A streak camera is disclosed which includes an entrance slit forming a first end of the streak camera, the entrance slit having a width and height, an image capture device forming a second send of the streak camera, and an epsilon-near-zero (ENZ) slab defined by a bandgap energy and plasma frequency disposed within the streak camera, the ENZ slab configured to receive two beams with a predetermined delay therebetween, the two beams include i) a witness beam through the entrance slit at an incident angle θ, and ii) a trigger beam at substantially a normal incident angle, wherein the trigger beam applies energy at a level below the bandgap energy of the ENZ slab to thereby modify refractive index of the ENZ slab, and thus generate a modified witness beam at a time-varying exit angle φ(t) onto the image capture device.

TECHNICAL FIELD

The present disclosure is generally related to time variant refraction, and in particular to a streak camera utilizing an epsilon-near-zero (ENZ) element.

BACKGROUND

Time-variant media in which material properties are not stationary in time, is a relatively new field in optical sciences that is gathering some interest as of late. Of particular interest, is exploration of time-reflection, time-refraction, and temporal steering to the temporal equivalent of the Brewster angle, temporal beamsplitters and light amplification based of photonic time crystals.

EPSILON-NEAR-ZERO (ENZ) materials host a region of wavelengths where the permittivity ε (permittivity describes the propagation of light in a medium) goes to zero resulting in extreme optical phenomenon such as enhanced nonlinearities and diverging phase velocities. Recently, these materials have also been shown to exhibit substantial modulations of optical properties under optical and electric excitation. Furthermore, these modulations can have ultrafast rise times on the order of 10 s of femtoseconds making them the ideal platform to study time-refraction and time-reflection.

Additionally, and more related to the present disclosure, streak cameras have been utilized for a long time to assist physicists and the like to investigate system dynamics, and particularly very fast dynamics, i.e., less than 10 nanoseconds, produced by lasers and the like. Streak cameras have been available for a long time, with one of the early such references dating back to early 1970s (see e.g., U.S. Pat. No. 3,586,260 to Looney et al., published Jun. 22, 1971). However, the modern streak cameras are much more sophisticated. A conventional streak camera system includes a streak tube having a photocathode with a slit formed thereon, on which a lens projects an image of an incident radiation pulse. The photocathode converts a portion of the incident radiation (photons) into electrons which are accelerated by a high voltage electrostatic field to form a beam of electrons. This beam is passed through a deflection field which is synchronized with the incident radiation pulse to rapidly sweep the electric field between the plates inducing a time-dependent directional change in the electron beam. For example, if the field is swept from low-to-high, electrons passing through the plates at the beginning of the electric field sweep are deflected less than those arriving at the plates later. This sweeping effect in turn affects the intensity of the electron beam prior to impacting a screen (e.g., a phosphor screen) to thereby generate an image which can be captured by a camera for later analysis.

Referring to U.S. Pat. Pub. 20200098817 for Opachich et al.,FIG.1illustrates a generalized diagram of a prior art streak camera. The ′817publication provides: As shown an energy source directs energy108to a slit112. The energy108may be photons. In some embodiments the camera may be angled such that the photons strike an inner wall or block. The slit plate112allows only a select portion of the photons to pass and may be aligned with an active region of a photocathode113, located directly behind the slit plate and also aligned with other components of the streak camera. Exiting the slit plate112are electrons114which are created by incident radiation upon the photocathode113. The photocathode113emits electrons114that pass between two charged plates116and through a focusing lens120(either magnetic or electro static), which focuses, directs or sweeps the electrons toward and across imaging detector124, such as a phosphor plate causing the imaging detector to illuminate. This illumination can be viewed and recorded with traditional optical recording devices.

Since in general streak cameras are used for fast-changing optical events, increasing time-sensitivity is of paramount importance. However, even with traditional streak cameras, there are many costly and sensitive elements which makes their deployment challenging, particularly when seeking to capture ever-faster optical events. In particular, in a traditional streak camera, since photons are converted to electrons with the use of a photocathode (see photocathode113inFIG.1) electronic components needed in a traditional streak camera, can become a bottleneck for higher speed streak cameras.

