The demand for tunable terahertz (THz) generating laser sources is significantly growing as they are used in a wide range of applications including THz imaging, spectroscopy, and metrology. However, the development of THz systems for the use in many practical applications is generally impeded by the limited availability of compact, sufficiently powerful and cost‐effective room‐temperature sources in the desired spectral ranges. Herein, the development of a compact, continuous‐wave, room‐temperature, tunable THz‐generating laser source in the 0.79–1.11 THz spectral region is reported. The laser source is based on intracavity difference‐frequency generation in an aperiodically poled lithium niobate (aPPLN) crystal within a dual‐wavelength vertical‐external‐cavity surface‐emitting laser. Furthermore, spectral coverage in the THz domain is compared for such a device utilizing a periodically poled lithium niobate (PPLN) and an aPPLN crystal. The demonstrated results pave the way to an effective approach for the development of high‐performance, room‐temperature, widely tunable THz lasers for a variety of applications in science and industry.

The demand for highpower verticalcavity surfaceemitting laser (VCSEL) arrays is increasing continuously due to the growing market for 3D sensing solutions. In these applications (e.g. face recognition or driveassistance systems), the distances of the objects of interest vary by orders of magnitude. For this reason, a flexible tailoring of the beam divergence is desired. In this work, we discuss methods to tune the emitted beam profile by only optimizing the epitaxial structure of a VCSEL. We show results of VCSEL arrays with beam divergences ranging between similar to 10 degrees and 45 degrees. This technique is also power scalable and multi-watt VCSEL arrays can be realized.

In this work, we discuss methods to adjust the beam divergence of vertical-cavity surface-emitting lasers (VCSELs) by controlling the transversal lasing modes using different epitaxial designs. We show results of high power, multi-watt VCSEL arrays with emission cones ranging from 10° to 40°.

We investigate the angular dependence of electro-optic sampling of terahertz radiation. We show that a non-perpendicular incident of copropagating terahertz and optical waves onto the electro-optic crystal only weakly influences the resulting electro-optic sensing efficacity while strongly reduces disturbing multiple reflections within the crystal. We found that close to Brewster's angle, the detectors response is most favorable. Experimental data support our theoretical discussion.

We present state of the art results obtained with numerically optimized VECSEL devices for the generation of multi-Watts single frequency and multi-color operation in the near infrared as well as for the direct generation of sub-100fs modelocked pulses at multi-GHz repetition rate.

A vertical external-cavity surface-emitting laser is presented that emits 2.6 Wof continuous-wave radiation centered at 1015 nm. The emission consists of a two-color spectrum with a difference frequency in the terahertz regime. The stability of the dual-wavelength operation is studied, utilizing a cavity design with multiple folds onto the gain medium. It is shown that spatial hole burning effects significantly help to improve the gain competition dynamics of the emitted wavelengths.

We present a novel Vertical External Cavity Surface Emitting Laser (VECSEL) cavity design which makes use of multiple interactions with the gain region under different angles of incidence in a single round trip. This design allows for optimization of the net, round-trip Group Delay Dispersion (GDD) by shifting the GDD of the gain via cavity fold angle while still maintaining the high gain of resonant structures. The effectiveness of this scheme is demonstrated with femtosecond-regime pulses from a resonant structure and record pulse energies for the VECSEL gain medium. In addition, we show that the interference pattern of the intracavity mode within the active region, resulting from the double-angle multifold, is advantageous for operating the laser in CW on multiple wavelengths simultaneously. Power, noise, and mode competition characterization is presented.

We present a novel cavity design for vertical external cavity surface emitting lasers (VECSELs) enabling multiple interactions with the gain structure under different angles in a single round trip. This allows for a low round-trip group delay dispersion (GDD) despite using high-gain resonant VECSEL structures possessing pronounced resonances in their reflective GDD profile. Femtosecond-regime pulses with an average output power of 1.14 W and record peak intensities for mode-locked VECSELs of 6.3 kW are presented with simulations demonstrating the GDD compensating mechanism employed in this scheme.

A dual-wavelength mode-locked semiconductor vertical-external-cavity-surface-emitting laser is demonstrated. A semiconductor saturable absorber mirror allows for simultaneous mode locking of pulses centered at two center wavelengths with variable frequency spacing. The difference-frequency control is achieved with an intracavity etalon. Changing the finesse of the etalon enables the adjustment of the pulse duration between 6 and 35 ps. The emitted two-color pulses are modulated by a beat frequency in the terahertz range. Self-starting mode-locking with 0.8-W average output power is demonstrated.

