Gabor Matthäus

Microlens coupled interdigital photoconductive switch

A large-area terahertz emitter based on an interdigital finger electrode photoconductive switch on low-temperature grown GaAs attached to a hexagonal microlens array is demonstrated. The hexagonal arranged microlenses direct the incident IR excitation pulses into specified electrode gaps, resulting in constructive interference in the terahertz far field. Using a Ti:sapphire oscillator running at 80MHz with 150fs pulses, 6.5μmTHz average power at 540mW optical excitation is obtained. The maximum IR-to-terahertz conversion efficiency achieved is ⩾1.35×10−5.

Terahertz line detection by a microlens array coupled photoconductive antenna array

We present THz ultrashort pulse detection by a photoconductive antenna array consisting of 16 photoconductive antennas. The efficient excitation of the photoconductive antennas has been realized by a microlens array which generates 16 single spots from the exciting fs-laser beam. This combination of optoelectronics and microoptics improves the detection efficiency by an order of magnitude in comparison to an excitation by a line focus.

Multichannel balanced electro-optic detection for Terahertz imaging

We present THz imaging with 1D electro-optic sampling of ultrashort THz pulses by multichannel balanced detection. Using a lock-in technique, it combines the advantage of a high signal to noise ratio along with the fast acquisition time of multichannel detection. The object is probed by a line focus and the resolution can be adjusted. The performance of the system is demonstrated exemplarily by imaging two objects.

InN as THz emitter excited at 1060 nm and 800 nm

InN, a novel semiconductor material, is used as THz surface emitter. The material is irradiated with fs-laser pulses at 1060 nm and 800 nm and the emitted ultrashort THz pulses are measured by phase sensitive detection. Pulsforms, amplitudes and spectra are compared to the THz emission of p-doped InAs, the standard material for THz surface emission.

Intracavity terahertz generation inside a high-energy ultrafast soliton fiber laser

Intracavity terahertz emission inside a high-energy ultrafast Yb-doped fiber laser is presented. The terahertz radiation is generated by a transient photocurrent induced at the surface of a saturable InGaAs multiquantum well grown by molecular beam epitaxy on top of a semiconductor Bragg reflector. This device simultaneously works as the saturable absorber mirror for initiating and managing the passive mode locking required for the ultrashort pulse operation of the laser system. The maximum terahertz average power achieved is 4.2μW, which reveals a net conversion efficiency of 3.1×10−5.

Induced terahertz emission as a probe for semiconductor devices

A nondestructive and contact free method for the characterization of semiconductor devices is presented using stimulated terahertz (THz) emission. For demonstration purposes, nanostructured semiconductor-insulator-semiconductor solar cells are investigated. These solar cells are based on indium tin oxide (ITO) upon black silicon (BS). During illumination with fs laser pulses, free charge carriers are generated at the junction between ITO and BS yielding the emission of broadband THz radiation. Since the THz field strength depends on the acceleration characteristics of the photoinduced charge carriers, phase sensitive detection of the emitted THz signal reflects the existing electric field distribution at the boundary zone. In contrast to existing methods where the sample is illuminated by an additional THz generator, here, the THz emission itself characterizes the sample. Moreover, only the region of THz generation is probed yielding a depth-resolved measurement setup that can be applied for the investiga...

Surface-emitted THz generation using a compact ultrashort pulse fiber amplifier at 1064 nm

Generation of InAs-surface-emitted terahertz radiation by application of an ultrashort pulse 1064 nm parabolic fiber amplifier source is reported for the first time. The fiber amplifier delivers 100 fs pulses at a repetition rate of 75 MHz and an average power of maximum 12 W. This new excitation laser for surface-emitters generates high brightness broadband THz radiation ranging from 100 GHz to over 2.5 THz. THz detection is demonstrated based on two-photon absorption at low-temperature-grown GaAs dipole receivers.

Fast THz Imaging of Styrofoam

Imaging of styrofoam with the help of ultrashort Terahertz pulses is investigated. With a combination of pulse amplitude and time delay imaging it is possible to speed up the measurement about two orders of magnitudes.

Femtosecond written buried waveguides in silicon

The laser inscription of waveguides into the volume of crystalline silicon is presented. By using sub-ps laser pulses at a wavelength of 1552 nm highly localized light guiding structures with an average diameter ranging from 1 – 3 μm are achieved. The generated waveguides are characterized in terms of mode field distribution, damping losses and permanent refractive index modification. First investigations indicate an induced increase of the refractive index in the order of 10-3 to 10-2. Depending on the applied laser pulse energy single-mode to multimode like propagation behavior can be observed. At optimized processing parameters, the damping losses can be estimated below 3 dB/mm.

Selective laser melting of glass using ultrashort laser pulses

Within the field of laser assisted additive manufacturing, the application of ultrashort pulse lasers for selective laser melting came into focus recently. In contrast to conventional lasers, these systems provide extremely high peak power at ultrashort interaction times and offer both the opportunity of nonlinear absorption and the potential to control the thermal impact at the vicinity of the processed region by tailoring the pulse repetition rate. Consequently, transparent materials like borosilicate glass or opaque materials with extremely high melting points like copper, tungsten or even special composites like AlSi40 can be processed. In this publication, we present the selective laser melting of glass by using 500 fs laser pulses at MHz repetition rates emitted at a center wavelength of 515 nm. In order to identify an appropriate processing window, a detailed parameter study was performed. We demonstrate the fabrication of porous bulk glass parts as well as the realization of structures featuring thicknesses below 30 μm, which is below typical achieved structural sizes using pulsed or CO2 laser [1]. In contrast to alternative approaches [2], due to the nonlinear absorption and therefore complete melting of the material, there was no need for binding materials. This work demonstrates the potential for 3D printing of glass using the powder bed approach.