering. 6 We note that in a recent experiment on the terahertz Hall effect a rotation of the terahertz polarization was observed when two orthogonally oriented photoconductive emitters were used. 7 Here, we report a method for measuring both the direction and the length of the transversal terahertz electric field vector by using electro-optic sampling in a (111)-oriented electro-optic crystal. We demonstrate the potential of this technique in terahertz imaging and spectroscopy with two examples. In the first example, we perform spectroscopic measurements on polystyrene foam. Surprisingly, this material shows an effective birefringence, which can be measured accurately with our new technique. In the second example, we show terahertz images of a plastic coin based on a measurement of the two transversal electric field components at each pixel. The images clearly show that scattering or reflection at the edges of the coin results in a change in the polarization state of the terahertz beam. Figure 1 shows a schematic drawing of the detection setup. We concentrate on the detection setup because the other details of our setup have been published previously. 8,9 The terahertz electric field is measured by using the electro-optic effect, which causes a birefringence of the detection crystal proportional to the electric field. The birefringence causes a polarization change of the optical probe pulse, which is measured with a differential detection setup. A quarter-wave plate is placed before the ZnTe detection crystal, oriented such that the originally linear polarization of the probe beam becomes circular. A key element in our setup is the use of a ZnTe detection crystal with a (111) crystal orie...

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The authors present measurements and calculations on the effect of thin dielectric coatings on the propagation of terahertz pulses along the surface of metal wires. Our measurements show that propagation over only a few centimeters of wire having a thin dielectric coating, strongly distorts the terahertz pulse, which results in a several tens of picoseconds long chirped signal. We demonstrate that the terahertz pulses propagate along the wire as surface waves, and show how a thin coating of a nondispersive material makes this propagation strongly dispersive, giving rise to the chirped signal observed in the measurements. Our results show the potential of terahertz surface plasmon polaritons on metal wires for the sensitive detection of thin dielectric layers. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2011773͔ Recently, there has been an increased interest in the search for a good waveguide for the transportation of terahertz radiation. [1][2][3][4][5][6] The latest development in this field is the propagation of terahertz waves along bare metal wires with very little absorption and dispersion. 7,8 Indeed, measurements were shown in which two metal wires were combined to form what could eventually become a medical probe. Many metal wires, however, are not bare. Thin dielectric layers can often be found on the surface of metal wires, applied intentionally or unintentionally through oxidation or contamination, and the effects of these layers on the propagation of terahertz pulses along the wire are not wellknown.Here we show measurements and calculations on the propagation of terahertz pulses over copper wires with and without a thin polyurethane coating. Our time-domain measurements of a terahertz pulse propagating along a 4 cm long wire show that a coating of tens of micrometers thickness strongly distorts the terahertz pulse resulting in a chirped terahertz signal that lasts tens of picoseconds. A comparison with calculations based on Maxwell's equations shows that the terahertz pulses propagate along the wire as surface plasmon polaritons, and that the distortion of the terahertz pulse originates from the dispersive propagation of these waves along the coated wire. Remarkably, the propagation is dispersive, although we assume that the coating material itself has a frequency-independent refractive index. Our work indicates that thin coatings can seriously distort terahertz pulses propagating along metal wires. At the same time, however, this offers the possibility of using metal wires as sensitive detectors of thin layers. Figure 1 shows a schematic drawing of our setup. The terahertz pulses from our emitter are focused onto one of two types of metal wires. The first wire is a bare copper wire with a diameter of 1 mm. The second wire is a copper wire with a diameter of 1 mm having a polyurethane coating, specified to be about 34 m thick.9 A sharp copper needle is used to couple the terahertz radiation onto the wire. 8 The terahertz surface plasmon polariton propagates over the wire toward the detecti...