Sources, Detectors and Receivers M9.2

Terahertz Transition Radiation A S Nikoghosyanac, E M Lazieva, R M Martirosyan", A A Hakhoumiana, J M Chamberlain',

R A Dudleyc, N N Zinov'ev'

'Department of Micmwaue Engineering, Yerevan State University, 1 Alez Manoogian Str., Yerevan 375025, Armenia 'Department of Physics, Rochester Building, Science Laboratories, University of Durham, Durham DHl 3LE

'National Physical Laboratory, Queens Road, Teddington, Middlesez T W l l OLW, UK

ABSTRACT We report on the observation of transition radia- tion generated by short optical pulses crossing the boundary between linear and non-linear materials. Unlike transition radiation observed with moving charges, we observe terahertz generation at the boundary between two media traversed by a fem- tosecond optical pulses. Contributions from bulk nonlinear difference frequency generation is also ob- served, but for the first t ime these effects can be separated.

1. INTRODUCTION Transition radiation (TR) has been originally proposed for moving charges', and it should have been observ- able with particles of any nature including phonons, plasmons and photons. For photons, and in particular for short optical pulses, the effect of T R should result from the formation and extinction of polarization space charge:x3 peff = -VPNL, where PNL = E(2)EE*, r e lated with a pulse crossing the boundary between linear and nonlinear media with ,$2) # 0 or two media, one with a greater nonlinearity than the other. TR does not require the group and phase velocities to match as for bulk non-linear effects, the Vavilov-Cherenkov (VC) or collinear difference frequency generation effects (CDFG), and, therefore, TR should be distinguishable when the phase matching conditions for VC or CDFG in the bulk are not met.

2. EXPERIMENT Utilizing a LiNbOs crystal mounted in free space and then confined within a metallic waveguide, variable CD-

herence length conditions within the sample were cre- ated, separating the effect of transition radiation with VC and CDFG contributions. Optical excitation of the crystals was with a 300 femtosecond pulse, X = 850 nm Tisapphire laser focused to a spot of 50-100 pm, with the polarization set parallel to the LiNbOs c-axis and an average power of 300 mW. The spectral components of the incident beam interact in the crystal resulting in nonlinear polarization and THz emission. The LiNbOs crystal with dimensions 300 pm by 1.1 mm and lengths L = 1 - 4 mm was mounted within W-band waveguide to generate dominant TEmo modes. THz radiation was collccted with a set of parabolic mirrors and detected using free space electreoptic ~ a m p l i n g . ~

Further author information: Send correspondence to N. N. Zinov'ev, Email: nick.sinovev&pl.co.uk

0 20 40 w time. ps

Figure 1. THz waveform detected from a lmm thickness LiNbOs placed in free space. The pulses labeled as A' and B' are the round trip reflection replicas of the original pulses A and B respectively. The mark C shows the expected a m plitude and position of the THz pulse that should have been produced by reflected pump pulse.

3. DISCUSSION The waveform of THz emission obtained from a 1 mm sample of LiNbOs placed in free space is depicted in Fig.1. Pulse A is generated at the exit surface of the sample and pulse B at the entrance surface experiencing attenuation inside the sample. For twin pulse structure, A and B, we suggest the mechanism of TR, the opti- cal analogue of transition radiation for charges crossing boundaria between materials with different dielectric constants. We suggest this mechanism, in contrast to multiple reflection of the pump in the sample: because the waveform of Fig.1 shows only a twin pulse structure with no high order reflections. Within the framework of general theory of transition radiation' the occurrence of THz A and B pulses is related to the optical recti- fication of the tangential component of the pump field at the surfaces z = 0 + U(%) and z = L - O(z) r+ spectively ( U ( z ) stands for the TR formation length). To remove the contribution of propagating THz phases, we analyze the spectra from fragments of the complete waveform. The spectra corresponding to both pulses, Fig.2, have similar profile but the spectral maximum of pulse B, is shifted by - 200 GHz to lower frequencies, a result of the rise in absorption coefficient with frequency within LiNbOs. To separate the effect of transition ra- diation from the effect of THz generation in the bulk, we studied the spectra under conditions of increased co-

0-7803-@490-3/04/$20.W 02004 IEEE 159

M9.2 Sources, Detectors and Receivers

frequency, THz

Figure 2. THz spectra of A (1) and B (2) pulses obtained in free space. (3) shows the dependence of the ratio of trans- mitted to incident electric field amplitudes

herence length using metallic waveguide structures with the same LiNbOs partially filling the cavity. Metallic waveguide creates a higher phase velocity for THz waves than in free space. The increase in phase velocity makes the wave vector mismatch smaller and coherence length larger. Hence, the efficiency of nonlinear conversion into THz in the bulk is expected to increase. If the mecha- nism of THz radiation on A and B pulses is transition radiation then these key measurements should develop the relation between two nonlinear effects, transition THz radiation and bulk THz emission due to the .VC ef- fect and CDFG. The data shown in Fig.3 demonstrates the changes obtained in the spectra of THz radiation. The major distinctive signature is a large increase of pulse B. The spectrum of pulse B shows a number of narrow lines below 1 THz which are significantly higher than the peak amplitude of the spectrum for pulse A. This proves that the nonlinear conversion responsible for pulse B has produced additional THz radiation com- pared to free space. Within the experimental estima- tion of the waveguide transmission the increase of the ratio I B ( ~ ) / I A ( Q ) occurs because of increase of I B ( ~ ) - Fig.3. The spectrum of pulse A in average remains the same spectral width as in free space, but only shows filtering introduced by the waveguide. The fall of this enhancement with frequency is explained by the fall of transmission of the waveguide at high frequencies rather than by the limits of the observed effect.

4. CONCLUSION In conclusion, our experimental data has shown the mechanism of transition THz generation created by an ultra short laser pulse crossing the boundary between nonlinear and linear media. Efficient transition THz generation can be obtained from the interface of nonliu- ear sample with high value of $'), even if the coherence length is small or the ratio of refractive indexes is un- favorable for the CDFG or VC effects. If the coherence

1.0

= s d 0.5

i

0.0 I 2 3 .

Figure 3. The spectra of A (1) and B (2) pulse obtained for placement the sample in waveguide. The inset: the spec- trum of pulse B (3) is shown in a high resolution mode

length is increased and the conditions for the CDFG mechanism of THz generation are met, the generation of THz changes over to the CDFG effect.

5. ACKNOWLEDGMENTS The authors acknowledge the financial support from the Royal Society and NPL (ASN), Ministry of Edu- cation and Science of Armenia according to the con- tract No.840 and INTAS-01-0397 (ASN, EML, RMM and AAH) and the DTI of the UK NMS Electrical Prw gram (RAD and NNZ).

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160 2004 Joint 2W Int. Conf. on Infrared and Millimeter Waves and 1 P Int. Coni. on Terahertz Electronics