Therefore, there is an unmet need for a novel approach in streak cameras with high levels of time sensitivity in the range of femtoseconds.

SUMMARY

A method of making a photonic time-varying medium is disclosed. The method includes depositing an epsilon-near-zero (ENZ) target material in a deposition chamber having a pressure, thereby causing ablation followed by accumulation in form of a film directly or indirectly onto a substrate until a predetermined thickness of the ENZ target material accumulates on the substrate. The thickness ranges from about 10 nanometer to about 50 μm, wherein the pressure of the deposition chambers is between about 1×10−12to about 1×10−2torr, and wherein the ablation occurs at a temperature of between about −20° C. and about 900° C. within the deposition chamber.

A streak camera is also disclosed. The streak camera includes an entrance slit forming a first end of the streak camera, the entrance slit having a width and height. The streak camera also includes an image capture device forming a second end of the streak camera. Additionally, the streak camera includes an epsilon-near-zero (ENZ) slab defined by a bandgap energy and plasma frequency disposed within the streak camera. The ENZ slab is configured to receive two beams with a predetermined delay therebetween. The two beams include i) a witness beam through the entrance slit at an incident angle θ, and ii) a trigger beam at substantially a normal incident angle. The trigger beam applies energy at a level below the bandgap energy of the ENZ slab to thereby modify refractive index of the ENZ slab, and thus modify the witness beam resulting in a time-varying exit angle φ(t) onto the image capture device.

DETAILED DESCRIPTION

A novel approach in streak cameras is disclosed herein with high levels of time sensitivity in the range of femtoseconds. Referring toFIG.2, a schematic of a novel streak camera is provided, according to the present disclosure. The streak camera shown inFIG.2, includes a source of an optical beam, referred to herein as the trigger beam. The streak camera shown inFIG.2also includes a source of an optical beam to be analyzed, hereinafter referred to as the witness beam. The witness beam enters the streak camera through an entrance slit formed on a plate. The streak camera shown inFIG.2also includes an epsilon-near-zero (ENZ) slab defined by a bandgap energy and plasma frequency, which is disposed in the path of both the witness beam and the trigger beam. The witness beam once passed through the entrance slit is incident on the ENZ slab at an incident angle θ, while the trigger beam is incident on the ENZ slab at substantially a normal incident angle. As indicated above, the incident witness beam arrives at the ENZ slab at an incident angle of θ, while the outgoing witness beam from the ENZ slab is at an output angle of φ(t) which is a time-varying output angle. Additionally, the streak camera of the present disclosure shown inFIG.2includes an image capture device denoted as the detector, e.g., a camera, which is adapted to capture the outgoing witness beam out of the ENZ slab in the form of an image.

To study the effect of the ENZ slab providing a medium with a time-varying refraction coefficient, a model was generated. The model is initially based on a classical experiment in optics, depicted inFIGS.3a-3bwhich provide a schematic of said experiment. According toFIG.3a, two antennae are placed a distance L from each other and made to emit a pulsed signal simultaneously. An observer in the same reference frame measures the time delay T between the two pulses. For a time-invariant medium, the result is simply:

where n0is the refractive index, and
c is the speed of light in vacuum. If the refractive index changes precisely at the arrival of the first pulse to a value of n1, the travel time becomes T=n1L/c. Extending further we allow the refractive index to change as the pulse travels a small step Δl and take the limit of infinitely many steps we find distance L expressed as:

From Equation (1) we can see that the time for the two pulses to arrive is related to the distance traveled and the specific function of refractive index. If instead of two pulses, we consider a wave traveling, it is easy to see that although the wavelength does not change, the frequency is modified by the changing medium leading to the more established equation n1ω1=n2ω2found in the literature. Although these two understandings are equivalent in the one-dimensional case, Eqn. (1) can be expanded to more dimensions.