Fully microscopic many-body calculations are used to study the influence of strong sub-picosecond pulses on the carrier distributions and corresponding optical response in saturable absorbers used for mode-locking—semiconductor (quantum well) saturable absorber mirrors (SESAMs) and single layer graphene based saturable absorber mirrors (GSAMs). Unlike in GSAMs, the saturation fluence and recovery time in SESAMs show a strong spectral dependence. While the saturation fluence in the SESAM is minimal at the excitonic bandgap, the optimal recovery time and least pulse distortion due to group delay dispersion are found for excitation higher in the first subband. For excitation near the SESAM bandgap, the saturation fluence is about one tenth of that in the GSAM. At energies above the bandgap, the fluences in both systems become similar. A strong dependence of the saturation fluence on the pulse width in both systems is caused by carrier relaxation during the pulse. The recovery time in graphene is found to be about two to four times faster than that in the SESAMs. The occurrence of negative differential transmission in graphene is shown to be caused by dopant related carriers. In SESAMs, a negative differential transmission is found when exciting below the excitonic resonance where excitation induced dephasing leads to an enhancement of the absorption. Comparisons of the simulation data to the experiment show a very good quantitative agreement.

We present a comprehensive characterization of semiconductor gain and absorber devices utilizing novel measurement techniques. Using a 20fs probe laser, a time resolution in the few femtosecond range is achieved in traditional pump and probe measurements performed on VECSELs and SESAMs. In-situ characterizations of VECSEL samples mode-locked in the sub-500fs regime reveal the fast and longtime recoveries of the gain present in real lasing conditions. Spectrally resolved probing gives further information about the properties of carriers in VECSEL gain media. Our results indicate that stable mode-locked operation is sustained by multiple carrier relaxation mechanisms ranging from a few femtoseconds to the pico- and nanosecond regimes.

We utilize an asynchronous optical sampling technique to study the gain dynamics of vertical-external-cavity-surface-emitting lasers (VECSELs) under mode-locked operation. This allows for an in situ characterization of the gain depletion and recovery over nanoseconds with femtosecond-scale resolution. Our method allows for a more direct study of intracavity gain dynamics than traditional pump/probe measurements. We observe a rapid depletion of the gain on the timescale of the intracavity pulse. Afterward, a rapid recovery over a few picoseconds due to intraband scattering and carrier heating takes place, followed by a long recovery attributed to the continuous supply of carriers by the pump laser.

Advanced Optical MaterialsVolume 3, Issue 5 p. 642-645 Communication Terahertz Metamaterials with Ultrahigh Angular Sensitivity Norman Born, Corresponding Author Norman Born Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, GermanyE-mail: norman.born@physik.uni-marburg.deSearch for more papers by this authorIbraheem Al-Naib, Ibraheem Al-Naib Department of Physics, Engineering Physics and Astronomy, Queen's University, Kingston, K7L 3N6 CanadaSearch for more papers by this authorChristian Jansen, Christian Jansen Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, GermanySearch for more papers by this authorRanjan Singh, Ranjan Singh Center for Disruptive Photonic Technologies, Nanyang Technological University, Singapore, 637371 SingaporeSearch for more papers by this authorJerome V. Moloney, Jerome V. Moloney College of Optical Sciences, University of Arizona, 1630 E University Boulevard, Tucson, AZ, 85721 USASearch for more papers by this authorMaik Scheller, Maik Scheller College of Optical Sciences, University of Arizona, 1630 E University Boulevard, Tucson, AZ, 85721 USASearch for more papers by this authorMartin Koch, Martin Koch Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, GermanySearch for more papers by this author Norman Born, Corresponding Author Norman Born Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, GermanyE-mail: norman.born@physik.uni-marburg.deSearch for more papers by this authorIbraheem Al-Naib, Ibraheem Al-Naib Department of Physics, Engineering Physics and Astronomy, Queen's University, Kingston, K7L 3N6 CanadaSearch for more papers by this authorChristian Jansen, Christian Jansen Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, GermanySearch for more papers by this authorRanjan Singh, Ranjan Singh Center for Disruptive Photonic Technologies, Nanyang Technological University, Singapore, 637371 SingaporeSearch for more papers by this authorJerome V. Moloney, Jerome V. Moloney College of Optical Sciences, University of Arizona, 1630 E University Boulevard, Tucson, AZ, 85721 USASearch for more papers by this authorMaik Scheller, Maik Scheller College of Optical Sciences, University of Arizona, 1630 E University Boulevard, Tucson, AZ, 85721 USASearch for more papers by this authorMartin Koch, Martin Koch Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, GermanySearch for more papers by this author First published: 21 January 2015 https://doi.org/10.1002/adom.201400469Citations: 28Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract A novel conceptual design methodology for strongly interacting metamaterials allows an ultrahigh angular sensitivity. Among others, this methodology enables precise sensing of the wave vector or the exclusion of unwanted directional radiation components. By diligently tailoring the dimensions of the primitive unit cell, an orders of magnitude enhanced sensitivity is achieved. Citing Literature Volume3, Issue5May 2015Pages 642-645 RelatedInformation

A broadband terahertz (THz) reflection imaging system with high spatial resolution over a large depth of field is reported. A THz axicon lens producing a truncated THz Bessel beam with a linear focus exceeding 120 mm in length, and with the diameter of the central lobe less than 2 mm was used for imaging in a reflection geometry employing a pulsed THz time-domain spectrometer. With numerical post-processing, it was possible to reconstruct the three dimensional shape of the scanned object.

