Hitherto known as a technique of generating or detecting terahertz waves is terahertz time-domain spectroscopy (TDS).  As known in the art, the terahertz time-domain spectroscopy is suitable for use in imaging samples because it utilizes terahertz waves that define ultra-short pulses, as short as about 100 femtoseconds.  Therefore, the terahertz time-domain spectroscopy attracts attention in various technical fields such as industry, medicine, biotechnology, agriculture and security. 
In the terahertz time-domain spectroscopy, a pulse light beam emitted from an ultra-short laser source is split into a pump beam and a probe beam.  The pump beam is focused on a terahertz-wave generating element.  In the terahertz-wave generating element, a current flow is generated or electrical polarization develops for about subpico seconds, generating a terahertz wave having an electric-field amplitude proportional to the time derivative.  The terahertz wave is focused by an optical system on a terahertz-wave detecting element.  At this point, the probe beam is applied to the terahertz-wave detecting element.  Then, the terahertz-wave detecting element generates a carrier.  The carrier is accelerated by the electrical field of the terahertz wave and changed to an electric signal.  The time at which the probe beam reaches the terahertz-wave detecting element is delayed, thereby measuring the time waveform the terahertz wave has in the amplification electric field.  The time waveform is Fourier-transformed, thereby determining the spectrum of the terahertz wave. 
The terahertz-wave generating element and the terahertz-wave detecting element can be identical in configuration.  Such an element is described in, for example, Jpn.  Pat.  Appln.  Laid-Open Publication No.  2002-223017.  This publication discloses, in paragraph [0036], a terahertz beam element 21 that has a base 22, an optically conductive film 23 formed on the base 22, and electrically conductive films 24 and 25 formed on the optically conductive film 23.  Note that a part of the base 22 functions as a lens. 
The publication teaches, in paragraph [0040], that three or more electrically conductive films are formed on the optically conductive film 23, each, isolated from another at intervals d, thus forming an array of optical switch elements, and that the base 22 may form an array of lenses associated with the optical switch elements, respectively. 
The terahertz time-domain spectroscopy is classified into so-called transmission type and so-called reflection type.  In the transmission type, the terahertz-wave generating element and the terahertz-wave detecting element are arranged, facing each other across a sample.  In the reflection type, the terahertz-wave generating element and the terahertz-wave detecting element are arranged, both facing a sample. 
To use the terahertz beam element 21 disclosed in the publication, in the reflection-type terahertz time-domain spectroscopy, the base 22 may form a lens array.  In this case, the terahertz wave applied from the terahertz beam element 21 used as a terahertz-wave generating element is reflected by the focusing plane of the sample and returns to the terahertz-wave generating element, without being applied to the terahertz beam element 21 used as terahertz-wave generating element.  Consequently, the sample may not be measured. 
An optical component such as a half mirror may be provided between the base 22 and the sample.  In this case, the apparatus is larger than in the case where the optical component is not used, and a loss is made in the energy of the terahertz wave. 
Two terahertz beam elements 21 may be used as a terahertz-wave generating element and a terahertz-wave detecting element, respectively, and arranged obliquely to the focusing plane of the sample.  In this case, the terahertz beam elements 21 can hardly be arranged in the same plane. 
Therefore desirable to propose a probe apparatus and a terahertz spectrometer that are simple in configuration and can yet measure samples.