For example, inFIG.1bwe depict an optical ray impinging at angle θ onto a time-varying slab of thickness δ with a time dependent refractive index n(t). If we consider a monotonically increasing refractive index increasing from n1to n2(right side), the angle of refraction α decreases during the interaction and consequently decreases the distance traveled L as shown inFIG.1c. This distance also depends on the initial angle and the refractive index of the surrounding environment nenv. The simple n1ω1=n2ω2model does not adequately describe this situation as it only predicts the redshift and not the blueshift.

With the model of the present disclosure, we investigate the exiting angle φ's dependance on the experimental parameters. We start with a simple model which only considers incident and exiting refractive index values. In a homogeneous media the momentum is conserved due to symmetry. A spatial interface breaks this symmetry in one direction but preserves in along the interface. In two dimensions x and y, with the interfaces along constants y=θ and y=δ, the momentum in the x direction is conserved. Therefore, we can write:

where − and + superscripts correspond to immediately above and below the y values where in the second lines of (2) and (3) we have used the relation of momentum and energy k0=n(t)ω0with the incident and exiting angles being θ and φ, respectively. We then rewrite this equation by scaling the refractive index at t1and t2in terms of their averagenand difference δn i.e., n(t1)=n+δn and n(t2)=n−δn. Notice that for δn>0 the refractive index change is from high-to-low and reversed for δn<0. The resulting equation is

The result of Equation (5) subtracted from the incident angle and is plotted inFIG.4(a graph of incident angle θ vs. δn/n) for different incident angles and values of δn/n. For large enough transitions in the low-to-high regime a critical angle condition occurs regardless of the absolute value of n and at the second interface. This critical angle only occurs for the time-varying case and we will refer to it as the time-varying critical angle. Along the time-varying critical angle curve, the exiting angle has the largest sensitivity to changes in refractive index or changes of incident angle: a fact exploited below for angular streaking.

Along the time-varying critical angle, the gradient of angular difference is evident from the quick change from green to white. To illustrate this, consider different temporal slices of a pulse incident on a time-varying slab. Each temporal slice will experience a different amount of refractive index change leading to different exiting angles. The difference between exiting angles of subsequent temporal slices will be largest near the time-varying critical angle. Importantly, the ultrafast rise times experimentally demonstrated coupled with the large angular difference implies an ultrafast angular streaking potential. There is also a significant angular change at higher incident angles and large positive changes in refractive index, but this region of experimental parameters space is less appealing because of the near-glancing angles of incidence and the significantly reduced gradient along the refractive index direction.

The simple model presented in Equation (5) can be improved upon by including equation (1). Consider a pulsed wave incident on the slab with the form

Ei⁢n(x,t)=A⁡(x,t)⁢e-i⁢ω0⁢t+ik0⁢sin⁢(θ)⁢x(6)where A(x, t) is the amplitude function,ω0is the carrier frequency related,k0is the wavevector, andx is the position along the slab-environment interface. Therefore, the exiting field is related to the incident field by the equation

In equation (7), the primed coordinates indicate coordinates at the exit plane of the time-varying media. To calculate the outgoing angle, we assume a functional form of the refractive index and use Snell's law, and equations (1), (6), (7) to calculate L(t) and T(t). These are then inverted to be functions of t′ resulting in an expression for the outgoing field

where Φ(t′) is the time dependent phase incurred by traversing the slab. Finally, we can determine the time-dependent exiting angle φ by determining the direction of constant phase fronts. This results in

where ω(t′)=ω0−dΦ/dt′ is the instantaneous frequency. This equation arises from the fact that transverse momentum kxis conserved and therefore the angular change is directly related to the instantaneous frequency shift. In the following section, we will employ this expression to explore the capability of temporal streaking.

We consider a pulsed plane wave witness beam with pulse duration pulse incident on a time-varying slab with thickness δ that changes from n1to n2as a sigmoid function with rise time τriseas shown inFIG.1b. The sigmoid is driven by the trigger beam where τriseis directly proportional to the pulse duration of the trigger beam. The relative delay between the center of the witness pulse and the refractive index change is tdelay. The streaking performance of this scheme is determined by the derivative of φ with respect to time averaged over the amplitude of the pulse. We perform multiple parameter sweeps to understand the effect of delay, rise time and sample thickness on the streaking performance. In all scans the initial parameters are τpulseis about 11 fs, τriseis about 10 fs, tdelay=−τpulse, n1=0.66, n2=1.41, and δ=5 μm unless otherwise stated. From Equation (5) andFIG.4, we set the optimal angle of incidence just below the time-varying critical angle θ=40°.