A metasurface with an ultrahigh angular sensitivity to the angle of incidence at a chosen frequency is presented. Strongly interacting metamolecules are arranged such that a novel functionality of a metamaterial structure with multiple Fano resonances arises. Tuning the lattice constant of the structure enables a precise control of the angular response. We experimentally demonstrate equal responses for changes in azimuthal and for altitude angles of incidence. The concept could be used to enhance angle of arrival measurements, to determine the direction of a propagating wave or enhance the sensitivity of detectors by shielding them from scattered or unwanted signals.

Due to their high spatial resolution, THz imaging systems (based on time domain spectrometers) have a broad variety of applications in scientific as well as industrial applications in the future—especially through the upcoming of reliable titanium sapphire lasers and fiber amplified femtosecond lasers. With such THz scanning highly sensitive measurements of materials as well as physical phenomena and simultaneously real-time imaging with real-time data extraction can be realized which can be used for production monitoring and/or more accurate and novel inline control systems. The article gives an overview on recent state of the art THz systems based on femtosecond lasers and their application scenarios, like detection of foreign bodies in foods, inline inspection of paper layer thickness and area mass as well as non-destroying impact or tensile tests or fiber orientation checks in Compound-Polymers.

While Vertical-External-Cavity-Surface-Emitting-Lasers (VECSELs) have been successfully used as ultrafast laser sources with pulse durations in the hundreds of femtosecond regime, the dynamics within the semiconductor gain structure are not yet completely understood. With the high carrier densities inside the semiconductor, nonequilibrium effects such as kinetic-hole burning are expected to play a major role in pulse formation dynamics. Moreover, the nonlinear phase change by the intense light field can induce a complex dispersion, which may potentially limit the achievable pulse durations. To shed light on such nonequilibrium dynamics, we perform in-situ characterization of mode-locked VECSELs. We probe the gain media as well as the intracavity absorber with a femtosecond fiber laser source. For measuring temporal characteristics, we employ an asynchronous optical sampling technique by phase-locking the repetition rate of the VECSEL to a multiple of the probe laser with an adjustable offset frequency. This allows for probing dynamics from femtosecond to nanosecond time scales with scan rates up to hundreds of Hertz without compromise of measurement precision which can be introduced by mechanical delays covering such large temporal windows. With a resolution in the femtosecond range, we characterize gain depletion by the intracavity pulse as well as the gain recovery timescales for different power levels and operation regimes.

The nitrogen laser is probably the simplest laser scheme that can be implemented by amateurs [1]. Using normal air and a high-voltage pulse from a homemade electrode system, one can generate a laserlike emission, called “superfluorescence,” without the need for a carefully aligned cavity or any doped crystals or glasses. Strictly speaking, the superfluorescent radiation in air is not coherent and does not formally qualify as lasing, but the emission is directed and amplified, so this “air lasing” effect has been considered for remote atmospheric sensing and light-based radar (called LIDAR). In these applications, a laser pulse—in place of a high-voltage pulse—would generate the superfluorescence in the upper atmosphere or at a distant target. However, optically induced air lasing has proven to be much more difficult than the electrically based nitrogen laser. Years of research and the use of sophisticated laser-pump schemes have not produced enough air lasing for practical purposes. But now Alexandre Laurain and colleagues at the College of Optical Sciences, Arizona, have developed a novel and substantially more efficient way to obtain remote lasing in air [2]. Their technique is based on two steps: an infrared laser pulse first dissociates air molecules, and then an ultraviolet pulse excites the resulting atoms to energy levels that emit fluorescent radiation. This two-color approach leads to higher lasing output and can work with substantially lower input power than previous techniques using only a UV pulse.

We present a concept for all optical Terahertz (THz) amplitude modulators based on a Fabry-Pérot (FP) filter design. By trapping the THz wave inside a cavity, an enhanced modulation can be achieved. The easy-to-handle and easy-to-fabricate design renders this concept very auspicious.

We present a systematic investigation of the longitudinal multi-mode emission in a vertical-external-cavity surface-emitting laser using both streak camera measurements and interferometric measurement techniques. We observe a steep increase of the emission bandwidth close to the laser threshold with only minor variances at higher pump powers. Additionally, we show that in our configurations the stability of a two-color emission process is linked to a sufficiently high number of longitudinal modes participating in the laser emission.