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One order of magnitude detection enhancement of bow-tie-shaped InGaAs-based terahertz detectors by on-chip incorporation of the secondary diffractive optics is reported. Zone plates were produced directly on the bottom surface of 500 µm-thick InP substrate using the direct laser write technique after an array of InGaAs detectors was formed on the front surface of the wafer. Measurements of detected signal dependence on the angle of the incident wave were performed at 0.76 THz and compared with finite-difference time-domain simulation results. Good correlation of the results shows that the observed enhancement is indeed caused by the focusing performance of the zone plate rather than by the exceptional sensitivity of the single detector.Introduction: Reduction of dimensions in terahertz (THz) imaging systems is one of the key-issues bridging them to direct and versatile implementation for security, medical and materials science aims [1]. As a rule, preferences are given to the development of compact room temperature THz sources [2] and sensing components [3], while routes to avoid relatively large size optical constituents like parabolic mirrors, splitters, waveguides etc. remain out of the margins of focal interest. Recently, we have demonstrated an option to minimise the size of passive optics by the use of zone plates with integrated crossshaped filters produced by means of the laser direct write technique or etching directly in the metal foil [4].In this Letter, we evidence a further step in the miniaturisation of the THz image recording setup via the on-chip integration of InGaAs-based bow-tie THz detectors [5] and focusing components -zone plates. The top surface of the InP wafer was used to process the InGaAs detector array using the previously reported procedure [5], whereas the bottom metalised surface was structured to provide the focusing elements. Integration of such focusing elements with detectors in a single chip allowed one to increase the signal by at least an order of magnitude, as was predicted by numerical simulations, and hence to benefit from the rigid alignment in the recording array.

We demonstrate a method to synthesize ultra-high repetition rate bursts of ultrashort laser pulses containing any number of pulses within a burst with identical pulse separation and adjustable amplitude. The key element to synthesize the GHz bursts of ultrashort laser pulses is an active fiber loop. The method was implemented in the fiber chirped pulse amplification system to obtain 72 nJ-energy bursts of 20 pulses with a 2.65 GHz intra-burst pulse repetition rate and a 500 kHz burst repetition rate. The dispersion compensation mechanism ensured a mean pulse duration of 570 fs within the bursts.

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