Spatial Terahertz Modulator

Abstract

Terahertz (THz) technology is a developing and promising candidate for biological imaging, security inspection and communications, due to the low photon energy, the high transparency and the broad band properties of the THz radiation 1-3 . However, a major encountered bottleneck is lack of efficient devices to manipulate the THz wave, especially to modulate the THz wave front. A wave front modulator should allow the optical or electrical control of the spatial transmission (or reflection) of an input THz wave and hence the ability to encode the information in a wave front 4 . Here we propose a spatial THz modulator (STM) to dynamically control the THz wave front with photo-generated carriers. A computer generated THz hologram is projected onto a silicon wafer by a conventional spatial light modulator (SLM). The corresponding photo-generated carrier spatial distribution will be induced, which forms an amplitude hologram to modulate the wave front of the input THz beam. Some special intensity patterns and vortex beams are generated by using this method. This all-optical controllable STM is structure free, high resolution and broadband. It is expected to be widely used in future THz imaging and communication systems.S andwiched between the microwave and infrared, THz radiation has been notoriously difficult to produce, modulate and detect [1][2][3] . Recent progresses such as quantum-cascade lasers 5,6 , terahertz wave generation through a nonlinear crystal 7 and THz time-domain spectroscopy 8 are promoting this subject into one of the most rapidly growing fields. High performance devices to control and manipulate the THz radiation are in urgent demand to develop sophisticated imaging and communication system. The filters, absorbers and polarizers based on graphene, frequency selective surface, metamaterials and photonic crystals have been reported [9][10][11][12][13][14][15][16][17][18] . However, the wave front modulation devices are still lacking. Recently, a novel technology based on the metasurface has been demonstrated to generate the desired wave front distribution 19,20 . Unfortunately, the specific function of this kind of devices has been determined at the moment of their design and could not be flexibly changed any more. The SLM which has been widely used in the visible light band can optically or electrically control the spatial transmission (or reflection) of an input light beam and encode information in the wave front 4 . The SLM always plays an important role in optical information processing, three dimensional image display, optical interconnections and real-time beam shaping. Usually, the SLM is realized through liquid crystals, magneto-optic materials or deformable mirrors. However, such mechanisms cannot work well in the THz regime due to the lack of suitable materials and the size mismatching between the micro-machined components and the THz wavelength 21 . In order to obtain the STM, a novel technology needs to be explored.The STM requires an array of small building blocks that can independentl...

Active terahertz beam steering by photo-generated graded index gratings in thin semiconductor films

“…Photo-generated antennas [18] and chiral structures [19] have been recently demonstrated using structured illumination of semiconductors. Also photo-generated gratings [20,21] and controlled THz diffraction has been demonstrated [22,23]. However, none of the previous works has shown an asymmetric scattering or diffraction as we do here, nor relates the results to the rapidly emerging field of metasurfaces.…”

All-integrated terahertz modulators

“…Normally, photons carrying energy higher than the material bandgap are employed, which can be emitted by pulsed or continuous wave (cw) sources. The material of choice is typically silicon [40][41][42][43] or GaAs, which suggests the implementation of optical or nearinfrared lasers, typically 800 nm centered broadband fs-pulsed lasers. In all these experimental situations, the main limits in modulation depth and speed are arising from the substrate material rather than the illumination sources.…”

Terahertz Imaging Progress at Capital Normal University

“…At the same time, the active THz pulse imaging is developed and applied for the quasi-near field measurement based on the terahertz time-domain spectroscopy with the probe-beam-expanded femtosecond pulse laser and an infrared CCD detection [16][17][18][19].…”

“…However, the imaging distance of the high frequency, such as 3.1 THz, of terahertz imaging is limited by THz attenuation due to the water vapor absorption. Moreover, the focal plane terahertz imaging is developed to obtain much more frequency domain of spectral information [16][17][18][19]. The focal plane imaging can be realized as a quasi-near field imaging so that it can obtain a higher resolution.…”