Referring toFIGS.5a-5d, output results are provided for numerical calculations of streaking performance. The streaking performance is characterized by the derivative of the exiting angle averaged over the probing pulse amplitude. In particular, referring toFIG.5a, which is a graph of change of exit angle (dφ/dt) vs. relative delay between probe pulse and refractive index change, we scan the relative delay between the index change from the trigger beam and the witness pulse. Two optimal delays are observed for the streaking centered nearly at ±τpulse. They are slightly shifted to later times due to the thickness of the sample and the pulse duration. Remarkably we see that at the maximum, the streaking is about 5 mrad/fs. To put that into context, the same angle is subtended by a 5 μm pixel—standard for many CCD and CMOS cameras—placed at 1 mm distance from the sample. Simply by increasing the camera distance by a factor of 10, the streaking performance reaches the attosecond regime.

Next, we examine the role of rise time on the effect of optical streaking. We set the delay to tdelay=−τpulsefor this scan. Referring toFIG.5b, the rise time is varied from 0.1 to 10 times the witness pulse duration. An almost threshold-like behavior of the streaking performance is observed at τrise=τpulse. Referring toFIG.5c, the optical streaking performance is plotted against the sample thickness δ. A similar threshold effect is visible near δ/c=τpulse, where c is the speed of light in vacuum. Increasing the sample thickness introduces resonant-like peaks in the streaking performance. As discussed above, the competition of path-length shortening in L(t) and longer transit times of T(t) can be thought of in this case as a temporal Fabry-Pérot resonance, known to a person having ordinary skill in the art. These resonances only appear for thick samples where large phases can be accumulated. Referring toFIG.5d, the trade-off for optical streaking performance between rise time and delay is plotted showing the evolution of the threshold effect and the temporal Fabry-Pérot resonances.

The above model was compared to standard Finite Difference Time Domain (FDTD) simulations. In the simulation, an 11 fs loosely focusing Gaussian laser pulse was incident on a slab at θ=40°. The refractive index was modulated from n1=0.66 to n2=1.41 over 10 fs and the delay was set to tdelay=−τpulse. After passing through the slab, the electric field was recorded as a function of time and transverse position as described in Equation (8). The results of theoretical calculations and numerical simulations are shown inFIGS.6a,6b,6c, and6d. InFIG.6athe theoretical and inFIG.6b, the frequency shift due to the time-varying refractive index is calculated theoretically and simulated against time as shown by curves identified as cone, coo. The pulse amplitude A(t) is shown in the curve identified as A(t) for reference. By this, one can understand the number of photons at a given frequency and a given time. Referring toFIGS.6cand6d, the optical streaking performance for both theory and simulation are shown in the curves identified as Angle. Again the curves identified as A(t) are the amplitudes which thereby allow the reader to understand the number of photons directed to a given angle in a given instant. The calculations and simulations are in excellent agreement. Most importantly, the ultrafast angular change across the pulse spans 20 degrees demonstrating capacity for attosecond angular streaking.

Those having ordinary skill in the art will recognize that numerous modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described. Other implementations may be possible including but not limited to additional optical imaging systems before and/or after the ENZ slab. Additionally, the use of a dimension that is orthogonal to streaking (e.g., coming out of the page inFIG.2) could serve auxiliary purposes such as spectroscopy (e.g., measuring a spectrum along that dimension). Finally, a general algorithm for feedback and optimization of the streaking camera performance can be implemented by varying parameters such as ENZ material, ENZ thickness, incidence angle of both the witness and trigger pulse, delay between witness and trigger pulse, trigger pulse intensity, and distance between ENZ slab and detector.