Demonstration of Submillimeter Radiation Generation from Static Field by Superluminous Ionization Front in Semiconductor Capacitor Array

Generation of frequency upshifted monochromatic radiation in the 100 GHz to FIR range by the interaction of a superluminous photoconducting front with an electrostatic “frozen wave” configuration in a semiconductor is reported. The interaction converts the energy contained in the “frozen wave” into monochromatic radiation, whose frequency depends on the energy in the laser pulse creating the superluminous front and the wavelength of the static wave. Monochromatic free space propagating waves with frequency between 100 GHz and 1.6 THz were generated using a 100 fsec Ti:Sapphire laser pulse incident obliquely on a ZnSe photoconducting slab containing a frozen wave field. A superluminous photoconducting front was formed by two photon absorption. Tunability was achieved by varying the laser pulse energy from .1 to 1 mJ. The electric field waveform of the emitted radiation pulse was measured by an optically gated miniature dipole antenna. The concept can be scaled to develop narrowband, tunable and powerful THz and FIR sources for spectroscopic imaging and other applications. In recent years the concept of frequency up shift of electromagnetic radiation propagating in the plasma medium by a rapid spatial and temporal modification of plasma parameters has received a considerable attention due to its potential use as tunable high power radiation source’. There is a special interest in a compact source of high power farinfrared radiation which may play an important role in diverse applications such as advanced radar systems, ultrafast chemical and biological imaging and etc. Several alternate schemes have been proposed and some of them were dem~nstrated~.~. One of the original schemes is up shift of radiation propagating through the gaseous medium by reflection out of fast ionization front. The front can be generated by laser beam that propagates inside the medium or by sweeping the medium with a laser beam from the side4. Frequency upshift is achieved due to Doppler shift from the overdense front ’. The frequency shift can be achieved even when the font is underdense ‘. It was shown theoretically that very large frequency up-shifts are possible by superluminous ionization front. Many of the schemes of frequency up-shift of an initially propagating wave share the same setback. Their output power is limited by the power of the initial wave , therefore requires an initial high power radiation sources. Recently it was suggested to use a laser produced ionization front to generate tunable radiation directly from a static electric field7 , thereby eliminating the need for the initial high power radiation source. In this scheme a dc to ac radiation converter(ARC), UltraWideband, Short-Pulse Electromagnetics 4 Edited by Heyman et al., Kluwer Academic /Plenum Publishers, New York, 1999 27 radiation is up shifted from zero frequency to a tunable value. In the proof of principle experiment of that scheme, an alternately biased capacitors produce a static filed inside working gas. An ionization front created by a short laser pulse move between the capacitors 12 -3 plate produced a 40 GHz when the plasma density was about 10 cm . In this work, we present the first measurements of tunable Far IR radiation obtained using a reflecting regime in semiconductor plasma. The plasma of electrons and holes was induced in ZnSe crystal by two photon ionization using lOOfsec Ti Sapphire laser. The crystal was placed in an alternatively DC biased capacitor array. The superluminous ionization front velocity kept fixed on 1.6 times the light speed, while the plasma frequency was changed monotonically by varying the laser energy. The emitted radiation frequency was observed in a wide range span from lOOGhz to a several THz . The measurement set up was designed to measure the full temporal shape of the emitted radiation electrical field amplitude. This feature was used for the detection of both amplitude and phase of the frequency spectrum up to a few THz. There are a several advantages in using a semiconductor as the capacitor substrate. a) The breakdown threshold is much larger in semiconductor, thus a very large electrical fields can be used.b). A low ionization energy, only -2ev per electron-hole pair production c.) The relatively short recombination time allows to obtain a high repetition rates and avoids the need for removal of the used plasma. d) The radiated frequency can be easily tuned by the varying laser energy and angle of incident. In interpreting and scaling the experimental results it is instructive to refer to the traditional frozen wave generator concept*. Such a device consists of segments of transmission lines sections with one way transit times equal to the desired microwave half period arranged in series or parallel and connected with optically activated switches. The sections are charged alternatively positive and negative to form a dc “frozen-wave”. Radiation is produced when the optically controlled switches are activated simultaneously. The critical differences in the physics controlling our device and the traditional frozen wave generator, is that the shorting of the capacitor array is sequential and the radiation produced by shorting it propagates inside the plasma which is generated at superluminous speeds. This, however has profound consequences since the frequency is upshifted to satisfy the appropriate dispersion relation and phase matching conditions. Following Lampe et al.’’ we work in the laboratory frame since the front is superluminous. There are three modes in the system. The first is the frozen wave, with zero frequency and wavelength k,=dd, where d is the distance between the capacitors (Fig. 1). The other two are the rigthward(+) and leftward (-) propagating waves in the semiconductor plasma. They must satisfy the usual dispersion relation where e is the dielectric constant of the semiconductor, we the plasma frequency and the g phenomenological dephasing rate. The continuity conditions require that the waves are in phase at the front,, z= -Ut, where U= c/sinq and q is the incidence of the laser on the semiconductor. This gives k,c/sinq = wI,‘ +k,,2c/sinq Neglecting the leftward wave since it never catches up with the front7 and droping the subscript from the frequency and wavenumber of the forward wave, we find from (1) and (2) the relationship between the emitted frequency w and the other parameters as (3) w (k,Cp w ) ~ w2@’ + wf@’ = O w-iv where b= l/sinq. It is easy to see that in the absence of plasma, i.e. w,->O (or w>> we), equation (3) reduces to the frozen generator one. However, for k,cb <<w equation (3) becomes